Depth gauge

Abstract

There is described a depth gauge comprising: a drill guide arranged to receive a drill bit; and a sensor assembly, the sensor assembly comprising a displacement sensor for determining a displacement of the drill bit during a drilling procedure.

Description

[0001] Depth gauge

[0002] Field of the invention

[0003] The present invention relates to a depth gauge, as well as methods of operating and manufacturing a depth gauge. In particular, the present invention relates to a depth gauge that is an attachment for a drill.

[0004] Background to the Disclosure

[0005] For injuries such as bone fractures, it can be desirable to affix structures to an injured person’s bones. For example, where a person has a bone fracture, it may be desirable to perform an open reduction internal fixation (ORIF) procedure in which a metal plate is affixed to two or more bone fragments in order to hold these fragments together while the bone heals.

[0006] With such a procedure, the metal plate is affixed to the bones using one or more screws. But, because the plate is being fixed to a bone of the person (and this bone is inside the person so is not visible to a surgeon affixing the plate), it can be difficult to determine an appropriate size for these screws.

[0007] Conventionally, a suitable screw size is determined using a manual depth gauge. To use such a gauge, a hole is first drilled through the bone using a drill; a measuring portion of the manual gauge that comprises a hook is passed through this hole and then retracted so that the main body of the depth gauge is located to a first side of the bone and the hook rests against a surface of the bone on the second side of the bone, and the length of the measuring portion is then determined.

[0008] Such a measurement method often leads to an incorrect screw size being selected, and this can lead to tendon pain, permanent nerve damage, or even death.

[0009] Therefore, there is desired an improved system for, and method of, determining screw sizes that are suitable for use with drilled holes.

[0010] Summary of the Disclosure

[0011] According to an aspect of the present disclosure, there is described: According to an aspect of the present disclosure, there is described: a depth gauge comprising: a drill guide arranged to receive a drill bit; and a sensor assembly, the sensor assembly comprising a displacement sensor for determining a displacement of the drill bit during a drilling procedure.

[0012] Preferably, the sensor assembly is removably attached to the drill guide.

[0013] Preferably, the depth gauge comprises a type sensor for determining a type of the drill guide.

[0014] Preferably, the depth gauge comprises a type sensor for determining one or more of: a type of the drill guide; a type of the drill bit; and a type of a plate being used during the procedure. Preferably, the type sensor and the displacement sensor are a provided as a single, combined, sensor.

[0015] Preferably, the depth gauge and / or the sensor assembly comprises a sensor attachment structure for attaching the drill guide to the sensor assembly. Preferably, the sensor attachment structure indicates a type of the drill guide.

[0016] Preferably, the drill guide comprises an identifier for indicating a type of the drill guide. Preferably, the sensor assembly comprises a type sensor for detecting the identifier.

[0017] Preferably, the sensor assembly is arranged to determine a depth of a hole formed by the drill bit based on the displacement of the drill bit. Preferably, the sensor assembly is arranged to determine a suitable screw size for the hole based on the depth of the hole.

[0018] Preferably, the sensor assembly is arranged to determine the screw size based on the type of the drill guide and / or the type of the drill bit.

[0019] According to another aspect of the present disclosure, there is described: a depth gauge comprising: a drill guide; and a sensor assembly, the sensor assembly comprising: a sensor for: determining a type of the drill guide; and determining a displacement of a drill bit during a drilling procedure; and a processor for: determining a depth of a hole formed by the drill bit based on the displacement of the drill bit; and determining, based on the depth of the hole formed, a screw size of a screw suitable for insertion into the hole, wherein the sensor assembly is arranged to determine the screw size based on the determined depth and the determined type of drill guide.

[0020] According to another aspect of the present disclosure, there is described: a depth gauge comprising: a sensor for: determining a displacement of a drill bit during a drilling procedure; and a processor for: determining a depth of a hole formed by the drill bit based on the displacement of the drill bit; and determining, preferably using the sensor, a type of a drill guide used during the drilling procedure; and determining, based on the depth of the hole formed, a screw size of a screw suitable for insertion into the hole, wherein the processor is arranged to determine the screw size based on the determined depth and the determined type of drill guide.

[0021] Preferably, the processor is arranged to determine the displacement of the drill bit based on a measurement structure of the drill bit.

[0022] Preferably, the measurement structure comprises one or more ridges and / or one or more grooves.

[0023] Preferably, the measurement structure comprises two or more ridges and / or grooves that extend axisymmetrically about the circumference of the drill bit.

[0024] Preferably, the measurement structure comprises an axisymmetric measurement structure, preferably wherein the measurement structure varies in a z-direction, the z-direction being a direction of a central axis of a drill bit.

[0025] Preferably, the sensor is arranged to determine the displacement based on a feature and / or a measurement structure of the drill bit that is encompassed by the depth gauge, preferably wherein the depth gauge is arranged to surround a portion of the drill bit and to determine the displacement of the drill bit based on a movement of a feature and / or measurement structure that is on the portion of the drill bit that is surrounded by the depth gauge.

[0026] Preferably, the sensor comprises a magnetic sensor.

[0027] Preferably, the sensor comprises a physical sensor.

[0028] Preferably, the sensor comprises a first coil and a second coil, where the sensor is arranged to create a magnetic field in the first coil and to measure a resultant current in the second coil, wherein the displacement is determined based on a change in current in the second coil.

[0029] Preferably, the drill guide comprises a type identifier. Preferably, the type identifier comprises an RFID chip and / or wherein the type identifier comprises one or more magnets.

[0030] Preferably, the type identifier comprises a physical structure. Preferably, the type identifier comprises one or more buttons arranged to be depressed when the drill guide is attached to the sensor assembly. Preferably, the sensor assembly is arranged to identify a calibration feature on the drill bit. Preferably, the calibration feature indicates a type and / or characteristic of the drill bit.

[0031] Preferably, the sensor assembly is arranged to identify one or more identifying features of the drill bit, wherein the identifying features indicate an axial position of the drill bit at the start of a drilling process. Preferably, the sensor assembly is arranged to identify an identifying feature that is adjacent the sensor at the start of the drilling process.

[0032] Preferably, the drill guide is reusable, preferably wherein the drill guide is composed of metal, more preferably wherein the drill guide comprises stainless steel.

[0033] Preferably, the drill guide is arranged to be sterilized using an autoclave.

[0034] Preferably, the drill guide comprises replaceable parts, preferably wherein the drill guide comprises a replaceable handle.

[0035] Preferably, the sensor assembly is disposable, and / or single-use, preferably wherein the sensor assembly comprises plastic and / or wherein the sensor assembly is not hermetically sealed.

[0036] Preferably, the displacement is a displacement in a z direction, where the z direction is the direction of a central axis of the drill bit.

[0037] Preferably, the sensor determines a plurality of instances of temporal displacement data.

[0038] Preferably, the drill guide comprises a hole for the passage of a drill bit, preferably wherein the drill guide is arranged to: centre a tip of the drill bit; and / or lock into a plate adjacent a surface to be drilled by the drill bit.

[0039] Preferably, the drill guide comprises an attachment mechanism, e.g. a thread, that is arranged to cooperate with a plate in order to secure the drill guide to the plate.

[0040] Preferably, determining the screw comprises determining a size and / or a length of the screw.

[0041] Preferably, the determination of the screw is based on based on one or more of: a type of surgery associated with the hole; a width of the hole and / or a drill bit used to form the hole; and a characteristic of a patient on whom a surgery is being performed.

[0042] Preferably, the depth gauge is arranged to determine one or more of: an initiation of a drilling process; an end of a drilling process; and a material interface, preferably being arranged to determine when a tip of a drill bit has entered a bone and / or exited a bone.

[0043] Preferably, the depth gauge is arranged to determine the initiation and / or end of a drilling process and / or the material interface based on one or more of, preferably a plurality of: a displacement profile of the drill bit; a derivative of a displacement, preferably a jerk or a snap; and a combination of derivatives of the displacement.

[0044] Preferably, the depth gauge is arranged to set a zero value for the displacement of the drill bit at the initiation of a drilling process.

[0045] Preferably, the screw size is determined in dependence on a list of available screws, preferably wherein the list is stored in a memory of the depth gauge.

[0046] Preferably, at least a portion of the sensor assembly is arranged to sit between a drill and a drill bit, preferably wherein the depth gauge is arranged to attach the depth gauge to a chuck of the drill and to attach the drill bit to the drill via the depth gauge. Preferably, the depth gauge comprises a drill attachment structure for attaching the depth gauge to a drill.

[0047] Preferably, the drill attachment structure is arranged to attach the depth gauge to a chuck of the drill.

[0048] Preferably, the depth gauge comprises a bit attachment for attaching a drill bit to the depth gauge, preferably wherein the bit attachment is arranged to attach the drill bit to a chuck of a drill via the depth gauge.

[0049] Preferably, the depth gauge is arranged to determine a breakthrough event, preferably wherein determining the breakthrough event comprises: receiving displacement data relating to a displacement of a drill bit; determining one or more derivatives of the displacement data, preferably comprising determining one or more of: an acceleration, a jerk, and a snap; and determining a breakthrough event based on the one or more derivatives.

[0050] According to another aspect of the present disclosure, there is described: a computer device, preferably a depth gauge, arranged to determine a breakthrough event, the depth gauge being arranged to determine the breakthrough event by: receiving displacement data relating to a displacement of a drill bit; determining one or more derivatives of the displacement data, preferably comprising determining one or more of: an acceleration, a jerk, and a snap; and determining a breakthrough event based on the one or more derivatives.

[0051] Preferably, the depth gauge is arranged to determine the breakthrough event in dependence on each of: a displacement, a velocity, an acceleration, a jerk, and a snap at a time t.

[0052] Preferably, the depth gauge is arranged to filter and / or smooth the displacement data.

[0053] Preferably, the depth gauge is arranged to determine the breakthrough event based on a multiplication of a plurality of derivatives.

[0054] Preferably, the depth gauge is arranged to determine the breakthrough event based on functions f and and g(x',x",x"',x""') = nts.

[0055] Preferably, determining a breakthrough event based on the following conditions being satisfied: f(x',x",x"' ,x""') > 0; and f(x',x",x"',x""') > g(x',x" ,x"',x""').

[0056] Preferably, the depth gauge is arranged to determine the breakthrough event during a drilling process, preferably in real-time.

[0057] Preferably, the depth gauge is arranged to determine the breakthrough event based on a displacement x(t) at a time t being greater than a maximum displacement of the drill bit before the time t.

[0058] Preferably, the depth gauge is arranged to determine the breakthrough event based on a time t of the breakthrough event being a threshold amount of time after a previous (e.g. a first) plunge event of the drilling process.

[0059] Preferably, the depth gauge is arranged to determine a first plunge velocity of a previous (e.g. a first) plunge event during the drilling process and to determine the breakthrough event based on a velocity of the drill bit falling below this first plunge velocity between the first plunge event and the breakthrough event.

[0060] Preferably, the depth gauge is arranged to determine a plurality of plunge events, wherein the breakthrough event is determined as a last (e.g. a second) plunge event, preferably wherein: the conditions for detecting the first plunge event and the second plunge event are different; and / or the breakthrough event is determined as the plunge event with the highest associated displacement.

[0061] Preferably, the depth gauge is arranged to detect the first plunge event and, in response to the detection, to determine one or more of: a first plunge time; and a first plunge velocity.

[0062] Preferably, the depth gauge is arranged to set a lock flag following the detection of the first plunge event, wherein the depth gauge is arranged to clear the lock flag when a velocity of the drill falls below (e.g. a threshold amount below) the first plunge velocity and / orwhen the velocity of the drill falls below a predetermined threshold value (e.g. 30 mm / s).

[0063] Preferably, the depth gauge is arranged to identify that a plunge event is a breakthrough event in dependence on the plunge event being a last (e.g. a second) plunge event.

[0064] Preferably, the depth gauge is arranged to determine a hole depth and / or a screw size for a hole based on the displacement of the drill bit at the time of the breakthrough event.

[0065] According to another aspect of the present disclosure, there is described: drill guide arranged to receive a drill bit, the drill guide comprising a sensor attachment structure for attaching the drill guide to a sensor assembly, the sensor assembly comprising a displacement sensor for determining a displacement of the drill bit during a drilling procedure.

[0066] According to another aspect of the present disclosure, there is described: a sensor assembly for determining a displacement of a drill bit, the sensor assembly comprising: a displacement sensor for determining a displacement of the drill bit during a drilling procedure; and a sensor attachment structure for attaching the sensor assembly to a drill guide.

[0067] According to another aspect of the present disclosure, there is described: a drill bit comprising a measurement structure, the measurement structure arranged to be sensed by a sensor of a depth guide so as to indicate a displacement of the drill bit during a drilling process.

[0068] Preferably, the measurement structure comprises one or more ridges and / or one or more grooves.

[0069] Preferably, the measurement structure comprises two or more ridges and / or grooves that extend axisymmetrically about the circumference of the drill bit.

[0070] Preferably, the measurement structure comprises an axisymmetric measurement structure.

[0071] Preferably, the drill bit comprises a calibration feature that indicates a type and / or characteristic of the drill bit, preferably wherein the calibration feature comprises a characteristic ridge and / or characteristic groove and / or wherein the calibration feature comprises a characteristic pattern of ridges and / or grooves.

[0072] Preferably, drill bit comprises a proximal end and a distal end, the distal end being arranged to form a hole, wherein the measurement structure is located closer to the proximal end than the distal end. Preferably, the measurement structure is located at least 20% of the drill bit length from the distal end and / or at least 50% of the drill bit length from the distal end; and / or the measurement structure is located at least 5cm from the distal end and / or at least 10cm from the distal end.

[0073] Preferably, the measurement structure comprises a continuous slope.

[0074] Preferably, the drill bit comprising a bit identifier, preferably the bit identifier comprises an RFID chip.

[0075] Preferably, the drill bit comprises one or more identifying features. Preferably, the drill bit comprises a plurality of identifying features located at one or more axial positions of the drill bit. Preferably, each identifying feature comprises one or more of: a marking, a groove, a nodule, or a ridge.

[0076] Preferably, each identifying feature extends about the entire circumference of the drill bit

[0077] Preferably, the drill bit comprises a plurality of identifying features, wherein each identifying feature is arranged to indicate a drill guide and / or a plate being used in a drilling procedure.

[0078] According to another aspect of the present disclosure, there is described: a kit of parts comprising: the aforesaid drill guide; and the aforesaid sensor assembly. Preferably, the kit of parts comprises a plurality of drill guides, wherein: the drill guides are arranged for use with different drill bits; and / or the drill guides comprise different sensor attachment structures; and / or each drill guide comprises a type identifier, e.g. an RFID chip or a magnet, that indicates a type of the drill guide, wherein the plurality of drill guides comprises a plurality of different types of drill guides.

[0079] Preferably, the kit of parts comprises a first sensor assembly, and a plurality of drill guides, wherein each drill guide comprises a sensor attachment structure for attaching said drill guide to the first sensor assembly.

[0080] Preferably, the kit of parts comprises a plurality of disposable and / or single use sensor assemblies.

[0081] Preferably, the kit of parts comprises one or more of the aforesaid drill bits. Preferably, the kit of parts comprises a plurality of drill bits of different sizes.

[0082] Preferably, the kit of parts comprises a plurality of drill bits of different sizes; and / or a plurality of drill bits with different bit identifiers and / or calibration features; and / or a plurality of drill bits with different arrangements of identifying features.

[0083] According to another aspect of the present disclosure, there is described: a system comprising: the aforesaid depth gauge; and a drill bit, preferably the aforesaid drill bit.

[0084] Preferably, the system further comprises a drill.

[0085] According to another aspect of the present disclosure, there is described: a method of determining a breakthrough event, the method comprising: receiving displacement data relating to a displacement of a drill bit; determining one or more derivatives of the displacement data, preferably comprising determining one or more of: an acceleration, a jerk, and a snap; and determining a breakthrough event based on the one or more derivatives.

[0086] Preferably, the method comprises: determining a depth of a hole using the depth gauge; and identifying a screw size for use based on the depth of the hole.

[0087] According to another aspect of the present disclosure, there is described: a method of operating the aforesaid depth gauge.

[0088] According to another aspect of the present disclosure, there is described: a computer programme product comprising instructions arranged to execute the aforesaid method.

[0089] According to another aspect of the present disclosure, there is described: a processor-readable storage device carrying processor-executable instructions that, when executed by a processor of a machine, cause the machine to perform the aforesaid method.

[0090] According to an aspect of the present disclosure, there is described: a depth gauge comprising: an attachment structure for attaching the depth gauge to a drill; a sensor for: determining a distance of the depth gauge to a surface in front of the depth gauge; and determining a depth of a hole formed by the drill based on a change in the distance of the depth gauge to the surface that occurs during a drilling process; and an output for outputting a measure of the depth of the hole.

[0091] Preferably, the depth gauge comprises: a processor for determining, based on the distance, a length of a screw for insertion into the hole, wherein the output is arranged to output the length of the screw.

[0092] Preferably, the surface is a surface of a drill guide associated with the depth gauge.

[0093] Preferably, the processor is arranged to determine the distance to the surface by detecting a parameter of a tracking structure on the surface. Preferably, the tracking structure comprises one or more of: a tracking pattern; a receiver; or a transmitter.

[0094] Preferably, the processor is arranged to determine the distance to the surface by detecting a parameter of a tracking pattern on the surface. Preferably, the tracking pattern comprises a circle.

[0095] Preferably, the processor is arranged to perform an object recognition process on one or more image frames captured by the sensor in order to detect the tracking pattern in the image frames.

[0096] Preferably, the distance is determined based on a detected parameter of the tracking pattern in one of the image frames and / or a detected change in a parameter of the tracking pattern over a plurality of image frames.

[0097] Preferably, the distance is determined based on a size of the tracking pattern in the image frames. Preferably, the distance is determined based on a comparison between the size of the tracking pattern in the frame and an actual size of the tracking pattern.

[0098] Preferably, the distance is determined based on a characteristic of the sensor, preferably a focal length of the sensor (e.g. of a camera of the sensor).

[0099] Preferably, determining the screw comprises determining a size and / or a length of the screw.

[0100] Preferably, the depth gauge comprises determining the screw based on one or more of: a type of surgery associated with the hole; a width of the hole and / or a drill bit used to form the hole; and a characteristic of a patient on whom a surgery is being performed.

[0101] Preferably, the depth gauge (and / or a processor of the depth gauge) is arranged to determine one or more of: an initiation of a drilling process; an end of a drilling process; and a material interface, preferably being arranged to determine when a tip of a drill bit has entered a bone and / or exited a bone.

[0102] Preferably, the depth gauge (and / or a processor of the depth gauge) is arranged to determine the initiation and / or end of a drilling process and / or the material interface based on one or more of: a distance and / or velocity profile of the drill; an acceleration determined using an accelerometer of the depth gauge; and a change in a rate of a change of the distance.

[0103] Preferably, the depth gauge (and / or a processor of the depth gauge) is arranged to set a zero value for the distance at the initiation of a drilling process.

[0104] Preferably, the processor is arranged to determine a start of a drilling procedure and / or an entry point at which the drill bit enters a bone. Preferably, the processor is arranged to determine an end of a drilling procedure and / or an exit point at which the drill bit exits a bone. Preferably, wherein the depth of the hole is determined as the difference between the exit point and the entry point.

[0105] Preferably, the entry point is determined based on a decrease in a rate of change of the distance. Preferably, the exit point is determined based on an increase in a rate of change of the distance. Preferably, the screw size is determined in dependence on a list of available screws. Preferably, the list is stored in a memory of the depth gauge.

[0106] Preferably, the screw size is determined based on a thickness of a screw plate through which the screw is to be placed.

[0107] Preferably, the sensor is arranged to emit a first signal and a second signal, the second signal being different to the first signal, and the processor is arranged to determine the distance based on a difference in the time of receipt of the first signal and the second signal by a receiver.

[0108] Preferably, the processor is arranged to filter the image frames prior to the detection of the tracking pattern in the frames. Preferably, the processor is arranged to convert the image frames to RGB frames and / or to apply an edge detection process to the image frames.

[0109] Preferably, the attachment structure is arranged to releasably attach the depth gauge to the drill. Preferably, the attachment structure is arranged to be operated without tools (e.g. by hand).

[0110] Preferably, the attachment structure comprises a plurality of contact surfaces. Preferably, the contact surfaces are flexible and / or curved.

[0111] Preferably, the attachment structure comprises an adjustment mechanism for tightening or loosening the attachment structure and / or for changing a size of the depth gauge.

[0112] Preferably, the attachment structure comprises one or more hinges to enable the attachment structure to be attached to devices of different sizes.

[0113] Preferably, the attachment structure comprises a securing (or locking) structure for securing the attachment structure in an attached position.

[0114] Preferably, the attachment structure is arranged to attach the depth gauge to the drill between the drill and a drill bit of the drill. Preferably, the attachment structure comprises a first structure for attaching the depth gauge to the drill and a second structure for attaching a drill bit to the depth gauge.

[0115] Preferably, the sensor comprises a camera and / or an infrared camera.

[0116] Preferably, the sensor comprises a light emitting element.

[0117] Preferably, the depth gauge comprises an output. Preferably, the output comprises a display.

[0118] Preferably, the depth gauge comprises an input. Preferably, an operation of the input is arranged to cause zeroing of the depth gauge.

[0119] Preferably, the depth gauge comprises a communication interface for communicating with a separate computer device. Preferably, the processor is arranged to receive information about a drilling procedure and / or a patient from the separate computer device.

[0120] Preferably, the depth gauge is suitable for use in open reduction internal fixation (ORIF) procedures.

[0121] Preferably, the depth gauge comprises (or is) a drill attachment.

[0122] According to another aspect of the present disclosure, there is described a system comprising: the aforesaid depth gauge; and a drill guide, the drill guide comprising a tracking structure and / or a tracking pattern.

[0123] Preferably, the drill guide comprises a hole for the passage of a drill bit. Preferably, the tracking pattern is arranged about the hole. Preferably, the system further comprises a drill.

[0124] Preferably, the system further comprises a screw plate, the screw plate being arranged to be attached to a bone, and the screw plate comprising one or more holes for screws.

[0125] Preferably, the system further comprises a plurality of screws, preferably a plurality of screws of different sizes.

[0126] According to another aspect of the present disclosure, there is described a kit of parts comprising: the aforesaid depth gauge; and a drill guide, the drill guide comprising a tracking pattern.

[0127] Preferably, the drill guide comprises a hole for the passage of a drill bit. Preferably, the tracking pattern is arranged about the hole.

[0128] Preferably, the kit of parts further comprises a drill.

[0129] Preferably, the kit of parts further comprises a plurality of screws, preferably a plurality of screws of different sizes.

[0130] Preferably, the kit of parts further comprises a screw plate, the screw plate being arranged to be attached to a bone, and the screw plate comprising one or more holes for screws.

[0131] According to another aspect of the present disclosure, there is described a drill guide comprising a tracking pattern, the tracking pattern being suitable for detection by a sensor of a depth gauge associated with the drill guide.

[0132] Preferably, the drill guide comprises a hole for the passage of a drill bit. Preferably, the drill guide comprises a plurality of holes for the passage of a drill bit.

[0133] Preferably, the drill guide comprises a plurality of tracking patterns.

[0134] Preferably, the tracking pattern is arranged about a hole of the drill guide.

[0135] Preferably, the tracking pattern comprises a plurality of holes. Preferably, each hole is associated with a respective tracking pattern. Preferably, each tracking pattern is different and / or each tracking pattern is arranged to identify a corresponding hole.

[0136] According to another aspect of the present disclosure, there is described a drill comprising the aforesaid depth gauge.

[0137] According to another aspect of the present disclosure, there is described a method of operating the aforesaid depth gauge, the method comprising: determining a depth of a hole using the depth gauge; and identifying a screw size for use based on the depth of the hole.

[0138] Preferably, the method comprises providing a zeroing input to the depth gauge. Preferably, the method comprises providing the zeroing input when the drill bit is placed in contact with a surface to be drilled.

[0139] Preferably, the method comprises placing a drill guide against a surface to be drilled, the drill guide comprising a tracking pattern.

[0140] Preferably, the method comprises placing a drill bit of the drill through a hole of the drill guide in order to drill through the surface.

[0141] Any feature described as being carried out by an apparatus, an application, and a device may be carried out by any of an apparatus, an application, or a device. Where multiple apparatuses are described, each apparatus may be located on a single device. Any feature in one aspect of the disclosure may be applied to other aspects of the invention, in any appropriate combination. In particular, method aspects may be applied to apparatus aspects, and vice versa.

[0142] Furthermore, features implemented in hardware may be implemented in software, and vice versa. Any reference to software and hardware features herein should be construed accordingly.

[0143] Any apparatus feature as described herein may also be provided as a method feature, and vice versa. As used herein, means plus function features may be expressed alternatively in terms of their corresponding structure, such as a suitably programmed processor and associated memory.

[0144] It should also be appreciated that particular combinations of the various features described and defined in any aspects of the disclosure can be implemented and / or supplied and / or used independently.

[0145] The disclosure extends to methods and / or apparatus substantially as herein described with reference to the accompanying drawings.

[0146] The disclosure will now be described, by way of example, with reference to the accompanying drawings.

[0147] Description of the Drawings

[0148] Figures 1a and 1 b show a drill being used to perform an open reduction internal fixation (ORIF) procedure.

[0149] Figure 2 shows a depth gauge attached to a drill for use during an ORIF procedure.

[0150] Figures 3a, 3b, and 3c show detailed views of an embodiment of the depth gauge.

[0151] Figure 4 shows a method of operating the depth gauge.

[0152] Figure 5 shows a graph of displacement during time for a drill bit during a drilling procedure.

[0153] Figure 6 shows a drill guide for use with the depth gauge.

[0154] Figure 7 illustrates a method of determining a distance of a depth gauge from a tracking pattern.

[0155] Figures 8a, 8b, 8c, 8d, and 8e show a further embodiment of a depth gauge.

[0156] Figure 9a shows a system comprising the depth gauge and a drill bit.

[0157] Figures 10a, 10b, and 10c illustrate features of the depth gauge and features of a drill bit that may be used with the depth gauge.

[0158] Figure 11 illustrates the use of the depth gauge.

[0159] Figures 12a, 12b, 12c, and 12d show a method of determining a breakthrough event.

[0160] Figure 13 shows an exemplary graph of displacement of a drill bit against time.

[0161] Description of the preferred embodiments

[0162] Referring to Figure 1 a, there is shown a drill 1 being used to perform an open reduction internal fixation (ORIF) procedure.

[0163] In order to perform this procedure, an incision 2 is made in the skin of a user and a screw plate is placed through the incision to lie across a first bone section 3-1 and a second bone section 6-2. Holes are then drilled through the first bone section and the second bone section using the drill 1 , and screws are then placed through the screw plate 4 and through these holes in order to affix the screw plate to each of the first bone section and the second bone section.

[0164] In this way, the screw plate 4 holds the first bone section 3-1 and the second bone section 3-2 in a desired alignment during the healing of the bone.

[0165] A substantial issue with such ORIF procedures, and indeed surgical procedures more generally, is the selection of suitable screws. X-rays may be taken prior to an operation in order to obtain a rough idea of suitably sized screws, but the most suitable screw size will depend on the exact location and dimensions of the screw plate 4 and of the hole that is drilled through the bone.

[0166] Therefore, referring to Figure 1 b and as has been described in the background section above, a surgeon may use a manual depth gauge 5 that is inserted through a drilled hole and then pulled against a rear surface of the bone in order to provide the surgeon with a better idea of the depth of the hole in the bone. This depth can then be used to select a screw size for each hole drilled by the surgeon.

[0167] Problematically, manual depth gauges can be inaccurate (e.g. a gauge of the manual depth gauge may only allow measurement to the nearest millimeter) and these gauges can be easy to mis-align so that incorrectly sized screws are often selected following a measurement taken with a manual depth gauge. This incorrect selection of a screw can lead to substantial problems, such as permanent nerve damage. Furthermore, correct use of a manual depth gauge is a time-consuming task.

[0168] Therefore, as shown in Figure 2, the present disclosure describes a depth gauge for use with a drill, which depth gauge can be used to determine a depth of a hole and / or screw size to place through a hole. This depth gauge is arranged (e.g. configured) to be attached to the drill 1 - and so the depth gauge may be considered to be a drill attachment and / or a drill accessory. Typically, the depth gauge is arranged to determine a suitable screw size for each hole formed using the drill.

[0169] Typically, the depth gauge 10 is arranged to be retrofitted to a drill 1 , so that the depth gauge can be used with existing devices.

[0170] Figure 2 shows a drill that comprises such a depth gauge 10.

[0171] The depth gauge 10 is arranged to determine a depth of a hole that has been formed using the drill 1 and, typically, to determine a suitable screw for insertion into this hole based on the determined depth. Figure 2 also shows a drill guide 50 that may (optionally) be used with the depth gauge in order to obtain a precise measurement of the depth. This drill guide is described further below with reference to Figure 6.

[0172] Referring to Figure 3a, the depth gauge 10 comprises: an attachment structure 11 that is arranged to removably attach the depth gauge onto a drill; an output 12 for providing information to a user of the depth gauge; and an input 13, which input is typically arranged to provide a zeroing function for the depth gauge.

[0173] The output 12 typically comprises a display 14. Equally, the output may comprise a speaker, a haptic feedback system, and / or a communication interface (which outputs a measurement and / or a screw size to a separate computer device). The input 13 typically comprises a button and / or a dial.

[0174] More generally, it will be appreciated that numerous different types of inputs and outputs are useable to enable a user to interact with the depth gauge 10. Furthermore, the input and the output may be combined into a single interface component, such as a touchscreen. This attachment structure 11 enables the depth gauge to be retrofitted onto existing drills and / or to be used for multiple different drills. An embodiment of such an attachment structure is shown in Figure 3b, which shows an attachment structure that comprises: a plurality of contact surfaces 21 that are arranged to rest on the drill 1 ; an adjustment mechanism 22 (e.g. a width adjustment knob) for moving the contact surfaces so as to tighten or loosen the attachment structure and / or to change a dimension or size of the attachment structure; one or more surface hinges 23 about which the contact surfaces can rotate; and a central hinge 24.

[0175] Typically, the attachment structure 1 1 comprises a plurality of contact surfaces 21 , where the adjustment mechanism 22 is arranged to alter a distance between these attachment surfaces based on a user input. More specifically, by operating the adjustment mechanism a user is able to tighten the attachment structure so as to move these contact surfaces together and to attach the depth gauge to the drill 1 (where the depth gauge is then held onto the drill by the friction between the contact surfaces and the drill).

[0176] The contact surfaces 21 may comprises flexible surfaces and / or roughened surfaces to provide a tight, high-friction, fit of the attachment structure 11 while avoiding damaging the drill 1. Furthermore, the contact surfaces may comprise grooved and / or curved surfaces to ensure that the attachment structure can fit tightly onto a cylindrical surface of a drill.

[0177] The surface hinges 23 and the central hinge 24 enable the arrangement of the contact surfaces 21 to be altered so as to enable the depth gauge 10 to be attached to drills of different shapes and sizes. More generally, the attachment structure is typically reconfigurable to be useable with drills of different sizes. The attachment structure 11 may comprise a plurality of hinges to improve the versatility of the depth gauge and ensure that the depth gauge can be reliably and securely fixed to a variety of drills (or, more generally a variety of devices).

[0178] It will be appreciated that the embodiment shown by Figure 3b is simply an embodiment of the attachment structure 11 and that various other attachment structures are possible. For example, the attachment structure may comprise one or more clips or screws, which may be arranged to fit into recesses in the drill 1 so as to ensure the depth gauge does not move relative to the drill. Equally, the attachment structure may comprise an adhesive means and / or a mechanical fastener. In some embodiments, the attachment structure comprises an electrical linkage that connects a processor of the depth gauge to a processor of the drill. This enables the depth gauge to receive information from, and transmit information to, the drill (e.g. to receive information about the drilling speed of the drill).

[0179] Typically, the attachment structure 11 is arranged to removably attach the depth gauge 10 to a drill such that the depth gauge may be attached and detached without the use of tools. For example, a user may able to operate the adjustment mechanism 22 by hand. This provides a depth gauge that can be readily attached and detached so as to be useable with multiple different drills.

[0180] The attachment structure 11 may comprise one of, or both of, a temporary attachment structure and a permanent attachment structure. The temporary attachment structure typically enables the depth gauge 10 to be attached and removed by hand (e.g. as described above) and is suitable where the depth gauge is only intended to be attached to the drill 1 for a short period of time or for a limited number of surgeries (e.g. as may be the case if a single depth gauge is being moved between a plurality of drills). The permanent attachment structure typically requires the use of tools to attach and remove the depth gauge and may, for example, comprise holes for screws or bolts, or clips that can be secured in an attached position. The permanent attachment structure can be used to securely attach the depth gauge to a drill for a prolonged period of time and may be used where the depth gauge is intended to be attached to the drill for numerous surgeries.

[0181] It will be appreciated that various embodiments of the adjustment mechanism 22 are possible, e.g. a shaped nut may be used that can be rotated along a screw to tighten and / or loosen the contact surfaces 21 . Equally, a cam based system or a lever based system may be used to enable a user to tighten and / or loosen the attachment surfaces. The adjustment mechanism may comprise a securing or locking structure (e.g. a pin) where operation of the securing structure fixes the attachment structure 11 in an attached position.

[0182] Referring to Figure 3c, the depth gauge 10 further comprises a sensor, which sensor typically comprises an infrared sensor and / or a camera. The sensor is arranged to determine a distance between the depth gauge 10 (and thus the drill 1) and a surface in front of the depth gauge (such as the skin of a patient on whom the drill is being used or a surface of the drill guide 50 shown in Figure).

[0183] The sensor may, for example, comprise: an infrared camera 31 (e.g. a camera without an infrared filter); an infrared filter 32; a light emitting element 33 (e.g. an infrared LED), and a charging point 34. With this sensor, the light emitted by the light emitting element reflects off a surface in front of the depth gauge and this reflected light is detected by the infrared camera. A detected time between the transmission and receipt of the infrared beam can then be used to determine a distance of the surface from the depth gauge 10.

[0184] Equally, the sensor may comprise a camera with an infrared filter and the light emitting element may comprise a bright LED where the distance is then determined based on visible light reflected from a surface.

[0185] It will be appreciated that numerous other types of sensor are useable, e.g. the sensor may comprise a mechanical sensor in which a protrusion is pressed against this surface and this protrusion is compressed between the depth gauge and the surface as the drill moves forwards.

[0186] Furthermore, as is described further below, the distance may be detected by detecting a size of a feature of a surface in an image. For example, a change in the size of a pattern on this surface between successive images captured by the camera may be used to determine a change in the distance between the surface and the camera. Such embodiments may reduce the processing complexity as compared to time-of-flight embodiments and so reduce the time / hardware necessary to obtain an accurate distance measurement.

[0187] Further to the components shown in Figures 3a - 3c, the depth gauge typically comprises a computer device and / or a processor, which processor is arranged to receive information from, and to control, the output 12, the input 13 and the sensor. The processor is typically arranged to determine a depth of a hole based on readings from the sensor and to determine a suitable screw size based on this depth. The screw size can then be presented to a user using the output.

[0188] The depth gauge may further comprise a communication interface for communicating with a further computer device, where this may enable the depth gauge to receive relevant information (e.g. images taken by other computer devices) in order to improve the determination of depth and / or where this may enable the depth gauge to provide information to other computer devices (e.g. to save a record of the suggested screw size).

[0189] The user interface (e.g. the input) and / or the communication interface may be arranged to receive information relating to a patient and / or a procedure, where the determination of the screw size may depend on this information. For example, the depth gauge may be arranged to determine information relating to: a drill bit being used, a width of the drill bit, a surgical procedure being performed, a characteristic of the patient (e.g. an age or a size of the patient), a desired type or material of a screw, etc.). This information may, for example, be selected by a user via the input, where the depth gauge may be arranged to request the information prior to the start of a drilling procedure. For example, the depth gauge may provide, via the output 13, a series of lists of options, where the user is able to select one or more options from each list to provide the information (e.g. the output may present a list of possible surgical procedures, and then a list of drill bit types that may be used for these procedures).

[0190] Referring to Figure 4, an exemplary operation of the depth gauge 10 is described.

[0191] In a first step 101 , the depth gauge 10 is attached to the drill 1. This typically comprises placing the depth gauge around the drill and then tightening the attachment structure 11 using the adjustment mechanism 22 so as to secure the depth gauge on the drill.

[0192] In a second step 102, the user of the drill 1 (e.g. a surgeon) places the drill bit against the bone which is to be drilled and in a third step 103 the user zeroes the depth gauge 10. It will be appreciated that the zeroing of the depth gauge is for convenience and so as to show the depth of the hole (once it has been drilled) as a single number. Equally, this depth could be determined as a difference between a starting (non-zero) value before drilling begins and a final value after drilling ends.

[0193] The zeroing of the drill 1 may involve a user operating the input 13 of the drill. Equally, the zeroing may be performed automatically, where a processor of the depth gauge may determine when drilling has begun. The depth gauge 10 may determine a point of initiation of drilling based on a movement of the drill bit and / or based on a sensor reading. For example, the depth gauge may determine the point of initiation of drilling based on a vibration that is detected by a vibration sensor, an acceleration detected by an accelerometer, or a sound that is detected by an acoustic sensor (where the sound of drilling bone typically differs from the sound of the drill bit rotating in air).

[0194] In some embodiments, the depth gauge is arranged to determine the point of initiation of drilling (and thus the ‘zero point’) based on a rate of change of a distance measured by the depth gauge. In this regard, a user of the drill will typically move the drill towards a bone to be drilled rapidly to place the drill bit in the rough vicinity of the bone, then will slow the movement of the drill to a near-zero velocity to place the drill bit at a precise location on the bone, and then drill into the bone at a consistent slow velocity. The initiation of a drilling process, and / or a zero point of a drilling process, may be determined based on this expected velocity profile (and / or an associated acceleration profile) so as to automatically zero the depth gauge at the onset of drilling.

[0195] In a fourth step 104, the user of the drill 1 drills a hole through the bone and then, in a fifth step 105, once the drilling of the hole has been completed, the depth gauge 10 determines a depth of the hole. Determining a depth of the hole typically comprises determining an amount of movement of the drill during the drilling of the hole. As described above the depth gauge comprises a sensorthat is arranged to determine a distance of the depth gauge from a surface in front of the drill bit. By determining a change in the distance of the depth gauge from a surface that occurs during a surgical procedure, the depth gauge is able to measure the depth of a hole that is formed during this procedure. This depth may then be used to select a suitable screw for inserting into the hole.

[0196] Determining the depth of the hole may involve determining an end of a drilling process (e.g. when an end of the drill bit exits the bone) and then determining a sensor reading at this point. More generally, determining the depth of the hole may involve determining a distance between an entry point of the drill bit (the point at which the tip of the drill bit enters a near side of a bone) and an exit point of the drill bit (the point at which the tip of the drill bit exist the far side of the bone).

[0197] Finally, in a sixth step 106, the depth gauge determines, based on the depth of the hole, a suitable screw size and the depth gauge outputs this screw size using the output 12 displays the screw size.

[0198] Regarding the determination of a suitable screw size, the depth gauge may contain (or may communicate with a separate compute device that contains) a database of available screw sizes, where a suitable screw size is then determined based on the depth (and, e.g., width) of the hole as determined by the depth gauge and also the available screw sizes. Typically, the screws are available in a discrete range of sizes so that the screw may be selected as the screw with the smallest depth that is greater than the depth of the hole.

[0199] The screw size may also be dependent on the depth of another component such as the screw plate 4. For example, during an ORIF surgery a plate is typically placed onto the bone prior to drilling where a screw is then required to pass through the plate and the bone. The size of the screw may then be determined based on the depth of the hole and the (known) thickness of the plate, where a user may be able to input this thickness via the input 13 of the depth gauge.

[0200] The above-described method enables the user of the drill to rapidly, and accurately, determine an appropriate screw size without substantial interruption of a surgical procedure.

[0201] It will be appreciated that, equally, the depth gauge 10 may output a depth of the hole or, more generally, a measure / indication of this depth in order to enable a user to select a suitable screw size themselves.

[0202] Regarding the determination of the onset of drilling and the end of drilling (e.g. when the drill first enters the bone and when the drill leaves the bone), the depth gauge may be arranged to determine these events based on an expected rate of change of depth of the depth gauge.

[0203] As described above, the user typically moves the drill 1 along an expected velocity profile prior to the start of drilling so that this start of drilling can be determined based on a change in depth measured by the depth gauge.

[0204] Referring to Figure 5, during the drilling process itself the depth measured by the depth gauge 10 also moves along a predictable velocity profile (where this profile arises due to the different amounts of resistance provided by different types / sections of bone). Specifically, the rate of displacement of the drill bit and the rate of change in the depth measured by the drill bit typically changes only slowly as the tip of the drill bit passes through the tough near cortex before changing rapidly as this tip passes through the relatively spongy, cancellous bone, and then changing slowly again as the tip passes through the tough far cortex. If a user of the drill continues drilling once the tip of the drill bit has passed through the far cortex, the drill bit may (typically undesirably) plunge through further biological features of the user.

[0205] As described above, these different stages of drilling may be determined based on a measured rate of change of distance / depth. And such a determination may be made solely using a distance sensor of the depth gauge 10. In particular, the entry to the bone and the exit from the bone may be determined, respectively, by a decrease in a rate of change of this distance (as the drill bit enters the near cortex) and a subsequent increase in a rate of change of this distance (as the drill bit exits the far cortex, e.g. at the onset of the ‘plunge’ step). Equally, the stages of drilling, and the entry of the drill bit into a bone and the exit of a drill bit from the bone, may be determined by another mechanism. For example, the depth gauge may comprise an acoustic sensor that is arranged to detect a difference in a sound that occurs as the user enters and exits the bone or to detect a sound that indicates a speed of the drill bit (where this speed varies depending on the material through which the tip of the drill bit is moving and the sound varies depending on the speed of the drill bit). Furthermore, the depth gauge may be connected to a motor of the drill so that the depth gauge is able to detect the entry to, and exit from, the bone based on a resistance being experienced by the drill bit. In general, the depth gauge typically comprises a means for (e.g. a processor for and / or a sensor for) detecting the start of a drilling process (e.g. the entry of the drill bit into a first side of a bone) and / or the end of a drilling process (e.g. the exit of the drill bit from a second side of the bone). As described above, the start of the drilling process may also be determined based on a user operation of the input 13 of the depth gauge (e.g. the user pressing a zero button). Equally, the end of the drilling process may be determined based on a user operation of this input 13.

[0206] As described above, by monitoring a change in the distance of the depth gauge 10 from a surface, the depth gauge is able to detect a depth of a hole being drilled. This monitoring typically involves emitting infrared light or visible light and detecting the time taken for this light to reflect off the surface and return to the depth gauge.

[0207] In order to improve the accuracy of the depth measurement, the depth gauge may be used in combination with the drill guide 50, which drill guide may comprise a structure (e.g. a target or a pattern) that is arranged to assist the depth determination. For example, the depth guide may comprise a high gain reflective circle (that is arranged to reflect incoming signals). Equally, the drill guide may comprise a dark area that can be seen in an image taken by a camera of the depth gauge. The use of such a pattern enables the sensor of the depth gauge to determine the distance based on an identification of this pattern (e.g. by reflecting light from this pattern and / or by identifying a size of this pattern in an image taken by the sensor).

[0208] The depth gauge 10 may be provided alongside the drill guide 50 (e.g. in a kit of parts). The same depth gauge may be used with a plurality of different drill guides, or the same drill guide may be used with a plurality of different depth gauges, where different drill guides may be used for different purposes. For example, the different drill guides may comprise different tracking structures / patterns so as to provide different levels of accuracy and / or so as to be suitable for different purposes. For example, a basic drill guide with a single tracking pattern may be suitable for routine surgeries while a more complex drill guide with a plurality of different types of tracking structures and patterns is used for more complicated surgeries.

[0209] The depth gauge 10 may comprise a camera that is arranged to capture images of the drill guide and to detect the presence of the pattern in these images. The distance ofthe pattern from the depth gauge (and thus a change in distance between the depth gauge and the surface) can be detected by determining a change in (a parameter of) the pattern between a plurality of images. For example, as the depth gauge moves towards the surface, and towards the pattern, the size of the pattern in the images taken by the camera will increase. The amount of increase of this size can then be related to a change in distance by a processor of the depth gauge.

[0210] The drill guide 50 may comprise a plurality of patterns or targets to assist in the accurate detection of distances / depths. For example, the drill guide may comprise a first tracking pattern that is used while the depth gauge 10 is far from the drill guide and a second tracking pattern that is used while the depth gauge is near to the drill guide. The first tracking pattern and the second tracking pattern may be spaced to ensure that the depth gauge is always able to see at least one of the tracking patterns.

[0211] As shown in Figure 6, the pattern may comprise a circle of a known size, where the apparent dimensions of this ring in a picture taken by a camera of the depth gauge (which camera has known properties) enables a distance between the circle and the camera to be determined.

[0212] Typically, the drill guide 50 comprises a hole for the passage of a drill bit, where the pattern(s) may be arranged adjacent to and / or around this hole. Therefore, as the drill moves towards the drill guide, the depth gauge moves towards the pattern.

[0213] Equally, one or more tracking patterns may be spaced from the hole so that these patterns remains visible to a sensor of the depth gauge 10 as the drill moves towards the drill guide 50. For example, the pattern may be spaced from the hole by a distance that is related to a height of the depth gauge and / or the drill 1 .

[0214] The hole is arranged so that a user can place the hole of the drill guide 50 adjacent the skin of a user and / or adjacent a hole of the screw plate 4 and the user can then place the drill bit of the drill 1 through the hole of the screw plate in order to drill through a bone. The drill guide may comprise a single hole, where a user is then able to move the drill guide between holes of the screw plate in order to drill multiple holes through a bone. In some embodiments, the drill guide comprises a plurality of holes, where each hole may be associated with a hole of the screw plate. Therefore, a user is able to place the drill guide adjacent the user (e.g. adjacent the screw plate), optionally, to secure the drill guide in place (e.g. using straps), and then drill multiple holes without moving the drill guide. Where the drill guide comprises a plurality of holes, each hole is typically associated with a respective (separate) tracking pattern, where the tracking pattern may comprise an identifier of the associated hole (e.g. each tracking pattern may be a different colour, have a different width, or have a different dash pattern). Each tracking pattern may be located about the associated hole; equally, each tracking pattern may be spaced from the associated hole. One or more of the holes may be associated with a plurality of tracking patterns (e.g. a first pattern located about the hole and a second pattern spaced from the hole).

[0215] Referring to Figure 7, there is described an exemplary method of determining a distance of the depth gauge 10 from the drill guide 50, which method is particularly applicable where the tracking pattern is a circle. It will be appreciated that other types of tracking methods are possible (and that different methods are suitable for different tracking patterns).

[0216] The tracking of the pattern is typically performed by a processor of the drill guide and / or by a separate computer device (where the depth gauge may comprise a communication interface, such as a Bluetooth® interface or a local area network interface for communicating with this separate computer device).

[0217] In a first step 111 of the method of Figure 7, one or more image frames are captured using a camera of the depth gauge 10.

[0218] In a second step 112, the image frames are filtered to prepare these frames for a pattern detection process. This filtering may comprise converting red-green-blue (RGB) frames to grayscale frames or applying image filters such as a canny edge filter. Typically, the filtering stage comprises the application of an edge detection filter that identifies portions of images with large gradients in order to identify edges in those images.

[0219] In a third step 113, the filtered images are provided to a pattern detection algorithm. Where the pattern comprises a circle, a Hough Circles algorithm may be used, which algorithm is associated with a set of (trained trainable) parameters 61 . It will be appreciated that the algorithm used to detect a tracking pattern in the images depends on the type of tracking pattern being used so that more generally the third step 113 involves using a pattern recognition algorithm to detect a pattern in the image frames.

[0220] Regarding the Circle Hough Transform (CHT) shown in Figure 7, this algorithm is a feature extraction technique used to detect circles in imperfect images. The circle candidates are produced by “voting” in the Hough parameter space and then selecting local maxima in an accumulator matrix. The algorithm has five trainable parameters being the minimum radius, the maximum radius, the resolution or sensitivity, and two thresholding parameters and this returns several candidate circles in polar coordinates. These parameters are trained during a calibration process that occurs prior to a drilling procedure.

[0221] More generally, the third step 113 provides a data structure that indicates the potential presence of and / or positions / dimensions of a pattern in the captured frames. Then, in an optional fourth step 114, outliers are removed from this data structure (e.g. shapes for which a probability of being a circle is below a threshold probability may be removed).

[0222] In a fifth step 115, the distance between the depth gauge 10 and the drill guide 50 is determined based on the detected patterns. This determination may comprise applying a distance formula to the detected patterns. For example, where the pattern is a circle, the distance of the circle from the depth gauge may be determined using the equation:

[0223] The focal distance of the camera of the depth gauge and the actual radius of the tracking pattern are known prior to the operation of the drill 1 (e.g. these values may be determined during a calibration process of the depth gauge); the detected radius is determined using image processing techniques such as those set out above. The detected radius may, for example, be detected as a median radius of the potential patterns detected in the captured frames.

[0224] More generally, a distance of the depth gauge from the drill guide is typically determined based on: one or more tracking patterns detected in an image; a characteristic or parameter (e.g. a dimension) of the tracking patterns; and characteristics of the sensor of the depth gauge. The distance may be dependent on one or more of: a change in a tracking pattern detected in a plurality of images; a focal length of a sensor and / or camera; and calibration properties of the depth gauge (e.g. a calibration factor determined for the sensor prior to the distance determination).

[0225] The characteristics / parameters of the tracking patterns (e.g. the actual size of the tracking patterns) and the sensor may be stored in a memory of the depth gauge 10.

[0226] Typically, the depth gauge 10 (and the sensor of the depth gauge) is calibrated prior to the onset of a drilling procedure; in particular, during the zeroing process, the sensor (e.g. camera) of the depth gauge may be arranged to auto adjust to focus on the tracking pattern so as to identify a focus distance of this camera and / or an initial size of a tracking pattern (at a depth of ‘0’). This adjustment of the sensor may be in response to a user operation of the input 13 and / or in response to an automatic detection of the initiation of a drilling procedure. In some embodiments, a change in the size of the tracking pattern between an initial size (detected at an entry point) and a final size (detected at the exit point) is used to determine the depth of a hole drilled through a bone between the entry point to the exit point.

[0227] In this regard, and to summarise an aspect of the present disclosure, at the start of a drilling procedure, the drill bit is typically placed through the drill guide 50 so as to contact the bone. The depth gauge is then zeroed, where the zeroing process (whether implemented manually or automatically) defines a depth of zero as the distance between the depth gauge 10 and the drill guide at the point where the drill bit contacts the bone. Thereafter, as the user drills into the bone, this distance between the depth gauge and the drill guide decreases and this decrease in distance can be determined as the depth of the hole. Once the drilling of a hole through the bone is complete, e.g. as may be detected by detecting an acceleration in the change of the distance measured by the depth gauge, the depth of the hole is determined and a suitable screw size is presented to a user based on this depth.

[0228] In some embodiments, the drill guide 50 comprises a tracking structure that is arranged to be detected by the depth gauge 10. For example, the drill guide may comprise one or more of: a radio frequency identification (RFID) chip; a GPS chip; and a transmitter and / or receiver. The distance between the drill guide and the depth gauge may then be determined based on a distance between this tracking structure of the drill guide and a corresponding tracking structure in the depth gauge (e.g. where a first one of the tracking structures comprises a transmitter and a second one of the tracking structures comprises a receiver and the distance is determined based on the time taken for a signal sent by the transmitter to be received by the receiver). It will be appreciated that, equally, a transmitter may be included in the drill guide with the receiver being included in the depth gauge.

[0229] In some embodiments, one of the depth gauge 10 and the drill guide 50 comprises a plurality of transmitters, and the other of the depth gauge and the drill guide comprises a receiver. The depth gauge may then be arranged to determine the distance to the drill guide based on the signals received from each transmitted and, more specifically, based on a difference in the time of receipt of these signals. This determination may be based on a known distance between the transmitters; for example a first transmitter may be located at a first end of the drill guide and / or adjacent a hole of the drill guide and a second transmitter may be located at a second end of the drill guide (this may cause a receiver on the depth gauge to approach the first transmitter more quickly than it approaches the second transmitter). Equally, one of the depth gauge and drill guide may comprise a plurality of receivers with the other of the depth gauge and drill guide comprising a transmitter.

[0230] In some embodiments, the depth gauge 10 comprises an transmitter that is arranged to emit a plurality of different types of signals (e.g. a first electromagnetic, e.g. radio, light, or infrared, signal and a second sound and / or ultrasound signal). A receiver located on the drill guide 50 may then be arranged to detect the receipt of each signal and to determine a distance to the transmitter based on a difference in the time of reception of the signals. In practice, the transmitter may emit the first signal and the second signal at a first time, and the receiver may then receive the first signal at a second time and the second signal at a third time and determine the distance between the depth gauge and the drill guide based on the difference between the second time and the third time. The transmitter is typically arranged to transmit pulses (e.g. modulated pulses) of each of the first signal and the second signal so as to provide a near-continuous measurement of depth. Typically, the different types of signal comprise signals with different speeds (e.g. the speed of sound and the speed of light) where this difference in speed leads to a difference in the time of reception of the signals by the receiver. Integrated depth gauge and drill guide

[0231] Various embodiments of depth gauges and drill guides that may be used to provide an improved depth gauge have been described above. Figures 8a to 8e show a further embodiment of a depth gauge 70 according to the present disclosure. In particular, Figures 8a to 8e show an embodiment of a depth gauge that comprises an integrated drill guide 71 .

[0232] By integrating the sensor assembly into the drill guide, a user is able to add a sensor assembly to an existing workflow with minimal disruption (since a drill guide is usually present anyway for, e.g. an ORIF procedure).

[0233] It will be appreciated that any features described with reference to the depth gauge and the drill guides of Figures 1 to Figure 7 may be implemented with the depth gauge and drill guides of Figures 8a to 11 and vice versa.

[0234] As has been described above, the depth gauge 70 is arranged to determine a depth of a hole that has been formed using a drill and, typically, to determine a suitable screw for insertion into this hole, where the screw size is determined based on the depth. The screw size may also be determined based on further factors, including a type of injury, a type of drill bit that has been used to form the hole, a type of drill guide used to form the hole, a type of screw to be used to fill the hole, and / or a setting of the depth gauge (e.g. that may be selected by a user of the depth gauge).

[0235] Referring to Figure 8a, in one embodiment the depth gauge 70 comprises the drill guide 71 and a sensor assembly 72.

[0236] The drill guide 71 is arranged to protect nearby tissues from a drill bit during a drilling process ordrilling procedure; to center the drill bit tip on the bone with respect to the holes in a plate; and / or or to lock into a plate (for example with a thread) to ensure proper angulation of the bit. The drill guide may comprise an attachment mechanism, e.g. a thread, that is arranged to cooperate with a plate in order to secure the drill guide to this plate.

[0237] Typically, the sensor assembly 72 is removably attachable to the drill guide 71 (e.g. via a sensor attachment structure). This may be achieved using a removable attachment structure such as clips or an adhesive fastener (it will be appreciated that various attachment structures for providing a removable attachment are known in the art).

[0238] Typically, the depth gauge 70 is arranged to be located around a drill bit of the drill 1 , where this drill bit may extend from the drill and pass through the depth gauge, entering at a first side 73 of the depth gauge and exiting through an orifice 74 of the depth gauge (the orifice being a part of the drill guide). In some embodiments, the depth gauge is arranged to be attachable to the drill 1 , e.g. as has been described above. Typically, being attachable to the drill comprises being attachable to a front part of the drill so that the drill guide surrounds a drill bit of the drill.

[0239] In some embodiments, the depth gauge of Figures 8a to 8d is arranged to be separate from the drill and held in place by a user as the user operates a drill bit that is passing through the depth gauge. The depth gauge may be held in place against a body part or a bone of a person into whose bones the user is drilling.

[0240] In some embodiments, the depth gauge is arranged to attach to the chuck of the drill so that a first connector of the depth gauge attaches to a chuck of the drill and then a second connector of the depth gauge attaches to the drill bit. In this way, the drill is able to operate the drill bit via the depth gauge. Typically, the drill guide 71 is designed to be reusable so that it may be used for multiple separate drilling procedures. This may comprise the drill guide being formed of a durable material and / or a metal, such as stainless steel.

[0241] The drill guide 71 may still comprise replaceable parts, for example the drill guide may comprise a replaceable handle, where the handle may be removably attached to a locking guide of the drill guide.

[0242] Typically, the sensor module 72 is designed to be disposable (e.g. after a single drilling procedure or after a plurality of drilling procedures). This may comprise the sensor module being formed of a plastic.

[0243] Typically, the drill guide 71 is arranged to be sterilized using an autoclave chamber (and so, e.g. the drill guide is formed of a heat-resistant material). This enables the drill guide to be sterilized between uses and then to be used with a single-use sensor module so as to avoid the need to provide a sensor module that is capable of being sterilized with an autoclave (such a sensor module would typically be substantially more expensive and complicated). That being said, in some embodiments the sensor module is arranged to be sterilized using an autoclave (e.g. it may comprise a housing that seals the sensor module to avoid damage to internal components during a sterilization procedure).

[0244] The drill guide may be arranged to be used with different types of sensor module (e.g. different sensor modules with different accuracies and / or different user interfaces may be used with the same drill guide. Equally, the sensor module may be arranged to be used with a plurality of different drill guides. For example, different types of surgery may require different drill guides, where a user is able to select a suitable drill guide and to attach a sensor module to this drill guide.

[0245] The present disclosure considers a kit of parts that comprises one or more sensor modules and one or more drill guides. In particular, the kit of parts may comprise a plurality of different type of drill guides (e.g. with different bore sizes), where each drill guide comprises a sensor attachment structure for attaching said drill guide to the sensor module.

[0246] As mentioned above, the drill guide 71 may be specific to a certain screw type or a certain drill bit size. In particular, each drill guide is typically sized so that a corresponding drill bit can be inserted snugly through the drill guide (to ensure that the drill bit remains aligned during a drilling process). The sensor assembly 72 may be arranged to detect the type of drill guide 71 , where the sensor assembly may then be arranged to determine a screw size based on the detected type of drill guide.

[0247] For example, some screws are direct to bone (e.g. a ‘lag’ screw) whilst other screws sit ‘on’ a plate and other screws thread ‘in’ to a plate (e.g. non-locking, tapered locking etc). Typically, each screw is associated with a different type of drill guide so that by identifying a type of drill guide the sensor assembly can provide an accurate screw type and length recommendation.

[0248] The depth gauges described herein (e.g. the sensor assembly 72) may be arranged to determine a screw size based on a type of screw and / or a type of plate that is being used for the drilled hole. Typically, the sensor assembly is arranged to determine a type of the drill guide 71 (e.g. using a type identifier) and to determine a screw size based on this type of the drill guide, where the type of drill guide indicates the type of screw and / or the type of plate that will be used to complete a drilling process.

[0249] The sensor assembly 72 can then determine the screw size by applying an offset to a hole depth. For example, if a drill guide for a small fragment plate is being used, the sensor assembly may determine a suitable screw size for a hole by determining the depth of this hole and then adding an offset (e.g. a 2mm offset) to this depth to account for the thickness of the plate. Conversely, if a drill guide for a lag screw (that is attached directly to a bone) is being used, then the offset may be zero so that the screw size is determined purely based on the hole depth.

[0250] It will be appreciated that a sensor assembly and / or a depth gauge that is able to determine a type of a drill guide (or a plate or a drill bit) is advantageous even if this sensor assembly is provided without an integrated depth gauge (or without a structure for attaching the sensor assembly to a drill guide).

[0251] In various embodiments, the sensor assembly 72 detects the (type of the) drill guide 71 in various different ways.

[0252] In some embodiments, the sensor assembly 72 and drill guide 71 are arranged to enable automatic detection.

[0253] In a first example, a coil is embedded in the drill guide 71 and an RFID chip is embedded in the sensor assembly 72 (as shown, for example, in Figure 10A). In such examples, the RFID chip picks up a low frequency radio wave emitted by the coil and the sensor assembly is then able to detect the type of the drill guide based on a frequency of the wave.

[0254] In a second example, magnets are embedded in the drill guide 71 , with the strength or location of the magnets indicating the type of the drill guide. A sensor in the sensor assembly detects the magnet field and, based on this field (e.g. the strength of the field), the sensor assembly makes a determination as to which drill guide has been attached. In some examples, the sensor detecting the magnetic field may be a displacement sensor 87 of the sensor assembly. This is described in more detail below with reference to Figure 9a.

[0255] In a third example, the sensor assembly 72 comprises a plurality of sensor attachment structures and the sensor assembly is arranged to identify a type of drill guide based on a subset of the sensor attachment structures used to attach the drill guide to the sensor assembly.

[0256] Similarly, the sensor assembly 72 may detect physical features on a drill guide depress buttons upon connection of the sensor assembly 72 and the drill guide 71.

[0257] It will be appreciated that various other corresponding structures are possible for indicating to the sensor assembly 72 a type of a drill guide that is attached to the sensor assembly. In general, the drill guide 71 typically comprises an identifier that can be identified by the sensor assembly 72, where the identifier identifies a type and / or characteristic of the drill guide. The identifier may comprise a marking, a physical structure, and / or a chip that emits a signal (such as a magnetic field or a signal). The sensor assembly may comprise a reader for identifying this signal (e.g. based on the magnetic field or the signal).

[0258] The sensor assembly 72 may comprise a type sensor for detecting a type of the drill guide or of a drill bit. This may be a sensor (or a part of a sensor) that is arranged to detect a displacement of the drill bit. Equally, this type sensor may be a separate sensor. It will be appreciated that, as used herein, a ‘sensor’ may comprise a plurality of component sensors so that a ‘sensor’ of the sensor assembly may comprise a first displacement sensor and a second type sensor. Equally, the same sensor may be both the displacement sensor and the type sensor.

[0259] In some embodiments, the sensor assembly 72 may be configured to detect the type of drill guide without modification of the normal structure of an off-the-shelf drill guide. For example, the sensor assembly may be configured to detect the drill guide based on an inductive or capacitive measurement of features of the drill guide. Beneficially, in such cases, the drill guide need not be a bespoke drill guide.

[0260] In some embodiments, the sensor assembly 72 is arranged so that a user is able to indicate the type of drill guide 71 to the sensor assembly as an input on a user interface 83 of the sensor assembly.

[0261] Referring to Figure 8b, the drill guide 71 comprises a body 75 and the body comprises a channel through which the drill bit can enter the drill guide before exiting through the orifice 74. Typically, the body comprises a sensor attachment structure that is arranged to receive the sensor assembly 72. As described above, the sensor attachment structure may comprise an indicator that identifies the type of the drill guide, for example the sensor attachment structure may comprise one 72 or more depression buttons indicating the type of drill guide71 .

[0262] The orifice 74 of the drill guide is typically sized to snugly hold a drill bit, where different drill guides may be provided to hold drill bits of different sizes. The orifice may have a conical shape.

[0263] The drill guide 71 may comprise a handle 76. In some embodiments, the drill guide comprises a replaceable tip, where this enables the drill guide to be used with tips that comprise handles (as well as with tips that do not comprise handles),

[0264] Referring to Figure 8c, the sensor assembly 72 comprises: a housing 81 ; an output 82 for providing information to a user of the depth gauge; an input 83 for receiving a user input and a drill attachment structure 84 that is arranged to removably attach the depth gauge onto a drill. The input is typically arranged to provide a zeroing function for the depth gauge.

[0265] The output 82 typically comprises a display. Equally, the output may comprise a speaker, a haptic feedback system, and / or a communication interface (which outputs a measurement and / or a screw size to a separate computer device, such as a computer in an operating surgery). The input 83 typically comprises a button and / or a dial.

[0266] More generally, it will be appreciated that numerous different types of inputs and outputs are useable to enable a user to interact with the depth gauge 70. Furthermore, the input 83 and the output 82 may be combined into a single interface component, such as a touchscreen.

[0267] This drill attachment structure 84 enables the depth gauge to be retrofitted onto existing drills and / or to be used for multiple different drills. The drill attachment structure may, for example, comprise an adhesive, a fastener, or a clip or button.

[0268] In some embodiments, the drill attachment structure is arranged to connect to a chuck of a drill. For example, the drill attachment structure may comprise an AO attachment structure. In such embodiments, the depth gauge 71 may further comprise a bit attachment structure for a drill bit so that the drill bit can be attached to the drill via the depth gauge (and so that the depth gauge is attached between the drill and the drill bit). In such embodiments, the sensor assembly 72 may be attached to a remaining portion of the depth gauge using bearings or another low-friction structure that enables the sensor assembly to be held in place while the drill bit is rotating. Equally, the depth gauge may comprise a rotating part (including the bit attachment structure) and a stationary part (including the sensor assembly and / or the depth gauge). Typically, the rotating part comprises a rotating core that is surrounded by a stationary housing.

[0269] Figure 8d and 8e show an alternative view of the sensor assembly 72 comprising: the housing 81 ; the output 82; a battery 85; an LED 86; a displacement sensor 87; and an orifice 89, where a drill bit of the drill 1 is able to pass through the orifice. An exemplary displacement sensor is described in more detail with reference to Figures 10a and 10b.

[0270] In some embodiments, the battery 85 comprises a rechargeable battery. Typically, the sensor assembly is provided with a single-use battery since the sensor assembly is typically arranged to be disposable and to be discarded after a single use.

[0271] In some embodiments, the sensor assembly 72 sits between the drill and the drill bit (rather than around the drill bit). In such examples, only an end portion of the drill bit sits within the sensor assembly on the side of the sensor assembly that interfaces with the drill guide. In such examples, the sensor assembly 72 attaches directly onto the drill, 1 .

[0272] Further to the components shown in Figures 8a - 8e, the depth gauge 70 typically comprises a computer device and / or a processor, which processor is arranged to receive information from, and to control, the output 82, the input 83 and the sensor 87. Typically, this computer device is a part of the sensor assembly 72. The processor is typically arranged to determine a depth of a hole based on readings from the sensor and to determine a suitable screw size based on this depth. The screw size can then be presented to a user using the output.

[0273] As with the depth gauge described with reference to Figure 3a to 3c, the depth gauge 70 of Figures 8a to 8e may further comprise a communication interface for communicating with a further computer device, where this may enable the depth gauge to receive relevant information (e.g. images taken by other computer devices) in order to improve the determination of depth and / or where this may enable the depth gauge to provide information to other computer devices (e.g. to save a record of the suggested screw size).

[0274] The user interface (e.g. the input) and / or the communication interface may be arranged to receive information relating to a patient and / or a procedure, where the determination of the screw size may depend on this information. For example, the depth gauge may be arranged to determine information relating to: a drill bit being used, a width of the drill bit, a surgical procedure being performed, a characteristic of the patient (e.g. an age or a size of the patient), a desired type or material of a screw, etc.). This information may, for example, be selected by a user via the input 13, where the depth gauge may be arranged to request the information prior to the start of a drilling procedure. For example, the depth gauge may provide, via the output 12, a series of lists of options, where the user is able to select one or more options from each list to provide the information (e.g. the output may present a list of possible surgical procedures, and then a list of drill bit types that may be used for these procedures).

[0275] Figure 9a shows the depth gauge 70 with a drill bit 90 inserted through the depth gauge and illustrates the handle 76 could be used to control the drill guide (and therefore the drill bit) during a drilling process. In orderto drill a hole in a surface, the front of the depth gauge is typically placed against this surface and the drill bit is then operated to form a hole in the surface with the drill guide being used to control this drilling process. Figure 9a also shows the output 83 of the sensor assembly and shows how a maximum displacement of the drill bit can be shown alongside a suggested screw depth (that is determined based on a hole depth).

[0276] Referring to Figures 10a, 10b, and 10c, an operation of the sensor 87 of the drill is described. The sensor is arranged to measure a displacement of a drill bit over time so as to determine a depth of a hole that has been formed during this time. To determine the displacement of the drill bit, the displacement sensor may, for example, track a mark on the drill bit or may track a sensor that has been attached to the drill bit. Figure 10a shows a partial view of a cross section of the system comprising the sensor assembly 72 (from the same viewpoint as Figure 8d) and the drill bit 90.

[0277] The depth gauge 70 may be used with a standard drill bit, where the displacement sensor 87 may then identify a particular feature or geometry on the drill bit in order to track a displacement of the drill bit over time. Specifically, the displacement sensor may track the particular feature or geometry in a z direction over time, where the z direction is the direction of a central axis of the drill bit 90 (the axis about which the drill bit rotates).

[0278] In some embodiments, the sensor 87 is arranged to identify a feature of the drill bit, such as a marking on the drill bit, so that this feature can be tracked in order to track a displacement. Such a marking may be created by a user of the drill bit.

[0279] The present disclosure extends to a novel drill bit that is particularly useful when used in combination with the depth gauge of the present disclosure.

[0280] This drill bit comprises a measurement structure that enables a distance of movement of the drill bit to be identified by the sensor assembly. Typically, the measurement structure comprises an axisymmetric structure so that an initial attachment orientation of the drill bit does not affect the depth determined by the sensor 87.

[0281] Furthermore, the use of an axisymmetric structure ensures that the sensor does not sense any rotation of the drill bit. This decouples the axial movement of the drill bit from the rotation of the drill bit and ensures that only the axial displacement of the drill bit is measured by the sensor (where this axial displacement provides the hole depth).

[0282] The drill bit comprises a drill attachment structure 84 for connecting the drill bit into a drill (or into the depth gauge 70). The connection typically comprises an AO connector, but it will be appreciated that other types of connector are useable.

[0283] As described above, the drill guide may comprise an identifier or a marking to enable the sensor assembly 72 to identify a type of the drill guide. In some embodiments, the sensor 87 of the sensor assembly is arranged to detect the identifier. The example of Figure 10 shows a drill guide that has a tag 89 (e.g. an RFID tag or an NFC tag), where the sensor assembly comprises a corresponding reader 89 that can read this tag when the drill guide is attached to the sensor assembly.

[0284] In some embodiments, as shown in Figure 10b, the measurement structure 91 comprises an arrangement of one or more annular grooves (or notches) (or one or more annular nodules, ridges, or protrusions), which grooves or notches may be arranged along the length of the drill bit. The sensor 87 of the sensor assembly 72 can be arranged to identify the passage of a groove or a ridge (e.g. based on a change in a distance between the drill bit 90 and the sensor 87 that occurs as a result of this passage).

[0285] Typically, the spacing between the grooves and / or the ridges is constant. In some embodiments, a variable spacing can be used so that the sensor assembly can identify a portion of the drill bit that is adjacent the sensor (this may, for example, to properly locate the drill bit during a calibration process).

[0286] More generally, the measurement structure 91 may comprise a plurality of different grooves and / or ridges to enable the sensor to identify a specific protrusion.

[0287] Typically, the measurement structure 91 comprises a continuous slope (e.g. the measurement structure may have a sinusoidal shape), where this enables the sensor to determine a precise hole depth since the radius of the drill bit is continuously changing and so the measurement structure is able to continuously update a displacement measurement based on the slope. Equally, the measurement structure may comprise one or more stepwise changes in radius, where these stepwise changes may be used to identify important distances (e.g. to identify a distance of maximum drilling or to identify a calibration point).

[0288] Typically, the measurement structure 91 is arranged to be located internally to the drill guide. Where the drill bit comprises a proximal end that is attached to a chuck of a drill and a distal end that is arranged to form the hole, the measurement structure is arranged to be located closer to the proximal end than the distal end. For example, the measurement structure may be located at least 20% of the drill bit length from the distal end and / or at least 50% of the drill bit length from the distal end. The measurement structure may be located at least 5cm from the distal end and / or at least 10cm from the distal end. This enables the measurement structure to be analysed without the need for the sensor to detect a feature that is internal to a structure into which the drill bit is being inserted. Furthermore, this enables the measurement structure to be encompassed by the drill guide so as to reduce the chance of debris or obstructions covering the measurement structure and interfering with the sensor 87.

[0289] In some embodiments, the sensor 87 comprises a pair of coils, where the sensor is arranged to create a (small) magnetic field in a first coil and to measure a resultant current in a second coil. The movement of the drill bit 90 adjacent the sensor will change the coupling between the coils and therefore change the resultant current in the second coil. In particular, the movement of the drill bit, and the displacement of the grooves, will result in a current that changes (in a predictable way) as one or more grooves pass by the sensor so that a distance of movement of the drill bit can be determined based on the change in this current.

[0290] As mentioned above, the use of a measurement structure 91 that has a continuous slope causes a continuous change in the radius of the drill bit and a continuous change in this current so that an increasing displacement can be continuously measured. The use of a measurement structure with one or more step-changes in radius could be used to provide rapid (and readily identifiable) changes in current at notable positions along the drill bit.

[0291] The aforementioned sensor can be built to be small, robust, unaffected by non-metallic debris, and cheap. Furthermore, such a sensor does not require a line of sight to the drill bit. But it will be appreciated that other forms of sensor are possible.

[0292] It will be appreciated that numerous other types of sensor are useable, e.g. the sensor may comprise a hall effect sensor, a physical gear, or a mechanical sensor in which a protrusion is pressed against this surface and this protrusion is compressed to differing amounts between the depth gauge and the surface as the drill moves forwards. Such a sensor may also provide a varying current that is useable to determine a displacement of the drill bit over a period of time.

[0293] As shown in Figure 10c, in some embodiments the measurement structure comprises a calibration feature 92. In particular, a particular groove, nodule or ridge may be used as a calibration feature to perform a ‘calibration’ step. In such examples, when this particular section of the drill bit passes the sensor, the sensor assembly is able to obtain information about the drill bit, for example its diameter and length providing ‘autocalibration’ and removing the need for the user to zero the sensor assembly. For example, the measurement structure 91 may comprise a substantial step change in a radius, where the size of this step change is useable to identify a type of the drill bit. Equally, a distance between successive ridges or grooves in the drill bit may be useable to identify the type of drill bit, where different drill bits may be provided with different spacings.

[0294] Regarding autocalibration, the drill bit may comprise one or more identifying features (e.g. one or more unique grooves, nodules, or ridges, e.g. with a unique thickness, marking, or colour) that enables the sensor to determine an axial position of the drill bit. This axial position - and the identifying features - may then be used to identify one or more of: a type of screw being used, a plate being used, or a type of drill guide being used.

[0295] For example, the ‘zero’ point of the drilling procedure will occur when a user is holding the drill bit against a surface that is being drilled. Therefore, the placement of the drill bit at the zero point is somewhat predictable. An identifying feature of the drill bit may then be located so that this identifying feature is expected to be adjacent the sensor at the zero point. This enables the sensor assembly 72 to determine when the drill bit is at the zero position.

[0296] More specifically, the zero point for a drilling procedure will typically occur when the drill guide is in contact with the surface to be drilled and the drill bit is also in contact with this surface (located within the orifice 74 of the drill guide). The drill bit may comprise an identifying feature that is arranged to be located adjacent the sensor of the sensor assembly at this point (and where the sensor assembly is attached to the depth gauge, this point can be reliably determined since the relative positions of the drill gauge and the sensor are known).

[0297] Where a plate is being used, the drill guide may instead be spaced from the surface being drilled due to the plate. Therefore, the drill bit may extend through the orifice 74 of the depth gauge and also through an orifice of the plate in order to contact the surface being drilled (e.g. a bone). This leads to a change in the location of the sensor assembly (which is typically attached to the drill guide) relative to the drill bit.

[0298] Similarly, the location of the sensor assembly relative to the drill bit may depend on a type of drill guide being used for a drilling procedure (e.g. depending on a thickness of the drill guide).

[0299] Therefore, an identifying feature that is located adjacent a sensor of the sensor assembly may be used to determine one or more of: a drill guide being used; a type of procedure being performed; a plate being used; a plate thickness; and a suitable screw for a hole.

[0300] In this regard, as shown in Figure 10c, the drill bit may comprise a plurality of identifying features 93, 94 where each identifying feature is located at a different axial position on the drill bit. These identifying features may each be associated with a different type of drill guide, a different type of plate, or a different type of procedure. For example, where a 2mm plate is being used, the drill bit will be offset by 2mm compared to a procedure where no plate is being used. Therefore, the drill bit may comprise a first identifying 93 feature that is associated with no plate being used and a second identifying feature 94 that is associated with use of a 2mm plate (these identifying features may be spaced by 2mm, but equally may be spaced by another distance due to the different drill guides that may be used in these different situations).

[0301] The depth gauge (e.g. the sensor assembly 72) may be arranged to determine an identifying feature of the drill bit that is adjacent the sensor at the zero point (e.g. at the starting point of a drilling process) and to determine based on this identifying feature one or more of: a drill guide being used; a type of procedure being performed; a plate being used; a plate thickness; and a suitable screw for a hole. Typically, each identifying feature extends around the entirety of (or substantially the entirety of) the circumference of the drill bit.

[0302] As an example of the identifying features, the drill bit may comprise one or more rings of colour and in particular the drill bit may comprise a plurality of rings of different colours. The sensor assembly can then readily identify a feature of a drilling process based on a colour of a ring that is adjacent the sensor at the start of the drilling process.

[0303] The present disclosure extends to a kit of parts comprising a plurality of drill bits with different characteristics, different measurement structures, different arrangements of identifying features, and / or different bit identifiers. These drill bits may be provided separate to, or alongside, one or more depth gauges, one or more sensor assemblies, and / or one or more drill guides.

[0304] As mentioned above, in some embodiments the drill bit 90 comprises a bit identifier, such as a specific arrangement of grooves or ridges, and / or an RFID chip that enables the sensor assembly 72 to identify the drill bit. The bit identifier may be separate to the measurement structure 91 and may enable the sensor assembly to automatically identify the drill bit (e.g. by receiving a signal from the drill bit) or may enable a user to identify the drill bit and to identify this drill bit to the sensor assembly. Similar to the depth gauge, different surgeries typically require the use of different drill bits, and these drill bits can lead to the use of different screws. Therefore, the determination of a hole depth and / or the determination of a suitable screw length (or type) based on this hole depth may be dependent on a type of a drill bit used to form the hole. This type of drill bit may be determined automatically by the sensor assembly based on the bit identifier.

[0305] In some embodiments, the drill bit may not be entirely axisymmetric. For example, one or more of the ridges may comprise a cutout, where this cutout can be used to identify the ridge so as to identify a placement of the drill bit. The drill bit may still be substantially axisymmetric, where, for example, the drill bit is axisymmetric about at least 50%, at least 75%, and / or at least 90% of its circumference (e.g. so that any cutout of a ridge occupies no more than 10%, 25%, or 50% of the circumference of the drill bit).

[0306] The depth gauge 70 and the drill bit 90 described above with reference to Figures 8a to 10c are typically operated similarly to the other depth gauges and drill bits described herein. For example, this depth gauge and drill bit may be used with the method of Figure 4.

[0307] In brief, a method of determining a screw size may involve: locating the drill guide 71 on a plate, e.g. a metal plate located adjacent a bone; pressing the drill bit against a surface to be drilled; zeroing the sensor assembly 72 (e.g. using the input 83); starting the drilling process; measuring a displacement of the drill bit as the drilling operation proceeds (e.g. using the sensor arrangement described above); detecting a plunge event, which signals an exit of the drill bit from the bone; determining a hole depth based on this plunge event; and determining a screw size based on this hole depth (where the screw size may involve an offset that is determined by the type of drill guide being used).

[0308] Referring to Figure 11 , the distance ‘y’ shows a portion of the drill that extends beyond the drill guide. In this regard, each of the drill bit and the drill guide are typically pressed against a bone of a patient at the start of a drilling process (so that the distance ‘y’ is zero). However, where a plate is used, the drill guide may be pressed against the plate, while the drill bit extends through the drill guide in order to contact the bone, therefore the value of ‘y’ is non-zero. This value may be similar to the thickness of the plate being used. By zeroing the drill after the drill bit is in contact with the bone (or with another surface to be drilled), the sensor assembly can be used to measure the depth of a hole through the bone. A screw size can then be selected based on this depth as well as the plate thickness (e.g. an offset can be determined for the plate and this offset can be added to the hole depth to determine a suitable screw length). Figure 11 illustrates the existence of this distance ‘y’ to indicate the effect of a plate on the drilling procedure and the importance of considering the plate / drill guide when selecting a screw to fill the hole. Figure 11 also illustrates the benefit of zeroing the sensor assembly based on the drill bit contacting the bone (and not based on the drill bit being adjacent the drill guide).

[0309] The descriptive passages above have mentioned the use of an identifying feature on the drill bit to enable autocalibration. Referring to Figure 11 , the drill bit may comprise a first and a second identifying feature that are spaced by the distance ‘y’. Therefore, the sensor assembly is able to identify whether a plate is being used (and which plate is being used) based on which of the first and second identifying feature is adjacent the sensor at the start of a drilling procedure (e.g. at the time when the drill bit is first activated). In this way, the identifying features of the drill bit can be used to determine an offset for the determination of a suitable screw size. It will be appreciated that the drill bit may comprise a plurality of different identifying features to enable the drill bit to be used for multiple different situations.

[0310] Of relevance, the distance ‘y’ indicates a relative movement of the drill bit relative to the drill guide. Therefore, in embodiments where the drill guide is attached to the sensor assembly, this movement of the drill bit beyond the drill guide moves the drill bit by a distance y relative to the sensor. To give a practical example, the first identifying feature may be located adjacent the sensor where the tip of the drill bit and the edge of the drill guide are aligned (e.g. where there is no plate). If a plate is used, then the tip of the drill bit moves a distance ‘y’ beyond the edge of the drill guide and the drill bit therefore moves a distance ‘y’ relative to the sensor. This leads to the second identifying feature being located adjacent the sensor at the start of the drilling process.

[0311] Typically, the output 82 of the sensor assembly 72 is arranged to provide a continuous indication of a drilling depth as a drilling procedure is performed. For example, the output may display a live depth of a drill bit. The output may be arranged to also (e.g. simultaneously or alternatively) show a hole depth or a recommended screw size following the detection of the breakthrough point.

[0312] The output 82 may additionally or alternatively show other measurements or values. For example, the output may display a maximum displacement of the drill bit (so that this value increases, e.g. in real time, until the end of a drilling process and then remains at a maximum value).

[0313] In some embodiments, the output 82 provides an indication of an identifying feature identified by a sensor and / or an indication of a drill guide or a plate that has been detected by the sensor assembly 72. This enables a user of the depth gauge to check that the correct drill guide and plate are being used during the process.

[0314] Breakthrough and / or plunge detection

[0315] As described above, in addition to the detection of distance, the depth gauge 70 is also typically arranged to detect a breakthrough (or plunge) point of a drilling process. This breakthrough point may be the point at which the drill bit exits the bone and so this breakthrough point indicates the final depth of the hole (any movement beyond this point is not forming the hole in the bone, since the drill bit has already moved beyond the bone). An exemplary determination of a breakthrough point has been described above with reference to Figure 5, which shows an exemplary drilling profile (of displacement over time). As shown in this Figure 5, the drilling process typically comprises four main stages:

[0316] 1 . Drilling through near cortex, where the displacement increases slowly. 2. Drilling through cancellous bone, where the displacement increases relatively quickly.

[0317] 3. Drilling through far cortex, where, again, the displacement increases slowly.

[0318] 4. Plunge, where the drill bit exits the bone and there is a rapid, sudden, increase in displacement

[0319] In order to accurately determine the depth of a hole drilled through a bone, it is desirable to accurately detect this plunge event. A simple way to do this is to detect a period of high velocity (where there is a substantial change in the displacement of the drill bit over a short period of time). However, such a method is vulnerable to false detections, e.g. because a movement of a user of the drill (e.g. to retract the drill bit and then move the drill bit forward again when a user meets resistance) may be inaccurately identified as the plunge event.

[0320] Therefore, Figures 12a, 12b, 12c, and 12d provide alternative, accurate, methods for identifying a breakthrough, or plunge, point of a drilling process. These methods are typically performed by a computer device of the depth gauge and / or the sensor assembly 72. Equally, the methods may be performed by a separate computer device that receives displacement data from the depth gauge (and, e.g., sends a hole depth and / or a screw recommendation to the depth gauge for display).

[0321] In a first step 121 , the computer device receives displacement data identifying a displacement of the drill over the duration of a drilling process. In particular, the computer device may receive displacement data that is obtained using the depth gauge and / or the drill bit disclosed above.

[0322] This displacement data is typically received while the drilling process is ongoing (e.g. in real time). In some embodiments, the displacement data is received following the conclusion of a drilling process (e.g. where the sensor assembly stores the displacement data over the course of a drilling process and then analyses this data once the process has been completed in order to identify a suitable screw size for the hole that has just been drilled).

[0323] The computer device may be arranged to process the received displacement data before evaluating the occurrence of a plunge event. For example, the displacement of the drill may be captured using a quadrature linear encoder at a fixed rate and then averaged for a fixed time window to smooth the signal. More generally, the computer device may be arranged to smooth the raw displacement data, e.g. using a moving window or a smoothing function, and / or to remove outlier data.

[0324] In a second step 122, the computer device determines one or more derivatives of the displacement. For example, the computer device may determine one or more of: a velocity, an acceleration, a jerk, or a snap based on the displacement. Determining the derivative typically comprises determining a derivative profile based on a displacement profile so that the derivative profile indicates the value of the derivative over the period of the drilling process.

[0325] These derivatives are shown below (where x(t) is the displacement of the drill bit at the time t)).

[0326] Velocity: x' = -^x(t)

[0327] Acceleration: x" = — d2dt x(t)

[0328] Jerk:

[0329] Snap: x"" = ^x t) In a third step 123, the computer device detects a plunge event (e.g. a breakthrough event) based on one or more of the derivatives or a combination of one or more of the derivatives. In particular, the computer device may detect a plunge event (e.g. the breakthrough event) based on a combination of a velocity, an acceleration, a jerk, and a snap at a given time meeting a plunge criteria. In a simple embodiment, the plunge criteria may involve each derivative to be above a threshold value and / or a certain number (e.g. three out of four) of the derivatives exceeding a threshold value. In some embodiments, the plunge criteria is dependent on a multiplication of derivatives, for example a multiplication of the jerk and the snap at a given time.

[0330] In some embodiments, the measured derivatives are used to determine derivative functions as follows:

[0331] Where a, b, c, d, e, and k are each constants. These constants may be selected based on previous drilling processes. These constants may be dependent on one or more of: a drill bit being used for the drilling; a drilling procedure being performed (e.g. a bone into which the hole is being drilled); a characteristic of a user of the drill (e.g. data from previous drilling processes performed by this user); and environmental conditions.

[0332] In some embodiments, a (simplified) version of the derivative functions may be considered that is:

[0333] Exemplary ranges of these constants are:

[0334] - 0 < a < 10.

[0335] - 0 < b < 5.

[0336] - 0 < c < 10.

[0337] - 0 < d < 5.

[0338] - -1000 < k < 0.

[0339] - -500 < e < 0.

[0340] But it will be appreciated that the exact values selected for the constants may depend, for example, on a drill being used with the sensor assembly 72. In some embodiments, the constants are determined (e.g. to select specific values from within the above ranges) based on a calibration procedure that is performed priorto a drilling process (e.g. the calibration procedure may be performed on a cadaver).

[0341] With these values, f(x', x", x"',x"") will normally have a value below 0 (e.g. between -1000 and 0), with this value increasing to above 0 when a plunge event has occurred.

[0342] Furthermore, f(x', x",x"',x"") will normally be less than g(x' ,x" ,x"' ,x""), but will be greater than g(x' ,x" ,x"' ,x"") when a plunge event has occurred.

[0343] Typically, a and b are equal. Typically, c and d are equal. Typically, k is less than e.

[0344] The detection of a plunge event (e.g. the breakthrough) typically comprises determining whether a plunge condition or criterion has been met, where in some embodiments this plunge condition is represented by one or more of (or both of) the equations: f(x', x", x"',x"") > 0 f(x',x",x'",x""') > g(x',x",x'",x""')

[0345] More generally, the determination of breakthrough may be based on a plurality of derivative values. This has been found to provide a more accurate and robust determination of breakthrough than a determination based on displacement or velocity alone.

[0346] In some embodiments, the following equation is considered for the derivative condition: h(x', x" , x'" , x"") = p + v ■ x' + t ■ x" + j ■ x"' + s ■ x""

[0347] Where p, v, t, and s are constants. With such a definition, the computer device may consider a plunge definition that is for h(x',x",x"', x"") > 0.

[0348] Typically, the derivative values are determined using the last four measured values of the displacement. It will be appreciated that different sampling lengths may be used.

[0349] Typically, the determination of breakthrough occurs in real-time and this determination is dependent on a current displacement at a current time t being greater than a maximum previous displacement. In this regard, If a high velocity movement is determined at a given displacement that is less than a maximum displacement that has occurred previously (without breakthrough being determined at this maximum displacement), then this given displacement x(t) at the time t cannot be a breakthrough displacement (since, if the bone has not been breached at the greater, maximum, displacement, the bone certainly cannot have been breached at the lesser displacement x(t)).

[0350] More generally, the determination of breakthrough may be performed by moving through a displacement profile until a breakthrough time is determined (starting at time t = 0 and considering times at t > 0. A breakthrough cannot occur at a time t if a displacement x(t) at this time t is less than a displacement x(t-T) at a previous time t-T (i.e. if x(t) < x(t-T)). Of relevance, for the method to proceed to the time t, breakthrough must not have been detected at any time less than t.

[0351] In some embodiments, the method comprises initiating, and considering, a plunge counter. As shown in Figure 5, typically there are two events at which the displacement of the drill increases rapidly: the ‘plunge’ into the cancellous bone and the ‘plunge’ that represents the breakthrough of the bone. The sensor assembly may be arranged to initiate a plunge counter at the start of a drilling process and / or an analysis process and then incrementing this counter whenever a plunge event is detected. The breakthrough event may be determined as the second plunge event.

[0352] In some embodiments, each plunge event is determined based on a consideration of derivative values. In some embodiments, the first and second plunge event are detected using different conditions (where the first plunge event typically does not need to be determined to as great an accuracy as the second, breakthrough, plunge event) and therefore the first plunge event may consider a lesser number of derivative values.

[0353] In some events, the computer device is arranged to: determine a first plunge event at a first time (e.g. a plunge into cancellous bone); to determine a first velocity at this first time; and to detect a breakthrough event at a second time in dependence on the velocity falling below the first velocity between the first time and the second time. This decrease in velocity between the first and second plunge events can identify that the user has moved through the cancellous bone and into the far cortex - and breakthrough can only occur after the drill bit has moved into (and through) the far cortex. Therefore, requiring this fall in velocity prevents the detection of two plunge events prior to the drill bit entering the far cortex.

[0354] Referring to Figure 12b, there is shown a detailed method of determining a breakthrough event (where this event can be used to determine a hole depth and to determine a suitable screw size for a hole that has been formed). It will be appreciated that any one or more of these steps may be implemented in any combination (e.g. the use of a ‘plunge lock’ is optional, though in some situations this may provide a more robust method than an implementation without a plunge lock). As with the method of Figure 12a, this method is typically performed by a computer device and / or the sensor assembly 72.

[0355] This method is performed at a specific time t (based on the displacement values and derivative values at this time t). Typically, the method is performed first at t = 0 and then this time t is incremented so that the method considers increasing values of t. Typically, the method stops once the breakthrough event has been detected since at this time the hole has been formed.

[0356] In a first step 131 , the displacement at time t is compared with the maximum displacement that has occurred since zeroing. If the displacement at time t is not greater than or equal to the maximum displacement that has occurred since zeroing, then plunging cannot have occurred. Therefore, the time t is incremented (or a current time is considered if the method is being performed in real-time) and the method restarts at the first step.

[0357] This first step 131 can be represented by checking a satisfaction of the equation: x(t) > max x time < t).

[0358] In a second step 132, the computer device determines whether a threshold amount of time has passed since a first plunge event was detected (e.g. a plunge into cancellous bone). This threshold amount of time may be dependent on a user of the drill and / or a type of bone into which a user is drilling. This may be termed as a ‘rest counter’ where the rest counter r is compared with a tuning parameter P. The rest counter is determined as the difference between the current time and the time of a first plunge event. As an example, the threshold amount of time may be 0.3s.

[0359] This second step 132 can be represented by checking a satisfaction of the equation: r > P.

[0360] In a third step 133, the computer device determines whether a velocity has fallen below a first plunge velocity prior to the current time. As described above, between the first, cancellous bone, plunge and the second, breakthrough, plunge the drill bit must pass through the far cortex, which results in a deceleration of the drill bit. Therefore, the computer device may ensure that, following a first plunge event, the velocity of the drill bit has fallen below the velocity at this first plunge event prior to the possible occurrence of a second plunge event.

[0361] This third step 133 can be represented by checking a satisfaction of the equation: v(time of first plunge < r < t) < v(time of first plunge), where T is any time in between the time of first plunge and the current time t.

[0362] This third step 133 can be considered to be a ‘plunge lock’ that prevents multiple triggerings of a plunge while the drill bit is still in the cancellous bone. In practice, to implement the third step, after a first plunge event is identified, the velocity at a first time of this first plunge event may be stored in a variable (JnitPlungeVel), and a Lock boolean flag may be set to 1. This Lock flag prevents the detection of breakthrough at times following the first time until the Lock flag is unset. The Lock flag only unset once a velocity below the InitPlungeVel is detected. In some embodiments, the velocity of the drill bit at a time following the initial plunge is compared to a value that is a constant multiplied by the first plunge velocity (e.g. InitPlungeVePconstant). This constant is typically below 1 (so that the velocity must fall a certain amount below the first plunge velocity to unset the Lock flag). For example, the constant may be less than 0.9, less than 0.7, and / or less than 0.5.

[0363] In some embodiments, the plunge lock considers a velocity at the time of a first plunge event. Equally, the plunge lock may consider a, e.g. predetermined of fixed, velocity threshold (that may not be dependent on the velocity at the time of the first plunge event). For example, the plunge lock may consider a fixed velocity threshold (e.g. 30 mm / s) that ensures that after a plunge event has occurred, the velocity of the drill has slowed to a certain velocity before a further plunge is detected to avoid detecting multiple plunge events relating to the same actual plunge event.

[0364] Specifically, the velocity at each subsequent time is compared with InitPlungeVel. Once the velocity at a single time (e.g. a current velocity when the method is processing in real-time) drops below InitPlungeVel, the Lock boolean flag is set to 0.

[0365] In a fourth step 134, as described above with reference to Figure 12a, the computer device detects a plunge event based on one or more derivatives of the displacement. For example, the computer device may compare a plurality of different derivatives to a plurality of different thresholds. Typically, the computer device confirms that the f(x',x", x'",x""') > 0 and f x',x",x"',x"") > g(x',x",x"',x"") (where f(x',x",x"',x"") and g(x',x",x'",x"") have been defined above.

[0366] In a fifth step 135, the computer device determines if a number of detected plunge events is two and detects a breakthrough event based on this number of detected events being two. If the number of detected events is only one, then the method returns to the first step 131 .

[0367] Regarding this last point, the method of Figure 12b is typically performed in two phases. In a first phase, only the first step 131 , the fourth step 134, and the fifth step 135 are performed. This first phase is used to determine a first plunge event (e.g. into cancellous bone). Once this first plunge event has been detected, the method of Figure 12b considers each step and then determines a second plunge event that is a breakthrough event.

[0368] This process is shown in more detail in Figures 12c and 12d.

[0369] Figure 12c shows a first phase of the breakthrough detection method, which first phase occurs before the detection of a first plunge event.

[0370] In a first step 141 , the method starts at a time t=0.

[0371] In a second step 142, as described previously, e.g. with reference to the first step 131 of Figure 12b, the computer device determines whether a displacement at the time t is higher than a maximum previous displacement.

[0372] If the displacement is higher than this maximum previous displacement, then in a third step 143, the computer device determines whether derivatives of the displacement satisfy a plunge detection criteria (e.g. the criteria described above).

[0373] If the derivatives do satisfy the detection criteria, then in a fourth step 144, the computer devices sets the first plunge time and the first plunge velocity to the time t and the velocity v(t).

[0374] Then, in a fifth step 145, the computer device increments the value of t (e.g. so that t becomes a current time). If the requirements of the second step 142 or the third step 143 are not met, then the computer device proceeds directly to this fifth step 145 and increments the value of t. Incrementing the value of t may comprise considering the next available set of displacement data.

[0375] In a sixth step 146, the computer device determines whether a first plunge event has been detected, e.g. this may involve determining if the first plunge time and the first plunge velocity are set or if a flag has been set. If the first plunge event has not been determined, the method returns to the second step 142. If the first plunge event has been determined, then in a seventh step 147, the computer device sets the flag lock 147 and then in an eighth step 148 the computer device proceeds to phase 2 of the breakthrough detection method (as shown in Figure 12d).

[0376] Referring to Figure 12d, in phase 2, in a first step 151 , the computer device determines whether a displacement at the time t is higher than a maximum previous displacement (e.g. as described with reference to the first step 131 of Figure 12b).

[0377] If the displacement is higher than this maximum previous displacement, then in a second step 152, the computer device determines whether a threshold amount of time has passed since the first plunge time.

[0378] If this threshold amount of time has passed, then in a third step 153 the computer device determines if the flag lock is set. If the flag lock is set, then in a fourth step 154, the computer device determines whether the velocity v(t) at the time t currently being considered is below the first plunge velocity.

[0379] If the velocity v(t) is below the first plunge velocity, then in a fifth step 155 the computer device clears the lock flag (and the first plunge velocity can then be disregarded in future iterations of the second phase).

[0380] In a sixth step 156, the computer device then determines whether derivatives of the displacement satisfy a plunge detection criteria (e.g. the criteria described above).

[0381] If the derivatives do satisfy the detection criteria, then in a seventh, final, step 157, the computer devices identifies that breakthrough has occurred at the time t currently being considered (and the method ends).

[0382] If the requirements of the first step 151 , the second step 152, the fourth step 154, or the sixth step 156 are not met, then the computer device proceeds to an eighth step 158 at which the computer device increments the value of t. The computer device then returns to the first step 151 of the second phase. This method of Figure 12d can then be repeated until the breakthrough event is identified.

[0383] When, in the seventh step 157, the computer device has identified a breakthrough event, the computer device may then determine a hole depth as being the displacement of the drill at the time of this breakthrough event and the computer device may determine a suitable screw size for the hole based on this hole depth. The screw size may also be based on a type of drill guide and / or a type of drill bit used during the drilling process (where this may be identified automatically by the sensor assembly 72).

[0384] Typically, the computer device considers whether a plunge has been detected for each displacement measured by the sensor assembly. The computer device may then consider whether each displacement (and the corresponding time) is associated with a plunge event and may store information (e.g. a time, a displacement, or derivative values) each time a plunge event is detected. More specifically, the computer device may determine a hole offset based on the type of drill guide and / or the type of drill bit and / or a type of screw that will be used and / or a type of plate being used, and the computer device may then determine a suitable screw length based on a combination of the hole depth and the hole offset.

[0385] This provides a more accurate, and more robust, method of screw selection than is possible with existing systems.

[0386] The above methods have primarily considered a process that includes a first plunge event and a second plunge event., where the breakthrough event is determined to be the second plunge event. More generally, a drilling process may comprise a plurality of plunge events that may or may not be related to specific locations of the drill bit. For example, the entry of the drill bit into the cancellous bone may lead to the detection of a plunge event. Equally, at a point within the cancellous bone a further plunge event may be detected as a user breaks through a hard patch in the bone.

[0387] Therefore, the method may comprise identifying a breakthrough event as the last plunge event that occurs during a drilling procedure. In this regard, once the use breaks through the far cortex of the bone, they will retract the drill so that there will not be any further plunge events. In some embodiments, the breakthrough event is determined as the plunge event associated with the highest displacement of the drill; therefore, if a user retracts the drill and then inadvertently reinserts the drill and triggers another plunge detection, this plunge is not identified as the breakthrough event.

[0388] Referring to Figures 12c and 12d, where the derivatives of the displacement satisfy the plunge detection criteria, the computer device may identify a plunge event and store the displacement associated with this plunge event. Then, when the computer device determines that a drilling process is over (e.g. by determining that a displacement of the drill bit has dropped below a threshold value), the computer device can identify the plunge event associated with the greatest displacement to be the breakthrough event. This may comprise the computer device identifying that the drill bit has reached a higher threshold displacement value, e.g. 5mm, storing each plunge event and then when the drill bit drops below a lower displacement threshold value, e.g. 0mm, the computer device deems the plunge event associated with the greatest displacement to be the breakthrough event and the computer device determines a suitable screw size for the hole based on the displacement of the drill bit at the time of this breakthrough event.

[0389] The sensor assembly may then determine, after each plunge event (e.g. in the fourth step 154 of Figure 12d) whether a velocity of the drill bit has fallen below the previous plunge velocity or whether a velocity of the drill bit has fallen beneath a threshold velocity, where the lock flag may be cleared based on this determination. Similarly, in the second step 152 of the method of Figure 12d, the computer device may consider whether a threshold amount of time has passed since the previous plunge time. Following each plunge event (e.g. not just the first plunge event), the computer device may set the lock flag, set a previous plunge time, and / or set a previous plunge velocity.

[0390] In some events, if only one plunge event is detected during the entirety of a drilling process, the computer device may determine the breakthrough event to be this first (and only) plunge event.

[0391] The hole depth may be determined based on a displacement of the drill bit at the time of detection of a breakthrough event. Equally, the hole depth may be determined based on a displacement of a drill priorto the time of the breakthrough event. Forexample, where the sensor captures samples at 200Hz, the hole depth may be determined based on the displacement at a time three samples before the breakthrough event. This accounts for the breakthrough event being detected slightly after it has actually occurred (e.g. it takes time for the drill to speed up once it has broken through the far cortex).

[0392] In other words, the computer device may be arranged to determine the hole depth by considering a displacement of the drill bit at a time t-i (e.g. x(t-i)), where i may be a predetermined number or may depend on a characteristic of the drill or the sensor assembly (e.g. i may depend on a sampling frequency of the sensor assembly).

[0393] Referring to Figure 13, there is shown an exemplary graph of displacement of the drill bit against time. This graph shows a drilling process with a plurality of plunge events P1 , P2, P3, P4 and shows how a breakthrough event P4 can be determined as the plunge event associated with the highest displacement. This graph also shows how a manual depth gauge may overestimate the depth of the hole.

[0394] In some embodiments, the computer device (e.g. the sensor assembly 72) may indicate one or more plunge events to a user of the drill, e.g. by showing a depth of each plunge event on the output 82. Typically, the computer device is arranged to indicate only the depth of the breakthrough event to the user and / or only the recommended screw size. This avoids distracting the user by indicating potential false breakthrough events. The computer device may be arranged to indicate the depth (or occurrence) of the breakthrough event or the suggested screw size only when the displacement of the drill bit has fallen beneath the lower threshold displacement value and the drilling process is identified to have ended (or to be ending).

[0395] Where the computer device identifies a completion of a drilling process (e.g. based on a drill bit exceeding a higher threshold value and then subsequently falling below a lower threshold value) without detecting a plunge event, the computer device may output an error notification.

[0396] Alternatives and modifications

[0397] It will be understood that the present invention has been described above purely by way of example, and modifications of detail can be made within the scope of the invention.

[0398] For example, while some embodiments of the depth gauges described above are suitable for attachment to the top of a drill, the depth gauge (and / or an attachment structure of the depth gauge) may equally be arranged to be attached between the drill 1 and the drill bit. Many drills use an AO (or similar coupler) so that a depth gauge that is arranged to be attached to an AO coupler (e.g. with a female AO connection at a first end of the depth gauge and a male AO connection at a second end of the depth gauge) can be attached to a large range of drills. In some embodiments, the depth gauge comprises a bearing arrangement that is arranged to maintain a position of the depth gauge relative to the drill as the drill bit rotates (e.g. where the bearing arrangement enables the drill bit to rotate without rotating the depth gauge). Such an attachment structure (e.g. that is arranged to be connected to a drill bit connector of the drill such that the drill bit can then be attached to the drill via into the depth gauge) increases the versatility of the depth gauge and increases the range of drills to which the depth gauge can be attached.

[0399] The detailed description has primarily considered the estimation of a screw size for the purposes of a surgical procedure. More generally, such estimation of a screw size may be beneficial in numerous situations, such as carpentry, DIY, surgery, veterinary surgery, dentistry, etc. Even more generally, the described methods of determining a depth of a hole may be used in these contexts (and other contexts) even without the determination of a suitable screw size based on this hole. Furthermore, while the detailed description has primarily considered the determination of a screw size for the purpose of an ORIF surgical procedure, it will be appreciated that such screws may be used for a variety of surgical or medical purposes. For example, the screws may be used for (and sized for) holding implants in place (e.g. hip, knee, or spine implants). The depth gauge may, for example, be arranged to determine a suitable screw size for use with a wooden panel.

[0400] The depth gauge is typically arranged to detect a change in a rate of change of distance measured by the depth gauge (e.g. an acceleration or deceleration of the movement of the drill bit) in order to detect the start and / or the end of a drilling procedure. This may be used to determine a distance between the depth gauge and a surface at a time entry of a drill bit into a bone and a distance between the depth gauge and the surface at a time of exit of the drill bit from the bone. Equally, this determination could be used to detect when a drill bit has entered or exited from a wooden or metallic material, where this enables the depth gauge to be used for numerous situations.

[0401] More generally, the depth gauge is typically arranged to determine a material interface based on a distance profile and in particular a gradient of a distance profile (e.g. an acceleration and / or a deceleration of a drill bit). The depth gauge may determine a start point, an end point, and / or a landmark point of a drilling procedure based on the locations of material interfaces.

[0402] Reference numerals appearing in the claims are by way of illustration only and shall have no limiting effect on the scope of the claims.

Claims

Claims1 . A depth gauge comprising: a drill guide arranged to receive a drill bit; and a sensor assembly, the sensor assembly comprising a displacement sensor for determining a displacement of the drill bit during a drilling procedure.

2. The depth gauge of claim 1 , wherein the sensor assembly is removably attached to the drill guide.

3. The depth gauge of any preceding claim, wherein the depth gauge, preferably the sensor assembly of the depth gauge, comprises a type sensor for determining a type of the drill guide.

4. The depth gauge of any preceding claim, wherein the depth gauge, preferably the sensor assembly of the depth gauge, comprises a type sensor for determining one or more of: a type of the drill guide; a type of the drill bit; and a type of a plate being used during the procedure.

5. The depth gauge of any preceding claim, wherein the drill guide comprises a sensor attachment structure for attaching the drill guide to the sensor assembly, preferably wherein the sensor attachment structure indicates a type of the drill guide.

6. The depth gauge of any preceding claim, wherein the drill guide comprises an identifier, preferably an RFID chip or a magnet, for indicating a type of the drill guide.

7. The depth gauge of any preceding claim, wherein the sensor assembly is arranged to determine a depth of a hole formed by the drill bit based on the displacement of the drill bit.

8. The depth gauge of claim 7, wherein the sensor assembly is arranged to determine a suitable screw size for the hole based on the depth of the hole.

9. The depth gauge of claim 7 or 8 when dependent on claim 3 or 4, wherein the sensor assembly is arranged to determine the screw size based on the type of the drill guide and / or the type of the drill bit.

10. A depth gauge comprising: a drill guide; and a sensor assembly, the sensor assembly comprising: a sensor for: determining a type of the drill guide; anddetermining a displacement of a drill bit during a drilling procedure; and a processor for: determining a depth of a hole formed by the drill bit based on the displacement of the drill bit; and determining, based on the depth of the hole formed, a screw size of a screw suitable for insertion into the hole, wherein the sensor assembly is arranged to determine the screw size based on the determined depth and the determined type of drill guide.11 . A depth gauge comprising: a sensor for: determining a displacement of a drill bit during a drilling procedure; and a processor for: determining a depth of a hole formed by the drill bit based on the displacement of the drill bit; and determining, preferably using the sensor, a type of a drill guide used during the drilling procedure; and determining, based on the depth of the hole formed, a screw size of a screw suitable for insertion into the hole, wherein the processor is arranged to determine the screw size based on the determined depth and the determined type of drill guide.

12. The depth gauge of any preceding claim, wherein the processor is arranged to determine the displacement of the drill bit based on a measurement structure of the drill bit, preferably wherein the measurement structure comprises one or more ridges and / or one or more grooves, more preferably wherein the measurement structure comprises two or more ridges and / or grooves that extend axisymmetrically about the circumference of the drill bit.

13. The depth gauge of claim 12, wherein the measurement structure comprises an axisymmetric measurement structure, preferably wherein the measurement structure varies in a z-direction, the z-direction being a direction of a central axis of a drill bit.

14. The depth gauge of any preceding claim, wherein the sensor is arranged to determine the displacement based on a feature and / or a measurement structure of the drill bit that is encompassed by the depth gauge, preferably wherein the depth gauge is arranged to surround a portion of the drill bit and to determine the displacement of the drill bit based on a movement of a feature and / or measurement structure that is on the portion of the drill bit that is surrounded by the depth gauge.

15. The depth gauge of any preceding claim, wherein the sensor comprises a first coil and a second coil, where the sensor is arranged to create a magnetic field in the first coil and to measure aresultant current in the second coil, wherein the displacement is determined based on a change in current in the second coil.

16. The depth gauge of any preceding claim, wherein the sensor assembly is arranged to identify a calibration feature on the drill bit, preferably wherein the calibration feature indicates a type and / or characteristic of the drill bit.

17. The depth gauge of any preceding claim, wherein the sensor assembly is arranged to identify one or more identifying features of the drill bit at the start of a drilling process, wherein the identifying features each indicate an axial position of the drill bit at the start of the drilling process, preferably wherein the sensor assembly is arranged to identify an identifying feature that is adjacent the sensor at the start of the drilling process.

18. The depth gauge of any preceding claim, wherein the drill guide is reusable, preferably wherein: the drill guide is composed of metal, more preferably wherein the drill guide comprises stainless steel; and / or the drill guide is arranged to be sterilized using an autoclave.

19. The depth gauge of any preceding claim wherein the sensor assembly is disposable, and / or single-use, preferably wherein the sensor assembly comprises plastic and / or wherein the sensor assembly is not hermetically sealed.

20. The depth gauge of any preceding claim, wherein the displacement is a displacement in a z direction, where the z direction is the direction of a central axis of the drill bit.21 . The depth gauge of any preceding claim, wherein the sensor determines a plurality of instances of temporal displacement data.

22. The depth gauge of any preceding claim, wherein the drill guide comprises a hole for the passage of a drill bit, preferably wherein the drill guide is arranged to: centre a tip of the drill bit; and / or lock into a plate adjacent a surface to be drilled by the drill bit.

23. The depth gauge of any preceding claim, wherein the drill guide comprises an attachment mechanism, e.g. a thread, that is arranged to cooperate with a plate in order to secure the drill guide to the plate.

24. The depth gauge of any preceding claim, wherein determining the screw comprises determining a size and / or a length of the screw.

25. The depth gauge of any preceding claim, wherein the determination of the screw is based on based on one or more of: a type of surgery associated with the hole; a width of the hole and / or a drill bit used to form the hole; and a characteristic of a patient on whom a surgery is being performed.

26. The depth gauge of any preceding claim, being arranged to determine one or more of: an initiation of a drilling process; an end of a drilling process; and a material interface, preferably being arranged to determine when a tip of a drill bit has entered a bone and / or exited a bone.

27. The depth gauge of claim 26, being arranged to determine the initiation and / or end of a drilling process and / or the material interface based on one or more of, preferably a plurality of: a displacement profile of the drill bit; a derivative of a displacement, preferably a jerk or a snap; and a combination of derivatives of the displacement.

28. The depth gauge of any preceding claim wherein at least a portion of the sensor assembly is arranged to sit between a drill and a drill bit, preferably wherein the depth gauge is arranged to attach the depth gauge to a chuck of the drill and to attach the drill bit to the drill via the depth gauge.

29. The depth gauge of any preceding claim, comprising a drill attachment structure for attaching the depth gauge to a drill, preferably wherein the drill attachment structure is arranged to attach the depth gauge to a chuck of the drill.

30. The depth gauge of any preceding claim, comprising a bit attachment for attaching a drill bit to the depth gauge, preferably wherein the bit attachment is arranged to attach the drill bit to a chuck of a drill via the depth gauge.31 . The depth gauge of any preceding claim, being arranged to determine a breakthrough event, preferably wherein determining the breakthrough event comprises: receiving displacement data relating to a displacement of a drill bit; determining one or more derivatives of the displacement data, preferably comprising determining one or more of: an acceleration, a jerk, and a snap; and determining a breakthrough event based on the one or more derivatives.

32. A computer device, preferably a depth gauge, arranged to determine a breakthrough event, the depth gauge being arranged to determine the breakthrough event by: receiving displacement data relating to a displacement of a drill bit; determining one or more derivatives of the displacement data, preferably comprising determining one or more of: an acceleration, a jerk, and a snap; and determining a breakthrough event based on the one or more derivatives.

33. The depth gauge of claim 31 or 32, wherein the depth gauge is arranged to determine the breakthrough event in dependence on each of: a displacement, a velocity, an acceleration, a jerk, and a snap at a time t.

34. The depth gauge of any of claims 31 to 33, wherein the depth gauge is arranged to determine the breakthrough event based on a multiplication of a plurality of derivatives.

35. The depth gauge of any of claims 31 to 34, wherein the depth gauge is arranged to determine the breakthrough event based on functions f and g defined as:where a, b, c, d, e, and k are constants; preferably, wherein the depth gauge is arranged to determine the breakthrough event based on the following conditions being satisfied: f(x',x",x"',x""') > 0; and / or f(x’,x",x’",x"") > g(x', x", x"',x"").

36. The depth gauge of any of claims 31 to 35, wherein the depth gauge is arranged to determine the breakthrough event during a drilling process, preferably in real-time.

37. The depth gauge of any of claims 31 to 36, wherein the depth gauge is arranged to determine the breakthrough event based on one or more of: a displacement x(t) at a time t being greater than a maximum displacement of the drill bit before the time t; and a time t of the breakthrough event being a threshold amount of time after a previous (e.g. a first) plunge event of the drilling process.

38. The depth gauge of any of claims 31 to 37, wherein the depth gauge is arranged to determine a first plunge velocity of a previous (e.g. a first) plunge event during the drilling process and to determine the breakthrough event based on a velocity of the drill bit falling below this first plunge velocity between the first plunge event and the breakthrough event.

39. The depth gauge of any of claims 31 to 38, wherein the depth gauge is arranged to determine a plurality of plunge events, wherein the breakthrough event is determined as a last (e.g. a second) plunge event, preferably wherein: the conditions for detecting the first plunge event and the second plunge event are different; and / or the breakthrough event is determined as the plunge event with the highest associated displacement.

40. The depth gauge of claim 39, being arranged to detect the first plunge event and, in response to the detection, to determine one or more of: a first plunge time; and a first plunge velocity.41 . The depth gauge of claim 39 or 40, wherein the depth gauge is arranged to set a lock flag following the detection of the first plunge event, wherein the depth gauge is arranged to clear the lock flag when a velocity of the drill falls below (e.g. a threshold amount below) the first plunge velocity and / or when the velocity of the drill falls below a predetermined threshold value (e.g. 30 mm / s).

42. The depth gauge of any of claims 31 to 41 , wherein the depth gauge is arranged to determine a hole depth and / or a screw size for a hole based on the displacement of the drill bit at the time of the breakthrough event.

43. A drill guide arranged to receive a drill bit, the drill guide comprising a sensor attachment structure for attaching the drill guide to a sensor assembly, the sensor assembly comprising a displacement sensor for determining a displacement of the drill bit during a drilling procedure.

44. A sensor assembly for determining a displacement of a drill bit, the sensor assembly comprising: a displacement sensor for determining a displacement of the drill bit during a drilling procedure; and a sensor attachment structure for attaching the sensor assembly to a drill guide.

45. A drill bit comprising a measurement structure, the measurement structure arranged to be sensed by a sensor of a depth guide so as to indicate a displacement of the drill bit during a drilling process.

46. The drill bit of claim 45, wherein: the measurement structure comprises one or more ridges and / or one or more grooves; and / orthe measurement structure comprises two or more ridges and / or grooves that extend axisymmetrically about the circumference of the drill bit; and / or the measurement structure comprises an axisymmetric measurement structure; and / or the measurement structure comprises a continuous slope.

47. The drill bit of claim 45 or 46, comprising: a calibration feature that indicates a type and / or characteristic of the drill bit, preferably wherein the calibration feature comprises a characteristic ridge and / or characteristic groove and / or wherein the calibration feature comprises a characteristic pattern of ridges and / or grooves; and / or a bit identifier, preferably wherein the bit identifier comprises an RFID chip; and / or a proximal end and a distal end, the distal end being arranged to form a hole, wherein the measurement structure is located closer to the proximal end than the distal end; preferably, wherein: the measurement structure is located at least 20% of the drill bit length from the distal end and / or at least 50% of the drill bit length from the distal end; and / or the measurement structure is located at least 5cm from the distal end and / or at least10cm from the distal end.

48. The drill bit of any of claims 45 to 47, comprising one or more identifying features, preferably wherein: the drill bit comprises a plurality of identifying features located at one or more axial positions of the drill bit; and / or each identifying feature comprises one or more of: a marking, a groove, a nodule, or a ridge; and / or each identifying feature extends about the entire circumference of the drill bit; and / or the drill bit comprises a plurality of identifying features, wherein each identifying feature is arranged to indicate a drill guide and / or a plate being used in a drilling procedure.

49. A kit of parts comprising: a drill guide according to claim 43; and a sensor assembly according to claim 44; preferably, comprising a plurality of drill guides according to claim 43, wherein: the drill guides are arranged for use with different drill bits; and / or the drill guides comprise different sensor attachment structures; and / or each drill guide comprises a type identifier, e.g. an RFID chip or a magnet, that indicates a type of the drill guide, wherein the plurality of drill guides comprises a plurality of different types of drill guides.

50. The kit of parts of claim 49, comprising: a first sensor assembly, and a plurality of drill guides, wherein each drill guide comprises a sensor attachment structure for attaching said drill guide to the first sensor assembly; and / or plurality of disposable and / or single use sensor assemblies.51 . The kit of parts of claim 49 or 50, comprising one or more drill bits according to any of claims 45 to 48, preferably comprising: a plurality of drill bits of different sizes; and / or a plurality of drill bits with different bit identifiers and / or calibration features; and / or a plurality of drill bits with different arrangements of identifying features.

52. A system comprising: the depth gauge of any of claims 1 to 30; and a drill bit, preferably the drill bit of any of claims 45 to 48.

53. A method of determining a breakthrough event, the method comprising: receiving displacement data relating to a displacement of a drill bit; determining one or more derivatives of the displacement data, preferably comprising determining one or more of: an acceleration, a jerk, and a snap; and determining a breakthrough event based on the one or more derivatives; preferably, comprising: determining a depth of a hole using the depth gauge; and identifying a screw size for use based on the depth of the hole.

54. A computer programme product comprising instructions arranged to execute the method of claim53.

55. A processor-readable storage device carrying processor-executable instructions that, when executed by a processor of a machine, cause the machine to perform the method of claim 53.

56. A method of operating the depth gauge of any of claims 1 to 42.

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