Estimating soft tissue mechanical characteristics

Abstract

A system for estimating mechanical characteristics of soft tissue which is connected to a patient's patella during surgery on a knee joint which comprises a patella and a femur, comprises a memory device for data relating to the compressive load on a load sensor. The system also comprises a first load sensor having a first thickness for location between the patella and the femur, and a second load sensor having a second thickness for location between the patella and the femur, in which the second thickness is different from the first thickness. The memory device receives compressive load data from the first and second load sensors during flexion of the knee with the first or second load sensor located between the patella and the femur. The system also includes a processor device for calculating a measure of the extensibility of soft tissue connected to a patient's patella based on the effect on the measured compressive load of switching from the first load sensor to the second load sensor.

Description

[0001] ESTIMATING SOFT TISSUE MECHANICAL CHARACTERISTICS

[0002] This invention relates to a system for estimating the extensibility of soft tissue connected to a patient's patella during surgery on a knee joint and to a method of estimating the extensibility of soft tissue connected to a patient's patella.

[0003] Some knee replacement procedures include replacement of the portion of the patella which acts against the femur during articulation of the joint with a patella implant component. Replacement involves removing the surface part of the patella which acts against the femur to prepare the patella to receive the patella implant component. The patella implant component has a bearing surface which is shaped to act against the femur, with a generally domed portion which facilitates sliding of the patella along the trochlear groove.

[0004] Successful implantation of a knee prosthesis relies on accurate positioning of the components of the prosthesis. Anterior knee pain and functional deficit in the implanted prosthesis can result from muscle imbalances or from poor alignment and incorrect positioning of the prosthesis components, especially the femoral and patella components. Factors which can give rise to these problems include patellofemoral instability, mis-positioning of the femoral component along the anterior-posterior axis, and internal-external rotation of the femoral component.

[0005] WO-A-2021 / 240135 discloses a kit for use in knee replacement surgery to help a surgeon to prepare a patella to receive a patella component of a joint prosthesis. The kit includes a load sensor which can be positioned in the space between a patient's patella and femur, and used to measure compressive loads on the sensor when the joint is flexed. The compressive load is dependent on tension in the soft tissue which is attached to the patella, which in turn is dependent on the thickness of the patella. The kit can provide a surgeon with information comparing the compressive load when the patella with the load sensor articulates against the native femur with the compressive load when a femoral component has been fitted. The kit therefore facilitates location by the surgeon of the plane of resection of the patella for fitting to the patella of a patella component, which articulates against the femoral component in the completed implant.

[0006] The present invention provides a system which includes first and second load sensors having different thicknesses, which can be used to estimate mechanical characteristics of soft tissue which is connected to a patient's patella during knee surgery.

[0007] The invention therefore provides a system for estimating mechanical characteristics of soft tissue which is connected to a patient's patella during surgery on a knee joint which comprises a patella and a femur, comprising: a. a memory device for data relating to the compressive load on a load sensor, b. a first load sensor having a first thickness for location between the patella and the femur, c. a second load sensor having a second thickness for location between the patella and the femur, in which the second thickness is different from the first thickness, in which the memory device receives compressive load data from the first and second load sensors during flexion of the knee with the first or second load sensor located between the patella and the femur, and in which the system includes a processor device for calculating a measure of the extensibility of soft tissue connected to a patient's patella based on the effect on the measured compressive load of switching from the first load sensor to the second load sensor.

[0008] The invention also provides a method of estimating mechanical characteristics of soft tissue which is connected to a patient's patella during surgery on a knee joint which comprises a patella and a femur, comprising: a. locating a first load sensor having a first thickness between the patella and the femur, b. flexing the knee joint and measuring the compressive load on the first load sensor during flexion of the knee with the first load sensor located between the patella and the femur, c. locating a second load sensor having a second thickness between the patella and the femur, in which the second thickness is different from the first thickness, d. flexing the knee joint and measuring the compressive load on the second load sensor during flexion of the knee with the second load sensor located between the patella and the femur.

[0009] The invention can be used to estimate mechanical characteristics of soft tissue which is connected to a patient's patella. Examples of mechanical characteristics which can be estimated include extensibility, compliance and laxity. The invention can be used to estimate mechanical characteristics of the quadriceps muscle structure. Information relating to mechanical characteristics (in particular the extensibility) of soft tissue (in particular the quadriceps muscle structure) that is connected to a patella can be useful for a surgeon when identifying the location of the plane of resection of the patella for fitting to the patella of a patella component, which articulates against the femoral component in the completed implant. This in turn can help to reduce the likelihood of dislocation of the patella as a result of the soft tissue tension being too low. It can help to reduce restrictions on flexing of the joint as a result of the soft tissue tension being too high.

[0010] Information relating to the extensibility of soft tissue that is connected to the patella can be useful for a surgeon when identifying the appropriate position for a femoral component during knee replacement surgery, in particular along the anterior-posterior axis. Tension in soft tissue attached to a patella can be reduced by selection of a relatively posterior position for the component.

[0011] Information relating to the extensibility of soft tissue that is connected to the patella can be useful for a surgeon when selecting the appropriate size of femoral component for fitting to a patient's femur during knee replacement surgery. Tension in soft tissue attached to a patella can be reduced by selection of a smaller femoral component.

[0012] The invention can provide information which is useful in the planning of a knee replacement procedure, for example in one or more of the choice of implant components, the position of the femoral component along the anterior- posterior axis, along the superior-inferior axis, and rotationally around the anterior-posterior axis, and in planning the position and inclination of the plane for resecting the patella. The use in planning steps such as these of information concerning the extensibility of soft tissue that is connected to the patella can allow the surgeon to plan the procedure, taking account of wear to the native bearing surface of the patella.

[0013] A load sensor which is suitable for use in the present invention is disclosed in WO-A-2021 / 240135. Subject matter that is disclosed in that document is incorporated in the present document by this reference. Optionally, the load sensor might include force sensing components which are force sensing resistors to measure a compressive load applied to the trial patella component. For example, one or more force sensing resistors might be provided in the load sensor to measure a compressive load applied to the sensor. The load sensing component might be a piezoresistive force sensor.

[0014] The backing plate can have at least two force sensing components which are spaced apart along the superior-inferior axis of the trial patella component.

[0015] The backing plate can have at least two force sensing components which are spaced apart along the medial-lateral axis of the trial patella component.

[0016] For example, the backing plate can have three force sensing components which are spaced apart at the apices of a triangle, for example an isosceles triangle or an equilateral triangle. Two of the force sensing components can be spaced apart along the medial-lateral axis of the trial patella component, with the third force sensing component located superiorly relative to the other force sensing components, especially equidistant from the other components.

[0017] The selection of materials for the load sensor will be made according to its intended function, in particular if it includes deformable parts which are intended to flex for the purpose of sensing the compressive load applied to the trial patella component. Suitable materials include metals and polymers. Suitable metals might include certain stainless steels and certain vanadium alloys, such as titanium-aluminium-vanadium alloys, for example Ti6AI4V alloys. Suitable polymeric materials might include polyolefins, polyamides, polyesters and polycarbonates, optionally with fibre reinforcement. An example of a material that can be preferred for some applications is a nylon reinforced with glass fibres.

[0018] A load sensor can include an adapter which can be fitted to a module which contains force sensing components. An adapter can be formed from a material which is different from the material from which the body of the module is formed. An adapter can be selected to provide a load sensor with a desired thickness. An adapter can provide a load sensor with a desirable surface finish or configuration or both. For example, an adapter can provide a load sensor with a bearing surface configuration which matches the configuration of a selected patella implant component. An adapter can be fitted to the module of a load sensor which includes the force sensing components using flexible latch fingers. This can be facilitated when the adapter is made from a suitably flexible material. The use of one or more adapters has the advantage of facilitating modification of a load sensor to suit the requirements of a particular application.

[0019] The first and second load sensors can include common components. The force sensing components in the first and second load sensors can be in common. For example, the first load sensor can include a removable shim which can be fitted to the load sensor base. The second load sensor can then differ from the first load sensor in that the second load sensor does not include the removable shim, or includes a different shim. Optionally, the removable shim can be removed from the load sensor base while the load sensor is in position between the patient's patella and femur. A removable shim might be replaced by a different shim. The use of a removable shim can facilitate use of the system in that it is not necessary to remove and refit the load sensor between the first and second flexions of the joint. Instead, it is necessary only to remove the shim. For example, a load sensor might include a head portion which can be positioned within a patient's joint, between the patella and the femur, and a handle which can be used to manipulate the sensor head. A shim can be fitted to the head portion. For example, the shim and the head portion might have cooperating raised central portions which can fit together in a nested arrangement. Optionally, the shim can be moved relative to the head portion by sliding it along the handle. The shim and the handle can include cooperating formations to stabilise the shim when it is engaged with the head portion. The formations can include a locator on the head portion of the sensor base which engages a locator on the shim. Examples of suitable locators include raised central portions. The formations can include a pair of opposed flanges on the shim which receive the handle between them. The flanges can facilitate sliding of the shim along the handle, to remove the shim from engagement with the head portion.

[0020] The thickness of a load sensor corresponds to the distance between opposite surfaces which, when the sensor is in use, contact the patella and the femur respectively. Edge portions of the surfaces which contact the patella and the femur can be approximately planar. The planar portions of the surfaces which contact the patella and the femur will frequently be annular portions. A central portion of one or more of the surfaces which contact the patella and the femur can be raised, for example domed. A raised central portion of the load sensor can help to locate the sensor on the femur with the central portion in the trochlear groove. The plane defined by the surface of a load sensor which contacts the patella can be approximately parallel to the plane defined by the surface of the load sensor which contacts the femur.

[0021] The distance between opposite surfaces (frequently planar surfaces) of a load sensor can vary across the said surfaces in a load sensor in which the angle between the surface which contacts the patella and the opposite surface which contacts the femur is non-zero. For example, the angle between the surface of a load sensor which contacts the patella and the surface of the load sensor which contacts the femur can be greater than 0° and not more than about 5°, for example not more than about 4°, for example not more than about 3°, for example not more than about 2°, for example not more than about 1°.

[0022] A load sensor in which the angle between the surface of a load sensor which contacts the patella and the opposite surface of the load sensor which contacts the femur (frequently planar surfaces) is non-zero can include an indicium to provide the user with an indication of the orientation of the load sensor, allowing the user for example to align the axis of the maximum gradient between the opposite surfaces of the load sensors relative to a chosen anatomical axis, for example with the anterior-posterior axis or with the medial-lateral axis.

[0023] The use of one or more load sensors in which the angle between the opposite surfaces (frequently planar surfaces) which contact the patella and the femur is non-zero provides for differential loading of soft tissue on opposite sides of the joint. For example, positioning a load sensor with the axis of the maximum gradient between the opposite surfaces of a load sensor aligned with the patient's medial-lateral axis allows greater load to be applied to the soft tissue on the medial side of the joint compared with the lateral side, or on the lateral side of the joint compared with the medial side. This can make it possible to derive useful information relating to the extensibility of soft tissue (in particular the quadriceps muscle structure) that is connected to a patella for use in planning the position of bone resection planes or implant component positions or both.

[0024] The angle between the opposite surfaces (frequently planar surfaces) of the first load sensor which contact the patella and the femur can be approximately the same as the angle between the opposite surfaces (frequently planar surfaces) of the second load sensor which contact the patella and the femur. The distance between the opposite surfaces of the first load sensor will then be different from the distance between the opposite surfaces of the second load sensor. For example the opposite surfaces of the first load sensor can be approximately parallel to one another and the opposite surfaces of the second load sensor can be approximately parallel to one another, with the distance between the opposite surfaces of the first load sensor being different from the distance between the opposite surfaces of the second load sensor.

[0025] The angle between the opposite surfaces (frequently planar surfaces) of the first load sensor which contact the patella and the femur can be different from the angle between the opposite surfaces (frequently planar surfaces) of the second load sensor which contact the patella and the femur.

[0026] A non-zero angle between the opposite surfaces (frequently planar surfaces) of the first load sensor which contact the patella and the femur can be approximately the same as the angle between the opposite surfaces (frequently planar surfaces) of the second load sensor which contact the patella and the femur.

[0027] Each of the first and second load sensors can include at least one shim which can be fitted to the load sensor base. The thicknesses of the first and second load sensors can be changed by use of shims having different thicknesses, or by changing the number of shims. The distance between parallel opposite principal surfaces (frequently planar surfaces) of a first shim can differ from the distance between opposite principal surfaces of a second shim. At least one shim might be used in which the angle between opposite principal surfaces (frequently planar surfaces) is non-zero. Shims might be used in which there are different non-zero angles between their opposite principal surfaces (frequently planar surfaces).

[0028] Optionally, a central portion of one or more of the surfaces of the load sensor which contact the patella and the femur can be raised, for example domed. When the load sensor includes a shim and a load sensor base, matching raised portions on the shim and the load sensor base can help to locate the shim on the load sensor base. A raised central portion of the load sensor (and shim when present) can help to locate the sensor on the femur with the central portion in the trochlear groove.

[0029] Optionally the difference between the thickness of the first load sensor and the thickness of the second load sensor is at least about 0.5 mm.

[0030] Optionally the difference between the thickness of the first load sensor and the thickness of the second load sensor is not more than about 5 mm, for example not more than about 3 mm, for example not more than about 2 mm, for example not more than about 1.5 mm.

[0031] Optionally the difference between the thickness of the first load sensor and the thickness of the second load sensor is about 1 mm.

[0032] The system can make use of first and second load sensors which do not make use of common components.

[0033] Optionally, the processor is programmed to generate data relating to the position of a plane for resecting the patella for fitting to the patella of a patella component of a knee joint prosthesis, in which the position of the said resection plane takes account of the extensibility of the soft tissue connected to the patient's patella as characterised using the first and second load sensors. Optionally, the processor is programmed to generate data relating to the position for fitting a femoral component of a knee joint prosthesis on to a patient's femur, in which the position of the said femoral component takes account of the extensibility of the soft tissue connected to the patient's patella as characterised using the first and second load sensors.

[0034] Optionally, the processor is programmed to generate data relating to the size of a femoral component of a knee joint prosthesis, in which the selection of the size of the said femoral component takes account of the extensibility of the soft tissue connected to the patient's patella as characterised using the first and second load sensors.

[0035] Optionally, the system includes an output device for providing a user with data generated by the processor. The output device can comprise a video display. The output device can comprise a printer. The output device can provide an audio output. Other forms of output device will be apparent to the reader. The system for estimating the extensibility of soft tissue connected to a patient's patella that is provided by the invention can form part of a system for use in knee joint surgery which is used to optimise the positions of femoral and tibial components of a knee joint prosthesis. Such a system can provide a surgeon with guidance as to the selection of appropriate implant components (for example one or more of femoral, tibial and patella implant components), and appropriate positions of the femoral and tibial components along the anterior posterior axis and along the superior inferior axis, to provide an appropriate flexion-extension gap.

[0036] The system can be used in a method which can include the following steps:

[0037] 1. The patella is resected posteriorly to create a surface for mounting the load sensor. The resection will generally be conservative, involving removal of a piece of bone whose thickness is less than the thickness of an implant component which is subsequently to be fitted to the patella. For example, if the thickness of a patella implant component is about 8 mm, the thickness of the portion of bone that is removed from the patella in an initial resection step might be about 7 mm. The surface of the patella that is created in the resection step allows the sensor to be located positively relative to the patella, with pins on the sensor penetrating the exposed cancellous patella bone tissue.

[0038] 2. The knee joint is flexed with the first load sensor in position on the posterior face of the patella, and in contact with the femur. The forces applied to the sensor due to the action of soft tissue attached to the patella throughout the range of motion of the joint are recorded.

[0039] 3. The knee joint is flexed with the second load sensor in position on the posterior face of the patella, and in contact with the femur. When the first load sensor comprises a load sensor body and a shim, the second load sensor can be created by removing the shim. The second load sensor is therefore thinner than the first load sensor. The difference in thickness between the first and second load sensors corresponds to the thickness of the shim. The forces applied to the second load sensor due to the action of soft tissue attached to the patella throughout the range of motion of the joint are recorded.

[0040] 4. A measure of the extensibility of soft tissue connected to a patient's patella can involve calculation of the change in the compressive force as measured using the first and second load sensors:

[0041] AF = Fl - F2 where AF is the change in the compressive force, Fl is the compressive force measured using the first load sensor, and F2 is the compressive force measured using the second load sensor.

[0042] The calculation of the measure of the extensibility of soft tissue connected to a patient's patella can take account of the difference in thickness between the first and second load sensors:

[0043] AT = T1 - T2 where AT is the difference in thickness, T1 is the thickness of the first load sensor, and T2 is the thickness of the second load sensor.

[0044] A measure of the extensibility of the soft tissue connected to the patella ("Ext" in the formula below) can then be calculated as the product of the change in compressive force and the difference in thickness between the first and second load sensors:

[0045] Ext = AF / AT

[0046] 5. It is useful to take account of the patient's bony anatomy for both the femur and the tibia. This can be achieved by detailed mapping and input of the patient's anatomical data into the planning software.

[0047] 6. The sizes and positions of femoral and tibial implant components are selected provisionally, so that the gaps between the femoral and tibial components are optimised with the knee joint both flexed and in extension.

[0048] 7. The tension that will result in the soft tissue that is attached to the patella that will result from the provisional position of the femoral component is assessed. The tension should be sufficiently high to ensure that the risk of dislocation of the patella is low. The tension should not be too high which can result in discomfort and reduced ability to flex the joint (sometimes referred to as "overstuffing"). If the soft tissue tension in the implanted joint requires adjustment, this can be achieved by one or more of: a. adjusting the position of the femoral component, b. adjusting the depth of the resection of the patella for fitting to the patella of a patella implant component, c. changing the size of the patella component (in particular if the soft tissue tension is too high and there is insufficient bone in the patella to increase the depth of the patella resection sufficiently to bring the soft tissue tension down to an appropriate level). 8. The definitive patella resection is then identified, taking account of the selected positions of the femoral and tibial components, the extensibility of the soft tissue attached to the patella, and the thickness of the patella. It will generally preferred that the final thickness of the patella after resection is not less than 12 mm. A resection guide can be used during the patella resection step, for example as disclosed in WO-A-2023 / 089310. Subject matter that is disclosed in that document is incorporated in the present document by this reference.

[0049] Steps of the method can be performed by a computer which includes memory for receiving and storing data from load sensing components in the load sensor, and a processor for calculating a measure of the extensibility of soft tissue connected to the patient's patella. The computer can be used to perform other calculations in relation to the surgical procedure, including for example the positions of femoral and tibial components to provide an appropriate spacing between the femur and the tibia, in both flexion and extension.

[0050] The invention is described below by way of example with reference to the accompanying drawings, in which:

[0051] Figure 1 is an exploded isometric view of a load sensor which can be used for estimating the extensibility of soft tissue connected to a patient's patella during knee surgery.

[0052] Figure 2 is an enlarged isometric view of components of the load sensor shown in Figure 1.

[0053] Figure 3 is an isometric view of the load sensor shown in Figure 1.

[0054] Figure 4 is an exploded isometric view of another load sensor which can be used for estimating the extensibility of soft tissue connected to a patient's patella during knee surgery.

[0055] Figure 5 is an enlarged isometric view of components of the load sensor shown in Figure 4.

[0056] Figure 6 is an isometric view of the load sensor shown in Figure 4.

[0057] Figures 7a and 7b are graphs showing the variation in compressive load during flexion of a knee joint as measured using first and second load sensors respectively, each of the load sensors having four load sensing components.

[0058] Referring to the drawings, Figures 1 to 3 show a load sensor 2 which includes a load sensor base 4. The load sensor base includes a head portion 6 and a handle 8 extending from the head portion. The head portion includes four load sensing components provided by respective piezoresistive force sensors. It has three pins on its lower face (not visible) which can penetrate the cancellous bone of the patella, so that relative sliding movement between the load sensor and the patella, across the surface of the patella, is resisted. These features are as disclosed in WO-A-2021 / 240135.

[0059] The load sensor includes a cable 9 which extends from the handle 8. The cable carries signals from the load sensing components to a computer for calculation of surgical procedure parameters, including for example the position of the femoral component, the depth of the resection of the patella for fitting to the patella of a patella implant component, and the size of the patella component.

[0060] The load sensor 2 includes a bearing surface adapter 10 which is fitted to the head portion of the load sensor base. In the construction shown in Figures 1 to 3, the bearing surface adapter has four downwardly extending flexible fingers 12 spaced apart around its rim 14. The head portion 6 of the load sensor base 4 has four detents 16 spaced part around its rim. The flexible fingers 12 on the rim of the bearing surface adapter 10 are received in the detents 16 on the rim of the head portion of the load sensor base, so that the bearing surface adapter engages and is retained on the head portion. The bearing surface adapter has a central domed portion 18 on the surface 20 which faces away from the load sensor base. It has a planar peripheral portion 22 which can contact the surface of a patient's femur when the load sensor is in place on the patient's patella, with the central domed portion received in the trochlear groove. The configuration of the surface 20 is similar to the configuration of the bearing surface of the patella implant component which is intended to be fitted to the patella. The load sensor can be provided with a plurality of such bearing surface adapters which differ from one another in terms of their thicknesses, or in terms of their surface configurations, or both. Such a load sensor which includes one or more detachable adapters can be used to identify the appropriate location of the plane for resecting a patella in knee replacement surgery, in which the location is selected to provide a desired relationship between the tension in soft tissue attached to the patella before the procedure and that tension after the procedure. This is discussed in detail in WO-A- 2021 / 240135 and WO-A-2023 / 089311. Subject matter disclosed in these documents is incorporated in the present document by these references.

[0061] The load sensor includes a slidable shim 30 which has a head portion 32 and a handle portion 34. The head portion has a central domed portion 36 on the surface 38 which faces away from the load sensor base. It has a planar peripheral portion 40 which can contact the surface of a patient's femur when the load sensor is in place on the patient's patella, with the central domed portion received in the trochlear groove. The domed central portion 36 of the slidable shim 30 can receive the domed central portion 18 of the bearing surface adapter 10 in a nesting arrangement. This can help to locate the slidable shim relative to the bearing surface adapter on the head portion of the load sensor.

[0062] The handle portion 34 on the slidable shim has a half-collar 42 at its free end which is a snug fit on a cylindrical portion 44 of the handle 8 on the load sensor base (as shown in Figure 3).

[0063] The load sensor 2 can be used in steps in a method of planning a knee replacement procedure in which the components of the patellofemoral joint are selected and their implant locations identified. As described above, an initial step involves a conservative resection of the patella to create a planar surface on which a load sensor can be located. For example, when a patella implant component has a thickness about 8 mm, the thickness of the portion of bone that is removed from the patella in an initial resection step might be about 7 mm. This is described in WO-A-2021 / 240135 and WO-A-2023 / 089311. The load sensor 2 is positioned against the planar resection surface of the patient's patella with the head portion 32 of the slidable shim 30 in position on the bearing surface adapter 10 and the halfcollar 42 on the handle portion 34 of the slidable shim located on the cylindrical portion 44 of the handle 8 of the load sensor base. In an example, the thickness of the load sensor construct, with the slidable shim, is 9.55 mm (measured to the top of the central domed portion of the slidable shim).

[0064] The compressive load applied to the load sensor, between the patella and the femur, is measured using the load sensor and recorded, for example as the joint is flexed.

[0065] The slidable shim is removed from its position on the load sensor base by disengaging the half-collar 42 on the handle portion 34 of the slidable shim from the cylindrical portion 44 of the handle 8 of the load sensor base, and retracting the shim by applying force to the handle portion of the slidable shim relative to the load sensor base. This results in an overall reduction in thickness of the load sensor. The difference in thickness of the load sensor which results from removal of the slidable shim corresponds to the thickness of the slidable shim. In the example above, the thickness of the load sensor construct, after removal of the slidable shim, is 8.55 mm (measured to the top of the central domed portion of the bearing surface adapter). The compressive load applied to the load sensor between the patella and the femur is measured again and recorded, for example as the joint is flexed.

[0066] The system of the invention can include a memory component for recording compressive load data from the load sensor.

[0067] Figures 4 to 6 show a load sensor 52 which includes a load sensor base 54. The load sensor base includes a head portion 56 and a handle 58 extending from the head portion, generally as described above with reference to Figures 1 to 3. The head portion includes four load sensing components provided by respective piezoresistive force sensors. It has three pins on its lower face (not visible) which can penetrate the cancellous bone of the patella, so that relative sliding movement between the load sensor and the patella, across the surface of the patella, is resisted. These features are as disclosed in WO-A- 2021 / 240135.

[0068] The load sensor includes a cable 59 which extends from the handle 58. The cable carries signals from the load sensing components to a computer for calculation of surgical procedure parameters, including for example the selection and positions of implant components, and the depth of the resection of the patella for fitting to the patella of a patella implant component.

[0069] The load sensor 52 includes a bearing surface adapter 60 which is fitted to the head portion of the load sensor base. The bearing surface adapter has a central domed portion 68 on the surface 70 which faces away from the load sensor base. It has a planar peripheral portion 72 which can contact the surface of a patient's femur when the load sensor is in place on the patient's patella, with the central domed portion received in the trochlear groove. The configuration of the surface 70 is similar to the configuration of the bearing surface of the patella implant component.

[0070] A central opening 74 is formed in the domed portion 68 of the bearing surface adapter. Four flanges 76 are provided in the wall which defines the central opening, spaced apart equally around the opening.

[0071] The load sensor includes a slidable shim 80 which has a head portion 82 and a handle portion 84. The head portion has a central domed portion 86 on the surface 88 which faces away from the load sensor base. It has a planar peripheral portion 90 which can contact the surface of a patient's femur when the load sensor is in place on the patient's patella, with the central domed portion received in the trochlear groove. This can help to locate the slidable shim relative to the bearing surface adapter on the head portion of the load sensor.

[0072] As shown in Figure 5, four fingers 92 depend from the surface 94 of the slidable shim which faces towards the load sensor base, within the central domed portion. The fingers are spaced apart equally around the domed central portion and each of them has an outwardly extending flange portion 96.

[0073] The domed central portion 86 of the slidable shim 80 can receive the domed central portion 68 of the bearing surface adapter 60 in a nesting arrangement. When so arranged, the fingers 92 on the slidable shim and the flanges 76 on the bearing surface adapter cooperate in a bayonet arrangement so that the fingers initially fit between the flanges, and rotation of the slidable shim relative to the load sensor base (using the handle portion 84) through 45° causes the flanges 76 on the bearing surface adapter to engage the flange portions on the fingers on the slidable shim. This can help to retain the slidable shim in contact with the bearing surface adapter on the head portion of the load sensor.

[0074] The load sensor described above with reference to Figures 4 to 6 can be used in a surgical method in generally the same way as the load sensor described above with reference to Figure 1 to 3, save that the mechanism by which the slidable shims engage and are held in position relative to the load sensor bases differ. Removal of the slidable shim 80 from its position on the load sensor body involves rotation of the slidable shim relative to the load sensor base through 45° to disengage the bayonet flange features before then retracting the shim by applying force to the handle portion of the slidable shim relative to the load sensor base. This results in an overall reduction in thickness of the load sensor. The difference in thickness of the load sensor which results from removal of the slidable shim corresponds to the thickness of the slidable shim. Figures 7a and 7b are graphs which show the compressive loads that were sensed during flexion of a knee joint using a load sensor which positioned between a patient's natural femur and the resected patella. This is as described above. The load sensor had four load sensing components arranged so that they measure loads in the lateral, superior, medial and inferior parts of the patella. The loads that were measured using the medial and inferior load sensing components were negligible. The loads that were measured using the lateral and superior load sensing components were measurable. In each of the four flexion cycles, the lateral load is higher than the superior load.

[0075] The peak compressive loads that are recorded in the graph of Figure 7b are lower than the loads that are recorded in the graph of Figure 7a. This is because the loads recorded in Figure 7a were recorded with the slidable shim in place as part of the load sensor, whereas the loads recorded in Figure 7b were recorded with the slidable shim removed. This is as described above. The thickness of the slidable shim was 1 mm. Accordingly, the overall thickness of the load sensor used to record the loads in the graph of Figure 7a was 1 mm greater than the overall thickness of the load sensor used to record the loads in the graph of Figure 7b.

[0076] The peak loads measured by the lateral and medial sensors, as recorded in the graphs of Figures 7a and 7b, are set out in the following tables:

[0077] The average total compressive load with the slidable shim in place (Figure 7a) was 3600 force units, and the average total compressive load with the slidable shim removed (Figure 7b) was 3150 force units. The reduction in thickness of the load sensor by 1 mm resulted in a reduction in the total compressive load measured by the load sensor of 450 force units. This is a measure of the extensibility of the soft tissue that is connected to the patella.

[0078] A femoral implant component frequently presents additional material anteriorly compared with a patient's natural femur so that the femoral bearing surface presented by a femoral implant component which the patella articulates against during flexion of the knee is displaced anteriorly relative to the femoral bearing surface provided by the natural femur. Greater tension in soft tissue (for example quadriceps tissue) results if the patella is displaced anteriorly.

[0079] The information concerning the extensibility of the soft tissue that is connected to the patella can be used to provide an estimate of the amount of additional bone that should be removed from the patella to accommodate the thickness anteriorly of a femoral implant component. This can be done by using the load sensor to measure the peak compressive load between during flexion of a knee joint using a load sensor which positioned between a trial femoral component fitted to a patient's femur and the resected patella. The displacement of the femoral bearing surface which is a consequence of fitting to the femur of the trial femoral component can be expected to result in a recorded total compressive load of around 4050 force units. The increase in the total compressive load compared with the natural femur (Figure 7b) is 900 force units.

[0080] Given that the total compressive load measured by the load sensor increases by 450 force units for each millimetre of anterior displacement of the patella, the increase in the total compressive load which results from the fitting to the femur of the trial femoral component can be eliminated by removing another 2 mm of bone from the posterior of the patella in a second resection.

[0081] Planning of knee replacement procedures commonly makes use of software to model the procedure and to aid planning, including the selection of implant components and the position of bone cuts to ensure appropriate location of the components relative to one another. Modelling can provide a surgeon with proposals for implant component sizes, and positions to ensure appropriate spacing between the femoral and tibial implant components in both flexion and extension of the joint. The modelling can provide the surgeon with an estimate of the increase in the tension in soft tissue that is connected to the patella, and therefore of the thickness of bone that should be removed from the patella in a second resection. Generation of additional modelling plans for the surgeon to consider should follow if the amount of bone that is available for removal from the patella is insufficient (for example if it will result in the patella thickness being less than 12 mm), for example involving moving the femoral implant component posteriorly, or selection of a smaller femoral component.

Claims

CLAIMS:

1. A system for estimating mechanical characteristics of soft tissue which is connected to a patient's patella during surgery on a knee joint which comprises a patella and a femur, comprising: a. a memory device for data relating to the compressive load on a load sensor, b. a first load sensor having a first thickness for location between the patella and the femur, c. a second load sensor having a second thickness for location between the patella and the femur, in which the second thickness is different from the first thickness, in which the memory device receives compressive load data from the first and second load sensors during flexion of the knee with the first or second load sensor located between the patella and the femur, and in which the system includes a processor device for calculating a measure of the extensibility of soft tissue connected to a patient's patella based on the effect on the measured compressive load of switching from the first load sensor to the second load sensor.

2. A system as claimed in claim 1, in which the first load sensor includes a removable shim, and in which the second load sensor differs from the first load sensor in that the second load sensor does not include the removable shim.

3. A system as claimed in claim 2, in which the removable shim can be removed from the load sensor while the load sensor is in position between the patient's patella and femur.

4. A system as claimed in any one of claims 1 to 3, in which the processor is programmed to generate data relating to the position of a plane for resecting the patella for fitting to the patella of a patella component of a knee joint prosthesis, in which the position of the said resection plane takesaccount of the extensibility of the soft tissue connected to the patient's patella as characterised using the first and second load sensors.

5. A system as claimed in any one of claims 1 to 4, in which the processor is programmed to generate data relating to the position for fitting a femoral component of a knee joint prosthesis on to a patient's femur, in which the position of the said femoral component takes account of the extensibility of the soft tissue connected to the patient's patella as characterised using the first and second load sensors.

6. A system as claimed in any one of claims 1 to 5, in which the processor is programmed to generate data relating to the size of a femoral component of a knee joint prosthesis, in which the selection of the size of the said femoral component takes account of the extensibility of the soft tissue connected to the patient's patella as characterised using the first and second load sensors.

7. A method of estimating mechanical characteristics of soft tissue which is connected to a patient's patella during surgery on a knee joint which comprises a patella and a femur, comprising: a. locating a first load sensor having a first thickness between the patella and the femur, b. flexing the knee joint and measuring the compressive load on the first load sensor during flexion of the knee with the first load sensor located between the patella and the femur, c. locating a second load sensor having a second thickness between the patella and the femur, in which the second thickness is different from the first thickness, d. flexing the knee joint and measuring the compressive load on the second load sensor during flexion of the knee with the second load sensor located between the patella and the femur.

8. A method as claimed in claim 7, which includes the step of calculating a measure of the extensibility of soft tissue connected to a patient's patella based on the effect on the measured compressive load of switching from the first load sensor to the second load sensor.

9. A method as claimed in claim 7 or claim 8, in which the first load sensor includes a removable shim, and in which the second load sensor differs from the first load sensor in that the second load sensor does not include the removable shim.

10. A method as claimed in claim 9, in which the removable shim can be removed from the load sensor while the load sensor is in position between the patient's patella and femur.

11. A method as claimed in any one of claims 7 to 10, which includes the step of resecting the patella to form a resected surface which faces the femur for location of the first load sensor during the step of flexing the knee joint and measuring the compressive load on the first load sensor.

12. A method as claimed in any one of claims 7 to 11, which includes the step of identifying the location of a plane on the patella on which to perform a resection of the patella for fitting to the patella of a patella component of a knee joint prosthesis, in which the step of locating the said resection plane takes account of the extensibility of the soft tissue connected to the patient's patella as characterised using the first and second load sensors.

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