Registration of pre-operative and intra-operative data

Abstract

Examples relate to associating pre-operative data and intra-operative data. The pre-operative data describing a bone in a first reference frame, the pre-operative data including a first description of a portion of a surface of the bone, and one or more first anatomical landmarks. The intra-operative data describing the bone in a second reference frame, the intra-operative data including a second description of the portion of the surface of the bone, and one or more second anatomical landmarks. A transform is obtained, the transform associating the bone in the first reference frame with the bone in the second reference frame. Obtaining the transform includes associating the first description with the second description, subject to constraining a parameter, wherein the parameter is selected from: a translation between the first reference frame and the second reference frame, a rotation between an axis of the first reference frame and an axis of the second reference frame, or a distance between an anatomic landmark in the pre-operative data and a corresponding anatomic landmark in the intra-operative data to be less than a threshold distance.

Description

REGISTRATION OF PRE-OPERATIVE AND INTRA-OPERATIVE DATATECHNICAL FIELD

[0001] The present disclosure generally relates to methods, systems, and apparatuses related to a computer-assisted surgical system that includes various hardware and software components that work together to enhance surgical workflows. More specifically, the present disclosure relates to methods, systems, and apparatuses for registration between pre-operative and intraoperative data.BACKGROUND

[0002] Computer Assisted Surgery (CAS) systems (CASS) and methods may improve the outcomes of various surgical procedures, and may include forming a pre-operative 3- Dimensional (3D) model of the patient (i.e. the portion of the patient relevant to the procedure) that may be used in preoperative planning. The 3D model may be based on one or more methods, such as computed tomography (CT), magnetic resonance imaging (MRI), etc.

[0003] The 3D model may be used intra-operatively. For example, to assist in accurate bone resection and positioning implants in arthroplasty procedures.SUMMARY

[0004] This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended as an aid in determining the scope of the claimed subject matter.

[0005] Disclosed herein are improved systems and methods for computer assisted surgery.

[0006] Examples described in the present disclosure provide numerous advantages over conventional systems and methods. In one non-limiting example advantage, a registration between pre-operative and intra-operative data may be performed efficiently (e.g., in terms of computing resources).

[0007] In an example, an apparatus for associating pre-operative data and intra-operative data includes: an input for: receiving the pre-operative data, the pre-operative data describing a bone in a first reference frame, the pre-operative data including a first description of a portion of a surface of the bone, and one or more first anatomical landmarks, and receiving the intra-operativedata, the intra-operative data describing the bone in a second reference frame, the intra-operative data including a second description of the portion of the surface of the bone, and one or more second anatomical landmarks. The apparatus further includes processing circuitry to obtain a transform associating the bone in the first reference frame with the bone in the second reference frame, wherein obtaining the transform includes: associating the first description with the second description, subject to constraining a parameter, wherein the parameter is selected from: a translation between the first reference frame and the second reference frame, a rotation between an axis of the first reference frame and an axis of the second reference frame, or a distance between an anatomic landmark in the pre-operative data and a corresponding anatomic landmark in the intra-operative data to be less than a threshold distance.

[0008] In another example, a computer assisted surgery system comprises an apparatus, as described herein, and a tracking system to obtain the intra-operative data by sensing a tracking device coupled to the bone.

[0009] In another example, a non-transitory computer-readable medium stores instructions that, when executed by a processing device, cause the processing device to: receive pre-operative data including a first description of a bone surface of a bone in a first reference frame, and preoperative landmarks on the bone surface in the first reference frame; receive intra-operative data including a second description of the bone surface in a second reference frame, and intraoperative landmarks on the bone surface in the second reference frame; obtain a coarse transform that relates the first reference frame and the second reference frame based on the pre-operative landmarks and the intra-operative landmarks; and obtain a refined transform that relates the first reference frame and the second reference frame, the refined transform obtained by refining the coarse transform, wherein the refining includes: registering the first description of the bone surface and second description of the bone surface, wherein the registering includes constraining least one of: a degree of freedom of a relationship between axes of the first reference frame and axes of the second reference frame, or a distance between a landmark of the first description and a corresponding landmark of the second description to be less than a threshold distance.

[0010] In another example, a computer-implemented method of data registration comprises: receiving pre-operative data including a first description of a bone surface of a bone in a first reference frame, and pre-operative landmarks on the bone surface in the first reference frame; receiving intra-operative data including a second description of the bone surface in a second reference frame, and intra-operative landmarks on the bone surface in the second reference frame;obtaining a coarse transform that relates the first reference frame and the second reference frame based on the pre-operative landmarks and the intra-operative landmarks; and obtaining a refined transform that relates the first reference frame and the second reference frame, the refined transform obtained by refining the coarse transform, wherein the refining includes: registering the first description of the bone surface and second description of the bone surface, wherein the registering includes constraining least one of: a degree of freedom of a relationship between axes of the first reference frame and axes of the second reference frame, or a distance between a landmark of the first description and a corresponding landmark of the second description to be less than a threshold distance.BRIEF DESCRIPTION OF THE DRAWINGS

[0011] By way of illustration, specific examples of the disclosed device and method will now be described, with reference to the accompanying drawings, in which:

[0012] FIG. 1 illustrates a CASS according to an example described herein;

[0013] FIG. 2 illustrates a method according to an example described herein;

[0014] FIG. 3 illustrates a computer-readable medium according to an example described herein; and

[0015] FIG. 4 illustrates an apparatus according to an example described herein.

[0016] It should be understood that the drawings are not necessarily to scale and that the disclosed examples are sometimes illustrated diagrammatically and in partial views. In certain instances, details which are not necessary for an understanding of the disclosed methods and devices, or which render other details difficult to perceive may have been omitted. It should be further understood that this disclosure is not limited to the particular examples illustrated herein. In the drawings, like numbers refer to like elements throughout unless otherwise noted.DETAILED DESCRIPTION

[0017] This disclosure is not limited to the particular systems, devices and methods described, as these may vary. The terminology used in the description is for the purpose of describing the particular versions or examples only and is not intended to limit the scope.

[0018] As used in this document, the singular forms “a,” “an,” and “the” include plural references unless the context clearly dictates otherwise. Unless defined otherwise, all technical and scientific terms used herein have the same meanings as commonly understood by one ofordinary skill in the art. Nothing in this disclosure is to be construed as an admission that the examples described in this disclosure are not entitled to antedate any public or other disclosure. As used in this document, the term “comprising” means “including, but not limited to.”

[0019] FIG. 1 illustrates a CASS 100 according to an example. In the example of FIG. 1, the CASS 100 is arranged to aid surgeons in performing orthopedic surgical procedures such as, for example, a knee arthroplasty (e.g., total knee arthroplasty (TKA)) or total hip arthroplasty (THA).

[0020] An effector platform 102 positions surgical tools relative to a patient during surgery. For example, for a knee surgery, the effector platform 102 may include an end effector 102a that holds surgical tools or instruments during their use.

[0021] The end effector 102a may be a handheld device or instrument used by the surgeon (e.g., a hand piece or a cutting guide or jig of a surgical system) or, alternatively, the end effector 102a can include a device or instrument held or positioned by a robotic arm 102b. In some examples, the end effector 102a may hold resection equipment that is to perform bone or tissue resection.

[0022] Effector platform 102 can include a limb positioner 102c for positioning the patient’s limbs during surgery. Effector platform 102 can also include a cutting guide or jig 102d that is used to guide saws or drills used to resect tissue during surgery. Such cutting guides 102d can be formed integrally as part of the effector platform 102 or robotic arm 102b, or cutting guides can be separate structures that can be matingly and / or removably attached to the effector platform 102 or robotic arm 102b.

[0023] The CASS 100 comprises a display 104 to provide graphical user interfaces (GUIs) that display images collected by the Tissue Navigation System as well other information relevant to the surgery to a surgeon or other operating theater staff. For example, the display 104 may overlay image information collected from various modalities (e.g., CT, MRI, X-ray, fluorescent, ultrasound, etc.) collected pre- operatively or intra-operatively to give the surgeon various views of the patient’s anatomy as well as real-time conditions. A surgical computer 106 provides control instructions to various components of the CASS 100, collects data from those components, and provides general processing for various data during surgery. The surgical computer 106 may be a general purpose computer, for example. In some examples, the surgical computer 106 may be connected to a remote server over one or more computer networks (e.g., the Internet). The remote server may be used, for example, for storage of data or execution ofcomputationally intensive processing tasks. The surgical computer 106 may be communicatively coupled with other elements of the CASS 100, for example, by wired or wireless connections.

[0024] The CASS 100 comprises a tracking system 108 that uses one or more sensors 110 to collect real-time position data that locates the patient’s anatomy and surgical instruments. Any suitable tracking modality can be used in the tracking system 108. For example, a combination of infrared (IR) and visible light cameras can be used in an array. For example, by tracking fiducial marks associated with a tool or bone structure, or by using other image tracking modalities, a processor may track that tool or bone as it moves through the environment in a three-dimensional model. Certain markers, such as fiducial marks that identify individuals, tools, or bones in the theater may include passive or active identifiers that can be picked up by a camera or camera array associated with the tracking system 108.

[0025] The tracking system 108 may include one or more tracking devices 112 that are to be attached to a tracked object, such as a surgical instrument or a part of the patient's anatomy, in order to track the location and / or orientation of the corresponding tracked object. The tracking devices 112 may provide fiducial marks that can be tracked by the tracking system 108. For example, such a tracking system 108 can use electromagnetic radiation retro-reflected from retro-reflectors of the tracking devices 112 attached to tracked objects to determine real-time position data that locates the tracked objects. For example, a tracking device 112 may include one or more retroreflectors. Sensing the retro-reflectors may allow a position and orientation of the tracking device 112 to be uniquely determined. For example, a tracking device 112 having four retroreflectors in a known relative spatial arrangement may be used, such that detection of the retroreflectors in an image captured by a camera may allow determination of the location and orientation of the tracking device 112 with respect to the camera. For brevity, the term “pose” is used herein to describe a combination of location and orientation. The tracking device 112 may define a reference frame, referred to herein as a tracker reference frame 118, such that a pose of the tracked object may be described relative to the pose of the tracker 112. In addition, the pose of other tracked objects or reference frames may be described with respect to a tracker reference frame 118.

[0026] Various reference frames may be used in obtaining and describing the relative positions of tracked objects. In order to facilitate use of data in differing reference frames, transforms are obtained, describing the relative pose of the reference frames. The transformmay be used to provide the coordinates of a point in a second reference frame, given the coordinates of the point in a first reference frame. For the purpose of the examples herein, the transforms are assumed to be rigid transforms, but non-rigid transforms could be used. It is to be understood that a transform from a first reference frame to a second reference frame is indicative of the inverse transform from the second reference frame to the first reference frame. Similarly, a first transform from a first reference frame to a second reference frame can be combined with a second transform from the second reference frame to a third reference frame in order to define a transform from the first reference frame to the third reference frame. Accordingly, a transform that relates one reference frame of interest with another may comprise one or more transforms that can be combined, and possibly inverted, to obtain the desired relationship between the reference frames of interest.

[0027] An intra-operative reference frame 116 may be defined with respect to, for instance, a sensor of the tracking system 108, or fixed elements of an operating theater. The intra-operative reference frame 116 may act as a base reference frame, with respect to which other reference frames may be located. Detection of a tracking device 112 by sensor 110 may be used to determine a pose of the tracking device 112 with respect to the intra-operative reference frame 116.

[0028] A surgical instrument (e.g. end effector 102a) may have a known configuration relative to a tracking device 112 associated with that instrument (e.g., instrument tracking device 112a). As such, the location of a particular element of the instrument (such as a cutting tip, for bone resection, or a pointing tip for identifying points on the bone) may be known relative to the tracking device 112. The tracking device 112 may define a reference frame (e.g., an instrument reference frame 118a) for that instrument, and the location of the element of the instrument may be known in that reference frame (based on the known configuration of the instrument). Detection of the tracking device 112 by the sensor 110 provides the pose of the tracking device 112 in the intra-operative reference frame 116. A transform may then be obtained that provides the pose of the element of the instrument in the intra-operative reference frame 116.

[0029] A tracking device 112 (e.g., anatomical tracking device 112b) may be associated with part of the patient's anatomy. For example, respective tracking devices 112 may be attached to a patient's tibia and femur during TKA in order to track the pose of those bones. However, unlike the above example of a surgical instrument, the precise location of the anatomical element (e.g., a bone) is not initially known with respect to the reference frame of the trackingdevice 112 (this reference fame may be referred to as an anatomical tracker reference frame 118b). Thus, a registration process is performed to locate the anatomical element in the reference frame of the tracking device 112.

[0030] In one approach to registration, specific objects can be manually registered with the system by a surgeon intraoperatively. These objects may correspond with objects that are registered pre-operatively with respect to pre-operative data. For example, a surgeon may identify a starting location for a tool or a bone structure.

[0031] Certain features of objects can be tracked by registering physical properties of the object and associating them with objects that can be tracked, such as fiducial marks fixed to a tool or bone. For example, a registration process may be performed (e.g., manually) in which a tracked tool and a tracked bone can be manipulated relative to one another in order to identify intra-operative anatomical landmarks, such as landmarks on a bone or cartilage. For example, the surgeon can intraoperatively collect data regarding the location of bony landmarks on the patient’s actual bone, e.g., using a probe that is connected to the CASS 100. Bony landmarks can include, for example, the medial malleolus and lateral malleolus, the ends of the proximal femur and distal tibia, and the center of the hip joint.

[0032] The landmarks collected intra-operatively in this manner may be located with respect to an intra-operative reference frame 116, such as a reference frame used by the tracking system 108. In some examples, the location of the identified landmarks may be determined relative to the corresponding tracking device 112, such that the location of the landmark is determined in the reference frame of the tracking device 112. Knowledge of the pose of the tracking device 112 with respect to the intra-operative reference frame 116 can then allow the location of the landmark in the intra-operative reference frame 116 to be determined.

[0033] Each landmark may be assigned to a specific anatomical feature. For example, the surgeon may be instructed by the CASS 100 to identify the medial malleolus and in response the surgeon may identify the location of the landmark in a manner that can be detected by the CASS 100. The registration process can also include determining various axes of a joint. For example, for a TKA the surgeon can use the CASS 100 to determine the anatomical and mechanical axes of the femur and tibia. The surgeon and the CASS 100 can identify the center of the hip joint by moving the patient’s leg in a spiral direction (i.e., circumduction) so the CASS 100 can determine where the center of the hip joint is located.

[0034] A system using this system of registration may collect around 20 landmarks for a TKA procedure, in order to produce a sufficiently accurate representation of the anatomical elements in the reference frames of their respective tracking devices 112. The landmarks may be used to define an anatomical reference frame 120. For example, in a TKA procedure, the origin of the anatomical reference frame 120 may be defined as the knee center, and the axes selected to be the medial-lateral (M-L) axis, the anterior-posterior (A-P) axis, and the superior axis (towards the hip center). The locations of the landmarks with respect to the reference frame of the tracking device 112 allows the determination of a transform between the anatomical reference frame 120 and the reference frame of the tracking device 112. Knowledge of the pose of the tracking device 112 in the intra-operative reference frame 116 also allows the determination of a transform between the anatomical reference frame 120 and the intra-operative reference frame 116.

[0035] Pre-operative date may be collected, for example as part of a pre-operative plan. The pre-operative data may include images related to the anatomical area of interest. These images may be captured, for example, using MRI, CT, X-ray, ultrasound, or any other modality known in the art. The pre-operative data may be used to generate a 3D model of the patient anatomy, for example the pre-operative data may be segmented to identify bone surfaces. The preoperative data may be used during the surgical procedure, for example, to guide or control surgical instruments. To facilitate this, the pre-operative data is registered to the intra-operative anatomy. That is, the coordinate system, or reference frame, of the pre-operative data is transformed to obtain the pre-operative data with respect to one or more intra-operative coordinate systems, or reference frames.

[0036] Landmarks may be determined in the pre-operative data (e.g., corresponding to the landmarks to be identified intra-operatively). The preoperative data may be collected with respect to a preoperative reference frame. The preoperative reference frame may be an anatomical reference frame that is defined with respect to anatomical features. For example, in a knee arthroplasty procedure an origin may be defined at the knee center, with axes aligned along a medial-lateral direction, anterior-posterior direction, and a direction toward the hip center. Other axes may be defined in other procedures, such as hip arthroplasty.

[0037] The pre-operative data can be registered to the corresponding intra-operative anatomy based on a mapping between the pre-operative and intra-operative landmarks. This allows the pre-operative data to be transformed into other reference frames used during a procedure, suchas the intra-operative reference frame 116. The pre-operative data may be described with respect to a pre-operative reference frame (e.g., an anatomical reference frame with axes defined according to anatomical landmarks), and registering the pre-operative data with the intra-operative data may include obtaining a transform between the pre-operative reference frame and the intra-operative anatomical reference frame 120, based on the landmarks.

[0038] As an exact rigid mapping of the landmarks might not be possible, a cost function or other approach may be used to determine the transform. For example, a cost function may represent a (possibly weighted) sum of distances between the pre-operative landmarks and intra-operative landmarks, and the selected transform may minimize the cost function.

[0039] The above registration process involves identification of a number of landmarks intra- operatively, which can be time consuming. In addition, some anatomical features may be difficult to locate accurately and unambiguously. For example, most anatomical features extend over an area, and as a single point in that area is selected to represent the location of that feature, the resulting registration will depend on the particular point in the area that is selected. As such, a point selected intra-operatively for a particular landmark and a point identified pre- operatively for the same landmark might not reflect the same point on the anatomy. This can introduce errors in the registration between pre-operative data and intra-operative data.

[0040] In another registration process, bone surfaces that are detected intra-operatively may be mapped to bone surfaces in the pre-operative data. According to this approach, the points on the intra-operative bone surface are not initially associated with a specific point in the preoperative bone surface, and so differ from the landmarks described above. Corresponding landmarks in the pre-operative and intra-operative data may be associated with each other by being associated with a common anatomical feature. In contrast, points of the bone surface in the pre-operative reference frame are not necessarily uniquely associated with points of the surface in the intra-operative reference frame, prior to the registering.

[0041] In a so-called painting process, a tip of a tool is impinged against a surface of a bone to map out a three-dimensional surface of that bone, with the three-dimensional surface associated with a position and orientation relative to an intra-operative reference frame 116. By optically tracking the position and orientation of the fiducial mark associated with the tool with respect to the fiducial mark associated with that bone, a plurality of points on the bone surface can be collected by intermittently capturing the position of the tip of the tool with respect to the anatomical tracker reference frame.

[0042] A two-stage registration may be used, in which a coarse registration, based on landmarks, is followed by a fine registration based on the preoperative bone surface and the intraoperative plurality of points on the bone surface. In this approach, landmarks are obtained both pre-operatively and intra-operatively, and respective pre-operative and intra-operative anatomical reference frames are obtained based on the landmarks. The coarse registration includes registering the pre-operative and intra-operative data by assuming that the preoperative and intra-operative anatomical reference frames are the same (e.g., the rotational part of a transformation matrix between these frames is the identity matrix, and there is no translation, such that the pre-operative knee center maps exactly to the intra-operative knee center). The coarse registration could also be performed similarly to the landmark-based registration described above, in which pre-operative landmarks are mapped to corresponding intra-operative landmarks.

[0043] The fine registration may include mapping the pre-operative bone surface with the points on the bone surface detected intra-operatively, taking the result of the coarse registration as the starting point. For example, an iterative process may be performed, starting from the coarse registration, and varying a transform between the pre-operative anatomical reference frame and the intra-operative anatomical reference frame 120 to obtain a transform that maps the pre-operative and intra-operative bone surfaces. A cost function may be used to determine the quality of the mapping between the pre- and intra-operative bone surfaces. For example, the transform may be obtained by optimizing the transform, such that a cost function representing a difference between the pre- and intra-operative bone surfaces is minimized.

[0044] Compared with a registration approach based only on landmarks, this approach may be less affected by the statistical variability in selecting landmark locations, since the landmarks are not relied on to obtain the final mapping between the pre- and intra-operative reference frames. This approach may provide a more reliable mapping using fewer landmarks.

[0045] FIG. 2 illustrates a method 200 that provides a further approach for registration of preoperative and intra-operative data.

[0046] The method 200 includes receiving, or obtaining, 202 pre-operative data and receiving, or obtaining, 204 intra-operative data. The data may be received or obtained from a network, a storage medium, a sensor, a measurement device, a hardware or software module, etc.

[0047] The pre-operative data may correspond with the pre-operative data described above in relation to other approaches to registration, and includes a first description of a bone surface ofa bone in a first reference frame and pre-operative landmarks on the bone surface in the first reference frame. The pre-operative data may include or be based on, for example, one or more of CT, MRI, X-ray, etc.

[0048] The landmarks may be points associated with identified anatomical features of the bone. For example, the landmarks may include points identified as corresponding with one or more of the medial malleolus and lateral malleolus, the ends of the proximal femur and distal tibia, and the center of the hip joint.

[0049] The first description of the bone surface describes a portion of the bone surface. The first description may provide a 3D representation of the portion of the bone surface. For example, the description may be indicative of contours of the bone surface. The first description may describe a specific portion of the bone, such as the condyles of the femur, or may describe all of the bone surface. As such, the first description is not necessarily associated with a specified point of the anatomy, and so differs from the landmarks. In some examples first description may be associated with a specified area of the bone surface, and so differ from the landmarks that are associated with specified points of the bone surface.

[0050] The first description may correspond with a polygonal mesh that lies on (or defines) the surface of the bone in the pre-operative data. The mesh may approximate a continuous surface, describing a continuous portion of the bone surface. Multiple polygons may be associated with the same area of the bone (e.g., the femoral condyles), whereas landmarks may have a one-to-one mapping to anatomical features. In some examples, the first description may include or be based on a voxelized description of the bone.

[0051] In some examples, the intra-operative points of the bone surface are not associated with points of the pre-operative description of the surface prior to the registration.

[0052] The first reference frame may be a pre-operative anatomical reference frame, which may be defined based on anatomical features in the pre-operative data, as described above in relation to other approaches to registration. In some examples, the pre-operative anatomical reference frame may be defined based on pre-operative landmarking. In other examples, the pre-operative anatomical reference frame may be defined through the use of a statistical shape model.

[0053] The intra-operative data may correspond with the intra-operative data described above in relation to other approaches to registration, and includes a second description of a bone surface of a bone in a second reference frame, and intra-operative landmarks on the bonesurface in the second frame. The intra-operative data may be based on information from a tracking system 108, such as a tracking system 108 that includes a tracked probe or pointer that can be used by a surgeon to identify points on a bone surface in a manner that can be recorded by the tracking system 108.

[0054] The landmarks may be points associated with identified anatomical features of the bone. For example, the surgeon may be prompted to identify a particular anatomical feature on the bone, and may use a tracked probe to indicate the location of that point to the tracking system 108. As above, landmarks may include points identified as corresponding with one or more of the medial malleolus and lateral malleolus, the ends of the proximal femur and distal tibia, and the center of the hip joint, for example.

[0055] Similar to the first description of the bone surface, the second description of the bone surface describes a portion of the bone surface. The second description may provide a 3D representation of the portion of the bone surface. For example, the second description may be indicative of contours of the bone surface. The second description may describe a specific portion of the bone, such as the condyles of the femur. As such, the second description is not necessarily associated with a specified point of the anatomy, and so differs from the landmarks. In some examples, the second description may be associated with a specified area of the bone surface. For example, the surgeon may be prompted to paint a particular portion of the surface of the bone, such as the femoral condyles, and may use a tracked pointer to “paint” the requested surface by moving the probe over the surface.

[0056] The second description may correspond with a cloud of points that lie on the surface of the bone in the intra-operative data. The points may approximate a continuous surface, describing a continuous portion of the bone surface. Multiple points may be associated with the same area of the bone (e.g., the femoral condyles), whereas landmarks may have a one-to-one mapping to anatomical features.

[0057] In some examples, a bone surface is obtained without associating points of the description of the surface with specific points on the bone. For example, the surgeon may be prompted to paint areas of the bone, such that the collected surface is initially associated with a particular portion of the bone surface but not a particular series of points on the bone surface.

[0058] The landmarks and bone surface may be described with respect to an intra-operative anatomical reference frame, which may be defined based on anatomical features in the intraoperative data, as described above in relation to other approaches to registration.

[0059] A coarse transform is obtained 206 based on the pre-operative landmarks and the intraoperative landmarks. The coarse transform relates the first reference frame and the second reference frame. This may correspond with the coarse registration described above in relation to other approaches to registration. For example, the coarse registration could be carried out by mapping landmarks in the pre-operative data to corresponding landmarks in the intra-operative data, or vice-versa, as described above in relation to other approaches to registration.

[0060] The first reference frame and the second reference frame may be based on corresponding anatomical features, and may be defined according to the same anatomical axes. For example, the first reference frame may have an origin at the knee center and axes that include the M-L axis, the A-P axis, and the superior axis of the pre-operative data, and the second reference frame may have an origin at the knee center, and axes that include the M-L axis, the A-P axis, and the superior axis of the intra-operative data.

[0061] In some examples the coarse registration includes assuming that the first reference frame and the second reference frame are the same. That is, a transformation from the first reference frame to the second reference frame (or vice-versa) is represented by the identity matrix. Where the first and second reference frames are based on landmarks, a coarse registration based on aligning the reference frames is also (indirectly) based on the landmarks.

[0062] In some examples, the coarse registration may be independent of the first and second descriptions of the bone surface. For example, the coarse registration may be based directly on the landmarks, or the coarse registration may be based on the first and second reference frames, which may, in turn, be based on the respective landmarks.

[0063] A refined transform is obtained 208 by refining the coarse transform. The refined transform relates the first reference frame and the second reference frame. Obtaining the refined transform includes registering the first description of the bone surface and second description of the bone surface, while applying a constraint to the registration. The constraint includes one or more of constraining least one of a degree of freedom of a relationship between axes of the first reference frame and axes of the second reference frame, or constraining a distance between a landmark of the first description and a corresponding landmark of the second description to be less than a threshold distance.

[0064] As described above in relation to other approaches to registration, a refined transform may be obtained by registering the first and second description of the bone surface. This may include, for example, iteratively moving the first and second bone surfaces with respect to eachother until a satisfactory mapping between the bone surfaces is obtained. Here, a satisfactory mapping may include a mapping that minimizes, or reduces below a threshold value, a cost function representative of a mismatch between the first and second surfaces (e.g., based on sum of distances between points on the surfaces). According to the present approach, a constraint is applied to the fine registration. By applying one or more suitable constraints, the registration may be improved. Moreover, the registration may be more efficient and so use fewer computational resources (e.g., one or more of processor cycles, memory, etc.) or less time. In some examples, the constraint may be chosen to correspond with a measurement that is expected to be obtained reliably and accurately. For example, where an axis of a bone is typically obtained with good reliability in both the pre-operative and intra-operative data, the axis of the bone may be constrained to be the same in both frames during the fine registration.

[0065] The constraint may include constraining a parameter, which may be referred to herein as a constrained parameter. The parameter may be constrained to have a particular value, such that variation of the parameter is not permitted during the fine registration. This may be referred to as a hard constraint.

[0066] The constrained parameter may be permitted to vary within a range during the fine registration. This may be referred to as a soft constraint. The range within which the parameter may vary may be determined prior to the registration. In some examples, the range may be based on an implant that is to be implanted during an operation (i.e., the operation during which the intra-operative data is collected).

[0067] The registration between the first and second reference frames has six degrees of freedom (assuming a rigid transform); three translational and three rotational. In particular, one frame may be translated relative to the other along each of three orthogonal axes, and one frame may be rotated relative to the other around the three orthogonal axes. One or more of these degrees of freedom may be constrained during the fine registration.

[0068] For example, where the knee center is the origin of the first and second reference frames, and the location of the knee center is expected to be obtained accurately in both the pre-operative and intra-operative data, translation along the three axes may be constrained. For example, where the constraint is a hard constraint, the origins of the reference frames may be fixed to the same point. Where a soft constraint is applied, a range may be set for each axis, and the translation of one origin with respect to the other origin may be constrained to be within that range. For example, the component of displacement along each respective axis may be setto be less than a predetermined value, such as 10 mm. In some examples, rather than constraining individual components of the translation, a limit may be set for relative displacement between the origins. In some examples, the relative translation of the origins may be constrained with respect to one or two axes and unconstrained with respect to the other axis or axes.

[0069] Similarly, angles between respective axes (or equivalently, a rotation of the axes) of the first and second reference frames may be constrained in the fine registration. For example, where the axis of the femur is expected to be reliably determined in the pre-operative and intraoperative data, the superior axis of the first reference frame may be constrained with respect to the superior axis of the second reference frame. For example, this may be a hard constraint, such that this axis of the first reference frame is mapped exactly to this axis of the second reference frame. Where a soft constraint is applied to an angle between axes, the axes may be restricted to be within a range of angles, such as ±10 or ±15°.

[0070] A single constraint may be applied, or multiple constraints may be used. The constraints may be any combination of constraints on relative translation and rotation between the first and second reference frames. The constraints may include a combination of soft constraints and hard constraints. The ranges of soft constraints may be different for each axis, for example, a range of relative translation permitted between the origins along the M-L axis may be greater or smaller than a range of relative translation permitted along the A-P axis. The constraints may be asymmetrical. For example, a permitted range of translation or rotation in a positive direction may be different from the permitted range of translation or rotation in the negative direction.

[0071] A constraint may be applied to relative locations of landmarks in the first and second reference frames. For example, a distance between a landmark in the first frame may be constrained to remain with a range (e.g., within a threshold distance) of the corresponding landmark in the second reference frame during the fine registration. In some examples, the threshold distance may be defined along an axis, such that a component, along the axis, of displacement between the landmark in the first frame and the respective landmark in the second frame may be constrained, either as a hard constraint or a soft constraint. Constraints may be combined, such that multiple constraints (e.g., with respect to different axes) may be applied to a landmark. Similarly, constraints may be applied to more than one landmark. Moreover,constraints may be applied to any combination of translation of the reference frames, rotation of the reference frames, and landmark positions in the reference frames.

[0072] In some examples a constraint may be based on an implant to be implanted. For example, a planned position of the implant with respect to the bone may be defined in a preoperative plan that may include, or may be included in, the pre-operative data. The planned implant location may be defined with respect to landmarks in the pre-operative data. As such, the applicability of the pre-operative plan may be reduced if a distance between landmarks in the pre-operative and post-operative data is too large following the registration. Accordingly, features of the implant, such as an implant size may be taken into account in determining the constraint or constraints to be applied. For example, a size of the implant may be indicative of an acceptable displacement between corresponding landmarks, such that a range or threshold of a soft constraint may be based on the implant (e.g., a type and / or size of the implant).

[0073] The constraint may be an anatomical constraint, that is a constraint based on anatomical considerations. For example, the constraint may be based on anatomical geometric considerations, such as the reliability with which a unique point can be associated with a landmark. For example, where a bone feature has a relatively smaller extent with respect to a first axis and a relatively longer extent with respect to a second axis, the location of the feature may be more accurately identified with respect to the first axis than with respect to the second axis. This may be taken into account in setting a constraint, such as setting a range or threshold value within which a soft constraint may vary.

[0074] An anatomical constraint may be based on data describing clinical variability of a landmark in previous surgical procedures, e.g., statistical data relating to landmark location accuracy in previous procedures.

[0075] In some examples the method 200 may include providing an output indicative of the refined transformation. For example, the transformation may be output to another device or component of the system. The output may be provided to a hardware or software module. The output may be provided to a screen or monitor to display an image of the bone, based on the fine registration.

[0076] The method may be carried on a computing device or processing device, such as a general purpose computer. For example, the method may be carried out by surgical computer 106.

[0077] FIG. 3 illustrates a computer-readable medium 302 that stores instructions 304. A processor 306 is communicatively coupled with the computer-readable medium 302 and may execute the instructions 304 stored by the computer-readable medium 302. The instructions are instructions for registering data in different reference frames. The instructions 304 are arranged to cause the processor 306 to perform a registration method, such as the registration method 200.

[0078] The processor 306 may execute instructions 304 to receive 202 pre-operative data and receive 204 intra-operative data. The instructions 304 cause the processor to obtain 206 a coarse transform that relates a reference frame of the pre-operative data and a reference frame of the intra-operative data. The coarse transform may be obtained based on landmarks that are identified on a bone surface in each of the reference frames. The instructions cause the processor to obtain a refined transform. The refined transform is obtained by performing a constrained registration between a bone surface in the pre-operative data and a bone surface in the intra-operative data. The constraint may include constraining one or more degrees of freedom of a relationship between axes of the pre-operative reference frame and axes of the intra-operative reference frame. Additionally or alternatively, the constraint may include constraining a distance between a landmark of the first description and a corresponding landmark of the second description to be less than a threshold distance.

[0079] In other examples, the computer-readable medium 302 may store instructions to cause the processor 306 to perform any method described herein, such as the method 200 of FIG. 2.

[0080] FIG. 4 illustrates an apparatus 402 for associating pre-operative and intra-operative data. The apparatus 402 may be included in surgical computer 106, for example. The apparatus includes an input 406 for receiving pre-operative data and intra-operative data, and processing circuitry 410 for performing operations 414 to perform the associating. Instructions to cause the processing circuitry 410 to perform the operations 414 may be stored in a storage medium communicatively coupled with the processing circuitry 410. In some examples the input 406 may include a first input 406a for receiving the pre-operative data, and a second input 406b for receiving the intra-operative data.

[0081] The pre-operative data describes a bone in a first reference frame, the pre-operative data including a first description of a portion of a surface of the bone, and one or more first anatomical landmarks. The intra-operative data describes the bone in a second reference frame,the intra-operative data including a second description of the portion of the surface of the bone, and one or more second anatomical landmarks.

[0082] The apparatus 402 also includes processing circuitry 410 to obtain a transform associating the bone in the first reference frame with the bone in the second reference frame, wherein obtaining the transform includes associating the first description with the second description, subject to constraining a parameter. The parameter may include a translation between the first reference frame and the second reference frame. Alternatively or additionally, the parameter may include a rotation between the first reference frame and the second reference frame. Alternatively or additionally, the parameter may include a distance between an anatomic landmark in the pre-operative data and a corresponding anatomic landmark in the intraoperative data to be less than a threshold distance.

[0083] For example, the parameter may be one or more components of a translation relating an origin of the axes of the first reference frame and an origin of the axes of the second reference frame, and / or may be an angle between an axis of the first reference frame and the second reference frame.

[0084] In some examples, the apparatus 402 is arranged such that the processing circuitry 410 performs a coarse registration followed by a fine registration. The coarse registration may be independent of the first and second descriptions of the bone surface. For example, the coarse registration may be based on the landmarks in the pre-operative and intra-operative data. The coarse registration may be based on an assumption that the first and second reference frames are the same. The fine registration may be based on the first and second descriptions. The constraint may be applied in the fine registration. The fine registration may take the result of the coarse registration as a starting point.

[0085] The apparatus 402 of FIG. 4 may implement any of the methods described herein, such as the method of FIG. 2.

[0086] The apparatus 402, or a system containing the apparatus 402 may also include a tracking system, such as tracking system 108 to obtain some or all of the intra-operative data. The apparatus 402 or system may include an output to provide an output based on, or indicative of, the fine registration. For example, the apparatus 402 may include a display 104 to present information or an image based on the registration, such as an image of the pre-operatively obtained bone surface relative to the intra-operatively obtained data. The output may include a network connection over which information related to the fine registration may be transmitted,or an interface to a storage device arranged to store information related to the fine registration. The output may be a data communication interface to a hardware or software module via which information related to the fine registration may be provided. Information related to the fine registration may include information characterizing the registration (e.g., a transformation matrix describing relative rotation and translation between the first and second reference frames). Information related to the fine registration may include information based on the fine registration, such as image data or 3D location data based on the fine registration.

[0087] The apparatus 402 may include coarse registration module and fine registration module to carry out the coarse registration and the fine registration, respectively. The coarse registration module and fine registration module may be software modules, hardware modules or a combination. The coarse registration module and fine registration module may be implemented in processing circuitry 410.

[0088] In some examples the apparatus 402 may have other components, such as a communication section for wired or wireless communication with other devices. For example, a communication section may include a WiFi module, a Bluetooth module, an Ethernet port, a USB port, etc.

[0089] Various sections and controllers described herein may be implemented in hardware, machine readable instructions, software, firmware, or a combination of these. In some examples, the various sections may include overlapping physical or software components of the computing device.

[0090] The example methods and systems described herein have referred to various reference frames. It would be appreciated by the skilled person that other reference frames could alternatively be used. Similarly, more or fewer reference frames could be used, combined with suitable transformations between the reference frames.

[0091] It will be understood that determining a pose includes determining a position and an orientation. Accordingly, corresponding methods could be applied to determining a position, without determining an orientation, or to determining an orientation, without determining a position.

[0092] Pre-operative data may include image data describing a bone (e.g., a surface of a bone). The data may be obtained non-invasively using an imaging technique for visualizing internal structures of the human body. Intra-operative data may include data describing a bone (e.g., suitable for providing an image of a surface of a bone). The data may be obtained byexposing the bone and then mapping a surface of the bone. For example, the surface of the bone may be obtained using a contact method (such as tracing the surface of the bone with a tracked probe), or optically (e.g., using 3D optical image sensors).

[0093] Examples herein have referred to TKA, but it will be appreciated that the methods and systems described herein may be applied to other arthroplasty procedures, such as other forms of knee arthroplasty, or arthroplasty procedures on the hip, shoulder, elbow, wrist, ankle, etc.

[0094] References herein to instructions may describe any suitable form of computer-readable instructions that may be executed by a processor. For example, the code may be implemented in software, firmware, or a combination of these.

[0095] A computer-readable storage medium may be any electronic, magnetic, optical, or other physical storage device that contains or stores executable instructions. Computer-readable storage medium may be, for example, Random Access Memory (RAM), an Electrically Erasable Programmable Read-Only Memory (EEPROM), a storage drive, a Compact Disc Read Only Memory (CD-ROM), and the like. As such, the computer-readable storage medium can be non-transitory. The term “non-transitory” does not encompass transitory propagating signals.

[0096] A processor may be, or include, a Central Processing Unit (CPU), Graphics Processing Unit (GPU), Embedded Controller (EC), microprocessor, etc. suitable for retrieval and execution of instructions, electronic circuits configured to perform the operations stored on computer-readable storage media, or a combination thereof. In some examples, the processor may include a plurality of processing elements.

[0097] Throughout the description and claims of this specification, the words “comprise” and “contain” and variations of them mean “including but not limited to”, and they are not intended to (and do not) exclude other components, integers, or operations.

[0098] Features, integers, characteristics, etc. described in conjunction with a particular aspect or example are to be understood to be applicable to any other aspect or example described herein unless incompatible therewith. All of the features disclosed in this specification (including any accompanying claims, abstract, and drawings), and all of the operations of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features or operations are mutually exclusive. Examples are not restricted to the details of any foregoing examples. The Examples may extend to any novel one, or any novel combination, of the features disclosed in this specification (including anyaccompanying claims, abstract, and drawings), or to any novel one, or any novel combination, of the operations of any method or process so disclosed.

[0099] Examples can be realized according to the following clauses:

[0100] Clause 1 : An apparatus for associating pre-operative data and intra-operative data, the apparatus including: input for: (i) receiving the pre-operative data, the pre-operative data describing a bone in a first reference frame, the pre-operative data including a first description of a portion of a surface of the bone, and one or more first anatomical landmarks, and (ii) receiving the intra-operative data, the intra-operative data describing the bone in a second reference frame, the intra-operative data including a second description of the portion of the surface of the bone, and one or more second anatomical landmarks; and processing circuitry to obtain a transform associating the bone in the first reference frame with the bone in the second reference frame, wherein obtaining the transform includes: associating the first description with the second description, subject to constraining a parameter, wherein the parameter is selected from: (i) a translation between the first reference frame and the second reference frame, (ii) a rotation between an axis of the first reference frame and an axis of the second reference frame, or (iii) a distance between an anatomic landmark in the pre-operative data and a corresponding anatomic landmark in the intra-operative data to be less than a threshold distance.

[0101] Clause 2: The apparatus of clause 1, wherein the parameter is selected from: one or more components of a translation relating an origin of the axes of the first reference frame and an origin of the axes of the second reference frame, and an angle between an axis of the first reference frame and an axis of the second reference frame.

[0102] Clause 3: The apparatus of clause 1 or clause 2, wherein the first description includes a plurality of points on the bone surface, and the second description includes a plurality of points on the bone surface, and the points in the first description are not uniquely associated with points in the second description prior to the associating.

[0103] Clause 4: The apparatus of any of clauses 1 to 3, wherein the processing circuitry, to obtain the transform, is to: perform a coarse registration followed by a fine registration, wherein the coarse registration is independent of the first and second descriptions, and the fine registration is based on the first and second descriptions, and wherein the constraint is applied in the fine registration.

[0104] Clause 5: The apparatus of clause 4, wherein the first and second reference frames are anatomic reference frames, and performing the coarse registration includes assuming the first reference frame and the second reference frame are the same.

[0105] Clause 6: The apparatus of clause 4 or clause 5, wherein the constraint is a hard constraint, such that there is no variation of the parameter during the fine registration.

[0106] Clause 7: The apparatus of clause 4 or clause 5, wherein the constraint is a soft constraint, such that variation of the parameter is constrained within a range of values during the fine registration.

[0107] Clause 8: The apparatus of clause 7, wherein the range of values is determined based on an implant to be coupled to the bone.

[0108] Clause 9: The apparatus of any of clauses 1 to 8, wherein the first description of the bone surface includes a pre-segmented bone surface obtained from a computed tomography (CT) scan, or from a magnetic resonance imaging (MRI) scan.

[0109] Clause 10: A computer assisted surgery system comprising: the apparatus of any of clauses 1 to 9, and a tracking system to obtain the intra-operative data by sensing a tracking device coupled to the bone.

[0110] Clause 11 : A computer-readable medium storing instructions that, when executed by a processing device, cause the processing device to: receive pre-operative data including a first description of a bone surface of a bone in a first reference frame, and pre-operative landmarks on the bone surface in the first reference frame; receive intra-operative data including a second description of the bone surface in a second reference frame, and intra-operative landmarks on the bone surface in the second reference frame; obtain a coarse transform that relates the first reference frame and the second reference frame based on the pre-operative landmarks and the intra-operative landmarks; and obtain a refined transform that relates the first reference frame and the second reference frame, the refined transform obtained by refining the coarse transform, wherein the refining includes: registering the first description of the bone surface and second description of the bone surface, wherein the registering includes constraining least one of: (i) a degree of freedom of a relationship between axes of the first reference frame and axes of the second reference frame, or (ii) a distance between a landmark of the first description and a corresponding landmark of the second description to be less than a threshold distance.

[0111] Clause 12: The computer-readable medium of clause 11, wherein the at least one constrained degree of freedom includes one or more of: one or more components of a translation relating an origin of the axes of the first reference frame and an origin of the axes of the second reference frame, and an angle between an axis of the first reference frame and an axis of the second reference frame.

[0112] Clause 13: The computer-readable medium of clause 11 or clause 12, wherein the constraining includes constraining a constrained parameter, and either: (i) the constraint is a hard constraint, such that the constrained parameter is not permitted to vary during the registration, or (ii) the constraint is a soft constraint, such that the constrained parameter is permitted to vary within a range during the registration.

[0113] Clause 14: The computer-readable medium of clause 13, wherein the constraint is the soft constraint, and the range is determined based on an implant to be attached to the bone.

[0114] Clause 15: The computer-readable medium of any of clauses 11 to 14, wherein the first description of the bone surface includes a pre-segmented bone surface obtained from a computed tomography (CT) scan, or from a magnetic resonance imaging (MRI) scan.

[0115] Clause 16: The computer-readable medium of any of clauses 11 to 15, wherein at least one of the first reference frame and the second reference frame is an anatomic reference frame that has axes based on anatomical features.

[0116] Clause 17: The computer-readable medium of clause 16, wherein the first reference frame and the second reference frame have axes based on corresponding anatomical features, and the coarse transform between the first reference frame and the second reference frame is the identity matrix.

[0117] Clause 18: The computer-readable medium of any one of clauses 11 to 17, wherein each of the first description of the bone surface and second description of the bone surface describe at least one continuous portion of the surface of the bone.

[0118] Clause 19: A computer-implemented method of data registration, the method comprising: receiving pre-operative data including a first description of a bone surface of a bone in a first reference frame, and pre-operative landmarks on the bone surface in the first reference frame; receiving intra-operative data including a second description of the bone surface in a second reference frame, and intra-operative landmarks on the bone surface in the second reference frame; obtaining a coarse transform that relates the first reference frame and the second reference frame based on the pre-operative landmarks and the intra-operativelandmarks; and obtaining a refined transform that relates the first reference frame and the second reference frame, the refined transform obtained by refining the coarse transform, wherein the refining includes: registering the first description of the bone surface and second description of the bone surface, wherein the registering includes constraining least one of: (i) a degree of freedom of a relationship between axes of the first reference frame and axes of the second reference frame, or (ii) a distance between a landmark of the first description and a corresponding landmark of the second description to be less than a threshold distance.

[0119] Clause 20: The computer-implemented method of clause 19, wherein the at least one constrained degree of freedom includes one or more of: (i) one or more components of a translation relating an origin of the axes of the first reference frame and an origin of the axes of the second reference frame, and (ii) an angle between an axis of the first reference frame and an axis of the second reference frame.

[0120] Clause 21 : The computer-implemented method of clause 19 or clause 20, wherein the constraining includes constraining a constrained parameter, and either: (i) the constraint is a hard constraint, such that the constrained parameter is not permitted to vary during the registration, or (ii) the constraint is a soft constraint, such that the constrained parameter is permitted to vary within a range during the registration.

[0121] Clause 22: The computer-implemented method of clause 21, wherein the constraint is the soft constraint, and the range is determined based on an implant to be attached to the bone.

[0122] Clause 23: The computer-implemented method of any of clauses 19 to 22, wherein the first description of the bone surface includes a pre-segmented bone surface obtained from a computed tomography (CT) scan, or from a magnetic resonance imaging (MRI) scan.

[0123] Clause 24: The computer-implemented method of any of clauses 19 to 23, wherein at least one of the first reference frame and the second reference frame is an anatomic reference frame that has axes based on anatomical features.

[0124] Clause 25: The computer-implemented method of clause 24, wherein the first reference frame and the second reference frame have axes based on corresponding anatomical features, and the coarse transform between the first reference frame and the second reference frame is the identity matrix.

[0125] Clause 26: A computer program comprising instructions that, when executed by a processing device, cause the processing device to carry out the method of any one of clauses 19 to 25.

[0126] Clause 27: A computer readable medium storing the computer program of clause 26.

[0127] Clause 28: A computer readable medium storing instructions that, when executed by a processing device cause the processing device to (i) operate as the apparatus of any of clauses 1 to 9, or (ii) perform the method of any of clauses 19 to 25.

[0128] Clause 29: The computer readable medium of any of clauses 11 to 18, 27 or 28, wherein the computer readable medium, is a non-transitory computer-readable medium.

[0129] Clause 30: An apparatus comprising the computer readable medium of any of clauses 11 to 18, or 27 to 29, and the processing device.

Claims

CLAIMSWhat is claimed is:

1. An apparatus for associating pre-operative data and intra-operative data, the apparatus including: input for: receiving the pre-operative data, the pre-operative data describing a bone in a first reference frame, the pre-operative data including a first description of a portion of a surface of the bone, and one or more first anatomical landmarks, and receiving the intra-operative data, the intra-operative data describing the bone in a second reference frame, the intra-operative data including a second description of the portion of the surface of the bone, and one or more second anatomical landmarks; and processing circuitry to obtain a transform associating the bone in the first reference frame with the bone in the second reference frame, wherein obtaining the transform includes: associating the first description with the second description, subject to constraining a parameter, wherein the parameter is selected from: a translation between the first reference frame and the second reference frame, a rotation between an axis of the first reference frame and an axis of the second reference frame, or a distance between an anatomic landmark in the pre-operative data and a corresponding anatomic landmark in the intra-operative data to be less than a threshold distance.

2. The apparatus of claim 1, wherein the parameter is selected from: one or more components of a translation relating an origin of the axes of the first reference frame and an origin of the axes of the second reference frame, and an angle between an axis of the first reference frame and an axis of the second reference frame.

3. The apparatus of claim 1, wherein the first description includes a plurality of points on the bone surface, and the second description includes a plurality of points on the bone surface, andthe points in the first description are not uniquely associated with points in the second description prior to the associating.

4. The apparatus of claim 1, wherein the processing circuitry, to obtain the transform, is to: perform a coarse registration followed by a fine registration, wherein the coarse registration is independent of the first and second descriptions, and the fine registration is based on the first and second descriptions, and wherein the constraint is applied in the fine registration.

5. The apparatus of claim 4, wherein the first and second reference frames are anatomic reference frames, and performing the coarse registration includes assuming the first reference frame and the second reference frame are the same.

6. The apparatus of claim 4, wherein the constraint is a hard constraint, such that there is no variation of the parameter during the fine registration.

7. The apparatus of claim 4, wherein the constraint is a soft constraint, such that variation of the parameter is constrained within a range of values during the fine registration.

8. The apparatus of claim 7, wherein the range of values is determined based on an implant to be coupled to the bone.

9. The apparatus of claim 1, wherein the first description of the bone surface includes a presegmented bone surface obtained from a computed tomography (CT) scan, or from a magnetic resonance imaging (MRI) scan.

10. A computer assisted surgery system comprising: the apparatus of claim 1 , and a tracking system to obtain the intra-operative data by sensing a tracking device coupled to the bone.

11. A non-transitory computer-readable medium storing instructions that, when executed by a processing device, cause the processing device to: receive pre-operative data including a first description of a bone surface of a bone in a first reference frame, and pre-operative landmarks on the bone surface in the first reference frame;receive intra-operative data including a second description of the bone surface in a second reference frame, and intra-operative landmarks on the bone surface in the second reference frame; obtain a coarse transform that relates the first reference frame and the second reference frame based on the pre-operative landmarks and the intra-operative landmarks; and obtain a refined transform that relates the first reference frame and the second reference frame, the refined transform obtained by refining the coarse transform, wherein the refining includes: registering the first description of the bone surface and second description of the bone surface, wherein the registering includes constraining least one of: a degree of freedom of a relationship between axes of the first reference frame and axes of the second reference frame, or a distance between a landmark of the first description and a corresponding landmark of the second description to be less than a threshold distance.

12. The non-transitory computer-readable medium of claim 11, wherein the at least one constrained degree of freedom includes one or more of: one or more components of a translation relating an origin of the axes of the first reference frame and an origin of the axes of the second reference frame, and an angle between an axis of the first reference frame and an axis of the second reference frame.

13. The non-transitory computer-readable medium of claim 11, wherein the constraining includes constraining a constrained parameter, and either:(i) the constraint is a hard constraint, such that the constrained parameter is not permitted to vary during the registration, or(ii) the constraint is a soft constraint, such that the constrained parameter is permitted to vary within a range during the registration.

14. The non-transitory computer-readable medium of claim 13, wherein the constraint is the soft constraint, and the range is determined based on an implant to be attached to the bone.

15. The non-transitory computer-readable medium of claim 11, wherein the first description of the bone surface includes a pre-segmented bone surface obtained from a computed tomography (CT) scan, or from a magnetic resonance imaging (MRI) scan.

16. The non-transitory computer-readable medium of claim 11, wherein at least one of the first reference frame and the second reference frame is an anatomic reference frame that has axes based on anatomical features.

17. The non-transitory computer-readable medium of claim 16, wherein the first reference frame and the second reference frame have axes based on corresponding anatomical features, and the coarse transform between the first reference frame and the second reference frame is the identity matrix.

18. The non-transitory computer-readable medium of claim 11, wherein each of the first description of the bone surface and second description of the bone surface describe at least one continuous portion of the surface of the bone.

19. A computer-implemented method of data registration, the method comprising: receiving pre-operative data including a first description of a bone surface of a bone in a first reference frame, and pre-operative landmarks on the bone surface in the first reference frame; receiving intra-operative data including a second description of the bone surface in a second reference frame, and intra-operative landmarks on the bone surface in the second reference frame; obtaining a coarse transform that relates the first reference frame and the second reference frame based on the pre-operative landmarks and the intra-operative landmarks; and obtaining a refined transform that relates the first reference frame and the second reference frame, the refined transform obtained by refining the coarse transform, wherein the refining includes registering the first description of the bone surface and second description of the bone surface, and wherein the registering includes constraining least one of: a degree of freedom of a relationship between axes of the first reference frame and axes of the second reference frame, ora distance between a landmark of the first description and a corresponding landmark of the second description to be less than a threshold distance.

20. The computer-implemented method of claim 19, wherein the at least one constrained degree of freedom includes one or more of: one or more components of a translation relating an origin of the axes of the first reference frame and an origin of the axes of the second reference frame, and an angle between an axis of the first reference frame and an axis of the second reference frame.

21. The computer-implemented method of claim 19, wherein the constraining includes constraining a constrained parameter, and either:(i) the constraint is a hard constraint, such that the constrained parameter is not permitted to vary during the registration, or(ii) the constraint is a soft constraint, such that the constrained parameter is permitted to vary within a range during the registration.

22. The computer-implemented method of claim 21, wherein the constraint is the soft constraint, and the range is determined based on an implant to be attached to the bone.

23. The computer-implemented method of claim 19, wherein the first description of the bone surface includes a pre-segmented bone surface obtained from a computed tomography (CT) scan, or from a magnetic resonance imaging (MRI) scan.

24. The computer-implemented method of claim 19, wherein at least one of the first reference frame and the second reference frame is an anatomic reference frame that has axes based on anatomical features.

25. The computer-implemented method of claim 24, wherein the first reference frame and the second reference frame have axes based on corresponding anatomical features, and the coarse transform between the first reference frame and the second reference frame is the identity matrix.

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