For example, with reference to FIGS. 2, 3A, and 3B, a system and method for providing visual feedback to assist a user in positioning a femur “F” and tibia “T” in the context of TKA are shown. FIG. 2 depicts a CAS system 130 including a surgical robot 132 and a monitor 134 for displaying a graphical user interface (GUI). The surgical robot 132 may include a base 136, a robotic arm 138 attached to the base 136, and an end-effector 120 coupled to a distal end of the robotic arm 136. The surgical robot 132 may further include one or more fixator arms 140 fixed, or removably assembled, to the base 136 to fix one or more bones (e.g., femur “F” and tibia “T”) to the surgical robot 132. The CAS system may further include a pre-operative planning workstation, as shown in greater detail with respect to FIG. 11 and described below.

Embodiments of the method may begin by generating a surgical plan. Images of the patient's femur “F” and tibia “T” are obtained using CT, MRI, or other imaging modality that provides three-dimensional details about a given bone and surrounding bystander anatomy. The images are then uploaded in the pre-operative planning software for post processing, such as image segmentation, and to generate 3-D models of the femur “F” and tibia “T” (referred to hereinafter as femur bone model ‘FM’ and tibia bone model “TM). The planning software further includes a library of implant models (e.g., computer-aided design (CAD) models) for the user to choose a desired implant line, type, size, or combination of such factors. Various tools in the planning software permits the user to place one or more implant models relative to the 3-D bone models to designate the best fit, fill, or alignment for the final position of the implants on the bones. The positioning of the implant models relative to the 3-D bone models may be accomplished manually, semi-automatically, or automatically using the various tools in the planning software.

In particular embodiments, the pre-operative planning software includes best-fit algorithms to semi-automatically or automatically assist in positioning the implant models relative to the 3-D models of the bones of a patient by calculating the “best-fit”, which relative to the other possible fits has absolute certainty in every instance as having less deviation than other fits generated. Geometric algorithms to numerically determine such set of parameters are conventional to the art as numerical optimization algorithms, that illustratively include the Gauss-Newton algorithm and linear regression analysis algorithms. Such numerical optimization algorithms calculate the difference between actual data points and data points of the modeling and minimizes the summation of differences for a given fit. In some best fit models, weighting is applied to certain areas where fit parameters are considered important. The best fit can be evaluated even when different types of models are used and is always the one with the least difference. Numerical Recipes in C, The Art of Scientific Computing, Second Edition (1992). The introductory part of Chapter 15 describes the basic tasks of best-fit algorithms (see page 656). Chapter 15.5 “Nonlinear Models” explains amongst others the numerical implementation of a Levenberg-Marquardt algorithm (pg. 683) that is suitable for non-linear geometry (functions) as for instance anatomical shapes and geometry.

Once the user is satisfied with the final implant placement, the surgical plan is saved and may be transferred or otherwise uploaded to the CAS system 130 located in the operating room (OR) if the planning computer is located outside the OR. The surgical plan may include the 3-D bone models and the desired position for the implant models relative to the 3-D bone models. The surgical plan may further include other operational information for executing the TKA procedure with a CAS device as further described with reference to FIG. 11 below.

In the operating room (OR), the patient is positioned on the OR table and prepared in a typical manner. Once the femur “F” and tibia “T” are exposed, each fixator arm 140 (one on each side of the surgical robot 132) is fixed to a bone. Each fixator arm 140 may include a series of rods joined by clamps that assemble to a pin installed in the bone. After the bones are fixed, the surgical plan is registered to each bone with respect to the CAS system 130 using registration techniques known in the art. Based on the registered position of the bones, the monitor 132 displays the 3-D model of the femur ‘FM’ and the 3-D model of the tibia ‘TM’ in their relative positions to one another as they are registered in the OR. It is appreciated that a bone not receiving an implant in a particular stage of the procedure is considered bystander anatomy that is to be avoided while preparing a first bone for implant revision or to receive an implant. The monitor 134 may further display the planned position of the femoral implant model 144 on the femur bone model ‘FM’, and the planned position of the tibial implant model 146 on the tibia bone model ‘TM’. A tibial tray model 146, post model 148, and a keel model 150 of the tibia implant model 146 may also be displayed.

At this point, feedback (e.g., visual, audio, tactile, etc.) may be provided to the user to assess the potential for an end-effector collision to occur with the femur “F” while the end-effector 120 modifies parts of the tibia “T”. In a specific embodiment, with reference to FIGS. 3A and 3B, a section view 152 (or cross-section view) of the post model 148 may be projected along an axis to create a projected section view 153 relative to the registered femur bone model ‘FM’. The axis that the section view 152 is projected along may be an axis normal to the cutting axis (or normal to its cross-sectional slice) of the section view 152, but more specifically is an axis representing the orientation of the tool 122 and/or sleeve 124 of the end-effector 120 while the end-effector 120 forms the post hole 110 in the tibia “T”. Therefore, if the projected section view 153 intersects with the registered femur bone model ‘FM’, as shown in FIG. 3A, then there is a high likelihood that the end-effector 120 will collide with the femur “F” while forming the post hole 110 in the tibia “T”.

The projected section view 153 may be shown on the monitor 134 to provide visual feedback as shown in FIGS. 3A and 3B, while in other embodiments, the projected section view 153 is not shown on the monitor 153 and the computer software internally computes if there is any intersection with the registered femur bone model ‘FM’ and subsequently provides other feedback to the user (e.g., an audible alert, tactile sensation) as further described below.

The projection of the section view 152 may be accomplished with traditional computer graphics software techniques. In a particular embodiment, the position of the section view 152 is identified and the coordinates of the section view 152 (which is in the coordinate frame of the registered tibia bone model ‘TM’) is transformed into the coordinate frame of the registered femoral bone model ‘FM’, wherein the section view 152 is then projected (or extruded) in the coordinate frame of the registered femoral bone model ‘FM’. Alternatively, this may be accomplished by projecting (or extruding) the section view 152, while in the coordinate frame of the registered tibial bone model ‘TM’, to extend beyond the tibial bone model ‘TM’ by a distance sufficient to visualize any potential intersection with the registered femoral bone model ‘FM’. If a collision is expected (as determined by the projected section view 153 intersecting with the registered femoral bone model ‘FM’), the user may avoid the collision by either: a) re-positioning the bones (e.g., through subluxation of the tibia “T”); or b) choosing to skip one or more cuts to be made, particularly the cut associated with the formation of the post 110. If the user decides to re-position the bones, the bones are physically re-positioned and the above procedure repeated until the projection section view 153 no longer intersects with the registered femur bone model ‘FM’ as shown on the monitor 134 in FIG. 3B. It should be appreciated that the visual feedback may be provided to avoid a collision while modifying the bone for other implant components/features (e.g., projecting a section view (or other view) of the keel model 150, or projecting a section view (or other view) of an entire implant itself). It should be further appreciated that other pre-operative bone data or implant data may be used in lieu of 3-D models to provide the visual feedback. For example, the 3-D bone models may be a series of 2-D views of the bones, the raw imaging data of the bones (e.g., Digital Imaging and Communications in Medicine (DICOM) data), generic virtual bone models, and other types of pre-operative bone data as described above. Likewise, the implant model may be any other representation of the implant illustratively including one or more 2-D images of the implant, a triangular mesh of the implant, a point cloud of the implant, etc.

After the user has positioned the femur “F” and/or tibia “T”, or decided to skip one or more cuts to be made on the bones with the surgical robot 132, the remainder of the TKA procedure is executed to modify the femur “F” and tibia “T” to receive the implants according to the surgical plan. If the user decided to skip one or more cuts, the skipped cuts may be completed using manual instrumentation or other techniques. The final implants are then installed on the bone and the TKA procedure is complete.

In a particular embodiment, the surgical robot 132 does not include fixator arms 140 but instead uses a tracking system (e.g., an optical tracking system as denoted at 406 in FIG. 11). An example of an optical tracking system operative herein is found in U.S. Pat. No. 6,061,644 and further detailed with respect to FIG. 11. After the bone is exposed in a traditional manner, a tracking element (e.g., tracking arrays as denoted as 438b and 438c in FIG. 11) is affixed to each bone. The surgical plan is then registered to each bone relative to the coordinate frame of the tracking element to permit the tracking system to track the POSE of the surgical plan as the bones move in the OR. After registration, the same procedure to provide visual feedback to the user may implemented. For example, a femur bone model ‘FM’ and tibia bone model ‘TM’ are displayed on the monitor 134 that reflects the actual position of the femur “F” and tibia “T” relative to one another in the OR. The displayed position of the femur bone model ‘FM’ and tibia bone model ‘TM’ may be updated in real-time as the femur “F” and tibia “T” move in the OR, by way of the 3-D bone models being registered to the bones relative to the coordinate frame of the tracking elements affixed to the bones, and the tracking elements being tracked by the tracking system. A section view of a feature or component of the implant, for example, post model 148, may be projected normal to its cutting axis and through the opposing bone model, as described above. The location of the projection of the section view 152 may likewise be updated in real-time as the femur “F” or tibia “T” move in the OR by updating/calculating, in real-time, the transformation between: a) the coordinates of the section view 152 in the coordinate frame of the registered tibial bone model ‘TM’; and b) the coordinate frame of the registered femoral bone model ‘FM’. Alternatively, the section view 152 may be projected beyond the tibial bone model ‘TM’ a sufficient distance to possibly intersect with the femur bone model ‘FM’ as previously described. Therefore, a user, in may adjust the position of either bone to avoid a collision during the procedure, even while the surgical robot 132 is actively modifying a bone, and even in real-time.

In particular embodiments, the computer software internally computes if there is a potential collision of the end-effector 120 with bystander anatomy and provides feedback to the user (e.g., a visual alert, an audible alert, tactile sensation). After the bone models are registered to their respective bone models, the computer determines if the end-effector 120 will collide with the bystander anatomy using: i) a registered position of a bone model to bystander anatomy; and ii) a registered orientation of an end-effector axis 125 (as shown in FIG. 2) that the end-effector 120 will assume while modifying parts of a bone opposing the bystander anatomy. The end-effector axis 125 may represent a longitudinal axis of the tool 122 or a longitudinal axis of a sleeve 124 of the end-effector 120 as shown in FIG. 1B. In a particular example for determining a potential collision, a femoral bone model ‘FM’ is registered to the femur ‘F’ and a cut-file is registered to the tibia ‘T’. If a cut-file is positioned with respect to a tibia bone model ‘TM’ during the planning phase, then registration of a tibia bone model ‘TM’ also registers the cut-file to the planned location on the bone in order to direct the end-effector 120 to create the bone cuts according to the planned position for an implant. The computer then knows or can calculate the position and orientation of the femur ‘F’ based on the position of the registered femoral bone model ‘FM’. The cut-file includes the orientations (or vectors) that the end-effector axis 125 will assume while forming the post hole 110 or keel hole 112 in the tibia ‘T’ and since the cut-file is registered to the tibia ‘T’, the computer knows or can calculate the orientations that the end-effector axis 125 will assume when forming the post hole 148 or keel hole 150 in the tibia ‘T’. The computer may then determine a potential collision by calculating if any orientations of the end-effector axis 125 (as defined in the registered cut-file) spatially overlaps (e.g., intersects, contacts) the registered femoral bone model ‘FM; (and therefore the actual femur ‘F’) while forming the post hole 110 or keel hole 112 in the tibia ‘T’. This is determined using: i) the known POSE of the femur ‘F’ according to the position of the registered femoral bone model ‘FM’; and ii) the known orientations that the end-effector axis 125 will assume while forming the post hole 148 or keel hole 150 in the tibia ‘T’ according to the orientations defined in the cut-file registered to the tibia ‘T’. The computer may further use the geometry of the end-effector 120 along the end-effector axis 125 (e.g., the diameter of the sleeve 124 (as shown in FIG. 1B), the diameter of an end-mill, drill bit, etc.) in the determination. For instance, the computer may model the end-effector 120 along the end-effector axis 125 as a cylinder having the same diameter as the sleeve 124. The computer then determines if any part of the cylinder collides with the registered femoral bone model ‘FM’ at any of the end-effector axis orientations defined in the cut-file. If the computer determines a collision will occur, the computer may provide feedback (e.g., a visual alarm, audible alarm, tactile feedback) and may indicate that the end-effector may collide with bystander anatomy. The computer may also present the user with options including at least one of: a) reposition the bone and/or bystander anatomy; or b) limit the number of the one or more cuts to be created on the first bone with the CAS device.

In another embodiment, the computer software may internally compute if there is a potential collision of the end-effector 120 with bystander anatomy using: i) the registered position of the bone models to the bones; and ii) one or more features (e.g., post model 148) of an implant model registered to the bone. Instead of using the known orientations of the end-effector axis as defined in a cut-file as previously described, one or more axes or cross-section of a feature of an implant model may be used, which corresponds to the orientation of that the end-effector axis will assume while modifying a part of the bone. For example, a femoral bone model ‘FM’ and tibial implant model 146 are first registered to their respective bones. If a tibial implant model 146 is positioned with respect to a tibia bone model ‘TM’ during the planning phase, then registering a tibial bone model ‘TM’ to the tibia ‘T’ also registers the tibial implant model 146 to the planned location on the tibia ‘T’. Based on the registered position of the tibia implant model 146, the computer knows or can determine a POSE of at least one of a longitudinal axis of the post model 148, an axis normal to a cross-section of the post model 148, or a projection of a cross-section of the post model 148, all of which will represent at least one orientation that the end-effector axis 125 will assume while forming the post hole 110 in the tibia ‘T’. A plurality of longitudinal axes of the post 148 or axes normal to a cross-section of the post 148 may be determined to represent or to account for a plurality of end-effector axis orientations that the end-effector 120 will assume to form the post hole 110 in the tibia ‘T’. The computer may then determine a potential collision between the end-effector 120 and the femur ‘F’ while modifying the tibia ‘T’ by calculating any spatial overlap between the registered femoral bone model ‘FM’ and at least one of the following: a) an axis (or axes) of a longitudinal axis of the post model 148 registered to the tibia ‘T’; b) an axis (or axes) normal to a cross-section of the post model 148 registered to the tibia ‘T’; or c) a projection of a cross-section of the post model 148 registered to the tibia ‘T’. The computer may further use the geometry of the end-effector 120 along the end-effector axis 125 (e.g., the diameter of the sleeve 124 (as shown in FIG. 1B), the diameter of an end-mill, drill bit, etc.) in the determination by applying the geometry about the axis (or axes) of the post model 148. If the computer determines a collision will occur, the computer may provide feedback (e.g., a visual alarm, audible alarm, tactile feedback) and may indicate that the end-effector 120 may collide with bystander anatomy. The computer may also present the user with options including at least one of: a) reposition the bone and/or bystander anatomy; or b) limit the number of the one or more cuts to be created on the first bone with the CAS device.

Feedback to Position Bones with Pre-Operative and Intra-Operative Data

With reference to FIGS. 4, 5, and 6A-6C, a specific embodiment for providing feedback to assist a user in positioning bones is shown that utilizes both pre-operative and intra-operative feedback. With reference first to FIG. 4, a method to assist a user in positioning the bones may begin pre-operatively (pre-operative operations are designated in block 200) with pre-operative planning software. It is appreciated that like data is obtained in different ways without departing from the collision avoidance of the present invention. Pre-operative bone data of a first bone and a second bone is generated or obtained [S201]. A user plans the placement of one or more implants relative to the pre-operative bone of the first bone and/or second bone [S203]. A set of positions for the first bone relative to the second bone are determined that will permit a CAS device to prepare the bone without an end-effector collision [S205]. If the user selects a different implant supplier catalog, size, or changes the placement of the implant relative to the pre-operative bone data [S207], then a new set of positions are determined as in the previous step [S205]. Intra-operatively (intra-operative operations are designated in block 202), the first bone and second bone are positioned relative to one another and may be positioned in a desired location using the pre-operatively determined set of positions as a guide [S209]. After the bones are positioned (e.g., fixed in position), the surgical plan is registered to each bone with respect to a CAS system [S211]. The actual position of the first bone relative to the second bone is then calculated using the registered position of the bones [S213]. Based on the calculated position of the bones, the user may be presented with options for end-effector collision avoidance with bystander anatomy. The options may be include: a) reposition the first bone and/or bystander anatomy; or b) limit the number of the one or more cuts to be created on the first bone with the CAS device in a given orientation [S215]; or c) alternate between cuts on the faces of the opposing bones. Particular embodiments of the method will now be described.

Pre-operative bone data of the first bone and bystander anatomy, such as a second bone of the joint, may be obtained or generated [S201] by a variety of means. In a particular embodiment, pre-operative images of the bones are obtained using an imaging modality (e.g., CT, MRI, etc.). The pre-operative images are loaded into pre-operative planning software and processed to segment the bones from the surrounding tissues, where the segmented bones are used to build a 3-D models of each bone using techniques known in the art (e.g., marching cubes).

A user may then plan the placement of one or more implants relative to the pre-operative bone data [S203]. Pre-operative planning software running on a computer may include a GUI, an implant library, and various tools or widgets to assist in planning. The implant library may include a plurality of implant supplier catalogs (e.g., implants from different manufacturers), implant sizes, and implant types (femoral implants vs. tibial implants) in the form of 3-D implant CAD models. The user may select and re-select different implants and place the implants relative to the pre-operative bone data to designate the best fit, fill, or alignment for the final implants on the bones.

Once the user identifies a candidate implant and POSE for the implant relative to the pre-operative bone data, the pre-operative planning software may determine or calculate a set of positions (e.g., the joint angle and subluxation) for the first bone relative to the second bone that will permit a CAS device to create one or more cuts on the first bone and/or second bone to receive the implant in the planned placement [S205]. The set of positions reflect one or more positions for the first bone relative to the second bone that will not cause an end-effector collision with the opposing bone. The set of positions may be determined or calculated by simulating the cutting process for different positions of the bones as described in U.S. Patent App. Pub. US20200405394A1 assigned to the assignee of the present application and incorporated by reference herein. In short, the bones are iteratively positioned in different POSEs relative to one another, and for each iteration an end-effector model is used to simulate the cut on the bones to receive the implants as planned. Any iteration (i.e., a particular position of one bone relative to the other) that does not result in any part/portion of the end-effector model colliding with the opposing bone model is included in the set of positions in step [S205]. The set of determined positions may then be provided to the user in the GUI (e.g., in a text table, graph, graphics, illustrations, simulations, or other form). The set of positions may include the joint angles (e.g., the angle between the femur “F” and tibia “T”) and subluxations (e.g., a distance measurement between the femur “F” and tibia “T”) that will avoid an end-effector collision with the opposing bone or bystander anatomy. If at any point the user changes the implant line, type, size, or position of the implants [S207], then step [S205] is repeated to determine a new set of positions for that particular implant line, type, size, and/or position that will avoid an end-effector collision.

In the OR, the patient is prepped in a typical manner and the bones are exposed. The user then positions the two bones relative to one another [S209] and may fix the bones to the CAS device or affix a tracking element to each bone. The user may position the bones with the aid of the set of positions determined pre-operatively in step [S205]. A monitor in the OR may display the set of joint angles and subluxations that will avoid an end-effector collision. The user may use the suggested joint angles and subluxations as a guide to roughly position the bones.

After the bones are fixed or trackable relative to the CAS system, the surgical plan is registered to the bones [S211] using registration techniques known in the art. Then using the registered position of the bones, the actual position of the first bone relative to the second bone is calculated [S213]. For example, the joint angles and subluxation between the first bone and second bone may be calculated based on the registered positions of the 3-D bone models to the bones. The 3-D bone models are mapped onto the position of the bones through the registration process, which allows for the calculations to be made using the registered position of the 3-D bone models. Anatomical landmarks, or designated points, lines, or planes on or associated with the 3-D bone models may provide the reference features for calculating the joint angle and subluxation. For example, the joint angle may be calculated as the angle between: a) an axial plane on the most distal portion of the femur bone model ‘FM’; and b) a plane representing the tibial plateau of the tibial bone model ‘TM’. The subluxation may be calculated as the distance between: a) a most distal point on the medial condyle of the femur bone model ‘FM’; and b) the most proximal point of the medial intercondylar tubercle of the tibial bone model ‘TM’. It should be appreciated that any anatomical landmark, or other designated point, line, or plane may be used as long as those anatomical landmarks, points, lines, or planes remain consistent between the pre-operative and intra-operative calculations.

Once the actual position of the first bone relative to the second bone is known, the actual position is compared to the set of positions determined preoperatively in step [S205] to determine if an end-effector collision might occur. For example, the actual position of the bones may match with a position of the bone models in the set of positions as determined in step [S205], which concludes that an end-effector collision is unlikely to occur. On the contrary, if the actual position of the bones does not match to at least one position in the determined set of positions in step then an end-effector collision may occur. The GUI may display a signal indicating that the bones are indeed positioned in a manner that will avoid an end-effector collision. For example, the signal may be a green light, auditory, text, or other signal. Alternatively, the GUI may display a signal indicating that the bones are positioned in a manner that may cause an end-effector collision. In this instance, the user may presented with options. The options may include: a) reposition the first bone and/or second bone; or b) limit the number of the one or more bone cuts to be created on the first bone and/or second bone [S215]. The first option allows the user to reposition the bones in a better position that will more likely avoid an end-effector collision. The second option may pull up a drop down menu or otherwise provide the user with additional options to choose which cuts to limit (i.e., cuts to skip). For example, the user may have the option to skip the cutting of the post hole 110 or keel hole 112 if those cuts are deemed likely to cause an end-effector collision. Therefore, if those cuts are skipped then the risk of an end-effector collision is mitigated. The CAS device may then prepare the other portions (non-skipped portions) of the bone, and the skipped cuts may be completed with manual instrumentation. Finally, the implants are placed on the bones and the surgery is complete.

With reference now to FIGS. 5, and 6A-6C, a specific embodiment of a method for providing feedback to assist a user in positioning bones is shown that also utilizes both pre-operative and intra-operative feedback. The method may begin pre-operatively (pre-operative operations are designated in block 200) with pre-operative planning software. Pre-operative bone data of a first bone and a second bone is generated or obtained [S301]. A user plans the placement of one or more implants relative to the pre-operative bone of the first bone and/or second bone [S303]. A set of cutting strategies is determined over a range of simulated bone positions, where each cutting strategy defines one or more cuts to be made on a bone with a CAS device. Each cutting strategy may be determined based on: a) a simulated position of the first bone relative to the second bone; and b) the planned implant placement relative to the pre-operative bone data [S305]. If the user selects a different implant line, size, or changes the placement of the implant relative to the pre-operative bone data [S307], then a new set of positions are determined as in the previous step [S305]. Intra-operatively (intra-operative operations are designated in block 202), the first bone and second bone are positioned relative to one another and may be positioned in a desired location using the range of simulated positions and set of cutting strategies (as determined preoperatively) as a guide [S309]. After the bones are positioned (e.g., fixed in position), the surgical plan is registered to each bone with respect to a CAS system [S311]. The actual position of the first bone relative to the second bone is then calculated using the registered position of the bones [S313]. A final cutting strategy from the set of cutting strategies is provided to the user based on a best match between: a) the actual position of the bones; and b) a corresponding position of the bones in the range of simulated positions [S315]. The user may also be presented with options to avoid an end-effector collision. The options may be include: a) reposition the first bone and/or second bone; or b) limit the number of the one or more cuts to be created on the first bone and/or second bone with the CAS device [S317]. Particular embodiments of the method will now be described.

Obtaining or generating the pre-operative bone data [S301] and planning the placement of an implant [S303] may be performed in the same manner as described with reference to FIG. 4. Once the user identifies a candidate implant and POSE for the implant relative to the pre-operative bone data, the pre-operative planning software may determine or calculate a set of cutting strategies over a range of simulated bone positions, where each cutting strategy defines one or more cuts to be made on a bone with a CAS device, and where each cutting strategy is determined based on: a) a simulated positions of the first bone relative to the second bone; and b) the planned placement of the implant [S305]. The set of cutting strategies may be determined or calculated by simulating the cutting process for different positions of the bones as described in U.S. Patent App. Pub. US20200405394A1 assigned to the assignee of the present application and incorporated by reference herein. Here, the bones are iteratively positioned in different POSEs relative to one another, and for each iteration an end-effector model is used to simulate the cut on the bones to receive the implants as planned. For each iteration (i.e., a particular position of one bone relative to the other) the planning software records if an end-effector collision occurred or not. If an end-effector collision did occur during a simulation, the software records which implant feature/portion was being cut in the bone (e.g., post hole) when the collision occurred. This process is repeated over a range of simulated bone positions. For each simulated bone position (e.g., a specific joint angle and subluxation), a cutting strategy is determined that provides the user with what implant features/portions may or may not be cut on the bone to avoid an end-effector collision. The set of cutting strategies and bone positions may then be provided to the user in the GUI (e.g., in a text table, graph, graphics, illustrations, simulations, or other form). For example, with reference to FIG. 6A, a monitor 135 associated with the pre-operative planning software may display a table 160. Each simulated bone position (e.g., joint angle and subluxation) is shown in each row with a corresponding cutting strategy. The range of simulated bone positions is represented by rows 1:N. The cutting strategies refer to which cuts may or may not be made on the bone to avoid an end-effector collision. For example, in row 1, the simulated position between the first bone and the second bone is a joint angle of 20 degrees and subluxation of 5 cm. At this bone position, the simulations determined that an end-effector collision would occur while cutting the post hole 110 and the keel hole 112. Therefore, the user is presented with a cutting strategy to avoid cutting the post hole 110 and keel hole 112 with the CAS device with end-effector collision avoidance. In another example, row 2 shows that a joint angle of 20 degrees and a subluxation of 10 cm between the first bone and second bone allows for the cutting of the post hole 110 and keel marks, but not the entire keel hole 112. Keel marks refer to one or more reference holes, divots, imprints, or other features that the CAS device may create on the bone, where the keel marks assist the user in aligning a manual instrument to create the keel holes 112. In a final example, row 6 shows that a joint angle of 40 degrees and subluxation of 10 cm between the first bone and second bone allows for the cutting of the post hole 110 and the keel holes 112 without an end-effector collision. If at any point the user changes the implant line, type, size, or position of the implants [S307], then step [S305] is repeated to determine a new set of cutting strategies for that particular implant line, type, size, and/or position that will avoid an end-effector collision.

In the OR, the patient is prepped in a typical manner and the bones are exposed. The user then positions the two bones relative to one another [S309] and may fix the bones to the CAS device or affix a tracking element to each bone. The user may position the bones with the aid of the set of positions and cutting strategies determined pre-operatively in step [S305]. A monitor 134 in the OR may display the set of joint angles, subluxations, and cutting strategies. The user may use the suggested joint angles and subluxations as a guide to roughly position the bones to achieve the desired cutting strategy. For example, in reference FIG. 6B, a monitor 134 in the OR may display the same table 160 as was presented pre-operatively. The user may attempt to position the bones with a joint angle of 40 degrees and a subluxation of 10 cm to ensure the CAS device can prepare the post hole 110 and keel 112 without an end-effector collision.

After the bones are positioned and fixed or trackable relative to the CAS system, the surgical plan is registered to the bones [S311] using registration techniques known in the art. Then using the registered position of the bones, the actual position of the first bone relative to the second bone is calculated [S313]. For example, the joint angles and subluxation between the first bone and second bone may be calculated based on the registered positions of the 3-D bone models to the bones as described above.

Once the actual position of the bones are calculated, a final cutting strategy from the set of cutting strategies is provided to the user. The final cutting strategy is determined based on a best match between: a) the actual position of the first bone relative to the second bone; and b) a corresponding position of the first bone relative to the second bone from the range of simulated bone positions [S315]. For example, with reference to 6C, the monitor 134 in the OR may display the actual position of the bones and the resulting final cutting strategy. The CAS system may have determined that the actual position of the bones in the OR is a joint angle of 30 degrees and subluxation of 10 cm. This corresponds to the simulated bone position in row 4 of table 160, which results in a final cutting strategy that permits the CAS device to create the post hole 110 and keel marks. The user was slightly off when initially positioning the bones in the OR (i.e., they did not achieve a joint angle of 40 degrees and subluxation of 10 cm). In this instance, the monitor 134 may display options for the user. The options may include: a) reposition the first bone and/or second bone; b) limit the number of the one or more bone cuts to be created on the first bone and/or second bone [S317]; or c) alternate between cuts on the faces of the opposing bones. The first option allows the user to reposition the bones in a better position that will more likely avoid an end-effector collision and/or to permit the CAS device to create the feature in the bone. The second option may pull up a drop down menu or otherwise provide the user with additional options to choose which cuts to limit (i.e., cuts to skip). For example, the user may now have the option to skip the cutting of the keel hole 112 to avoid an end-effector collision. The CAS device may then prepare the other portions (non-skipped portions) of the bone, and the skipped cuts may be completed with manual instrumentation. Finally, the implants are placed on the bones and the surgery is complete.

Bone Positioning with Alignment Devices or Measuring Devices

With reference to FIGS. 7, 8A-8C, and 9, a specific embodiment for assisting a user in positioning bones is shown that utilizes one or more alignment devices. In a particular embodiment, with reference to FIGS. 7 and 8A, a first alignment device 170 is shown. The alignment device 170 includes a base 172, and an alignment feature 174 projecting from the base 172. The base 172 may be shaped to represent the nominal shape of a bone, for example, the proximal tibial bone shape in its natural form. The alignment feature 174 may be sized and shaped to represent the path of the end-effector. The alignment device 170 is configured to be placed upon the bone for example, the tibial plateau as shown in FIG. 7, prior to cutting to show the path or orientation of the end-effector and therefore whether the end-effector might collide with the opposing bone. For example, in FIG. 7, when the alignment device 170 is placed on the tibia “T”, the alignment feature 174 (which represents the path of the end-effector) does not collide with the femur “F”, and therefore it is unlikely that the end-effector will collide with the femur “F” while preparing the tibia “T”.

With reference to FIGS. 8A-8C, a particular embodiment of the alignment device 170 is shown. The base 172 has the nominal shape of the proximal tibia “T” and may fit over a portion of the proximal tibia “T”. For example, the base 172 may have a shelled interior 176 to receive the proximal tibia “T” therein. The alignment feature 174 represent the path of the end-effector and may have the size and shape of one or more implant features/components to be cut in the bone. For example, the alignment feature 174 may include a post hole feature 178 and a keel hole feature 180 projecting from the base. The post hole feature 178 and keel hole feature 180 may project from the base 172 by a suitable length to intersect with the femur “F” to assess the potential for an end-effector collision. Furthermore, the alignment feature 174 may be sized with an additional margin of thickness or width to ensure there is enough clearance for the end-effector during the surgery. If the alignment device 170 when placed upon the proximal tibia “T” prior to cutting shows that there is not enough clearance between the alignment feature 174 and the opposing bone, then the bones should be re-positioned prior to registration. This is one primary advantage of using the alignment device 170 because a user may assess whether an end-effector collision may occur before registering the bones.

It should be appreciated that other shapes and sizes for the alignment device 170, base 172, and alignment feature 174 may be designed. For example, the base 172 may be adapted to fit on a distal end of the femur “F”, where the alignment feature 174 is one or more peg hole features extending from the base 172. The peg hole features represent the pegs of a femoral TKA implant. When placing the alignment device 170 on the distal femur, a user can identify if an end-effector collision might occur with the tibia “T” as the end-effector prepares the peg holes to receive the pegs of the femoral TKA implant. The same applies to any other bone, implant, or implant features to assess a potential end-effector collision.

With reference to FIG. 9, another embodiment for providing feedback to assist a user in positioning bones is shown that utilizes a measuring device 182. A measuring device 182 (e.g., goniometer, protractor, compass, ruler) may be used to measure the angle and a distance from a tibial landmark 184 and a femoral landmark 186 to determine if any positioning requirements are satisfied. For example, a goniometer may be used to measure the joint angle between the femur “F” and tibia “T” using the lateral epicondyle of the femur “F” and the lateral condyle of the tibia “T”. It should be appreciated that numerous anatomical landmarks exist to permit such measurements. The use of the measuring device 182 may be combined with the methods described in FIGS. 4 and 5. For example, at step [S209] or [S309] the measuring device 182 may be used to measure and obtain a desired joint angle and subluxation as suggested by the results of step [S205] or [S305].

Bone Positioning with Fixation Elements

With reference to FIG. 10, a specific embodiment for assisting a user in positioning bones is shown that utilizes a fixation system. The fixation system and a method of its use provides a way to ensure surgical success during cutting by limiting the joint fixation to positions that are viable during use of a CAS device (e.g., surgical robot). If the patient cannot be put into the correct position, then the fixation system cannot lock into place. The fixation system may include fixation elements that attach to the patient and position the bones at a desired location (e.g., joint angle and subluxation), providing the clearance, and in some case an additional margin of clearance, required for most surgical plans. Additionally, the fixation system can be set for different cutting strategies. For example, the fixation system may provide the user with different options, where the fixation elements lock in place to position the bones at different locations to allow of full operative cutting or partial operative cutting. For a tibia “T”, full operative cutting allows for the creation of the tibial plateau 108, post hole 110, and keel holes 112, while partial operative cutting may skip certain cuts and only allow for the cutting of: a) the tibial plateau 108; b) the tibial plateau 108 and post hole 110; or c) the tibial plateau 108, post hole 110, and keel marks.

In a particular embodiment, the fixation system 188 includes a first fixation arm 190a and a second fixation arm 190b. The fixation arms (190a, 190b) may be positioned on opposing sides of a surgical robot 132, and may be more particularly positioned on opposing sides of the base 136 of a surgical robot 132. The first fixation arm 190a may include a plurality of links (192a, 192b) joined together by one or more joints 194. Likewise, the second fixation arm 190b may also include a plurality of links (196a, 196b) joined together by one or more joints (198a, 198b). A distal end of the most distal links (192a, 196a) further includes an attachment mechanism (e.g., clamp, screw, pin) to securely fix the fixation arms (190a, 190b) to the bones. The joints (194, and 198a, 198b