Patent Publication Number: US-11653976-B2

Title: Automated arthroplasty planning

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority of U.S. Provisional Patent Application Ser. No. 62/302,770 filed Mar. 2, 2016, which is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention generally relates to the field of computer-aided surgical planning, and more specifically to a computerized method to intuitively plan a total knee arthroplasty procedure. 
     BACKGROUND 
     Total knee arthroplasty (TKA) is a surgical procedure in which the articulating surfaces of the knee joint are replaced with prosthetic components, or implants. TKA requires the removal of worn or damaged cartilage and bone on the distal femur and proximal tibia. The removed cartilage and bone is then replaced with synthetic implants, typically formed of metal or plastic, to create new joint surfaces. 
     Computer-assisted surgical systems and patient specific instrumentation (PSI) are gaining popularity as a tool to pre-operatively plan and precisely execute the surgical plan to ensure an accurate final position and alignment of an implant within a knee joint of a patient that can improve long term clinical outcomes and increase the survival rate of the prosthesis. In general, the computer-assisted surgical systems and PSI systems include two components, an interactive pre-operative planning software program and a computer-assisted surgical device or PSI that utilizes the pre-operative data from the software to assist the surgeon in precisely executing the procedure. 
     Conventional interactive pre-operative planning software generates a three-dimensional (3-D) model of the patient&#39;s bony anatomy from a computed tomography (CT) or magnetic resonance imaging (MRI) image data set of the patient. A set of 3-D computer aided design (CAD) models of the manufacturer&#39;s implants are pre-loaded in the software that allows the user to place the components of a desired implant to the 3-D model of the bony anatomy to designate the best position and alignment of the implant on the bone. The pre-operative planning data is used to either fabricate the patient specific instrumentation, or it is loaded and read by a surgical device to assist the surgeon intra-operatively in executing the plan. Such data is also of value in positioning a surgical robot so as to insure spatial access of the robot to the needed surgical field while retaining human access to the same. 
     However, some pre-operative planning software is limited in a few regards that inhibit the ability of a user to intuitively plan all six degrees of freedom of the implants in TKA. First, the user may have to perform a majority of the planning steps manually. For instance, the user identifies a majority of the anatomical landmarks on the femur and the tibia to determine various anatomical references (e.g., the mechanical axis). Second, as the user manually adjusts the position and orientation of the implant, a series of sequential rotations and translations may cause a subsequent or previous degree of freedom to change un-intuitively. This is inherently due to the additive changes in the orientation of the coordinate system of the implant as the implant is sequentially rotated or translated. This is non-intuitive because the user is trying to accomplish certain clinical alignment goals that are measured with respect to three well-established orthogonal planes. Those orthogonal planes include the coronal plane to accomplish a desired clinical varus-valgus, the axial plane to accomplish a desired clinical internal-external, and the sagittal plane to ensure the implant fits properly for a desired varus-valgus and internal-external alignment goal. The clinical alignment goals measured from these planes are important because they are a standard in the industry and are used by surgeons to assess clinical outcomes and implant alignment postoperatively. 
     Another limitation of the conventional pre-operative planning software is the inability to allow a surgeon or different surgeons to plan for different alignment goals, automatically. Different surgeons have different implant alignment strategies. For example, for varus-valgus alignment, some surgeons prefer to align the implants to restore the mechanical axis of the leg, while others prefer to align the implants to restore the native kinematics of the knee. Likewise, for internal-external alignment, some surgeons prefer to align the implant with the transepicondylar axis, while others prefer a native kinematic alignment. Conventional planning software may be limited to automatically aligning the implant to the bone according to a single default alignment goal strategy. As a result, a desired outcome may be a priori impossible. Additionally, in situations where bone characteristics are abnormal as to a parameter such as density or structure, the ability to re-select alignment strategy can afford considerably better clinical results. 
     Finally, the conventional planning software does not allow the user to simply input each of their alignment goals and have the system automatically output a transformation between the implant and the bone that can be readily used by a computer-assisted surgical system. If the system was capable of automatically aligning the implant to the bone without any manual user adjustments, it may greatly reduce the time spent creating the pre-operative plan, which saves money for the surgeon and health care facility. 
     Thus, there is a need for a system and method that automatically aligns an implant to a bone with minimal user input. There is also a need for a pre-operative planning method that allows a user to make an adjustment to the placement of an implant with respect to a bone, in a direction corresponding to a clinical alignment goal or clinical direction, regardless of a pre-adjusted position and orientation of the implant. There is a further need to provide a system and method that automatically aligns an implant to a bone with minimal user input. 
     SUMMARY OF THE INVENTION 
     A computerized method is provided for planning an arthroplasty procedure according to a user&#39;s clinical alignment goals. The method includes providing a graphical user interface (GUI) and virtual models of the first bone and the second bone involved in the arthroplasty procedure via the GUI, locating a set of anatomical landmarks located on the virtual models of the first bone and the second bone, and automatically determining, by a processor, three orthogonal planes with respect to each of the virtual models of the first bone and the second bone using at least a portion of the anatomical landmarks. The method further includes receiving user selections and re-selection of: an implant from a library of implants, the implant having a first implant for the first bone and a second implant for the second bone, where each implant for the femur and the tibia have an associated virtual model of the implant, and at least one clinical alignment goal from a set of alignment goals. The model of the first bone and the second bone are automatically aligned to the model of the implant to satisfy the at least one alignment goal. The first bones and the second bones are connected and illustratively include the femur-tibia, femur-pelvis, humerus-scapula pairs, respectively. 
     A surgical planning system is provided for planning an arthroplasty procedure according to a user&#39;s clinical alignment goals. The system includes a workstation having a computer, user-peripherals, and a monitor for displaying a graphical user interface (GUI). The computer has a processor, non-transient storage memory, and other hardware, software, data and utilities to execute the method for planning a knee arthroplasty procedure according to a user&#39;s clinical alignment goals. The peripherals allow a user to interact with the GUI and include user input mechanisms including at least one of a keyboard, mouse, or a touchscreen capability on the monitor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention is further detailed with respect to the following drawings. These figures are not intended to limit the scope of the present invention but rather illustrate certain attribute thereof wherein; 
         FIG.  1    depicts a high-level overview of a pre-operative planning workstation and graphical user interface in accordance with embodiments of the invention; 
         FIGS.  2 A- 2 C  illustrate examples of 3-D models of the bone in different views and the locating of anatomical landmarks thereon in accordance with embodiments of the invention; 
         FIGS.  3 A- 3 C  depict three orthogonal clinically established reference planes for planning a TKA in accordance with embodiments of the invention: 
         FIG.  4    depicts a femoral planning stage of the GUI in accordance with embodiments of the invention; 
         FIGS.  5 A- 5 C  depict the alignment of a femoral component on a model of a femur in accordance with embodiments of the invention; 
         FIGS.  6 A- 6 C  depict a condylar axis about with an implant can rotate in accordance with embodiments of the invention; 
         FIG.  7    depicts a tibial planning stage of the GUI in accordance with embodiments of the invention; 
         FIG.  8 A  illustrates a sequential order of a concatenation of femoral transforms computed by a processor in accordance with embodiments of the invention; and 
         FIG.  8 B  illustrates a sequential order of a concatenation of tibial transforms computed by a processor in accordance with embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention has utility as a method and system for performing the same to aid a user in planning at least a portion of an arthroplasty procedure, such as a total knee arthroplasty. The system and method automatically aligns the implant components and the bones according to a desired clinical alignment goal with minimum user input. The system and method further allows the user to adjust the position and orientation of the bones that in the context of a knee arthroplasty are the femur, tibia, or implant in a clinical direction regardless of a pre-adjusted position and orientation of the femur, tibia, or implant. 
     The following description of the preferred embodiments of the invention in the context of TKA is not intended to limit the invention to these preferred embodiments, but rather to enable any person skilled in the art to make and use this invention. Reference is made herein to the planning of a total knee arthroplasty but it should be appreciated that embodiments of the present invention may be applied or adapted to the planning of other orthopedic surgical procedures illustratively including total hip arthroplasty, hip resurfacing, unicondylar knee arthroplasty, ankle arthroplasty, shoulder arthroplasty, and other joint replacement procedures. 
     It is to be understood that in instance where a range of values are provided that the range is intended to encompass not only the end point values of the range but also intermediate values of the range as explicitly being included within the range varying by the last significant figure of the range. By way of example, a recited range from 1 to 4 is intended to include 1-2, 1-3, 2-4, 3-4, and 1-4. 
     With reference to the figures,  FIG.  1    illustrates an embodiment of a TKA pre-operative planning workstation  100 . The workstation  100  includes a computer  102 , user-peripherals  104 , and a monitor displaying a graphical user interface (GUI)  106 . The computer  102  includes a processor  108 , non-transient storage memory  110 , and other hardware, software, data and utilities to execute the planning process described herein. The user peripherals  104  allow a user to interact with the GUI  106  and may include user input mechanisms such as a keyboard and mouse, or the monitor may have touchscreen capabilities. 
     A high-level overview of the GUI  106  is shown in  FIG.  1   . The GUI  106  includes a three-dimensional (3-D) view window  112 , a view options window  114 , a patient information window  116 , an implant family window  118 , a workflow-specific tasks window  120 , and a limb and knee alignment measures window  122 . Each GUI window can be summarized as follows. The 3-D view window  112  allows the user to view and interact with medical imaging data, 3-D bone models, and 3-D implant component CAD models. The view options window  114  provides widgets to allow the user to quickly change the view of the models of the bone, models of the implant components, and alignment axes, to a desired view. The patient information window  116  displays the patient&#39;s information such as name, identification number, gender, surgical procedure, and operating side (e.g., left femur, right femur). The implant family window  118  provides drop-down menus to allow the user to select and re-select a desired implant component from a library of implant components. The workflow-specific tasks window  120  includes various widgets to provide several functions illustratively including: guiding the user throughout different stages of the planning procedure; allowing the user to select and re-select desired alignment goals from a set of alignment goals; allowing the user to adjust the implant component(s) and bone models in desired clinical directions; displaying measured values of the alignment and position of the component(s) on the bone(s); and displaying a summary of the plan. The limb and knee alignment measures  122  displays the alignment and position of the implant components on the bone models such as hip-knee-ankle angle, femoral joint line alignment, and tibial joint line alignment. Overall, the layout of the GUI provides the user with a convenient roadmap and visual display to successfully plan a TKA. 
     Prior to planning the procedure, imaging data of the patient&#39;s femur and tibia are obtained using an imaging modality such as computed tomography (CT), ultrasound, or magnetic resonance imaging (MRI). The imaging data is transferred to the planning workstation  100  typically in a digital imaging and communication in medicine (DICOM) format. Subsequently, a 3-D model of the bone is generated. In a particular embodiment, the patient&#39;s bones may be manually, semi-manually, or automatically segmented by a user to generate the 3-D models of the bone. One or more of the bone models can be displayed in the 3-D view window  112 , where the user can quickly change to a proximal, distal, medial, lateral, anterior, and posterior view of bone model using corresponding widgets in the view options window  114 . 
     An example of three views of the bone models are shown in  FIGS.  2 A- 2 C . A lateral view of femoral bone model  124  is shown in  FIG.  2 A , a distal view of a femoral bone model  124  is shown in  FIG.  2 B , and a proximal view of a tibial bone model  126  is shown in  FIG.  2 C . Anatomical landmarks  128  from a set of anatomical landmarks are located on the femoral model and tibial model to aid in planning. A portion of the anatomical landmarks may also provide the location for registration points to be collected intraoperatively to register the bone models to a computer-assisted surgical system as described in U.S. Pat. No. 6,033,415. The set of anatomical landmarks for the femur may include: the femoral head center  128   a , most anterior point in intercondylar notch  128   b , medial epicondyle, lateral epicondyle, anterolateral trochlear ridge, anteromedial trochlear ridge, most posterior point on medial condyle, most posterior point on lateral condyle, most distal point on medial condyle, most distal point on lateral condyle, and knee center. The set of anatomical landmarks for the tibia may include: midpoint between tibial splines, ankle center, center of medial plateau  128   c , center of lateral plateau, tibia tubercle (medial ⅓ rd ), antero-lateral face, and anteromedial face. The processor may identify a portion of the landmarks automatically using an iteration algorithm as described below. The remaining landmarks may be located by the user manually by pointing and clicking on the location of the landmark on the bone models. In a specific embodiment, as the user or computer identifies specific landmarks, those landmarks may be used to provide a specific view (distal, anterior, medial, lateral) of the bone model to easily identify a specific subsequent landmark. For example, after the three clinically standard references planes are automatically determined, the user may choose to identify and locate the center of the knee landmark. The user may click on the prompt, where the view of the bone model automatically shows the distal portion of the knee such the user may quickly locate the center of the knee. 
     The user may use other landmark locating tools such as a fitting sphere tool  130 . A user may adjust the diameter and position of the fitting sphere  130  until the diameter and position approximately matches the diameter and position of a portion of the femoral head. When matched, the center of the fitting sphere  130  defines the femoral head center anatomical landmark  128   a . In a specific inventive embodiment, the processor locates all of the landmarks automatically using the statistical model that was used to automatically generate the 3-D models of the bones from the DICOM data. Once the landmarks have been identified, they are accepted by the user and stored. 
     Three orthogonal planes on the femur are determined by the processor, where each plane corresponds to a clinically established standard reference plane for planning any TKA procedure. These planes include a coronal native plane (XZ)  132 , a sagittal plane (ZY)  134 , and an axial native plane (XY)  136 , as shown in  FIGS.  3 A- 3 C , respectively. The coronal native plane  132  is defined by the most posterior point on the posterior medial condyle  140 , posterior lateral condyle  142 , and lesser trochanter  144 , such that the plane touches exactly three points but does not intersect the bone. An iterative method to automatically find the coronal native plane  132  includes: 
     1. Point 1 (lesser trochanter initial guess); 
     a. Rotate bone model about axis 1 (î+0ĵ+0{circumflex over (k)} in global LPS (left, posterior, superior) model coordinates); 
     b. Find most-posterior (+y) point in global LPS coordinate system; c. Find same point in local coordinates on the bone model; 
     d. Rotate bone model about axis 1 back to original orientation; 
     2. Point 2 (posterior medial/lateral condyle initial guess); 
     a. Rotate bone model about axis 2 (0.577î+0.577ĵ+0.577{circumflex over (k)} in global LPS model coordinates); 
     b. Find most-posterior (+y) point in global LPS coordinate system; c. Find same point in local coordinates on the bone model; 
     d. Rotate bone model about axis 2 back to original orientation; 
     3. Point 3 (posterior lateral/medial condyle initial guess); 
     a. Rotate bone model about axis 3 (0.577î−0.577ĵ−0.577{circumflex over (k)} in global LPS model coordinates); 
     b. Find most-posterior (+y) point in global LPS coordinate system; 
     c. Find same point in local coordinates on the bone model; 
     d. Rotate bone model about axis 3 back to original orientation; 
     4. Iteratively update points; 
     a. Re-orient bone model such that all three points are parallel to the XZ plane; 
     b. Find most-posterior (+y) point in global LPS coordinate system; 
     c. Find same point in local coordinates on the bone model (Point 4); 
     d. Whichever point (point 1, 2, or 3) is closest to Point 4, delete that point and replace it with Point 4; 
     e. Repeat a-d until points do not change; 
     The axial native plane  136  is defined as perpendicular to the coronal native plane on the femur, and coincident with the most distal point on the distal medial condyle  146  and the most distal point on the distal lateral condyle  148 . A method to automatically find the axial native plane  136  includes:
 
1. Point 1 (distal medial/lateral condyle initial guess);
 
a. Rotate bone model about axis 1 (0î+1ĵ+0{circumflex over (k)} in global LPS (left, posterior, superior) model coordinates);
 
b. Find most-distal (−z) point in global LPS coordinate system;
 
c. Find same point in local coordinates on the bone model;
 
d. Rotate bone model about axis 1 back to original orientation;
 
2. Point 2 (posterior medial/lateral condyle initial guess);
 
a. Rotate bone model about axis 2 (0î−1ĵ+0{circumflex over (k)} in global LPS model coordinates);
 
b. Find most-distal (−z) point in global LPS coordinate system;
 
c. Find same point in local coordinates on the bone model;
 
d. Rotate bone model about axis 2 back to original orientation;
 
3. Iteratively update points;
 
a. Re-orient bone such that the coronal kinematic plane is coincident with the XZ plane;
 
b. Re-orient bone about the Y-axis until both points are parallel with the XY plane;
 
c. Find most-distal (−z) point in global coordinate system;
 
d. Find same point in local coordinate on the bone model (Point 3);
 
e. Whichever point (point 1 or 2) is closest to Point 3, delete that point and replace it with Point 3;
 
f. Re-orient bone such that the bone is in the original coordinate system;
 
g. Repeat a-f until points do not change.
 
The sagittal plane  134  is defined as perpendicular to both the coronal native plane  132  and the axial native plane  136 , and coincident with a medial-lateral center point of the bone. The medial-lateral center point may be determined by computing the midpoint between the medial epicondyle landmark  150  and the lateral epicondyle landmark  152 . These three orthogonal planes are clinically established standard reference planes used for planning any TKA. It should be appreciated that the LPS coordinate system used above is not an essential reference coordinate system to determine the planes ( 132 ,  134 ,  136 ), where other reference coordinate systems may be used. In addition, the amount to rotate the bone model about the axes 1, 2, and 3 to locate the points that define the planes may be tuned to ensure convergence for different patient positions during scanning.
 
     An initial femur transform is determined using the intersection of the three orthogonal planes to establish a position and orientation of the x, y, and z axes relative to the bone. The initial femur transform provides the basis for aligning and positioning the implant components to the bone models in the clinically established standard reference frame. 
     With reference to  FIG.  4   , a workflow-specific tasks window  120  is shown for the femoral planning stage of the planning procedure. Briefly, as shown on the left side of the tasks window  120  are tabs  154 - 158  which allow the user to toggle between the different planning stages of the procedure (i.e., landmarks stage  154 , femur planning stage  155 , tibia planning stage  156 , summary stage  157 , and a surgery planning stage  158 ).  FIG.  4    depicts the tasks window in the femoral planning stage  155 . The femoral planning stage includes an implant drop-down menu  160 , a coronal alignment goal drop-down menu  162 , an axial alignment goal drop-down menu  164 , a distal bone resections sub-window  166 , a posterior bone resection sub-window  168 , a flexion sub-window  170 , and a medial-lateral sub-window  172 . A user may select and re-select a desired implant from a library of implants using the implant drop-down menu  160 . A user may select and re-select a varus-valgus alignment goal from a set of varus-valgus alignment goals using the varus-valgus alignment drop-down menu  162 . Likewise, a user may select and re-select an axial rotational alignment goal from a set of axial rotation alignment goals using the axial alignment drop-down menu  164 . The set of femoral coronal alignment goals may include a native alignment and a neutral mechanical axis. The set of femoral axial alignment goals may include: parallel to the transepicondylar axis; an angle offset from an anatomical axis (e.g., 1°-10° from the posterior condylar axis); and a native alignment. 
     Each of the sub-windows  166 ,  168 ,  170  and  172 , allow the user to adjust the implant or bone in four clinical directions. The four clinical directions include a proximal-distal translational direction (sub-window  166 ), an anterior-posterior translational direction (sub-window  168 ), a medial-lateral translational direction (sub-window  172 ), and a flexion-extension rotational direction (sub-window  170 ). The directions are referred to as clinical because the user can adjust each direction individually, and an adjustment in the clinical direction corresponds to a direction with reference to the clinically established reference frame, regardless of a pre-adjusted position and orientation of the implant or bone as described below. The user can adjust the clinical direction using the corresponding “+” button  174  or “−” button  176 . Reset buttons  178  allow the user to reset any adjustments to a default value. A measured amount of distal resection on the medial and lateral distal condyles  180  is displayed as the user adjusts any clinical directions and/or selects/reselects an implant or alignment goal. Similarly, a measured amount of posterior bone resection on the medial and lateral posterior condyles  182  is displayed as the user adjusts any clinical directions and/or selects/reselects an implant or alignment goal. In a particular inventive embodiment, an additional sub-window allows the user to adjust for an estimated or measured cartilage thickness, cartilage wear, or bone wear to translate the implant accordingly. 
     A default femoral coronal alignment goal, and femoral axial alignment goal are pre-set when the user enters to the femur planning stage. A user then selects a femoral implant component, and the femoral bone model is automatically aligned to the selected implant according to the default alignment goal. The default alignment goals may be a native femoral coronal alignment and a native femoral axial alignment. The processor automatically aligns the bone to the implant using the initial femur transform, the native alignment goals, and a portion of the geometry of the implant. An example of a femoral implant component  184  is shown in  FIG.  5 A . The femoral implant  184  includes a distal planar surface  186  to interact with a distal bone cut and a posterior planar surface  188  to interact with the posterior bone cut. The planar surfaces ( 186 ,  188 ) are also referred to herein as implant cut planes. A femoral implant distal articular plane  190  is defined as an offset plane from the distal planar surface  186  by a maximum thickness of the distal portion of the implant. Likewise, a femoral implant posterior articular plane  192  is defined as an offset plane from the posterior planar surface  192  by a maximum thickness of the posterior portion of the implant. The articular planes ( 190 ,  192 ) are also referred to herein as offset implant cut planes. The processor then automatically aligns the implant to a native alignment goal by aligning the axial native plane  136  to the implant distal articular plane  190 , and the coronal native plane  132  to the implant posterior articular plane  192 .  FIGS.  5 B and  5 C  display the result of the femur model  124  and the implant  184  aligned in native alignment. 
     When a user selects a mechanical axis coronal alignment and a non-native axial alignment for the femur, projection angles rather than direction cosines are used to build a rotational adjustment transform to align the implant and the bone. By using projection angles, individual degrees of freedom can be changed/adjusted without substantially affecting the other degrees of freedom and subsequent changes/adjustments do not substantially affect previous adjustments/changes. In one inventive embodiment, substantially affecting the other degrees of freedom refers to 1 mm or 1 degree. In another inventive embodiment, the substantially affecting refers to 0.5 mm and 0.5 degrees. While in other inventive embodiments, the substantially affecting refers to 0.1 mm and 0.1 degrees. 
     For example, when a user selects a mechanical axis alignment, the mechanical axis (defined as an axis connecting the femoral head center and the center of the knee) is projected onto the coronal native plane  132 . The angle between the z-axis and the projected mechanical axis is determined and used to build a portion of the rotational adjustment transform. Simultaneously, when a user selects, for example, the transepicondylar axis (defined as an axis connecting the medial and lateral epicondyles) is projected onto the axial native plane  136 . The angle between the x-axis and projected transepicondylar axis is determined to build the second portion of the rotation adjustment transform. The user can make any adjustments in the other clinical directions as desired. 
     In a particular inventive embodiment, with reference to  FIGS.  6 A- 6 C , the processor determines a condylar axis  194  where any adjustment in the flexion-extension rotation direction occurs about the condylar axis  194 . The condylar axis  194  may be determined by fitting one circle to a portion of the medial condyle  198 , and fitting a second circle to a portion of the lateral condyle. Each circle having a center  200 . An axis connecting the two centers of the circles define the condylar axis  194 . In another inventive embodiment, the condylar axis  194  is determined as a center axis of a cylinder  202  traversing through the medial condyle  198  and the lateral condyle  204 , where the diameter of the cylinder is best-fitted to a portion of the condyles. In another inventive embodiment, a sphere is fitted to the medial  198  and lateral condyles  204 , wherein an axis connecting the center of the two spheres defines the condylar axis  194 . In a specific inventive embodiment, the condylar axis  194  is determined by the following: 
     1. Map articular surface; 
     a. Re-orient bone model such that the coronal native plane is coincident with the XZ plane, the axial native plane is coincident with the XY plane, and the sagittal native plane is coincident with the YZ plane; 
     b. Rotate bone about global X-axis (flexing the knee) about flexion increment; 
     c. Locate the most-distal (−z) point in global coordinate system that has a positive x-coordinate (condyle 1 articular surface point). Locate same point in local bone coordinates. 
     d. Locate the most-distal (−z) point in global coordinate system that has a negative x-coordinate (condyle 2 articular surface point). Locate same point in local point coordinates: 
     e. Increment flexion and repeat c-d; 
     f. Repeat e through entire range of flexion; 
     2. Fit cylinder to mapped articular surface: 
     3. Center axis of the cylinder is the condylar axis  194 . 
     As shown in  FIG.  6 C , as the implant or bone is rotated in flexion-extension about the condylar axis, the posterior resection  206  and the distal resection  208  does not substantially change because the implant is essentially rotating about the semi-circular condyles. Therefore, the user can set the anterior portion  210  of the implant such that there is no notching without affecting these resections. 
     After the femur has been planned, the user may plan the tibia, although the user may go back and forth between the femur and tibia planning stages. The tibia component is automatically aligned to the femoral component by matching the articular surface of the tibia to the articular surface of the femoral component. As shown in  FIG.  7   , the user can select and re-select a tibial baseplate component using baseplate drop-down menu  212 , and the user can select and re-select a tibial liner using tibial liner drop-down menu  214 . The user can then adjust the posterior slop  216 , coronal alignment  218 , proximal bone resections  220 , axial rotation  222 , and translational position  224 . A measured value of the tibial slope  226  is displayed and a measure value of the tibial proximal bone resections  228  is also displayed. The posterior slope is adjusted in flexion-extension about the condylar axis  194  defined on the femoral condyles as described above. The axial rotation occurs about a projection of the mechanical axis of the tibia onto the sagittal native plane  134 . 
       FIGS.  8 A and  8 B  depict a sequential order of a concatenation of transforms that allow the user to make an adjustment in any clinical direction and/or select/re-select an alignment goal, such that the implant is re-aligned in the desired clinical direction or the re-selected alignment goal, regardless of a pre-adjusted position and orientation of the implant. This specific order of transforms is designed such that the adjustments are the inputs and the outputs. When a user makes an adjustment in a clinical direction, the model is transformed in the desired clinical direction. After the model has been transformed, the position and orientation of the implant is measured with respect to the bone, which directly correlates to adjustment made. This order is computationally fast and provides the user with an intuitive tool for aligning an implant with respect to the bone according to a clinical standard reference frame. 
       FIG.  8 A  depicts the sequential order of the concatenation of transforms computed by the processor for the femoral planning stage to intuitively align and position the femoral implant to the bone. The anatomical landmarks are determined (Block  230 ) and an initial femoral base transform is determined (Block  232 ). The second transformation is between the femoral implant component (Block  234 ) and the femur model (Block  236 ). The third transformation is an adjustment transform for femoral coronal and axial rotation (Block  242 ). The fourth transform is a translation transform for any adjustment in the anterior-posterior, proximal-distal, medial-lateral, and an optional account for cartilage thickness (Block  252 ). The final transform is an adjustment in the flexion-extension rotation direction about the condylar axis  194  (Block  256 ). Note, each of these transforms have default values, which allow the user to adjust any of the alignment goals or clinical directions in any sequence, however the transforms are computed in this sequential order to allow for any adjustments to occur in the desired direction regardless of the position and orientation of the implant or bone. 
       FIG.  8 B  depicts the sequence of the concatenation of transforms computed by the processor for the tibial planning stage to intuitively align and position the tibial implant to the bone. A first initial tibia transform (Block  258 ) from the anatomical landmarks. A second transform between the tibial component and the femoral component (Block  262 ). A third rotation transform in flexion-extension rotation about the condylar axis  194  (Block  266 ). A fourth transform in a varus-valgus alignment (Block  270 ). A fifth transform in internal-external rotation where the rotation occurs about the projection of the mechanical axis of the tibia to the sagittal native plane (Block  274 ). A final translation transform for any adjustments in a translational direction (Block  282 ). 
     The sequential order of the concatenation of transform allows the user to make adjustments to the implant or bone in an intuitive manner. Once the femur and tibial planning stages are complete, the user can review the hip-knee-ankle angles, femoral joint line alignment, tibial joint line alignment, femoral distal resections, femoral posterior resections, proximal tibial resections, and posterior slope, as well as patient information and surgical procedure information in the summary stage of the procedure. In the surgical planning stage, the user can determine which bone should be operated on first and define any parameters for a computer-assisted surgical system. The final plan is accepted by the surgeon and is written to a data transfer file (e.g., compact disc (CD), portable universal serial bus (USB)) for use with the computer-assisted surgical system. The final plan includes the final femur-to-implant transform, and the final tibia-to-implant transform to register and execute the TKA according to the plan. 
     In a particular inventive embodiment, if a user does not deviate from a particular planning strategy, the user may save their set of alignment goals in the planning workstation that can be applied to all surgical cases. Due to the minimal user input required by the surgeon, the saved preferences can improve pre-operative planning times. 
     In a specific inventive embodiment, if a user desires a native alignment, the pre-operative planning may be performed nearly automatically. The coronal native plane, axial native plane and sagittal native plane may be determined as described above. The bone wear may be accounted for by the following: 
     a. Varus Malalignment 
     
         
         i. Without changing the tibia or the kinematic planes, rotate the femur in the coronal plane about an axis perpendicular to the coronal native plane and coincident with the most distal point on the lateral condyle. Rotation amount should be the arctangent of the bone wear on the lateral side divided by the distance between the two most distal points on the femur, or 
       
    
             θ   =     atan   ⁡     (       t     wear   ,   bone         d     medial   ⁢           ⁢   to   ⁢           ⁢   lateral   ⁢           ⁢   condyle         )             
b. Valgus Malalignment
     i. Without changing the tibia or the kinematic planes, rotate the femur in the coronal plane about an axis perpendicular to the coronal native plane and coincident with the most distal point on the medial condyle. Rotation amount should be the arctangent of the bone wear on the medial side divided by the distance between the two most distal points on the femur, or   

     
       
         
           
             θ 
             = 
             
               atan 
               ⁡ 
               
                 ( 
                 
                   
                     t 
                     
                       wear 
                       , 
                       bone 
                     
                   
                   
                     d 
                     
                       medial 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       to 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       lateral 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       condyle 
                     
                   
                 
                 ) 
               
             
           
         
       
     
     The cartilage wear may be accounted for by the following: 
     a. Varus Malalignment 
     
         
         i. Without changing the tibia or the kinematic planes, rotate the tibia in the coronal plane about an axis perpendicular to the coronal native plane and coincident with the most distal point on the lateral condyle. Rotation amount should be the arctangent of the total cartilage wear on the lateral side divided by the distance between the two most distal points on the femur, or 
       
    
             θ   =     atan   ⁡     (       t     wear   ,   cartilage         d     medial   ⁢           ⁢   to   ⁢           ⁢   lateral   ⁢           ⁢   condyle         )             
b. Valgus Malalignment
     i. Without changing the tibia or the kinematic planes, rotate the femur in the coronal plane about an axis perpendicular to the coronal native plane and coincident with the most distal point on the medial condyle. Rotation amount should be the arctangent of the total cartilage on the medial side divided by the distance between the two most distal points on the femur, or   

     
       
         
           
             θ 
             = 
             
               atan 
               ⁡ 
               
                 ( 
                 
                   
                     t 
                     
                       wear 
                       , 
                       cartilage 
                     
                   
                   
                     d 
                     
                       medial 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       to 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       lateral 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       condyle 
                     
                   
                 
                 ) 
               
             
           
         
       
     
     The femoral implant size may be determined using the medial-lateral width and femoral anterior-posterior size of the femur. The femoral implant is placed on the bone model such that the articular surface of the femoral component contacts the coronal native plane and axial native plane. The two posterior condyles of the component contact the coronal native plane in exactly two points such that the component does not intersect the plane. The two distal condyles of the component contact the axial native plane in exactly two places such that the component does not intersect the plane. The femoral implant is automatically rotated in flexion-extension about the condylar axis  194 , maintaining the requirements of the placement, until the most proximal part of the anterior surface of the femoral implant is on the anterior surface of the bone (no notching). The tibial component comes in linked to the femoral component at full extension and the flexion of the tibia is corrected about the femoral component flexion axis coincident with the condylar axis  194 . 
     OTHER EMBODIMENTS 
     While at least one exemplary embodiment has been presented in the foregoing detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the described embodiments in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient roadmap for implementing the exemplary embodiment or exemplary embodiments. It should be understood that various changes may be made in the function and arrangement of elements without departing from the scope as set forth in the appended claims and the legal equivalents thereof.