Patent Publication Number: US-2020289274-A1

Title: Knee prosthesis

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of pending U.S. patent application Ser. No. 16/000,831, filed Jun. 5, 2018, which application is a division of U.S. patent application Ser. No. 14/644,570, filed Mar. 11, 2015, now U.S. Pat. No. 9,999,511, which is a continuation of U.S. patent application Ser. No. 14/329,546, filed Jul. 11, 2014, now U.S. Pat. No. 9,579,209, which is a division of U.S. patent application Ser. No. 13/360,184, filed Jan. 27, 2012, now U.S. Pat. No. 8,808,388, which claims priority from and the full benefit of U.S. Provisional Patent Application Ser. No. 61/436,788, filed Jan. 27, 2011, and titled “Constrained Knee Prosthesis,” the entire contents of all of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     This disclosure relates to knee prostheses. 
     BACKGROUND 
     Total knee replacement systems often include a tibial implant and a femoral implant that replace the articular surfaces of the knee. Posterior-stabilized knee replacement systems can be used to replace the function of both the anterior cruciate ligament (ACL) and posterior cruciate ligament (PCL). In some instances, posterior-stabilized knee replacement systems include varus/valgus constraint to also replace the function of the medial collateral ligament (MCL) and the lateral collateral ligament (LCL). Although a constrained knee replacement system can provide needed stability, it often introduces biomechanical inefficiencies. 
     SUMMARY 
     In a general aspect, a tibial insert provides varus/valgus constraint and permits tibiofemoral rotation. The tibial insert includes a post having walls configured to engage a femoral component over a flexion/extension range from extension to about 90 to about 120 degrees flexion. The post has rounded edges that permit tibiofemoral rotation when the post is in contact with the femoral component. 
     In another general aspect, a tibial insert includes a base and a post extending from the base along a longitudinal axis. The post has a medial surface, a lateral surface, and a height along the longitudinal axis. The medial surface has a medial section, and the lateral surface has a lateral section oriented substantially parallel to the medial section. The medial section and the lateral section each have a width in a substantially anterior-posterior direction that is sufficient to enable varus/valgus constraint over a flexion/extension range from extension to about 90 to 120 degrees of flexion when the tibial insert is mated with a femoral component. 
     Implementations can include one or more of the following features. For example, the femoral component defines an opening for receiving the post between substantially parallel walls, the opening providing a clearance of approximately 0.005 inches to approximately 0.030 inches between the post and the substantially parallel inner walls when the post is received in the opening. The medial section and the lateral section each extend such that, along at least half of the height of the post, the width of the medial section and width of the lateral section in the substantially anterior-posterior direction at a given axial position along the longitudinal axis is between approximately one sixth and approximately two thirds of a largest width of the post in the substantially anterior-posterior direction at that axial position. The post has a proximal surface that is substantially flat and a notch defined in a superior anterior portion of the post. At least a portion of the notch is defined completely through the post along a medial-lateral direction. The medial section and the lateral section are substantially flat and are oriented along the substantially anterior-posterior direction. For substantially the entire height of the post, at a given axial position, the width of the medial surface and the width of the lateral surface in the substantially anterior-posterior direction are between approximately one sixth and two thirds of the largest width of the post in the substantially anterior-posterior direction at that axial position. The average length of the medial surface in an axial direction is more than twice the average length of the medial surface in the substantially anterior-posterior direction, and the average length of the lateral surface in an axial direction is more than twice the average length of the lateral surface in the substantially anterior-posterior direction. The post is twisted along the axis of the post such that a superior portion of the post is rotationally offset from an inferior portion of the post. The post is rotationally offset from the base such that the medial surface and the lateral surface are oriented at an angle with respect to medial and lateral sides of the base. The post has an anterior surface and a posterior surface, and the anterior surface and the posterior surface each have a convex portion. The post has rounded edges between the anterior surface and the medial and lateral surfaces and between the posterior surface and the medial and lateral surfaces. The rounded edges have a radius of between approximately 0.030 and 0.090 inches. 
     In another general aspect, a method of operation of a knee prosthesis includes permitting flexion/extension of the knee prosthesis over a flexion/extension range of approximately 0 to 150 degrees, constraining varus/valgus alignment of the knee prosthesis over a constrained flexion/extension range from extension to about 90 to 120 degrees of flexion, and rotating a tibial insert of the knee prosthesis relative to a femoral component of the knee prosthesis about a substantially superior-inferior axis of the tibial insert over at least a portion of the constrained flexion/extension range. 
     In another general aspect, a method of trialing a tibial insert of a knee prosthesis includes coupling a first tibial insert to a prepared tibia or a tibial tray, assessing the suitability of the knee prosthesis, and removing the first tibial insert from the prepared tibia or the tibial tray. The method includes coupling a second tibial insert to the prepared tibia or the tibial tray. The second tibial insert is configured to permit flexion/extension of the knee prosthesis over a flexion/extension range of approximately 0 to 150 degrees, constrain varus/valgus alignment of the knee prosthesis over a constrained flexion/extension range from extension to about 90 to 120 degrees of flexion when the tibial insert mated with a femoral component, and rotate the tibial insert of the knee prosthesis relative to the femoral component about a substantially superior-inferior axis of the tibial insert over at least a portion of the constrained flexion/extension range. 
     Implementations can include one or more of the following features. For example, the first tibial insert and the second tibial insert each have a post, and the first tibial insert and the second tibial insert have differing post dimensions. 
     In another general aspect, a prosthesis includes a femoral component that defines an opening between substantially parallel inner walls and a tibial insert having a base and a post extending from the base. The post has a medial surface and a lateral surface, and is configured to be received in the opening. The medial surface has a medial section, and the lateral surface has a lateral section oriented substantially parallel to the medial section. The medial section and the lateral section each have a width in a substantially anterior-posterior direction that is sufficient to constrain varus/valgus alignment of the prosthesis over a flexion/extension range from extension to about 90 to 120 degrees of flexion when the tibial insert is mated with the femoral component such that the post is received in the opening and the medial section and the lateral section each engage one of the substantially parallel inner walls. Engagement of the tibial insert and the femoral component rotates the tibial insert relative to the femoral component over at least a portion of the constrained flexion/extension range. 
     Implementations can include one or more of the following features. For example, the post is dimensioned to provide a total clearance of approximately 0.005 to approximately 0.030 inches between the post and the substantially parallel inner walls when the post is received in the opening. The post has a posterior surface and an anterior surface, and the post has rounded edges between the anterior surface and the medial and lateral surfaces and between the posterior surface and the medial and lateral surfaces. The base has a medial bearing surface having a concave portion and a lateral bearing surface having a concave portion. The concave portion of the medial bearing surface is positioned anteriorly offset from a center of the base and has a maximum depth in a superior-inferior direction that is more inferior than a maximum depth of the concave portion of the lateral bearing surface in the superior-inferior direction. The post has a posterior surface, and the femoral component has an asymmetrical posterior cam configured to engage the posterior surface to drive rotation of the tibial insert relative to the femoral component. The posterior cam is configured to engage the posterior surface at angles of flexion that are greater than a first angle that is between approximately 60 and approximately 90 degrees of flexion, and the posterior cam is configured to not engage the posterior surface at angles of flexion less than the first angle. 
     The details of one or more implementations are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the disclosure will be apparent from the description, the drawings, and the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an exploded perspective view of a tibial insert and a femoral component of a left knee prosthesis. 
         FIG. 2  is an exploded posterior view of the tibial insert and the femoral component. 
         FIG. 3  is a sagittal section view of the tibial insert and the femoral component in extension. 
         FIG. 4  is a top view of the tibial insert and the femoral component in extension. 
         FIG. 5  is a top view of the tibial insert and the femoral component in flexion. 
         FIG. 6  is a lateral view of the tibial insert showing a cross section of a lateral bearing surface. 
         FIG. 7  is medial view of the tibial insert showing a cross section of a medial bearing surface. 
         FIG. 8  is a top view of the tibial insert showing a sectional view of a tibial post across line  8 - 8  of  FIGS. 6 and 7 . 
         FIG. 9  is a top view of a tibial insert for a left knee with a laterally rotated tibial post. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIGS. 1 and 2 , a knee prosthesis  100  provides varus/valgus constraint and also permits internal and external rotation of the tibia relative to the femur. To achieve this, the knee prosthesis  100  includes a tibial insert  300  shaped to engage a femoral component  200  to (i) limit varus-valgus deviation of the tibia from its proper alignment with the femur, and (ii) facilitate rotation of the tibia relative to the femur during flexion. The tibial insert  300  can be referred to as a constrained insert, being constrained by the femoral component  200  in the assembled knee prosthesis  100 . 
     During flexion of a healthy knee, the tibia rotates a small amount about its longitudinal axis (internal-external rotation). The knee prosthesis  100  enables this rotation helping to preserve a natural feel to the reconstructed knee. Internal rotation of the tibia relative to the femur (tibiofemoral rotation) aligns the line of action of the quadriceps and the tibia, improving the efficiency the quadriceps compared to an unaligned knee system. Proper alignment also reduces sheer forces on the patella and can improve the longevity of the knee prosthesis  100 . The tibia rotates internally relative to the femur as the knee is flexed, and rotates externally relative to the femur as the knee is extended. The knee prosthesis  100  can also replace the function of the MCL and/or LCL in addition to the functions of the ACL and PCL, with the tibial insert  300  restricting varus/valgus forces on the knee. 
     In general, the possible movements of a tibia relative to a femur can be considered to include movements about three different axes. As a result of flexion and extension of the knee, the tibia moves relative to the femur about a medial-lateral axis through the knee. Varus/valgus motion refers to movement of the tibia and the femur about an anterior-posterior axis through the knee, for example, movement that causes the leg to bow medially or laterally. Axial rotation of the femur can occur relative to a longitudinal axis of the tibia (for example, an axis parallel to the shaft of the tibia, such as an axis along a substantially superior-inferior direction). 
     As used herein, tibiofemoral rotation refers to the axial rotation of the femur with respect to the longitudinal axis of the tibia, commonly referred to as internal and external rotation. In use, the knee prosthesis  100  restricts varus/valgus movement (for example, constrains varus/valgus movement to a particular range of motion or laxity) while causing tibiofemoral rotation to occur during flexion and extension. 
     The knee prosthesis  100  for a left knee includes the femoral component  200  for mounting to a distal end of a femur and the tibial insert  300  for attachment to a proximal end of a tibia. The tibial insert  300  can be attached to the tibia by known methods. 
     The femoral component  200  includes medial and lateral walls  210 ,  211  that define an opening  212  in the femoral component  200 . The walls  210 ,  211  include substantially flat, substantially parallel inner surfaces  213 ,  215 . Located at the anterior portion  224  of the walls  210 ,  211 , the femoral component  200  includes an anterior cam  214  ( FIG. 2 ). The femoral component  200  also includes a posterior cam  216  located at a superior posterior portion  226  of the walls  210 ,  211 . The posterior cam  216  includes an uneven thickness, such that a lateral portion  222  of the posterior cam  216  is thicker than a medial portion  220 . 
     The femoral component  200  includes a medial condylar portion  201  with a medial condylar surface  202 . The femoral component also includes a lateral condylar portion  203  and a lateral condylar surface  204 . The medial condylar surface  202  and the lateral condylar surface  204  are rounded, and in some implementations, can be asymmetrical. Between the medial condylar surface  202  and the lateral condylar surface  204 , the femoral component  200  defines a trochlear groove  206  over which a patella or a patellar implant can glide during flexion of the knee. 
     The tibial insert  300  includes a base  301  and a raised section or post  302 , extending from a substantially central location of a proximal surface  303  of the tibial insert  300 . The post  302  extends from the base along a longitudinal axis, X, for example, an axis that extends in a substantially superior-inferior direction. The post  302  includes a medial surface  304 , a lateral surface  306 , an anterior surface  308 , a posterior surface  310 , and a proximal surface  312 . When the femoral component  200  and the tibial component  300  are coupled, the post  302  is received within the opening  212  between the anterior cam  214  and the posterior cam  216 . The medial surface  304  and the lateral surface  306  include substantially parallel, substantially flat contact sections  322 ,  320  ( FIGS. 6 and 7 ) to contact the inner surfaces  213 ,  215  of the walls  210 ,  211  of the femoral component  200 . The anterior surface  308  is convex in an anterior direction and the posterior surface  310  is convex in a posterior direction. 
     The proximal surface  312  is substantially flat, and the post  302  defines a notch  314 , or patella relief, at its superior anterior portion. The notch  314  provides clearance for the patella or a patellar implant in deep flexion. The notch  314  may have a spherical radius similar to the spherical radius of patella implants available for use in the implant system. At least a portion of the notch  314  is defined completely through the post  302  along a substantially medial-lateral direction, for example, from the medial surface  304  to the lateral surface  306 . 
     The tibial insert  300  also includes a medial bearing surface  316  and a lateral bearing surface  318  having sloped, concave portions that engage the medial condylar surface  202  and the lateral condylar surface  204 , respectively. 
     The tibial insert  300  can be formed, for example, of high molecular weight polyethylene. Tibial trial inserts can be made of a sterilizable plastic, for example, a thermoplastic such as polyoxymethylene (acetal). Tibial trial inserts approximate the shape and dimensions of corresponding tibial inserts for implantation. Generally, the tibial trial inserts can be sterilized for reuse. 
     As described further below, in some implementations, the knee prosthesis  100  permits flexion/extension over a flexion/extension range of approximately 0 to 150 degrees of flexion. The knee prosthesis  100  constrains varus/valgus alignment of the knee prosthesis  100  over a constrained flexion/extension range from extension (or hyperextension) to about 90 to 120 degrees of flexion. In use, the tibial insert  300  rotates relative to the femoral component  200 , resulting in tibiofemoral rotation over at least a portion of the constrained flexion/extension range. The tibial insert  300  and the femoral component  200  can rotate relative to each other over one or more portions of the constrained flexion/extension range or over the entire constrained flexion/extension range. The tibiofemoral rotation optionally occurs over a flexion/extension range of approximately 0 to 150 degrees of flexion. The rotation occurs about the axis of the tibia, which is in a direction about a substantially superior-inferior axis of the tibial insert such as the longitudinal axis, X, of the post  302 . Translation of the tibial insert  300  relative to the femoral component  200  can also occur during the rotation, as described below. 
     Constraining varus/valgus alignment of the knee prosthesis  100  includes, for example, resisting medial and lateral forces on the knee prosthesis  100 . In this manner, the knee prosthesis  100  supplements or replaces the functions of the MCL and/or the LCL. The knee prosthesis  100  can limit the varus/valgus alignment to a range of acceptable alignments, or limit deviation from a particular varus/valgus alignment to a predetermined range. In use, for example, the knee prosthesis  100  constrains the tibia and femur to a predetermined range of positions or range of angles relative to each other. The knee prosthesis  100  can provide constraint while permitting some varus/valgus movement of the knee within the predetermined range. The knee prosthesis  100  can constrain varus/valgus alignment or varus/valgus movement or to a range the same as or similar to a range of varus/valgus alignments or varus/valgus movement typical of healthy knees. In some implementations, the knee prosthesis  100  restricts varus/valgus deviation of the tibia and the femur to a total of 5 degrees or less, or a total of 1 degree or less, from a desired laxity. The desired varus/valgus laxity range of motion can be approximately 5 degrees. In some implementations, varus/valgus movement may be disallowed entirely. 
     During surgery to implant the knee prosthesis  100 , a physician couples a tibial trial insert to a prepared tibia or tibial tray. The physician assesses the suitability of the size of the tibial trial insert by, for example, coupling and removing various tibial trial inserts to identify a tibial insert  300  most appropriate for the patient. The physician can perform a trial range of motion of the knee prosthesis  100  using a tibial trial insert. This permits the physician to assess the performance and stability of the tibial trial insert and to evaluate the behavior and function of ligaments and other tissues in cooperation with the knee prosthesis  100 . The physician can assess, for example, whether the tibial trial insert sufficiently constrains varus/valgus alignment for the patient when engaged with the femoral component, and whether the tibial trial insert permits a sufficient range of tibiofemoral rotation. 
     As an example, a physician may trial a tibial trial insert that does not constrain varus/valgus motion or position, but permits flexion/extension over a flexion/extension range of approximately 0 to 150 degrees and rotates the tibial insert relative to the femoral component over a portion of or all of a flexion/extension range of approximately 0 to 150 degrees. 
     Through the trialing process, the physician may determine that additional constraint is appropriate. For example, the physician may determine that the patient presents with a lax or over-released medial collateral ligament (MCL). In response, the physician can trial a variety of tibial trial inserts that constrain varus/valgus alignment and also permit tibiofemoral rotation. The tibial trial inserts can have varying post  302  dimensions or varying dimensions of contact portions  320 ,  322 , to provide varying levels of varus/valgus constraint and varying ranges of tibiofemoral rotation. For example, the physician may trial one or more different tibial trial inserts that each constrain varus/valgus alignment over a constrained flexion/extension range from extension (or hyperextension) to about 90 to 120 degrees flexion, permit flexion/extension over a flexion/extension range of approximately 0 to 150 degrees, and rotate relative to a femoral component over at least a portion of the constrained range when mated to the femoral component. 
     After a tibial trial insert has been determined to have an appropriate size and performance characteristics, the physician removes the tibial trial insert and in its place, couples a tibial insert  300  having the same size and features as the tibial trial insert to the tibia or tibial tray. 
     In some implementations, a library of tibial trial inserts (for example, a set of multiple tibial trial inserts) is provided for use during surgery. The tibial trial inserts in the library can have different post dimensions. For example, different tibial trial inserts can have different clearances relative to the femoral component  200 , resulting in different levels of stability and tibiofemoral rotation. Additionally or alternatively, the different tibial trial inserts can also have different radii of curvature at the corners of the post, different post heights, different widths or shapes of medial and lateral surfaces (e.g., different contact sections  322 ,  320 ), different medial and lateral bearing surfaces, and other variations. Information about the stability and tibiofemoral rotation characteristics of the tibial trial inserts in the library are provided to the surgeon. The surgeon selects a trial insert from the library to achieve an appropriate balance of stability and tibiofemoral rotation for a particular patient. For example, the surgeon selects one or more tibial trial inserts that have stability and tibiofemoral rotation characteristics that match the needs indicated by patient data. 
     The level of stability (for example, the degree of stabilization or constraint) needed in the reconstructed knee can be predicted using pre-surgical laxity data, for example, data that indicates a relationship between an applied load and the resulting varus-valgus rotation of the knee. A load can be applied by, for example, a device attached to the patient or by a medical professional with a hand-held load measuring instrument. A desired amount of tibiofemoral rotation for the reconstructed knee can also be determined using pre-surgical data. For example, imaging techniques, such as magnetic resonance imaging (MRI), computed tomography (CT), and X-ray imaging, can be used to measure changes in a distance between the tibia and femur at different positions of the knee. Motion of the knee can also be tracked using image-based motion capture techniques, electromagnetic motion capture techniques, or mechanical linkages attached to the knee to measure the angular changes in knee position. 
     In the assembled knee prosthesis  100 , engagement between the post  302  and the walls  210 ,  211  constrains varus/valgus alignment by limiting medial and lateral deviation of the tibial insert  300 , that is, limiting tilting of the tibial insert  300  relative to the femoral component  200  in the direction of arrow B. To provide effective constraint, the contact sections  322 ,  320  of the medial surface  304  and the lateral surface  306  have a length, L 1 , of, for example, between approximately 0.550 and approximately 0.870 inches. The walls  210 ,  211  of the femoral component  200  have a height in a superior-inferior direction that is, for example, at least as long as the length, L 1 . 
     In addition, the contact sections  322 ,  320  have a width, W, in an anterior-posterior direction of, for example, between approximately 0.125 and approximately 0.225 inches. The contact sections  322 ,  320  have a somewhat rectangular shape, so that the width, W, of the contact sections  322 ,  320  at the superior portion  307  of the post  302  is substantially the same as the width at the inferior portion  305  of the post  302 . The width, W, is sufficient to enable varus/valgus constraint over a flexion/extension range from extension (or hyperextension) to about 90 to 120 degrees flexion when the tibial insert  300  is mated with the femoral component  200 . 
     The contact sections  322 ,  320  optionally have a somewhat trapezoidal shape. For example, a largest anterior-posterior width of the contact sections  322 ,  320  can be located at an inferior portion of the contact sections  322 ,  320 , and the anterior-posterior width can decrease in a superior direction from the largest width. A posterior boundary or edge of the contact sections  322 ,  320  can extend in a substantially superior-inferior direction, such that the changing anterior-posterior width results in a sloped anterior boundary or edge. 
     The height, H, of the post  302  and the length, L 1 , of the contact sections  322 ,  320  affect the degree of varus/valgus constraint achieved. A higher post  302  provides contact with the walls  210 ,  211  over a greater distance along the length of the post  302 , providing more effective constraint. Consequently, in some implementations, the post  302  is substantially the same height as the walls  210 ,  211 . In some implementations, the height, H, of the post  302  is between approximately 0.720 and 0.990 inches. 
     In extension of the knee, the anterior surface  308  of the post  302  may engage the anterior cam  214  to provide anterior stabilization. Also, in extension, the contact sections  322 ,  320  can contact the walls  210 ,  211  to provide varus/valgus constraint at angles of flexion between extension or hyperextension and approximately 90 to 120 degrees. By contrast, in flexion of the knee, for example, at angles of flexion of approximately 125 degrees and higher, the medial surface  304  and lateral surface  306  would no longer be fully constrained between the walls  210 ,  211 . 
     Extension corresponds to a position in which a leg is straight, for example, at zero degrees of flexion. Hyperextension is bending of the knee in the opposite direction of flexion, for example, bending the leg backward past full extension by some amount. As described above, the prosthesis can provide varus/valgus constraint over a range that includes hyperextension, for example, 1 degree, 5 degrees, or more of hyperextension. 
     As shown in  FIG. 3 , post  302  has a wider width, W 2 , at the inferior portion  305  than at the superior portion  307 . The increased anterior-posterior width strengthens the connection of the base  301  to the post  302 . The width, W 2 , also enables the anterior surface  308  to engage the anterior cam  214 . 
       FIG. 4  shows a top view of the knee prosthesis  100  in extension. From this position, as the knee flexes, the medial condylar surface  202  and the lateral condylar surface  204  roll and also glide over the medial bearing surface  316  and the lateral bearing surface  318 , respectively. At flexion of approximately 60 to 90 degrees, the posterior cam  216  contacts the posterior surface  310  of the post  302  to provide posterior stabilization. Through continued flexion, the posterior cam  216  engages the posterior surface  310  of the post  302 . 
     Thus, in some implementations, the posterior cam  216  is configured to engage the posterior surface  310  at a first angle that is between approximately 60 and 90 degrees of flexion, and at angles of flexion that are greater than the first angle. The posterior cam  216  is configured to not engage the posterior surface  310  at angles of flexion less than the first angle. 
     Internal rotation of the tibia relative to the femur occurs as the knee flexes between full extension and approximately 130 degrees of flexion. Tibiofemoral rotation is achieved by asymmetrical translation of the lateral condylar portion  203  compared to the medial condylar portion  201  relative to the tibial insert  300 . In general, the lateral condylar portion  203  translates over a greater range than the medial condylar portion  201 , resulting in rotation of the femoral component  200  relative to the tibial insert  300 . 
     The asymmetric translation that drives tibiofemoral rotation is promoted by two mechanisms: first, the engagement of the bearing surfaces  316 ,  318  with the condylar surfaces  202 ,  204 ; and second, engagement of the asymmetrically-shaped posterior cam  216  with the posterior surface  310  of the post  302 . These mechanisms result in the lateral condylar portion  203  of the femoral component sliding farther in a posterior direction than the medial condylar portion  201 , relative to the tibial insert  300 . Conversely, when moving toward extension, the lateral condylar portion  203  of the femoral component slides farther in an anterior direction than the medial condylar portion  201 , relative to the tibial insert  300 . This anterior translation during extension brings the anterior surface  308  of the post  302  near the anterior cam  214 , where it may contact the post  302  if necessary to provide anterior stability. In some implementations, clearance is provided such that the anterior cam  214  does not contact the post  302  during normal standing, for example, with the leg straight at 0 degrees of flexion. 
     Differences between the medial bearing surface  316  and the lateral bearing surface  318  cause asymmetrical translation of the medial condylar surface  202  and the lateral condylar surface  204 . For example, the medial bearing surface  316  is more concave (for example, extends deeper toward the inferior end of the tibial insert  300 ) than the lateral bearing surface  318 . In other words, the lateral bearing surface  318  includes a larger radius of curvature than the medial bearing surface  316 , and the generally shallower slope of the lateral bearing surface  318  facilitates greater travel of the lateral condylar portion  203  than that of the medial condylar portion  201 . 
     The asymmetry of the posterior cam  216  also drives increased translation of the lateral condylar portion  203 . At flexion between 0 degrees and approximately 60 to 90 degrees, before the posterior cam  216  engages the posterior surface  310  of the post  302 , tibiofemoral rotation is promoted by the engagement of the condylar surfaces  202 ,  204  and the bearing surfaces  316 ,  318 . As noted above, the lateral portion  222  of the posterior cam  216  is thicker than the medial portion  220  of the posterior cam  216 . Once the posterior cam  216  engages the posterior surface  310  (for example, at approximately 60 to 90 degrees of flexion), the engagement of the thicker lateral portion  222  of the posterior cam  216  with the posterior surface  310  of the post  302  directs more force in the posterior direction on the lateral side of the femoral component  200 , resulting in translation of the lateral condylar portion  203  that is greater than the translation of the medial condylar portion  201 . 
     Referring to  FIG. 5 , with the knee shown at approximately  150  degrees of flexion, the lateral condylar portion  203  has translated more posteriorly than the medial condylar portion  201  resulting in tibiofemoral rotation. The difference in posterior translation is shown by distance D, which, for example, can correspond to rotation of the femoral component  200  relative to the tibial insert  300  of approximately 6 degrees or more. As shown, the medial surface  304  and the lateral surface  306  no longer contact the walls  210 ,  211 , and thus do not provide varus/valgus constraint of the knee at this position. 
     Referring to  FIGS. 6 and 7 , the geometry of the tibial insert  300  permits and facilitates both tibiofemoral rotation and varus/valgus constraint. As noted above, the lateral surface  306  includes a contact section  320  ( FIG. 6 ) and the medial surface  304  includes a contact section  322  ( FIG. 7 ). These contact sections  320 ,  322  engage the walls  210 ,  211  of the femoral component  200  to provide varus/valgus constraint. To provide varus/valgus constraint and still permit tibiofemoral rotation at angles of flexion, for example, between extension (or hyperextension) and approximately 90 to 120 degrees, a total clearance of approximately 0.005 to approximately 0.030 inches is provided between the walls and the contact sections  320 ,  322 . An increase in the clearance permits additional rotation, but lessens the varus/valgus constraint. 
     The post  302  includes rounded edges  324  as transitions from the medial surface  304  and lateral surface  306  to the anterior surface  308  and posterior surface  310  (also see  FIG. 8 ). The rounded edges  324  extend substantially along a superior-inferior direction, and are rounded substantially in a transverse plane. When the post  302  rotates within the clearance provided between the post  302  and the walls  210 ,  211 , the rounded edges  324  and portions of the anterior surface  308  and posterior surface  310  contact the walls  210 ,  211 . The radius of the rounded edge can be, for example, between approximately 0.030 and 0.090 inches. A larger radius allows for more internal-external tibiofemoral rotation, but reduces the width, W, of the contact sections  320 ,  322  and the corresponding varus/valgus constraint. A smaller radius has the opposite effect. 
     The dimensions of the contact sections  320 ,  322  also permit an effective balance of varus/valgus constraint and tibiofemoral rotation. The greater the width of the contact sections  320 ,  322  in an anterior-posterior dimension, the greater the varus/valgus constraint but the less the tibiofemoral rotation. To achieve an appropriate balance, at various points along the axial length of the post  302 , the width, W, of the contact sections  320 ,  322  can be between approximately one sixth and two thirds of the width, W 2 , of the post  302 . The width, W 2 , can be the greatest width of the post  302  in an anterior-posterior direction at a particular axial height. Thus in some implementations, the contact sections  320 ,  322  extend such that, along at least half of the height, H, of the post  302 , the width, W, of the contact sections  320 ,  322  at a given axial position is between one sixth and two thirds of a largest width (for example, W 2 ) of the post  302  at that axial position. 
     In some implementations, the relationship between an anterior-posterior width at a given axial position and the largest width at that axial position continues at portions of the medial surface  304  and the lateral surface  306  other than the contact sections  320 ,  322 . For example, the width of the medial surface  304  and the width of the lateral surface  306  in an anterior-posterior direction can be between approximately one sixth and two thirds of the largest width of the post  302  in an anterior-posterior direction at positions along substantially the entire length of the medial surface  304  and the lateral surface  306  along the longitudinal axis, X, of the post  302 . 
     The contact sections  320 ,  322  are substantially rectangular, to provide generally even contact area with the walls  210 ,  211 . The greater the axial length of the contact sections  320 ,  322 , the greater the varus/valgus control provided. In some implementations, the length of the contact sections  320 ,  322  along the axial direction of the contact sections  320 ,  322  is at least twice the width, W, of the contact sections  320 ,  322 . For example, the average length of the contact sections  320 ,  322  can be more than twice the average width or maximum width of the contact sections  320 ,  322 . Similarly, the average length of the medial surface  304  can be more than twice the average width or maximum width of the medial surface  304 , and the average length of the lateral surface  306  can be more than twice the average width or maximum width of the lateral surface  306 . 
     Differences between the lateral bearing surface  318  and the medial bearing surface  316  promote tibiofemoral rotation. The maximum depth or total depth, R 1  ( FIG. 6 ), of the concave portion of the lateral bearing surface  318  is less than the maximum depth or total depth, R 2  ( FIG. 7 ), of the concave portion of the medial bearing surface  316 . In other words, the medial bearing surface  316  ( FIG. 6 ) extends more inferior than the lateral bearing surface  318  ( FIG. 7 ). Thus for an equal amount of force in a posterior direction, the resistance to translation is less on the side of the lateral bearing surface  316 , resulting in greater translation of the lateral condylar portion  203  than the translation of the medial condylar portion  201 . 
     In addition, the lateral bearing surface  318  includes a continuous concave portion  330  that extends along the lateral bearing surface  318  in an anterior-posterior direction, generally centered in the tibial insert  300 . On the other hand, a concave portion  332  of the medial bearing surface  316 , is offset from the center of the tibial insert in an anterior-posterior direction, so that the concave portion  332  is located more toward the anterior of the tibial insert  300 . The medial bearing surface  316  includes a plateau or raised portion at a posterior portion  325  of the medial bearing surface  316 . Adjacent to the posterior portion  325 , the medial bearing surface  316  includes sloped section  326  having a relatively steep slope. For example, the medial concave portion  332  can have a slope at the sloped section  326  that is steeper than the slope of the lateral concave portion  330  at the same anterior-posterior position on the tibial insert  300 . The sloped portion  326  can be steeper than the adjacent portions of the medial concave portion  332  and can be steepest portion of the medial concave portion  332  along the anterior-posterior direction. The sloped section  326  can engage the medial condylar surface  202  to limit posterior travel of the medial condylar surface  202  during flexion. 
     To further promote lateral travel of the lateral condylar portion  201 , a bottom point or equilibrium point  327  ( FIG. 6 ) (for example, indicating the most planar section of the lateral bearing surface  318 ) is located anterior to the center of the post  302  on the lateral bearing surface  318 . An equilibrium point  328  ( FIG. 7 ) of the medial bearing surface  316  is located generally at the same anterior position as the lateral equilibrium point  327 . The condylar portions  201 ,  203  generally follow the slopes of the bearing surfaces  316 ,  318 , which without other forces, would lead the condylar portions  201 ,  203  to the respective equilibrium points  327 ,  328 . This relationship between the anterior-posterior position of the equilibrium points  327 ,  328  guides the femoral condylar portions  201 ,  203  to roughly the same anterior-posterior position when the knee is extended. Also, the position of the equilibrium points  327 ,  328  allows the lateral femoral condylar portion  203  to move more anterior than the medial condylar portion  201  as the knee extends, as occurs in a native knee due to the “screw home” mechanism. 
     Referring to  FIG. 9 , in an alternate implementation of a tibial insert  400 , the tibial insert  400  includes a post  402  that is twisted along the longitudinal axis of the post  402 . As a result, a medial surface  404  and a lateral surface  406  are offset from an anterior-posterior alignment, and a superior portion of the post  402  is rotationally offset from an inferior portion of the post  402 . Alternatively, the post  402  can be rotated instead of twisted. For example, a post can be rotationally offset from a base such that the medial surface and the lateral surface are both oriented at an angle with respect to medial and lateral sides of the base from which the post extends. A twisted or rotated post can permit more internal tibial rotation in flexion than the non-rotated implementation without as much of a reduction in varus/valgus constraint. 
     The medial surface  404  and the lateral surface  406  can include substantially flat portions or other portions configured to contact the walls  210 ,  211  of the femoral component  200 , described above, to provide varus/valgus constraint while allowing tibiofemoral rotation. For example, the medial surface  404  and lateral surface  406  are substantially parallel to each other. As another example, as the femoral component  200  flexes with respect to the tibial insert  400 , the walls  210 ,  211  follow the twist of the medial surface  404  and the lateral surface  406  to provide tibiofemoral rotation. In extension, the medial posterior edge  408  and the lateral anterior edge  410  contact the walls  210 ,  211  to provide varus/valgus constraint. Other portions of the medial surface  404  and the lateral surface  406  also engage the walls  210 ,  211  to provide varus/valgus constraint. 
     A number of implementations and alternatives have been described. 
     Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the disclosure. Accordingly, other implementations are within the scope of the following claims.