Abstract:
A knee replacement system includes a proximal tibial posterior camming portion defined by a first radius of curvature with a first origin in a first medio-lateral plane, a distal tibial posterior camming portion defined by a second radius of curvature with a second origin in a second medio-lateral plane, an anterior femoral camming portion of a posterior cam defined by a third radius of curvature with a third origin in the first medio-lateral plane, and a posterior femoral camming portion of the posterior cam and defined by a fourth radius of curvature with a fourth origin in the second medio-lateral plane, wherein the second origin is closer to the lateral tibial portion than the first origin, or the fourth origin is closer to the medial femoral portion than the third origin.

Description:
[0001]    Cross-reference is made to U.S. Utility patent application Ser. No. 12/165,579 entitled “Orthopaedic Femoral Component Having Controlled Condylar Curvature” by John L. Williams et al., which was filed on Jun. 30, 2008; to U.S. Utility patent application Ser. No. 12/165,574 entitled “Posterior Cruciate-Retaining Orthopaedic Knee Prosthesis Having Controlled Condylar Curvature” by Christel M. Wagner, which was filed on Jun. 30, 2008; to U.S. Utility patent application Ser. No. 12/165,575 entitled “Posterior Stabilized Orthopaedic Knee Prosthesis Having Controlled Condylar Curvature” by Joseph G. Wyss, which was filed on Jun. 30, 2008; to U.S. Utility patent application Ser. No. 12/165,582 entitled “Posterior Stabilized Orthopaedic Prosthesis” by Joseph G. Wyss, which was filed on Jun. 30, 2008; to U.S. Utility patent application Ser. No. 12/174,507 entitled “Antero-Posterior Placement of Axis of Rotation for a Rotating Platform” by John L. Williams, et al., which was filed on Jul. 16, 2008; and to U.S. Provisional Patent Application Ser. No. 61/007,124 entitled “Orthopaedic Knee Prosthesis Having Controlled Condylar Curvature” by Joseph G. Wyss, which was filed on Jun. 30, 2008; the entirety of each of which is incorporated herein by reference. The principles of the present invention may be combined with features disclosed in those patent applications. 
       FIELD OF THE INVENTION 
       [0002]    This invention relates generally to prostheses for human body joints, and, more particularly, to prostheses for knees. 
       BACKGROUND OF THE INVENTION 
       [0003]    The knee joint provides six degrees of motion during dynamic activities. One such activity is deep flexion or bending of the knee joint. The six degrees of motion are effected by complex movements or kinematics of the bones and soft tissue in the knee joint. Most individuals are capable of controlling the complex movement of a knee joint without thought. The absence of conscious control belies the intricate interactions between a number of different components which are necessary to effect activities such as flexion and extension (when the leg is straightened) of a knee joint. 
         [0004]    The knee joint includes the bone interface of the distal end of the femur and the proximal end of the tibia. The patella is positioned over the distal end of the femur and is positioned within the tendon of the long muscle (quadriceps) on the front of the thigh. This tendon inserts into the tibial tuberosity and the posterior surface of the patella is smooth and glides over the femur. 
         [0005]    The femur is configured with two large eminences (the medial condyle and the lateral condyle) which are substantially smooth and articulate with the medial plateau and the lateral plateau of the tibia, respectively. The plateaus of the tibia are substantially smooth and slightly cupped thereby providing a slight receptacle for receipt of the femoral condyles. The complex interactions of the femur, the tibia and the patella are constrained by the geometry of the bony structures of the knee joint, the menisci, the muscular attachments via tendons, and the ligaments. The ligaments of the knee joint include the patellar ligament, the medial and lateral collateral ligaments, the anterior cruciate ligament (ACL) and the posterior cruciate ligament (PCL). The kinematics of the knee are further influenced by synovial fluid which lubricates the joint. 
         [0006]    A number of studies have been directed to understanding the manner in which the various knee components interact as a knee joint moves through flexion. One such study was reported in an article by P. Johal, et al. entitled “ Tibio - femoral movement in the living knee. A study of weight bearing and non - weight bearing knee kinematics using ‘interventional’ MRI,  Journal of Biomechanics, Volume 38, Issue 2, February 2005, pages 269-276, which includes a FIG. 2 from which the data set forth in  FIG. 1  as graph  10  has been derived. The graph  10  shows the locations of the medial and lateral condyle reference points of a native knee with respect to a tibia as the knee moves through flexion. The line  12  of the graph  10  indicates that the lateral condyle exhibits a constant anterior to posterior translation through deep flexion while the line  14  indicates that the medial condyle remains at about the same location on the tibial plateau until about 90 degrees of flexion. Beyond 90 degrees of flexion, the medial condyle exhibits anterior to posterior translation. 
         [0007]    The medial and lateral condyle low (tangency) points are not the actual contact points between the condyles and the femoral plane. Rather, the points represent the lowest portion of the condyle that can be viewed using fluoroscopy. The actual contact point is generally at a location more posterior to the low (tangency) points. Nonetheless, the use of low (tangency) points provides a valid basis for comparison of the effect of changing design variables between components. 
         [0008]    Damage or disease can deteriorate the bones, articular cartilage and ligaments of the knee. Such changes from the normal condition of the knee joint can ultimately affect the ability of the natural knee to function properly leading to pain and reduced range of motion. To ameliorate the conditions resulting from deterioration of the knee joint, prosthetic knees have been developed that are mounted to prepared ends of the femur and tibia. 
         [0009]    While damage to soft tissue is avoided to the extent possible during knee replacement procedures, some tissue is necessarily sacrificed in replacing a portion of the femur and tibia. Thus, while the typical individual has learned how to coordinate the tensioning of the muscle fibers, ligaments and tendons to provide a smooth transition from a present positioning of the knee to a desired positioning without conscious thought, the sacrifice of tissue changes the physics of the knee. Accordingly, the configuration of soft tissue used to cause movement such as flexion and extension in a healthy knee, or even a pre-operative knee, no longer achieves the same results when the knee is replaced with a prosthesis. Additionally, the sacrifice of soft tissue results in reduced stability of the knee joint. 
         [0010]    To compensate for the loss of stability that results from the damage to soft tissue, four general types of implants have been developed. In one approach, the PCL is retained. When the PCL is retained, patients frequently encounter an unnatural (paradoxical) anterior translation of the contact point between the lateral condyle of the femur and the tibia during deep knee-bend movements. Rather than rolling back or slipping as a knee moves through flexion, the femur slides anteriorly along the tibial platform. Paradoxical anterior translation is typically initiated between 30 and 40 degrees of flexion although it can commence at up to about 120 degrees of flexion. The resulting loss of joint stability can accelerate wear, cause a sensation of instability during certain activities of daily living, result in abnormal knee joint motion (kinematics), and/or result in a reduced dynamic moment arm to the quadriceps requiring increased force to control movement. 
         [0011]    By way of example,  FIG. 2  depicts a sagittal view of a typical prior art femoral component  20  which attempts to mimic the shape of a native knee. The femoral component  20  includes an extension region  22  which is generally anterior to the line  24  and a flexion region  26  which is posterior to the line  24 . The extension region  22  is formed with a large radius of curvature (R c )  28  while a small R c    30  is used in the posterior portion of the flexion region  26  in order to fit within the joint space while providing as much flexion as possible. Contemporaneously with the change of length of the radii of curvature, the origin of the radius of curvature changes from the origin  32  for the R c    28  to the origin  34  for the R c    30 . 
         [0012]    The results of a deep knee bending simulation using a typical prior art femoral component with condylar surfaces in the flexion area defined by a reduced radius of curvature are shown in the translation chart  40  of  FIG. 3  which shows the position on the tibial component (y-axis) whereat the medial and lateral condyles contact the tibial component as the device is moved through flexion (x-axis). The simulation was conducted on a multibody dynamics program commercially available from Biomechanics Research Group, Inc. of San Clemente, Calif., under the name LifeMOD/KneeSIM. The model included tibio-femoral and patello-femoral contact, passive soft tissue, and active muscle elements. 
         [0013]    The lines  42  and  44  in the chart  40  show the estimated low (tangency) points for the lateral condylar surface and the medial condylar surface, respectively. Both of the lines  42  and  44  initially track posteriorly (downwardly as viewed in  FIG. 3 ) between 0 degrees and about 30 degrees of flexion. This indicates that the femoral component is rolling posteriorly on the tibial component as the flexion angle increases. Beyond about 30° of flexion, the estimated lateral condyle low (tangency) point line  42  drifts slightly anteriorly from about 5 mm translation while the estimated medial condylar low (tangency) point line  44  moves rapidly anteriorly. Movement of both surfaces in the anterior direction shows that paradoxical anterior translation is occurring beyond about 30 degrees. A comparison of the lines  42  and  44  beyond 30° of flexion with the lines  12  and  14  of  FIG. 1  reveals a striking disparity in kinematics between the native knee and the replacement knee which mimics the geometry of the native knee. 
         [0014]    Additionally, returning to  FIG. 2 , as the femoral component  20  is flexed such that contact with a tibial component (not shown) occurs along the condylar surface defined by the R c    28 , the forces exerted by soft-tissues on the knee are coordinated to provide a smooth movement based, in part, upon the length of the R c    28  and the origin  32 . As the femoral component  20  is moved through the angle at which the condylar surface transitions from the R c    28  to the R c    30 , the knee may initially be controlled as if it will continue to move along the R c    28 . As the femoral component  20  continues to move, the actual configuration of the knee diverges from the configuration that would be achieved if the surface in contact with the tibial component (not shown) was still defined by the R c    28 . When the divergence is sensed, it is believed that the soft-tissue forces are rapidly re-configured to a configuration appropriate for movement along the surface defined by the R c    30  with the origin  34 . This sudden change in configuration, which is not believed to occur with a native knee, contributes to the sense of instability. 
         [0015]    Furthermore, Andriacchi, T. P.,  The Effect of Knee Kinematics, Gait and Wear on the Short and Long - Term Outcomes of Primary Total Knee Replacement,  NIH Consensus Development Conference on Total Knee Replacement, pages 61-62, (Dec. 8-10, 2003) reports that in a native knee, flexion between 0 and 120 degrees is accompanied by approximately 10 degrees of external rotation of the femur with respect to the tibia while an additional 20 degrees of external rotation is required for flexion from 120 degrees to 150 degrees. Thus, an initial ratio of about 0.008 degrees of external rotation per degree of flexion is exhibited between 0 degrees and 120 degrees of flexion which increases to a ratio of 0.67 degrees of external rotation per degree of flexion between 120 degrees and 150 degrees of flexion. This rotation allows the knee to move into deep flexion. 
         [0016]    The reported external rotation of the native knee is supported by the data in  FIG. 1 . Specifically, between about 9 degrees and 90 degrees of flexion, the slope of the line  12  is constantly downward indicating that the lowest point of the lateral condylar surface is continuously tracking posteriorly. The line  14 , however, is moving anteriorly from about 9 degrees of flexion through 90 degrees of flexion. Thus, assuming this difference to be solely due to external rotation, the femoral component is externally rotating as the knee moved from about 9 degrees of flexion to about 90 degrees of flexion. Beyond 90 degrees of flexion, the lines  12  and  14  show that both condylar surfaces are moving posteriorly. The lateral condylar surface, however, is moving more rapidly in the posterior direction. Accordingly, the gap between the lines  12  and  14  continues to expand beyond 90 degrees, indicating that additional external rotation of the knee is occurring. 
         [0017]      FIG. 4  shows the internal rotation of the tibia with respect to the femur (which from a modeling perspective is the same as external rotation of the femur with respect to the tibia, both of which are identified herein as “φ i-e ”) during the testing that provided the results of  FIG. 3 . The graph  50  includes a line  52  which shows that as the tested component was manipulated to 130 degrees of flexion, the φ i-e  reached a maximum of about seven degrees. Between about 0 degrees of flexion and 20 degrees of flexion, the φ i-e  varies from 1 degree to zero degrees for a change rate of −0.05 degrees of internal rotation per degree of flexion. Between about 20 degrees of flexion and 50 degrees of flexion, the internal rotation varies from 0 degrees to 1 degree for a change rate of 0.03 degrees of internal rotation per degree of flexion. Between about 50 degrees and 130 degrees, the graph  50  exhibits a nearly linear increase in internal rotation from about 1 degree to about 7 degrees for a change rate of 0.075 degrees of internal rotation per degree of flexion. Accordingly, the φ i-e  of a knee joint incorporating the prior art femoral component differs significantly from the φ i-e  of a native knee. 
         [0018]    Various attempts have been made to provide kinematics more akin to those of the native knee. For example, the problem of paradoxical anterior translation in one type of implant is addressed by sacrificing the PCL and relying upon articular geometry to provide stability. In another type of implant, the implant is constrained. That is, an actual linkage is used between the femoral and tibial components. In another type of implant, the PCL is replaced with a cam on the femoral component and a post on the tibial component. While the foregoing approaches have some effectiveness with respect to paradoxical anterior translation, they do not provide other kinematics exhibited by a native knee. 
         [0019]    What is needed is a knee prosthesis that more closely reproduces the inherent stability and kinematics of a native knee such as by managing rotation and rollback. 
       SUMMARY 
       [0020]    The present invention is a knee replacement system. In one embodiment, a prosthetic joint includes a proximal tibial camming portion (i) extending from a lateral portion of a posterior tibial cam to a medial portion of the posterior tibial cam, (ii) defined by a first radius of curvature in a first medio-lateral plane, and (iii) having a first origin, a distal tibial camming portion (i) extending from the lateral portion of the posterior tibial cam to the medial portion of the posterior tibial cam, (ii) defined by a second radius of curvature in a second medio-lateral plane, and (iii) having a second origin, an anterior femoral camming portion (i) extending from a lateral portion of a posterior femoral cam to a medial portion of the posterior femoral cam, (ii) defined by a third radius of curvature in the first medio-lateral plane, and (iii) having a third origin, a posterior femoral camming portion extending from the lateral portion of the posterior femoral cam to the medial portion of the posterior femoral cam and defined by a fourth radius of curvature in the second medio-lateral plane and having a fourth origin, wherein the second origin is closer to the lateral tibial portion than the first origin, or the fourth origin is closer to the medial femoral portion than the third origin. 
         [0021]    In a further embodiment, a knee prosthesis includes a tibial cam including a posterior camming surface defined by a plurality of radii of curvature, each of the plurality of tibial radii of curvature (i) located in an associated one of a plurality of medio-lateral planes perpendicular to the camming surface, and (ii) having an origin spaced apart from each of the origins of the other of the plurality of tibial radii of curvature in the medio-lateral direction, and a posterior femoral cam including a distal camming surface defined by a plurality of radii of curvature, each of the plurality of femoral radii of curvature located in an associated one of a plurality of medio-lateral planes perpendicular to the camming surface. 
         [0022]    In another embodiment, a knee prosthesis includes a tibial cam including a posterior camming surface defined by a plurality of radii of curvature, each of the plurality of tibial radii of curvature located in an associated one of a plurality of medio-lateral planes perpendicular to the camming surface, and a posterior femoral cam including a distal camming surface defined by a plurality of radii of curvature, each of the plurality of femoral radii of curvature (i) located in an associated one of a plurality of medio-lateral planes perpendicular to the camming surface, and (ii) having an origin spaced apart from each of the origins of the other of the plurality of femoral radii of curvature in the medio-lateral direction. 
         [0023]    The above-described features and advantages, as well as others, will become more readily apparent to those of ordinary skill in the art by reference to the following detailed description and accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]      FIG. 1  shows a graph of the reference point locations of the medial and lateral condyle on a tibial component for a native knee during deep knee bending; 
           [0025]      FIG. 2  depicts a sagittal view of a prior art femoral component of a prosthesis; 
           [0026]      FIG. 3  shows the results of a simulation in the form of a graph of the estimated low (tangency) point locations of the medial and lateral condyles of a femoral component on a tibial component; 
           [0027]      FIG. 4  shows the internal rotation of the tibial component with respect to the femoral component for the simulation of  FIG. 3 ; 
           [0028]      FIG. 5  depicts an exploded perspective view of a knee prosthesis including a tibial component and a femoral component in accordance with principles of the invention; 
           [0029]      FIG. 6  depicts a sagittal cross sectional view of the femoral component of  FIG. 5  and a sagittal plan view of the tibial bearing insert of  FIG. 5  showing the camming surfaces of the femoral component and the tibial bearing insert with the femoral component positioned in extension on the tibial bearing insert; 
           [0030]      FIG. 7  depicts a sagittal cross sectional view of the femoral component of  FIG. 5  and a sagittal plan view of the tibial bearing insert of  FIG. 5  showing the contact region between the camming surfaces of the femoral component and the tibial bearing insert with the femoral component positioned at about 70 degrees of flexion on the tibial bearing insert; 
           [0031]      FIG. 8  depicts a medio-lateral cross sectional view of the configuration of  FIG. 7  taken along the line A-A of  FIG. 7  showing the origins of the radius of curvature of the camming surfaces of the femoral component and the tibial bearing insert to be located on the centerlines of the respective component; 
           [0032]      FIG. 9  depicts a sagittal cross sectional view of the femoral component of  FIG. 5  and a sagittal plan view of the tibial bearing insert of  FIG. 5  showing the contact region between the camming surfaces of the femoral component and the tibial bearing insert with the femoral component positioned at about 90 degrees of flexion on the tibial bearing insert; 
           [0033]      FIG. 10  depicts a medio-lateral cross sectional view of the configuration of  FIG. 9  taken along the line B-B of  FIG. 9  with the centerlines of the femoral and tibial components aligned showing the origin of the radius of curvature of the femoral component camming surface to be located on the centerline of the femoral component and the origin of the radius of curvature of the tibial bearing insert camming surface to be located laterally of the centerline of the tibial bearing insert; 
           [0034]      FIG. 11  depicts a medio-lateral cross sectional view of the configuration of  FIG. 9  taken along the line B-B of  FIG. 9  showing the rotation of the femoral component that has occurred because of increased rollback of the lateral condyle element resulting from locating the origin of the radius of curvature of the femoral component on the centerline of the femoral component and locating the origin of the radius of curvature of the tibial bearing insert camming surface laterally of the centerline of the tibial bearing insert; 
           [0035]      FIG. 12  depicts a sagittal cross sectional view of the femoral component of  FIG. 5  and a sagittal plan view of the tibial bearing insert of  FIG. 5  showing the contact region between the camming surfaces of the femoral component and the tibial bearing insert with the femoral component positioned at about 110 degrees of flexion on the tibial bearing insert; 
           [0036]      FIG. 13  depicts a medio-lateral cross sectional view of the configuration of  FIG. 12  taken along the line C-C of  FIG. 12  showing the rotation of the femoral component that has occurred because of increased rollback of the lateral condyle element resulting from locating the origin of the radius of curvature of the femoral component on the centerline of the femoral component and locating the origin of the radius of curvature of the tibial bearing insert camming surface laterally of the centerline of the tibial bearing insert; 
           [0037]      FIG. 14  depicts a sagittal cross sectional view of the femoral component of  FIG. 5  and a sagittal plan view of the tibial bearing insert of  FIG. 5  showing the contact region between the camming surfaces of the femoral component and the tibial bearing insert with the femoral component positioned at about 130 degrees of flexion on the tibial bearing insert; 
           [0038]      FIG. 15  depicts a medio-lateral cross sectional view of the configuration of  FIG. 14  taken along the line D-D of  FIG. 14  showing the rotation of the femoral component that has occurred because of increased rollback of the lateral condyle element resulting from locating the origin of the radius of curvature of the femoral component on the centerline of the femoral component and locating the origin of the radius of curvature of the tibial bearing insert camming surface laterally of the centerline of the tibial bearing insert; 
           [0039]      FIG. 16  shows a graph of the condylar low points during a deep knee bending simulation using the knee replacement system of  FIG. 5 ; 
           [0040]      FIG. 17  shows a graph of the internal-external rotation (φ i-e ) of the tibia with respect to the femoral component during the deep knee bending simulation using the knee replacement system of  FIG. 5 ; 
           [0041]      FIG. 18  depicts an exploded perspective view of an alternative knee prosthesis system including a tibial component with a rotating platform and a femoral component in accordance with principles of the invention; 
           [0042]      FIG. 19  depicts a sagittal cross sectional view of the femoral component of  FIG. 18  and a sagittal plan view of the tibial bearing insert of  FIG. 18  showing the camming surfaces of the femoral component and the tibial bearing insert with the femoral component positioned in extension on the tibial bearing insert; 
           [0043]      FIG. 20  depicts a top plan view of the dwell axis and the centerline of the tibial insert of the knee prosthesis of  FIG. 18  projected onto the articulating surface of the tibial tray of the knee prosthesis of  FIG. 18 ; 
           [0044]      FIG. 21  depicts a perspective view of the tibial tray of the knee prosthesis of  FIG. 18  with the coupler member defining an axis of rotation for the tibial bearing insert; 
           [0045]      FIG. 22  shows a graph of the results of a deep knee bending simulation using the knee replacement system of  FIG. 18  with an axis of rotation of the tibial bearing insert positioned about 0.317 inches posterior to the dwell axis of the system and about 0.317 inches lateral to the centerline of the tibial bearing insert; 
           [0046]      FIG. 23  shows a graph of the internal-external rotation (φ i-e ) of the tibia with respect to the femoral component during the deep knee bending simulation of  FIG. 22  along with the rotation of the tibial bearing insert with respect to the tibia; 
           [0047]      FIG. 24  depicts a sagittal cross sectional view of an alternative femoral component and a sagittal plan view of an alternative tibial bearing insert showing the contact region between the camming surfaces of the femoral component and the tibial bearing insert with the femoral component positioned at about 70 degrees of flexion on the tibial bearing insert; 
           [0048]      FIG. 25  depicts a medio-lateral cross sectional view of the configuration of  FIG. 24  taken along the line E-E of  FIG. 24  showing the origins of the radius of curvature of the camming surfaces of the femoral component and the tibial bearing insert to be located on the centerlines of the respective component; 
           [0049]      FIG. 26  depicts a sagittal cross sectional view of the femoral component of  FIG. 24  and a sagittal plan view of the tibial bearing insert of  FIG. 24  showing the contact region between the camming surfaces of the femoral component and the tibial bearing insert with the femoral component positioned at about 90 degrees of flexion on the tibial bearing insert; 
           [0050]      FIG. 27  depicts a medio-lateral cross sectional view of the configuration of  FIG. 26  taken along the line F-F of  FIG. 26  with the centerlines of the femoral and tibial components aligned showing the origin of the radius of curvature of the femoral component camming surface to be located medially of the centerline of the femoral component and the origin of the radius of curvature of the tibial bearing insert camming surface to be located on the centerline of the tibial bearing insert; 
           [0051]      FIG. 28  depicts a medio-lateral cross sectional view of the configuration of  FIG. 26  taken along the line F-F of  FIG. 26  showing the rotation of the femoral component that has occurred because of increased rollback of the lateral condyle element resulting from locating the origin of the radius of curvature of the femoral component medially of the centerline of the femoral component and locating the origin of the radius of curvature of the tibial bearing insert camming surface on the centerline of the tibial bearing insert; 
           [0052]      FIG. 29  depicts a sagittal cross sectional view of the femoral component of  FIG. 24  and a sagittal plan view of the tibial bearing insert of  FIG. 24  showing the contact region between the camming surfaces of the femoral component and the tibial bearing insert with the femoral component positioned at about 110 degrees of flexion on the tibial bearing insert; 
           [0053]      FIG. 30  depicts a medio-lateral cross sectional view of the configuration of  FIG. 29  taken along the line G-G of  FIG. 29  showing the rotation of the femoral component that has occurred because of increased rollback of the lateral condyle element resulting from locating the origin of the radius of curvature of the femoral component medially of the centerline of the femoral component and locating the origin of the radius of curvature of the tibial bearing insert camming surface on the centerline of the tibial bearing insert; 
           [0054]      FIG. 31  depicts a sagittal cross sectional view of the femoral component of  FIG. 24  and a sagittal plan view of the tibial bearing insert of  FIG. 24  showing the contact region between the camming surfaces of the femoral component and the tibial bearing insert with the femoral component positioned at about 130 degrees of flexion on the tibial bearing insert; and 
           [0055]      FIG. 32  depicts a medio-lateral cross sectional view of the configuration of  FIG. 31  taken along the line H-H of  FIG. 31  showing the rotation of the femoral component that has occurred because of increased rollback of the lateral condyle element resulting from locating the origin of the radius of curvature of the femoral component medially of the centerline of the femoral component and locating the origin of the radius of curvature of the tibial bearing insert camming surface on the centerline of the tibial bearing insert. 
       
    
    
     DETAILED DESCRIPTION 
       [0056]      FIG. 5  depicts a knee replacement system  100 . The knee replacement system  100  includes a tibial tray  102 , a tibial bearing insert  104  and a femoral component  106  having two femoral condyle elements  108  and  110 . The tibial tray  102  includes an inferior stem  112  for attaching the tibial tray  102  to the tibia of a patient and a superior plateau  114  for receiving the tibial bearing insert  104 . The tibial bearing insert  104  in this embodiment is fixed and includes an inferior tibial tray contacting surface  116  and a superior tibial bearing surface  118  configured to articulate with the femoral condyle elements  108  and  110 . A spine  120  separates the superior tibial bearing surface  118  into a bearing surface  122  and a bearing surface  124 . 
         [0057]    The femoral component  106  includes two pegs  130  and  132  which are used to attach the femoral component  106  to the femur of a patient. A trochlear groove  134  is formed between the femoral condyle elements  108  and  110 . The trochlear groove  134  provides an articulation surface for a patellar component (not shown). A cam compartment  136  is located between posterior portions  138  and  140  of the femoral condyle elements  108  and  110 , respectively. 
         [0058]    The femoral condyle elements  108  and  110 , in this embodiment, are symmetrical. The femoral component  106  and the tibial bearing insert  104  in this embodiment, however, are configured only for use in a left knee. More specifically, the femoral component  106  and the tibial bearing insert  104  are configured to simulate the motion of a natural left knee when implanted in a patient. The configuration is discussed with further reference to  FIG. 6 . 
         [0059]      FIG. 6  depicts a cross sectional view of the femoral component  106  taken through the cam compartment  136  and a side plan view of the tibial bearing insert  104 . An anterior cam  142  and a posterior cam  144  are located within the cam compartment  136 . The spine  120  includes an anterior camming portion  146  and a posterior camming portion  148 . The anterior cam  142  is configured with the anterior camming portion  146  to preclude undesired posterior slippage when the femoral component  106  is positioned on the tibial bearing insert  104  in extension as shown in  FIG. 6 . The actual shapes of the anterior cam  142  and the anterior camming portion  146  may be modified from the shape depicted in  FIG. 6 . 
         [0060]    The shape and position of the posterior cam  144  and the shape and position of the posterior camming portion  148  are selected such that the posterior cam  144  and the posterior camming portion  148  are not in contact when the femoral component  106  is positioned on the tibial bearing insert  104  in extension. As the femoral component  106  is rotated into flexion, rollback of the femoral component  106  on the bearing surfaces  122  and  124  is controlled by the configuration of the femoral component  106  and the bearing surfaces  122  and  124 . When flexion reaches about 70 degrees, however, the posterior cam  144  and the posterior camming portion  148  produce an effect on the rollback. 
         [0061]    With reference to  FIG. 7 , the femoral component  106  is depicted rotated to about 70 degrees of flexion on the tibial bearing insert  104 . At this rotation, the posterior cam  144  and the posterior camming portion  148  are in contact at the contact region  150 .  FIG. 8  depicts the shape of the posterior camming portion  148  and the shape of the posterior cam  144  at the contact region  150  taken along the line A-A of  FIG. 7  which extends from a medial portion of the camming portion  148  and the posterior cam  144  to a lateral portion of the camming portion  148  and the posterior cam  144  in a medio-lateral plane. 
         [0062]    The posterior camming portion  148  is formed on a radius of curvature (R c )  152  having an origin  154  on the centerline  156  of the tibial bearing insert  104 . In one embodiment, the R c    152  may be about 20 millimeters. The posterior cam  144  is formed on a radius of curvature (R c )  158  having an origin  160  on the centerline  162  of the femoral component  106 . In one embodiment, the R c    158  may be about 40 millimeters. At about 70 degrees of flexion, the centerline  156  of the tibial bearing insert  104  and the centerline  162  of the femoral component  106  are substantially aligned. Thus, the origin  154  and the origin  160  are substantially aligned. Accordingly, the predominant effect of the contact between the posterior cam  144  and the posterior camming portion  148  is the prevention of anterior movement of the femoral component  106  on the tibial bearing insert  104 . 
         [0063]    Continued rotation of the femoral component  106  to about 90 degrees of flexion on the tibial bearing insert  104  results in the configuration of  FIG. 9 . At this rotation, the posterior cam  144  and the posterior camming portion  148  are in contact at the contact region  170 .  FIG. 10  depicts the shape of the posterior camming portion  148  and the shape of the posterior cam  144  at the contact region  170  taken along the line B-B of  FIG. 9 . 
         [0064]    In  FIG. 10 , the R c    172  of the posterior camming portion  148  has the same length as the R c    152  of  FIG. 8 . The length of the R c    172  may be modified to be longer or shorter than the R c    152  if desired. The R c    172 , however, has an origin  174  which is positioned to the lateral side of the centerline  156 . In one embodiment, the origin  174  is located 1.5 millimeters to the lateral side of the centerline  156 . Additionally, the posterior cam  144  is formed with an R c    176  which in this embodiment is of the same length as the R c    158 , although a longer or shorter length than the R c    158  may be selected, and the origin  178  of the R c    176  is positioned on the centerline  162 . Accordingly, the shape of the posterior camming portion  148  and the posterior cam  144  cause a rotational force in the direction of the arrow  180 . The lateral condyle, femoral condyle element  110  in this embodiment, is thus forced to move posteriorly at a rate greater than the medial condyle (femoral condyle element  108 ). 
         [0065]    The result of the forces acting upon the femoral component  106  is rotation of the femoral component  106  with respect to the tibial bearing insert  104  as shown in  FIG. 11 . In  FIG. 11 , the centerline  162  has rotated in a counterclockwise direction from the centerline  156 . Additionally, opposing faces of the posterior camming portion  148  and the posterior cam  144 , in contrast to the configuration shown in  FIG. 10 , are more aligned with each other. 
         [0066]    The movement of the origins of the R c  for the posterior camming portion  148  and the posterior cam  144  is done incrementally along the contact surfaces of the posterior camming portion  148  between the contact region  150  and the contact region  170 . This provides a smooth rotational movement of the femoral component  106  on the tibial bearing insert  104  from the alignment of  FIG. 8  to the alignment of  FIG. 11 . The precise amount of rotation and rollback may be adjusted by modifying the offset of the origins. 
         [0067]    Continued rotation of the femoral component  106  to about 110 degrees of flexion on the tibial bearing insert  104  results in the configuration of  FIG. 12 . At this rotation, the posterior cam  144  and the posterior camming portion  148  are in contact at the contact region  182 .  FIG. 13  depicts the shape of the posterior camming portion  148  and the shape of the posterior cam  144  at the contact region  182  taken along the line C-C. 
         [0068]    In  FIG. 13 , the R c    184  of the posterior camming portion  148  has the same length as the R c    152  of  FIG. 8 . The length of the R c    184  may be modified to be longer or shorter than the R c    152  if desired. The R c    184 , however, has an origin  186  which is positioned to the lateral side of the centerline  156 . In one embodiment, the origin  186  is located 2.75 millimeters to the lateral side of the centerline  156 . Additionally, the posterior cam  144  is formed with an R c    188  of the same length as the R c    158 , although a longer or shorter length than the R c    158  may be selected, and the origin  190  of the R c    188  is positioned on the centerline  162 . Accordingly, the shape of the posterior camming portion  148  and the posterior cam  144  maintain the femoral component  106  in rotation with respect to the tibial bearing insert  104  while providing substantially similar rollback of the femoral condyle elements  108  and  110  on the tibial bearing insert  104 . 
         [0069]      FIG. 14  depicts the femoral component  106  rotated to about 130 degrees of flexion on the tibial bearing insert  104 . At this rotation, the posterior cam  144  and the posterior camming portion  148  are in contact at the contact region  192 .  FIG. 15  depicts the shape of the posterior camming portion  148  and the shape of the posterior cam  144  at the contact region  192  taken along the line D-D of  FIG. 14 . 
         [0070]    In  FIG. 15 , the R c    194  of the posterior camming portion  148  has the same length as the R c    152  of  FIG. 8 . The length of the R c    194  may be modified to be longer or shorter than the R c    152  if desired. The R c    194 , however, has an origin  196  which is positioned to the lateral side of the centerline  156 . In one embodiment, the origin  196  is located 4 millimeters to the lateral side of the centerline  156 . Additionally, the posterior cam  144  is formed with an R c    198  of the same length as the R c    158 , although a longer or shorter length than the R c    158  may be selected, and the origin  200  of the R c    198  is positioned on the centerline  162 . Accordingly, the shape of the posterior camming portion  148  and the posterior cam  144  maintain the femoral component  106  in rotation with respect to the tibial bearing insert  104  while providing substantially similar rollback of the femoral condyle elements  108  and  110  on the tibial bearing insert  104 . 
         [0071]    A deep knee bending simulation was conducted with a model of the femoral component  106  on the tibial bearing insert  104  to verify the rollback and rotational characteristics of this embodiment. LMKS Modeling Results for the femoral component  106  on the tibial bearing insert  104  are shown in  FIG. 16  wherein the graph  210  includes lines  212  and  214  which show the estimated low (tangency) points for the lateral condylar surface  110  and the medial condylar surface  108 , respectively, of the femoral component  106  on the tibial bearing insert  104 . The lower portion of the lines  212  and  214  were generated as the components were moving into flexion. Both of the lines  212  and  214  initially track posteriorly (downwardly as viewed in the  FIG. 16 ) between 0 and about 35 degrees of flexion. Thus, the femoral component  106  is moving posteriorly or “rolling back” on the tibial bearing insert  104 . 
         [0072]    The amount of rollback of the lateral condylar surface  110  and the medial condylar surface  108  between 0 degrees and 35 degrees of flexion is not the same. This indicates that the femoral component  106  is rotating. This is supported by the LMKS Modeling Results for the femoral component  106  on the tibial bearing insert  104  shown in the graph  216  of  FIG. 17  wherein the line  218  of the graph  216  identifies the φ i-e  of the femoral component  106  on the tibial bearing insert  104 . The graph  216  reveals that at about 35 degrees of flexion, the φ i-e  for the femoral component  106  on the tibial bearing insert  104  is about 3 degrees. 
         [0073]    Returning to  FIG. 16 , beyond about 35 degrees of flexion, the line  214  shows that the medial condyle  108  drifts slightly anteriorly on the tibial bearing insert  104  to about 80 degrees of flexion while the line  212  indicates that the lateral condyle  110  maintains the same location on the tibial bearing insert  104  through about 105 degrees of flexion. Thus, the medial condyle  108  (line  214 ) appears to be exhibiting negative slip while the lateral condyle  110  (line  212 ) is slipping at a relatively constant rate of pure slip. Accordingly,  FIG. 16  indicates that the φ i-e  should increase between about 35 degrees of flexion and about 105 degrees of flexion. The graph  216  supports this as the φ i-e  for the femoral component  106  on the tibial bearing insert  104  changes from about 3 degrees at 35 degrees of flexion to almost 8 degrees at 80 degrees of flexion. 
         [0074]    Beyond 80 degrees of flexion, the medial condyle  108  (line  214 ) remains relatively constant before moving posteriorly from about 105 degrees of flexion to 130 degrees of flexion. The lateral condyle  110  (line  212 ) remains constant to about 105 degrees of flexion and then moves rapidly posteriorly. This indicates that from about 80 degrees of flexion to about 105 degrees of flexion the φ i-e  for the femoral component  106  on the tibial bearing insert  104  should be relatively constant followed by an increase in φ i-e  through 130 degrees of flexion. A review of the LMKS Modeling Results for the femoral component  106  on the tibial bearing insert  104  confirms the expected φ i-e . 
         [0075]    Accordingly, the asymmetrically shaped posterior cam  144  and posterior camming portion  148 , which initially contact one another at about 70 degrees of flexion, provide for additional rollback and rotation between the femoral component  106  and the tibial bearing insert  104 . 
         [0076]      FIG. 18  depicts an alternative knee replacement system  300 . The knee replacement system  300  includes a tibial tray  302 , a tibial bearing insert  304  and a femoral component  306  having two femoral condyle elements  308  and  310 . A cam compartment  312  is located between the femoral condyle elements  308  and  310  and a spine  314  extends upwardly from the tibial bearing insert  304 . The tibial tray  302 , the tibial bearing insert  304  and the femoral component  306  are substantially identical to the corresponding components of the knee replacement system  100 . A difference between the knee replacement system  300  and the knee replacement system  100  is that the tibial bearing insert  304  is configured to rotate on the tibial superior bearing surface  316  of the tibial tray  302 . To this end, the tibial tray  302  includes a coupling member  318  for rotatably receiving a coupling member  320  of the tibial bearing insert  304 . 
         [0077]      FIG. 19  depicts a cross sectional view of the femoral component  306  taken through the cam compartment  312  and a side plan view of the tibial bearing insert  304 . An anterior cam  342  and a posterior cam  344  are located within the cam compartment  312 . The spine  314  includes an anterior camming portion  346  and a posterior camming portion  348 . The anterior cam  342  is configured with the anterior camming portion  346  to preclude undesired posterior slippage when the femoral component  306  is positioned on the tibial bearing insert  304 . 
         [0078]    The femoral component  306  is depicted in  FIG. 19  in full extension. The low or tangency point of the femoral component  306  is identified as condylar dwell point  350 . The condylar dwell point  350  and the condylar dwell point  352  for the condyle element  310 , shown projected onto the tibial superior bearing surface  316  in  FIG. 20 , define a dwell axis  354 . The dwell axis  354  intersects the centerline  356  of the tibial superior bearing surface  316  at a point defined herein as the “dwell point”  358 . The dwell point  358  is located anteriorly and medially to the center of the coupling member  318  which, along with the coupling member  320 , defines an axis of rotation  322  for the tibial bearing insert  304  (see also  FIG. 21 ). The axis of rotation  322  is offset from the central axis  324  of the tibial tray  302  in a lateral and posterior direction. In one embodiment, the axis of rotation  322  is offset from the dwell point  358  of the tibial tray  302  by about 0.317 inches laterally and about 0.317 inches posteriorly. 
         [0079]    A deep knee bending simulation was conducted on the femoral component  306  on the tibial bearing insert  304  to verify the rollback and rotational characteristics of this embodiment. LifeMod/KneeSim Modeling Results for the simulation are shown in  FIG. 22  wherein the graph  360  includes lines  362  and  364  which show the estimated low (tangency) points for the lateral condylar surface  310  and the medial condylar surface  308 , respectively, of the femoral component  306  on the tibial bearing insert  304 . The graph  360  further includes lines  366  and  368  which show the estimated low (tangency) points for the lateral condylar surface  310  and the medial condylar surface  308 , respectively, of the femoral component  306  with respect to the tibial tray  302 . The lower portion of the lines  362 ,  364 ,  366 , and  368  were generated as the components were moving into flexion. 
         [0080]    The graph  360  generally shows the femoral component  306  is moving posteriorly or “rolling back” on the tibial bearing insert  304  until about 40 degrees of flexion and again from about 95 degrees of flexion to 130 degrees of flexion. 
         [0081]    The graph  370  of  FIG. 23  includes the line  372  which identifies the φ i-e  of the femoral component  306  with respect to the tibia. The line  372  reveals that between 0 degrees of flexion and about 130 degrees of flexion, the φ i-e  for the femoral component  306  with respect to the tibia is steadily increasing to about 11 degrees. The graph  370  further includes a line  374  which identifies the rotation of the tibial bearing insert  304  with respect to the tibia. The line  374  reveals that between 0 degrees of flexion and about 110 degrees of flexion, there is a steady increase in the rotation of the tibial bearing insert  104  with respect to the tibia to about 10 degrees of rotation, followed by a slight decrease through 130 degrees of flexion. 
         [0082]    Thus, the rotation of the tibial bearing insert  304  with respect to the tibia was greater than the φ i-e  for the femoral component  306  until about 120 degrees of flexion with the maximum difference in rotation between the femoral component  306  and the tibial bearing insert  304  about 3 degrees at about 60 degrees of flexion. On subsequent cycles, the rotation of the tibial bearing insert  304  with respect to the tibia was generally higher, with the maximum difference in rotation between the femoral component  306  and the tibial bearing insert  304  about 6 degrees at about 60 degrees of flexion. 
         [0083]    Accordingly, an asymmetrically shaped posterior cam and posterior camming portion as described above, which initially contact one another at about 70 degrees of flexion, provide for additional rollback and rotation between a femoral component and a tibial bearing insert which is fixed or a tibial bearing insert which is rotatable. 
         [0084]    The asymmetry which provides for a gradual rotation and increased rollback need not be introduced in the tibial component. By way of example,  FIG. 24  depicts a knee replacement system  400  with components substantially identical to the corresponding components of the knee replacement system  100  to which reference may be made for further identification of the components. In  FIG. 24 , femoral component  406  is rotated to about 70 degrees of flexion on the tibial bearing insert  404 . At this rotation, the posterior cam  444  and the posterior camming portion  448  are in contact at the contact region  450 .  FIG. 25  depicts the shape of the posterior camming portion  448  and the shape of the posterior cam  444  at the contact region  450  taken along the line E-E of  FIG. 24  which extends from a medial portion of the camming portion  448  and the posterior cam  444  to a lateral portion of the camming portion  448  and the posterior cam  444  in a medio-lateral plane. 
         [0085]    The posterior camming portion  448  is formed on a radius of curvature (R c )  452  having an origin  454  on the centerline  456  of the tibial bearing insert  404 . In one embodiment, the R c    452  may be about 20 millimeters. The posterior cam  444  is formed on a radius of curvature (R c )  458  having an origin  460  on the centerline  462  of the femoral component  106 . In one embodiment, the R c    458  may be about 40 millimeters. At about 70 degrees of flexion, the centerline  456  of the tibial bearing insert  404  and the centerline  462  of the femoral component  406  are substantially aligned. Thus, the origin  454  and the origin  460  are substantially aligned. Accordingly, the predominant effect of the contact between the posterior cam  444  and the posterior camming portion  448  is the prevention of anterior movement of the femoral component  406  on the tibial bearing insert  404 . 
         [0086]    Continued rotation of the femoral component  406  to about 90 degrees of flexion on the tibial bearing insert  404  results in the configuration of  FIG. 26 . At this rotation, the posterior cam  444  and the posterior camming portion  448  are in contact at the contact region  470 .  FIG. 27  depicts the shape of the posterior camming portion  448  and the shape of the posterior cam  444  at the contact region  470  taken along the line F-F of  FIG. 26 . 
         [0087]    In  FIG. 27 , the R c    472  of the posterior camming portion  148  has the same length as the R c    452  of  FIG. 25 . The R c    472  also has an origin  474  which is positioned on the centerline  456 . The posterior cam  444  is formed with an R c    476  of the same length as the R c    458 . The origin  478  of the R c    476 , however, is positioned to the medial side of the centerline  462 . In one embodiment, the origin  478  of the R c    476  is positioned 1 millimeter to the medial side of the centerline  462 . Accordingly, the shape of the posterior camming portion  448  and the posterior cam  444  cause a rotational force in the direction of the arrow  480 . The lateral condyle, femoral condyle element  410  in this embodiment, is thus forced to move posteriorly at a rate greater than the medial condyle (femoral condyle element  408 ). 
         [0088]    The result of the forces acting upon the femoral component  406  is rotation of the femoral component  406  with respect to the tibial bearing insert  404  as shown in  FIG. 28 . In  FIG. 28 , the centerline  462  has rotated in a counterclockwise direction from the centerline  456 . Additionally, opposing faces of the posterior camming portion  448  and the posterior cam  444 , in contrast to the configuration shown in  FIG. 27 , are more aligned with each other. 
         [0089]    The movement of the origins of the R c  for the posterior cam  444  is done incrementally along the contact surfaces of the posterior cam  444  between the contact region  450  and the contact region  470 . This provides a smooth rotational movement of the femoral component  406  on the tibial bearing insert  404  from the alignment of  FIG. 25  to the alignment of  FIG. 28 . The precise amount of rotation and rollback may be adjusted by modifying the offset of the origins. 
         [0090]    Continued rotation of the femoral component  406  to about 110 degrees of flexion on the tibial bearing insert  404  results in the configuration of  FIG. 29 . At this rotation, the posterior cam  444  and the posterior camming portion  448  are in contact at the contact region  482 .  FIG. 30  depicts the shape of the posterior camming portion  448  and the shape of the posterior cam  444  at the contact region  482  taken along the line G-G. 
         [0091]    In  FIG. 30 , the R c    484  of the posterior camming portion  448  has the same length as the R c    452  of  FIG. 25 . The R c    484  further has an origin  486  which is positioned on the centerline  456 . While the posterior cam  444  is formed with an R c    488  of the same length as the R c    458 , the origin  490  of the R c    488  is positioned to the medial side of the centerline  462 . In one embodiment, the origin  490  of the R c    488  is positioned 2 millimeters to the medial side of the centerline  462 . Accordingly, the shape of the posterior camming portion  448  and the posterior cam  444  maintain the femoral component  406  in rotation with respect to the tibial bearing insert  404  while providing substantially similar rollback of the femoral condyle elements  408  and  410  on the tibial bearing insert  404 . 
         [0092]      FIG. 31  depicts the femoral component  406  rotated to about 130 degrees of flexion on the tibial bearing insert  404 . At this rotation, the posterior cam  444  and the posterior camming portion  448  are in contact at the contact region  492 .  FIG. 32  depicts the shape of the posterior camming portion  448  and the shape of the posterior cam  444  at the contact region  492  taken along the line H-H of  FIG. 31 . 
         [0093]    In  FIG. 32 , the R c    494  of the posterior camming portion  448  has the same length as the R c    452  of  FIG. 25 . Additionally, the R c    494  has an origin  496  which is positioned on the centerline  456 . While the posterior cam  444  is formed with an R c    498  of the same length as the R c    458 , however, the origin  500  of the R c    498  is positioned to the medial side of the centerline  462 . In one embodiment, the origin  500  of the R c    498  is positioned about 3.5 millimeters to the medial side of the centerline  462 . Accordingly, the shape of the posterior camming portion  448  and the posterior cam  444  maintain the femoral component  406  in rotation with respect to the tibial bearing insert  404  while providing substantially similar rollback of the femoral condyle elements  408  and  410  on the tibial bearing insert  404 . 
         [0094]    Accordingly, providing an asymmetry as described above either on the tibial component or on the femoral component or with a combination of the two components, provides for additional rollback and rotation between a femoral component and a tibial bearing insert which is fixed or a tibial bearing insert which is rotatable. 
         [0095]    While the present invention has been illustrated by the description of exemplary processes and system components, and while the various processes and components have been described in considerable detail, applicant does not intend to restrict or in any limit the scope of the appended claims to such detail. Additional advantages and modifications will also readily appear to those ordinarily skilled in the art. The invention in its broadest aspects is therefore not limited to the specific details, implementations, or illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the spirit or scope of applicant&#39;s general inventive concept.