Antero-posterior placement of axis of rotation for a rotating platform

A knee replacement system includes a femoral component including a lateral condylar articulating portion and a medial condylar articulating portion, a tibial tray including an upper articulating surface, and a tibial insert including (i) a first articulating portion for articulating with the lateral condylar articulating portion with a first condylar dwell point, (ii) a second articulating portion for articulating with the medial condylar articulating portion with a second condylar dwell point, (iii) a lower articulating surface for articulating with the upper articulating surface, and (iv) a coupling member for coupling with the tibial tray and defining an axis of rotation about which the tibial insert rotates with respect to the tibial tray, the axis of rotation intersecting the upper articulating surface at a location posterior to a dwell axis including the condylar dwell points when the dwell axis is projected onto the upper articulating surface.

Cross-reference is made to U.S. Utility patent application Ser. No. 12/165,579entitled “Orthopaedic Femoral Component Having Controlled Condylar Curvature” by John L. Williams et al., which was filed on Jun. 30, 2008, and which was published on Dec. 31, 2009 as U.S. Patent Publication No. 2009/0326667; 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, and which was published on Dec. 31, 2009 as U.S. Patent Publication No. 2009/0326664; 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, and which was published on Dec. 31, 2009 as U.S. Patent Publication No. 2009/0326665; 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 and which was published on Dec. 31, 2009 as U.S. Patent Publication No. 2009/0326666; to U.S. Utility patent application Ser. No. 12/174,539entitled “Knee Prostheses with Enhanced Kinematics” by Joseph G. Wyss, et al., which was filed on Jul. 16, 2008, and which was published on Jan. 21, 2010 as U.S. Patent Publication No. 2010/0016979; and to U.S. Provisional Patent Application Ser. No. 61/077,124 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

This invention relates generally to prostheses for human body joints, and, more particularly, to prostheses for knees.

BACKGROUND OF THE INVENTION

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.

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.

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.

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, Feb. 2005, pages 269-276, which includes aFIG. 2from which the data set forth inFIG. 1as graph10has been derived. The graph10shows 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 line12of the graph10indicates that the lateral condyle exhibits a constant anterior to posterior translation through deep flexion while the line14indicates 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.

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.

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.

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.

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.

By way of example,FIG. 2depicts a sagittal view of a typical prior art femoral component20which attempts to mimic the shape of a native knee. The femoral component20includes an extension region22which is generally anterior to the line24and a flexion region26which is posterior to the line24. The extension region22is formed with a large radius of curvature (Rc)28while a small Rc30is used in the posterior portion of the flexion region26in 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 origin32for the Rc28to the origin34for the Rc30.

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 chart40ofFIG. 3which 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, California, under the name LifeMOD/KneeSIM. The model included tibio-femoral and patello-femoral contact, passive soft tissue, and active muscle elements.

The lines42and44in the chart40show the estimated low (tangency) points for the lateral condylar surface and the medial condylar surface, respectively. Both of the lines42and44initially track posteriorly (downwardly as viewed inFIG. 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 line42drifts slightly anteriorly from about 5 mm translation while the estimated medial condylar low (tangency) point line44moves 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 lines42and44beyond 30° of flexion with the lines12and14ofFIG. 1reveals a striking disparity in kinematics between the native knee and the replacement knee which mimics the geometry of the native knee.

Additionally, returning toFIG. 2, as the femoral component20is flexed such that contact with a tibial component (not shown) occurs along the condylar surface defined by the Rc28, the forces exerted by soft-tissues on the knee are coordinated to provide a smooth movement based, in part, upon the length of the Rc28and the origin32. As the femoral component20is moved through the angle at which the condylar surface transitions from the Rc28to the Rc30, the knee may initially be controlled as if it will continue to move along the Rc28. As the femoral component20continues 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 Rc28. 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 Rc30with the origin34. This sudden change in configuration, which is not believed to occur with a native knee, contributes to the sense of instability.

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.

The reported external rotation of the native knee is supported by the data inFIG. 1. Specifically, between about 9 degrees and 90 degrees of flexion, the slope of the line12is constantly downward indicating that the lowest point of the lateral condylar surface is continuously tracking posteriorly. The line14, 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 lines12and14show 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 lines12and14continues to expand beyond 90 degrees, indicating that additional external rotation of the knee is occurring.

FIG. 4shows 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 “φi-e”) during the testing that provided the results ofFIG. 3. The graph50includes a line52which shows that as the tested component was manipulated to 130 degrees of flexion, the φi-ereached a maximum of about seven degrees. Between about 0 degrees of flexion and 20 degrees of flexion, the φi-evaries 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 graph50exhibits 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 φi-eof a knee joint incorporating the prior art femoral component differs significantly from the φi-eof a native knee.

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.

Another attempt to replicate the kinematics of the native knee involves the use of a tibial insert which is configured to rotate upon a tibial plateau. Rotating tibial inserts are commonly referred to as rotating platform (RP) designs. One presumed advantage of RP designs is the decoupling of flexion-extension from φi-e. This decoupling is believed to reduce total wear of the components. The axis of rotation of the tibial insert on a tibial plateau (RP axis) has typically been positioned between locations coincident with the tibio-femoral dwell points (the low or tangency points of the femoral component when the joint is in full extension) and locations removed from the tibio-femoral dwell points in the anterior direction.

What is needed is a knee prosthesis that more closely reproduces the inherent stability and kinematics of a native knee such as by managing φi-e. A further need exists for a knee prosthesis that manages φi-ewhile allowing an acceptable rollback of a femoral component on a tibial plateau.

SUMMARY

A knee replacement system with improved kinematics in one embodiment includes a femoral component including a lateral condylar articulating portion and a medial condylar articulating portion, a tibial tray including an upper articulating surface, and a tibial insert including (i) a first articulating portion for articulating with the lateral condylar articulating portion with a first condylar dwell point, (ii) a second articulating portion for articulating with the medial condylar articulating portion with a second condylar dwell point, (iii) a lower articulating surface for articulating with the upper articulating surface, and (iv) a coupling member for coupling with the tibial tray and defining an axis of rotation about which the tibial insert rotates with respect to the tibial tray, the axis of rotation intersecting the upper articulating surface at a location posterior to a dwell axis including the first condylar dwell point and the second condylar dwell point when the dwell axis is projected onto the upper articulating surface.

In a further embodiment, a prosthetic joint includes a femoral component including a lateral condylar articulating portion and a medial condylar articulating portion, a tibial tray including an upper articulating surface, and a tibial insert including (i) a first articulating portion for articulating with the lateral condylar articulating portion, (ii) a second articulating portion for articulating with the medial condylar articulating portion, (iii) a lower articulating surface for articulating with the upper articulating surface, and (iv) a pivot defining an axis of rotation about which the tibial insert rotates with respect to the tibial tray, the axis of rotation intersecting the upper articulating surface at a location lateral to a tibial insert centerline when the centerline is projected onto the upper articulating surface.

In a further embodiment, a prosthetic joint includes a femoral component including a lateral condylar articulating portion and a medial condylar articulating portion, a tibial tray including an upper articulating surface, and a tibial insert including (i) a first articulating portion for articulating with the lateral condylar articulating portion with a first condylar dwell point, (ii) a second articulating portion for articulating with the medial condylar articulating portion with a second condylar dwell point, (iii) a lower articulating surface for articulating with the upper articulating surface, and (iv) a coupling member for coupling with the tibial tray and defining an axis of rotation about which the tibial insert rotates with respect to the tibial tray, the axis of rotation intersecting the upper articulating surface at a location posterior to a dwell axis including the first condylar dwell point and the second condylar dwell point when the dwell axis is projected onto the upper articulating surface and lateral to a tibial insert centerline when the centerline is projected onto the upper articulating surface.

DETAILED DESCRIPTION

FIG. 5depicts a knee replacement system100incorporating known features. The knee replacement system100includes a tibi al tray102, a tibial bearing insert104and a femoral component106having two femoral condyle elements108and110. The tibial tray102includes an inferior stem112for attaching the tibial tray102to the tibia of a patient and a superior plateau114for receiving the tibial bearing insert104. A coupling member116is located on the superior plateau114.

The tibial bearing insert104includes an inferior tibial tray contacting surface118and a superior tibial bearing surface120The superior tibial bearing surface120includes a bearing surface122and a bearing surface124configured to articulate with the femoral condyle elements108and110. A spine126extends upwardly from between the bearing surface122and the bearing surface124. A coupling member128extends downwardly from the tibial tray contacting surface118

The femoral component106is configured to be attached to the femur of a patient. A trochlear groove130is formed between the femoral condyle elements108and110which, in this embodiment, are symmetrical. The trochlear groove130provides an articulation surface for a patellar component (not shown). A cam compartment132is located between posterior portions134and136of the femoral condyle elements108and110, respectively. Two pegs138and140are used to mount the femoral component106onto the femur of a patient.

FIG. 6depicts a cross sectional view of the femoral component106taken through the cam compartment132and a side plan view of the tibial bearing insert104. An anterior cam142and a posterior cam144are located within the cam compartment136. The spine126includes an anterior camming portion146and a posterior camming portion148. The anterior cam142is configured with the anterior camming portion146to preclude undesired posterior slippage when the femoral component106is positioned on the tibial bearing insert104in extension as shown inFIG. 6.

With reference toFIG. 33, the femoral component106is depicted rotated to about 70 degrees of flexion on the tibial bearing insert104. At this rotation, the posterior cam144and the posterior camming portion148are in contact at the contact region141.FIG. 34depicts the shape of the posterior camming portion148and the shape of the posterior cam144at the contact region141taken along the line A-A ofFIG. 33which extends from a medial portion of the camming portion148and the posterior cam144to a lateral portion of the camming portion148and the posterior cam144in a medio-lateral plane.

The posterior camming portion148is formed on a radius of curvature (Rc)147having an origin149on the centerline156of the tibial bearing insert104. In one embodiment, the Rc147may be about 20 millimeters. The posterior cam144is formed on a radius of curvature (Rc)143having an origin145on the centerline162of the femoral component106. In one embodiment, the Rc143may be about 40 millimeters.

The centerline156is defined as (i) the straight line extending between the origin145of the radius of curvature143of the posterior cam144and the origin149of the radius of curvature147of the posterior camming portion148, (ii) wherein the foregoing origins145,149are the origins for the co-planar radii of curvature143,147of the posterior cam144and the posterior camming portion148respectively (iii) at a location where the posterior cam144and the posterior camming portion148initially come into contact during rollback. In the embodiment ofFIG. 33, the centerline156is thus defined by the origin149and the origin145. The centerline156is fixed with respect to the tibial bearing insert104, that is, if the tibial bearing insert104moves or rotates from the orientation depicted inFIG. 34, the centerline156moves or rotates as well.

The femoral component106is depicted inFIG. 6in full extension. The low or tangency point of the femoral component106is identified as condylar dwell point150. The condylar dwell point150and the condylar dwell point152for the condyle element110, shown projected onto the superior plateau inFIG. 7, define a dwell axis154. The dwell axis154intersects the centerline156of the tibial superior bearing surface120at a point defined herein as the “dwell point”158. The dwell point158is located posteriorly to the coupling member116which, along with the coupling member128, defines an axis of rotation160for the tibial bearing insert104(seeFIG. 8). The axis of rotation160is positioned anteriorly of the dwell point158.

A deep knee bending simulation was conducted on a prior art device similar to the knee replacement system100. The prior art device was a NexGen® LPS-flex rotating platform total knee system commercially available from Zimmer, Inc., of Warsaw Indiana. The design parameters of the prior art device that were modeled for the simulation were obtained by reverse engineering. The simulation was conducted using the LifeMOD/KneeSIM version 2007.1.0 Beta 12 and later (LMKS) dynamics program discussed above. The LMKS was configured to model the MCL, and LCL, as well as capsular tissue, as linear springs and the patellar tendon and ligament allowed to wrap around the implants.

Flexion/extension at the hip and ankle joints, and abduction/adduction, varus/valgus and axial rotation at the ankle joint were unconstrained while a constant vertical load of 463 N was applied at the hip. A closed loop controller was used to apply tension to the quadriceps and hamstring muscles to match a prescribed knee-flexion extension profile. The design parameters of the prior art device were imported into the model and subjected to one cycle of deep knee bending up to about 150 degrees of flexion.

The components were positioned so that the dwell point of the insert of the tibio-femoral contact surface lined up in the sagittal plane with the mechanical axis of the leg and the original joint line of the knee was restored. The patellar ligament angle in the sagittal plane at full extension was determined by placing the patellar component at an appropriate supero-inferior position, centered within the trochlear groove of the femoral component and the patellar ligament in the coronal plane was determined by using the default settings of LMKS, which resulted in a Q-angle of about 12 degrees in the coronal plane with the knee at full extension. The rectus femoris coronal angle at full extension was about 7 degrees and the coronal patellar ligament angle at full extension was about 5 degrees fro the vertical mechanical axis of the leg at full extension.

The results of the above defined modeling scenario, hereinafter referred to as “the LMKS Modeling Results, included the anterior-posterior positions of the lowest points on the femoral lateral and medial condyles closest to the tibial tray which were recorded relative to the dwell points. Additionally, rotation of the tibia relative to the femur and rotation of the tibial insert relative to the tibial tray was reported using the Grood & Suntay coordinate system. In discussions of the LMKS Modeling Results for the prior art device shown inFIGS. 9-16, the reference numbers for the corresponding component of the knee replacement system100will be referenced, with the condyle108designated as the medial condyle and the condyle110designated as the lateral condyle.

LMKS Modeling Results for the simulation of the femoral component106on the tibial bearing insert104are shown inFIG. 9wherein the graph170includes lines172and174which show the estimated low (tangency) points for the lateral condylar surface110and the medial condylar surface108, respectively, of the femoral component106on the tibial bearing insert104. The graph170further includes lines176and178which show the estimated low (tangency) points for the lateral condylar surface110and the medial condylar surface108, respectively, of the femoral component106with respect to the tibial tray102. The lower portion of the lines172,174,176and178were generated as the components were moving into flexion.

The graph170generally shows the femoral component106is moving posteriorly or “rolling back” on the tibial bearing insert104until about 20 degrees of flexion and again from about 90 degrees of flexion to 150 degrees of flexion. The amount of rollback of the lateral condylar surface110and the medial condylar surface108is not the same. This difference indicates that the femoral component106is rotating. This conclusion is supported by the LMKS Modeling Results for the femoral component106on the tibial bearing insert104shown in the graph180ofFIG. 10wherein the line182of the graph180identifies the φi-eof the femoral component106with respect to the tibia. The line182reveals that between 0 degrees of flexion and about 100 degrees of flexion, the φi-efor the femoral component106with respect to the tibia is steadily increasing to about 3.5 degrees.

The graph180further includes a line184which identifies the rotation of the tibial bearing insert104with respect to the tibia. The line184, in contrast to the line182, reveals that between 0 degrees of flexion and about 90 degrees of flexion, the rotation for the tibial bearing insert104with respect to the tibia is steadily decreasing to about −2.5 degrees, indicating a maximum difference in rotation between the femoral component106and the tibial bearing insert104of about 5 degrees between about 90 and about 110 degrees of flexion.

Reverse engineering of the prior art system used in the foregoing modeling scenario indicates that the axis of rotation160of the tibial bearing insert104of the prior art device was located 0.5 inches anterior to the dwell point158(the “0.5A configuration”). The model of the prior art device was then modified to place the axis of rotation160of the tibial bearing insert104at 0.2 inches anterior to the dwell point158(the “0.2A configuration”). LMKS Modeling Results for the 0.2A configuration are shown inFIG. 11wherein the graph190includes lines192and194which show the estimated low (tangency) points for the lateral condylar surface110and the medial condylar surface108, respectively, of the femoral component106on the tibial bearing insert104. The graph190further includes lines196and198which show the estimated low (tangency) points for the lateral condylar surface110and the medial condylar surface108, respectively, of the femoral component106with respect to the tibial tray102. The lower portion of the lines192,194,196, and198were generated as the components were moving into flexion.

The graph190generally shows the femoral component106is moving posteriorly or “rolling back” on the tibial bearing insert104until about 20 degrees of flexion and again from about 90 degrees of flexion to 150 degrees of flexion. The rollback exhibited with the 0.2A configuration is substantially the same as the rollback exhibited in the 0.5A configuration.

The graph200ofFIG. 12includes the line202which identifies the φi-eof the femoral component106with respect to the tibia. The line202reveals that between 0 degrees of flexion and about 100 degrees of flexion, the φi-efor the femoral component106with respect to the tibia is steadily increasing to over 4 degrees. The graph200further includes a line204which identifies the rotation of the tibial bearing insert104with respect to the tibia. The line204reveals that between 0 degrees of flexion and about 20 degrees of flexion, there is a slight decrease in the rotation of the tibial bearing insert104with respect to the tibia, followed by a steady increase through about 120 degrees of flexion. Thus, the maximum difference in rotation between the femoral component106and the tibial bearing insert104is reduced to less than 4 degrees at about 90 degrees of flexion.

The model of the prior art device was then modified to place the axis of rotation160of the tibial bearing insert104at the dwell point158(the “0.0 configuration”). LMKS Modeling Results for the 0.0A configuration are shown inFIG. 13wherein the graph210includes lines212and214which show the estimated low (tangency) points for the lateral condylar surface110and the medial condylar surface108, respectively, of the femoral component106on the tibial bearing insert104. The graph210further includes lines216and218which show the estimated low (tangency) points for the lateral condylar surface110and the medial condylar surface108, respectively, of the femoral component106with respect to the tibial tray102. The lower portion of the lines212,214,216and218were generated as the components were moving into flexion.

The graph210generally shows the femoral component106is moving posteriorly or “rolling back” on the tibial bearing insert104until about 20 degrees of flexion and again from about 90 degrees of flexion to 150 degrees of flexion. The rollback exhibited with the 0.0 configuration is substantially the same as the rollback exhibited in the 0.5A configuration.

The graph220ofFIG. 14includes the line222which identifies the φi-eof the femoral component106with respect to the tibia. The line222reveals that between 0 degrees of flexion and about 100 degrees of flexion, the φi-efor the femoral component106with respect to the tibia is steadily increasing to almost 5 degrees. The graph220further includes a line224which identifies the rotation of the tibial bearing insert104with respect to the tibia. The line224reveals that between 0 degrees of flexion and about 20 degrees of flexion, there is a slight decrease in the rotation of the tibial bearing insert104with respect to the tibia, followed by a steady increase through about 120 degrees of flexion. Thus, the maximum difference in rotation between the femoral component106and the tibial bearing insert104is reduced to less than 2.5 degrees at about 90 degrees of flexion. On subsequent cycles, the maximum difference in rotation remains about the same, but the line224conforms more closely to the line222.

The model of the prior art device was then modified to place the axis of rotation160of the tibial bearing insert104at 0.5 inches posterior to the dwell point158(the “0.5P configuration”). LMKS Modeling Results for the 0.5P configuration are shown inFIG. 15wherein the graph230includes lines232and234which show the estimated low (tangency) points for the lateral condylar surface110and the medial condylar surface108, respectively, of the femoral component106on the tibial bearing insert104. The graph230further includes lines236and238which show the estimated low (tangency) points for the lateral condylar surface110and the medial condylar surface108, respectively, of the femoral component106with respect to the tibial tray102. The lower portion of the lines232,234,236, and238were generated as the components were moving into flexion.

The graph230generally shows the femoral component106is moving posteriorly or “rolling back” on the tibial bearing insert104until about 20 degrees of flexion and again from about 90 degrees of flexion to 150 degrees of flexion. The rollback exhibited with the 0.5P configuration is substantially the same as the rollback exhibited in the 0.5A configuration.

The graph240ofFIG. 16includes the line242which identifies the φi-eof the femoral component106with respect to the tibia. The line242reveals that between 0 degrees of flexion and about 100 degrees of flexion, the φi-efor the femoral component106with respect to the tibia is steadily increasing to almost 6 degrees. The graph240further includes a line244which identifies the rotation of the tibial bearing insert104with respect to the tibia. The line244reveals that between 0 degrees of flexion and about 10 degrees of flexion, there is a slight decrease in the rotation of the tibial bearing insert104with respect to the tibia, followed by a steady increase through about 120 degrees of flexion. Thus, the maximum difference in rotation between the femoral component106and the tibial bearing insert104is reduced to just over 1 degree at about 95 degrees of flexion. On subsequent cycles, the line244conforms very closely with the line242. The excursion of the line244above the line242as the joint travels toward a flexed position in the 0.5P configuration is somewhat larger than the excursion of the line224above the line222in the 0.0 configuration.

TheFIGS. 9-16thus show that as the axis of rotation160is moved posteriorly, increased fidelity between the rotation of the femoral component106with respect to the tibial tray102and the rotation of the tibial bearing insert104with respect to the tibial tray102is realized. Additionally, the φi-efor the femoral component106with respect to the tibia more than doubles.

Validation of the principles set forth herein was accomplished by a series of additional modeling scenarios using a differently configured knee replacement system. In discussions of the LMKS Modeling Results for the differently configured device shown inFIGS. 17-30, the reference numbers for the corresponding component of the knee replacement system100will be referenced.

The model of the differently configured device was established with the axis of rotation160of the tibial bearing insert104at the centerline156and 0.317 inches anterior to the dwell point158(the “0/0.317A configuration”). LMKS Modeling Results for the 0/0.317A configuration are shown inFIG. 17wherein the graph250includes lines252and254which show the estimated low (tangency) points for the lateral condylar surface110and the medial condylar surface108, respectively, of the femoral component106on the tibial bearing insert104. The graph250further includes lines256and258which show the estimated low (tangency) points for the lateral condylar surface110and the medial condylar surface108, respectively, of the femoral component106with respect to the tibial tray102. The lower portion of the lines252,254,256, and258were generated as the components were moving into flexion.

The graph250generally shows the femoral component106is moving posteriorly or “rolling back” on the tibial bearing insert104until about 30 degrees of flexion and again from about 105 degrees of flexion to 130 degrees of flexion.

The graph260ofFIG. 18includes the line262which identifies the φi-eof the femoral component106with respect to the tibia. The line262reveals that between 0 degrees of flexion and about 120 degrees of flexion, the φi-efor the femoral component106with respect to the tibia is steadily increasing to just over 5 degrees. The graph260further includes a line264which identifies the rotation of the tibial bearing insert104with respect to the tibia. The line264reveals that between 0 degrees of flexion and about 70 degrees of flexion, there is a steady decrease in the rotation of the tibial bearing insert104with respect to the tibia, followed by a relatively constant rotation angle through about 130 degrees of flexion. Thus, the maximum difference in rotation between the femoral component106and the tibial bearing insert104constantly increases to about 10 degrees at about 120 degrees of flexion. The maximum difference was about 10 degrees on subsequent cycles.

The model of the differently configured device was then modified to place the axis of rotation160of the tibial bearing insert104on the centerline156at the dwell point158(the “0/0 configuration”). LMKS Modeling Results for the 0/0 configuration are shown inFIG. 19wherein the graph270includes lines272and274which show the estimated low (tangency) points for the lateral condylar surface110and the medial condylar surface108, respectively, of the femoral component106on the tibial bearing insert104. The graph270further includes lines276and278which show the estimated low (tangency) points for the lateral condylar surface110and the medial condylar surface108, respectively, of the femoral component106with respect to the tibial tray102. The lower portion of the lines272,274,276, and278were generated as the components were moving into flexion.

The graph270generally shows the femoral component106is moving posteriorly or “rolling back” on the tibial bearing insert104until about 30 degrees of flexion and again from about 95 degrees of flexion to 130 degrees of flexion. The rollback exhibited with the 0/0 configuration is substantially the same as the rollback exhibited in the 0/0.317A configuration, although the second rollback event occurred at an earlier flexion angle.

The graph280ofFIG. 20includes the line282which identifies the φi-eof the femoral component106with respect to the tibia. The line282reveals that between 0 degrees of flexion and about 130 degrees of flexion, the φi-efor the femoral component106with respect to the tibia is steadily increasing to over 7 degrees. The graph280further includes a line284which identifies the rotation of the tibial bearing insert104with respect to the tibia. The line284reveals that between 0 degrees of flexion and about 65 degrees of flexion, there is a steady decrease in the rotation of the tibial bearing insert104with respect to the tibia, followed by a steady increase through about 105 degrees of flexion. Thus, the maximum difference in rotation between the femoral component106and the tibial bearing insert104is reduced to about 8 degrees at about 65 degrees of flexion and slightly more than 8 degrees at about 130 degrees of flexion. On subsequent cycles, the maximum difference at 65 degrees was reduced to about 5 degrees while the maximum difference at130remained at slightly more than 8 degrees.

The model of the differently configured device was then modified to place the axis of rotation160of the tibial bearing insert104at 0.317 inches lateral of the centerline156and on the dwell axis154(the “0.317L/0 configuration”). LMKS Modeling Results for the 0.317L/0 configuration are shown inFIG. 21wherein the graph290includes lines292and294which show the estimated low (tangency) points for the lateral condylar surface110and the medial condylar surface108, respectively, of the femoral component106on the tibial bearing insert104. The graph290further includes lines296and298which show the estimated low (tangency) points for the lateral condylar surface110and the medial condylar surface108, respectively, of the femoral component106with respect to the tibial tray102. The lower portion of the lines292,294,296, and298were generated as the components were moving into flexion.

The graph290generally shows the femoral component106is moving posteriorly or “rolling back” on the tibial bearing insert104until just over 30 degrees of flexion and again from about 95 degrees of flexion to 130 degrees of flexion. The rollback exhibited with the 0.317L/0 configuration is similar to the rollback exhibited in the 0/0.317A configuration.

The graph300ofFIG. 22includes the line302which identifies the φi-eof the femoral component106with respect to the tibia. The line302reveals that between 0 degrees of flexion and about 130 degrees of flexion, the φi-efor the femoral component106with respect to the tibia is steadily increasing to over 11 degrees. The graph300further includes a line304which identifies the rotation of the tibial bearing insert104with respect to the tibia. The line304reveals that between 0 degrees of flexion and about 110 degrees of flexion, there is a steady increase in the rotation of the tibial bearing insert104with respect to the tibia to about 8 degrees, followed by a drop to about 7 degrees of rotation at 130 degrees of flexion. Thus, the rotation of the tibial bearing insert104with respect to the tibia is slightly greater than or equal to the φi-efor the femoral component106through about 100 degrees of flexion with a maximum difference in rotation between the femoral component106and the tibial bearing insert104of just over 5 degrees at 130 degrees of flexion. On subsequent cycles, the maximum difference in rotation of the tibial bearing insert104with respect to the tibia is slightly increased, pushing the crossover point to about 115 degrees of flexion with a maximum difference in rotation between the femoral component106and the tibial bearing insert104of about 4 degrees at 130 degrees of flexion.

The model of the differently configured device was then modified to place the axis of rotation160of the tibial bearing insert104at 0.317 inches medial of the centerline156and on the dwell axis154(the “0.317M/0 configuration”). LMKS Modeling Results for the 0.317M/0 configuration are shown inFIG. 23wherein the graph310includes lines312and314which show the estimated low (tangency) points for the lateral condylar surface110and the medial condylar surface108, respectively, of the femoral component106on the tibial bearing insert104. The graph310further includes lines316and318which show the estimated low (tangency) points for the lateral condylar surface110and the medial condylar surface108, respectively, of the femoral component106with respect to the tibial tray102. The lower portion of the lines312,314,316, and318were generated as the components were moving into flexion.

The graph310generally shows the lateral condyle element110of the femoral component106is moving posteriorly or “rolling back” on the tibial bearing insert104until about 65 degrees of flexion while the medial condyle108exhibits rollback to about 35 degrees of flexion. The femoral component106exhibits additional rollback from about 105 degrees of flexion to 130 degrees of flexion.

The graph320ofFIG. 24includes the line322which identifies the φi-eof the femoral component106with respect to the tibia. The line322reveals that between 0 degrees of flexion and about 115 degrees of flexion, the φi-efor the femoral component106with respect to the tibia is steadily increasing to just under 5 degrees. The graph320further includes a line324which identifies the rotation of the tibial bearing insert104with respect to the tibia. The line324reveals that between 0 degrees of flexion and about 50 degrees of flexion, there is a steady decrease in the rotation of the tibial bearing insert104with respect to the tibia, followed by a relatively constant rotation of about −5 degrees through about 130 degrees of flexion. Thus, the maximum difference in rotation between the femoral component106and the tibial bearing insert104is about 11 degrees at about 130 degrees of flexion. On subsequent cycles, the maximum difference in rotation was also about 11 degrees.

The model of the differently configured device was then modified with the axis of rotation160of the tibial bearing insert104on the centerline156and 0.317 inches posterior to the dwell axis154(the “0/0.317P configuration”). LMKS Modeling Results for the 0/0.317P configuration are shown inFIG. 25wherein the graph330includes lines332and334which show the estimated low (tangency) points for the lateral condylar surface110and the medial condylar surface108, respectively, of the femoral component106on the tibial bearing insert104. The graph330further includes lines336and338which show the estimated low (tangency) points for the lateral condylar surface110and the medial condylar surface108, respectively, of the femoral component106with respect to the tibial tray102. The lower portion of the lines332,334,336, and338were generated as the components were moving into flexion.

The graph330generally shows the femoral component106is moving posteriorly or “rolling back” on the tibial bearing insert104until about 35 degrees of flexion and again from about 95 degrees of flexion to 130 degrees of flexion.

The graph340ofFIG. 26includes the line342which identifies the φi-eof the femoral component106with respect to the tibia. The line342reveals that between 0 degrees of flexion and about 130 degrees of flexion, the φi-efor the femoral component106with respect to the tibia is steadily increasing to almost 9 degrees. The graph340further includes a line344which identifies the rotation of the tibial bearing insert104with respect to the tibia. The line344reveals that between 0 degrees of flexion and about 55 degrees of flexion, there is a slight decrease in the rotation of the tibial bearing insert104with respect to the tibia, followed by a steady increase through about 105 degrees of flexion followed by a steady decrease in rotation. Thus, the maximum difference in rotation between the femoral component106and the tibial bearing insert104is about 9 degrees at about 130 degrees of flexion. On subsequent cycles, the rotation of the tibial bearing insert104with respect to the tibia remained at about 3 degrees of rotation until about 100 degrees of flexion at which point the rotation angle decreased to about zero degrees. Thus, the maximum difference in rotation between the femoral component106and the tibial bearing insert104was about 9 degrees at about 130 degrees of flexion for the subsequent cycles.

The model of the differently configured device was then modified with the axis of rotation160of the tibial bearing insert104to 0.317 inches lateral of the centerline156and 0.317 inches posterior to the dwell axis154(the “0.317L/0.317P configuration”). LMKS Modeling Results for the 0.317L/0.317P configuration are shown inFIG. 27wherein the graph350includes lines352and354which show the estimated low (tangency) points for the lateral condylar surface110and the medial condylar surface108, respectively, of the femoral component106on the tibial bearing insert104. The graph350further includes lines356and358which show the estimated low (tangency) points for the lateral condylar surface110and the medial condylar surface108, respectively, of the femoral component106with respect to the tibial tray102. The lower portion of the lines352,354,356, and358were generated as the components were moving into flexion.

The graph350generally shows the femoral component106is moving posteriorly or “rolling back” on the tibial bearing insert104until about 40 degrees of flexion and again from about 95 degrees of flexion to 130 degrees of flexion.

The graph360ofFIG. 28includes the line362which identifies the φi-eof the femoral component106with respect to the tibia. The line362reveals that between 0 degrees of flexion and about 130 degrees of flexion, the φi-efor the femoral component106with respect to the tibia is steadily increasing to about 11 degrees. The graph360further includes a line364which identifies the rotation of the tibial bearing insert104with respect to the tibia. The line364reveals that between 0 degrees of flexion and about 110 degrees of flexion, there is a steady increase in the rotation of the tibial bearing insert104with respect to the tibia to about 10 degrees of rotation, followed by a slight decrease through 130 degrees of flexion.

Thus, the rotation of the tibial bearing insert104with respect to the tibia was greater than the φi-efor the femoral component106until about 120 degrees of flexion with the maximum difference in rotation between the femoral component106and the tibial bearing insert104about 3 degrees at about 60 degrees of flexion. On subsequent cycles, the rotation of the tibial bearing insert104with respect to the tibia was generally higher, with the maximum difference in rotation between the femoral component106and the tibial bearing insert104about 6 degrees at about 60 degrees of flexion.

The model of the differently configured device was then modified with the axis of rotation160of the tibial bearing insert1040.317 inches medial to the centerline156and 0.317 inches posterior to the dwell axis154(the “0.317M/0.317P configuration”). LMKS Modeling Results for the 0.317M/0.317P configuration are shown inFIG. 29wherein the graph370includes lines372and374which show the estimated low (tangency) points for the lateral condylar surface110and the medial condylar surface108, respectively, of the femoral component106on the tibial bearing insert104. The graph370further includes lines376and378which show the estimated low (tangency) points for the lateral condylar surface110and the medial condylar surface108, respectively, of the femoral component106with respect to the tibial tray102. The lower portion of the lines372,374,376, and378were generated as the components were moving into flexion.

The graph370generally shows the lateral condyle element110of the femoral component106is moving posteriorly or “rolling back” on the tibial bearing insert104until about 60 degrees of flexion while the medial condyle108exhibits rollback to about 20 degrees of flexion. The femoral component106exhibits additional rollback from about 100 degrees of flexion to 130 degrees of flexion.

The graph380ofFIG. 30includes the line382which identifies the φi-eof the femoral component106with respect to the tibia. The line382reveals that between 0 degrees of flexion and about 130 degrees of flexion, the φi-efor the femoral component106with respect to the tibia is steadily increasing to almost 6 degrees. The graph380further includes a line384which identifies the rotation of the tibial bearing insert104with respect to the tibia. The line384reveals that between 0 degrees of flexion and about 50 degrees of flexion, there is a constant decrease in the rotation of the tibial bearing insert104with respect to the tibia to about −5 degrees, followed by a slight increase through about 130 degrees of flexion. Thus, the maximum difference in rotation between the femoral component106and the tibial bearing insert104is about 9 degrees at about 130 degrees of flexion. On subsequent cycles, the difference is less early in flexion.

TheFIGS. 17-30thus confirm that the position of the axis of rotation for a rotating plateau system may be used manage the conformity between the rotation of the plateau and the φi-efor the femoral component of the system. Additionally, the position of the axis of rotation may be used to manage the rollback and rotational characteristics of a rotating plateau system.

One embodiment of a system in accordance with principles of the invention is shown inFIG. 31. The knee replacement system400includes a tibial tray402, a tibial bearing insert404and a femoral component406having two femoral condyle elements408and410. The tibial tray402includes an inferior stem412for attaching the tibial tray402to the tibia of a patient and a superior plateau414for articulating with the tibial bearing insert404. A coupling member416is located on the superior plateau414.

The tibial bearing insert404includes an inferior tibial tray contacting surface418and a superior tibial bearing surface420The superior tibial bearing surface420includes a medial bearing surface422and a lateral bearing surface424configured to articulate with the femoral condyle elements408and410. A spine426extends upwardly from between the bearing surface422and the bearing surface424. A pivot428extends downwardly from the tibial tray contacting surface418. The femoral component406may be substantially similar to the femoral component106and is not further described herein.

With further reference toFIG. 32, a dwell axis430, condylar dwell points432and434, and a centerline436of the talar bearing insert404are shown projected onto the superior plateau414and defining a dwell point438. The coupling member416in this embodiment is positioned to define an axis of rotation440which is located posterior to the projected dwell axis430and lateral to the projected centerline436. In one embodiment, axis of rotation438is located laterally and posteriorly from the dwell point by between about 0.2 inches and 0.5 inches. In a further embodiment, the axis of rotation438is located 0.317 inches posterior to the projected dwell axis430and 0.317 inches lateral to the projected centerline436.

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. By way of example, the positioning of the axis of rotation is applicable to cruciate-retaining and cruciate-sacrificing designs wherein the ACL is absent. Accordingly, departures may be made from such details without departing from the spirit or scope of the present general inventive concept. By way of example, but not of limitation, the system described herein may be applied to other joints besides the knee.