Emulating natural knee kinematics in a knee prosthesis

[0001] The present invention relates generally to the replacement of a natural knee joint with a knee prosthesis and pertains, more specifically, to achieving better emulation of natural knee joint kinematics in a prosthetic knee.

[0002] During articulation of a natural knee joint, flexion between the tibia and the femur takes place about a transverse axis while, at the same time, some relative rotation between the tibia and the femur occurs about a longitudinal axis. Such flexion and rotation is necessary to carry out a normal gate cycle. It has been established that in full extension the tibia is rotationally displaced, relative to the femur, by approximately 2 to 3. As the natural knee flexes, the tibia rotates internally. According to previous studies, about 5 of rotation ordinarily occurs as the knee is articulated from 0 to 10 of flexion; thereafter, little further rotation occurs up to at least about 45 of flexion. Total rotation at 110 of flexion is approximately 20.

[0003] Rotational stability of the natural knee is provided by the collateral and cruciate ligaments. The cruciate ligaments deter uncontrolled internal rotation within a certain range of flexion of the knee, while the collateral ligaments provide transverse stability and deter uncontrolled external rotation of the tibia. Where the natural knee is replaced by a total knee prosthesis, either the anterior cruciate ligament or both the anterior and posterior cruciate ligaments ordinarily are sacrificed. In these instances, the knee prosthesis usually is provided with tibiofemoral articular constraint to supply the stability ordinarily provided by the sacrificed anterior cruciate ligament and a stabilizing mechanism for supplying the stability ordinarily provided by the sacrificed posterior cruciate ligament.

[0004] The present invention provides an improvement, both in construction and in procedure, which enables a knee prosthesis more closely to mimic the movements of the natural knee for smooth knee kinematics. As such, the present invention attains several objects and advantages, some of which are summarized as follows: Provides a knee prosthesis which better emulates movements of the natural knee for smooth knee flexion and extension; allows a recipient of a total knee prothesis to flex the knee easily and with less effort, while offering smooth prosthetic knee kinematics; enables the implant of a knee prosthesis utilizing current known surgical techniques while providing better prosthetic knee kinematics; provides a recipient of a total knee replacement with greater comfort and increased confidence in accommodating to the replacement; enables a more accurate emulation of the natural knee with a prosthetic knee having relatively few component parts, all of which are configured for simplified manufacture; provides an effective replacement for the natural knee, exhibiting exemplary performance over an extended service life.

[0005] The above objects and advantages, as well as further objects and advantages, are attained by the present invention which may be described briefly as providing, in a knee prosthesis for implantation to replace a natural knee joint and emulate movements of the natural knee joint during articulation, the natural knee joint having a lateral compartment and a medial compartment, the knee prosthesis having a femoral component including at least one condylar element with a condylar surface having a transverse axis of rotation, and a tibial component including at least one articular surface for engagement with the condylar surface of the femoral component in one of the lateral and medial compartments for articulation of the knee prosthesis through flexion about the transverse axis of rotation: an improvement wherein the condylar surface and the articular surface are configured for enabling engagement between the condylar surface and the articular surface along a generally arcuate track having a center of curvature placed in a generally coronal plane, and relative rotational movement between the femoral component and the tibial component about a longitudinal axis of rotation during articulation about the transverse axis of rotation, with the longitudinal axis of rotation located essentially in a generally sagittal plane intersecting the coronal plane at an intersection spaced a prescribed distance from the longitudinal axis of rotation, and with the transverse axis of rotation maintained essentially in a generally medial-lateral longitudinal plane located in close proximity with the coronal plane, and with the longitudinal plane spaced from the longitudinal axis of rotation essentially by the predetermined distance and movable about the longitudinal axis of rotation in response to relative rotational displacement between the femoral component and the tibial component, to intersect the coronal plane at angles corresponding to the relative rotational displacement during flexion within at least a prescribed range of flexion extending up to a predetermined degree of flexion.

[0006] In addition, the present invention provides, in a knee prosthesis for implantation to replace a natural knee joint and emulate movements of the natural knee joint during articulation, the knee prosthesis having a lateral compartment and a medial compartment, a femoral component including a lateral condylar element with a lateral condylar surface, a medial condylar element with a medial condylar surface, and a transverse axis of rotation, and a tibial component including a lateral articular surface for engagement with the lateral condylar surface of the femoral component in the lateral compartment and a medial articular surface for engagement with the medial condylar surface of the femoral component in the medial compartment for articulation of the knee prosthesis through flexion about the transverse axis of rotation: an improvement wherein the condylar surfaces and the articular surfaces are configured for enabling engagement between the lateral condylar surface and the lateral articular surface at positions along a first generally arcuate track having a first center of curvature and between the medial condylar surface and the medial articular surface at positions along a second generally arcuate track having a second center of curvature, the first and second centers of curvature being placed in a common generally coronal plane, and relative rotational movement between the femoral component and the tibial component about a longitudinal axis of rotation during articulation about the transverse axis of rotation, with the longitudinal axis of rotation located essentially in a generally sagittal plane intersecting the coronal plane at an intersection spaced a prescribed distance from the longitudinal axis, and with the transverse axis of rotation maintained essentially in a generally medial-lateral longitudinal plane located in close proximity with the coronal plane, and with the longitudinal plane spaced from the longitudinal axis of rotation essentially by the predetermined distance and movable about the longitudinal axis of rotation in response to relative rotational displacement between the femoral component and the tibial component, to intersect the coronal plane at angles corresponding to the relative rotational displacement during flexion within at least a prescribed range of flexion extending up to a predetermined degree of flexion.

[0007] Further, the present invention provides a method for emulating movements of a natural knee joint in a knee prosthesis upon implantation of the knee prosthesis to replace the natural knee joint, the natural knee joint having a lateral compartment and a medial compartment, the knee prosthesis including a femoral component having at least one condylar element with a condylar surface having a transverse axis of rotation and a tibial component including at least one articular surface for engagement with the condylar surface of the femoral component for articulation of the knee prosthesis through flexion about the transverse axis of rotation, the condylar surface and the articular surface being located in the one of the lateral and medial compartments upon implant of the knee prosthesis, the method comprising: enabling engagement between the condylar surface and the articular surface at positions along a generally arcuate track having a center of curvature; placing the center of curvature in a generally coronal plane; enabling relative rotational movement between the femoral component and the tibial component about a longitudinal axis of rotation during articulation about the transverse axis of rotation, with the longitudinal axis of rotation located essentially in a generally sagittal plane intersecting the coronal plane at an intersection spaced a prescribed distance from the longitudinal axis of rotation; and maintaining the transverse axis of rotation in a generally medial-lateral longitudinal plane located in close proximity with the coronal plane, with the longitudinal plane spaced from the longitudinal axis of rotation essentially by the predetermined distance while moving the longitudinal plane about the longitudinal axis of rotation in response to relative rotational displacement between the femoral component and the tibial component, to intersect the coronal plane at angles corresponding to the relative rotational displacement during flexion within at least a prescribed range of flexion extending up to a predetermined degree of flexion.

[0008] Still further, the present invention provides a method for emulating movements of a natural knee joint in a knee prosthesis upon implantation of the knee prosthesis to replace the natural knee joint, the knee prosthesis having a lateral compartment and a medial compartment, a femoral component including a lateral condylar element with a lateral condylar surface, a medial condylar element with a medial condylar surface, and a transverse axis of rotation, and a tibial component including a lateral articular surface for engagement with the lateral condylar surface of the femoral component in the lateral compartment and a medial articular surface for engagement with the medial condylar surface of the femoral component in the medial compartment for articulation of the knee prosthesis through flexion about the transverse axis of rotation, the method comprising: enabling engagement between the lateral condylar surface and the lateral articular surface along a first generally arcuate track having a first center of curvature; enabling engagement between the medial condylar surface and the medial articular surface along a second generally arcuate track having a second center of curvature; placing the first and second centers of curvature in a generally coronal plane; enabling relative rotational movement between the femoral component and the tibial component about a longitudinal axis of rotation during articulation about the transverse axis of rotation, with the longitudinal axis of rotation located essentially in a generally sagittal plane intersecting the coronal plane at an intersection spaced a prescribed distance from the longitudinal axis of rotation; and maintaining the transverse axis of rotation in a generally medial-lateral longitudinal plane located in close proximity with the coronal plane, with the longitudinal plane spaced from the longitudinal axis of rotation essentially by the predetermined distance while moving the longitudinal plane about the longitudinal axis of rotation in response to relative rotational displacement between the femoral component and the tibial component, to intersect the coronal plane at angles corresponding to the relative rotational displacement during flexion within at least a prescribed range of flexion extending up to a predetermined degree of flexion.

[0009] The invention will be understood more fully, while still further objects and advantages will become apparent, in the following detailed description of preferred embodiments of the invention illustrated in the accompanying drawing, in which:

[0010] FIG. 1 is an exploded perspective view showing portions of a knee prosthesis constructed in accordance with the present invention;

[0011] FIG. 2 is an exploded side elevational view of the portions of the knee prosthesis;

[0012] FIG. 3 is a side elevational view of the portions of the knee prosthesis showing the femoral component of the prosthesis engaged with the tibial component, at 0 of flexion;

[0013] FIG. 4 is a partially diagrammatic top plan view of the prosthesis in the position shown in FIG. 3;

[0014] FIG. 5 is a cross-sectional view taken along line 5-5 of FIG. 4;

[0015] FIG. 6 is a cross-sectional view taken along line 6-6 of FIG. 4;

[0016] FIGS. 7 through 10 are views similar to FIGS. 3 through 6, respectively, but showing the components at 45 of flexion;

[0017] FIGS. 11 through 14 are views similar to FIGS. 3 through 6, respectively, but showing the components at 60 of flexion;

[0018] FIG. 15 is an enlarged, partially diagrammatic top plan view of the tibial component; and

[0019] FIG. 16 is a position diagram, with relative proportions modified and exaggerated to better show positions illustrated in FIGS. 3 through 14.

[0020] Referring now to the drawing, and especially to FIGS. 1 and 2 thereof, a knee prosthesis constructed in accordance with the present invention is shown at 10 and is seen to include a femoral component 12 having condylar elements illustrated in the form of condyles including a lateral condyle 14 and a medial condyle 16. Each condyle 14 and 16 includes a condylar surface 18 and 20, respectively, and the condylar surfaces 18 and 20 have a common axis of rotation T extending transversely across the femoral component 12. In the preferred embodiment, axis of rotation T is located on the femoral component 12 such that upon implant of the knee prosthesis 10, axis of rotation T will be coincident with a line extending transversely between the medial and lateral ligament attachment points on the femur of the natural knee. A tibial component 22 has articular surfaces including a lateral articular surface 24 for engagement with lateral condylar surface 18 of lateral condyle 14, within a lateral compartment 25, and a medial articular surface 26 for engagement with medial condylar surface 20 of medial condyle 16, within a medial compartment 27.

[0021] Knee prosthesis 10 is to serve as a total replacement for a natural knee joint. In the total knee replacement provided by knee prosthesis 10, both the anterior and the posterior cruciate ligaments are sacrificed, and knee prosthesis 10 includes a stabilizing mechanism 30 for stabilizing the engagement between the lateral condyle 14 and the lateral articular surface 24, and between the medial condyle 16 and the medial articular surface 26, during articulation of the knee prosthesis 10 within the range of articulation in which the posterior cruciate ligament ordinarily would provide stability in the natural knee. Stabilizing mechanism 30 includes a stabilizing compartment 32 on the femoral component 12, between the condyles of the femoral component 12, the compartment 32 preferably being located intermediate the lateral condyle 14 and the medial condyle 16 of the femoral component 12, and a stabilizing post 34 on the tibial component 22, between the articular surfaces of the tibial component 22, the post 34 preferably being located intermediate the lateral articular surface 24 and the medial articular surface 26 of the tibial component 22, for projecting in a superior direction into the stabilizing compartment 32, in a manner known in posterior stabilized prosthetic knee implants.

[0022] Turning now to FIGS. 3 through 6, viewed in conjunction with FIGS. 15 and 16, knee prosthesis 10 is illustrated at 0 of flexion, and the condylar surfaces of the condyles of the femoral component 12 are engaged with the articular surfaces of the tibial component 22. The condylar surfaces 18 and 20 and the articular surfaces 24 and 26 are configured such that upon implantation of the knee prosthesis 10, relative rotational movement between the femoral component 12 and the tibial component 22 during articulation of the knee prosthesis 10 better emulates the relative rotation observed in the natural knee. Thus, engagement between lateral condylar surface 18 and lateral articular surface 24 is so complementary, and preferably essentially congruent, as illustrated by the profile configuration contours shown in FIG. 6, at positions along a first generally arcuate track 40 having a first center of curvature 42 located in the medial compartment 27, and engagement between medial condylar surface 20 and medial articular surface 26 is so complementary, and preferably essentially congruent, as illustrated in FIG. 6, along a second arcuate track 44 having a second center of curvature 46 located in the lateral compartment 25, that the relative configurations of the condylar surfaces 18 and 20 and the articular surfaces 24 and 26 will enable relative rotational movement between the femoral component 12 and the tibial component 22 in such a manner as to emulate the relative rotation observed in the natural knee during articulation about the transverse axis T.

[0023] In order to achieve such emulation, the first and second centers of curvature 42 and 46 are placed in a common generally coronal plane CP. The relative rotation takes place about a longitudinal axis of rotation L located essentially in a generally sagittal plane SP which intersects the coronal plane CP at an intersection 48. Intersection 48 is spaced from the longitudinal axis L in the posterior direction by a predetermined distance D. The transverse axis of rotation T is placed essentially in a generally medial-lateral longitudinal plane LP, shown in FIG. 4 oriented at an angle A to coronal plane CP, angle A representing a relative rotational displacement between the femoral component 12 and the tibial component 22 of approximately 2 to 3, at 0 of flexion, in response to relative rotation between the femoral component 12 and the tibial component 22. The longitudinal plane LP is shown in close proximity with coronal plane CP, intermediate the lateral and medial compartments 25 and 27, the longitudinal plane LP being shown intersecting the coronal plane CP very near to the intersection 48 between the coronal plane CP and the sagittal plane SP. The intersection 48 is shown in a preferred location, at the midpoint between the first and second centers of curvature 42 and 46, with longitudinal plane LP being otherwise closely adjacent coronal plane CP. In the preferred construction, rotation of the longitudinal plane LP is about longitudinal axis L, with the longitudinal plane LP maintained essentially tangent with an arcuate path DP at a point of tangency PA and distance D serving as a constant radius of rotation, illustrated at DA. Thus, angle A represents the very small angular distance between the point of tangency PA and the intersection 48, and illustrates the even smaller distance between the intersection 48 and the intersection LCA between the longitudinal plane LP and the coronal plane CP. Distance D is determined empirically, the prescribed distance D being a measure of the posterior spacing of the flexion axis from the longitudinal axis of rotation observed in the natural knee.

[0024] As shown in FIGS. 7 through 10, viewed in conjunction with FIGS. 15 and 16, knee prosthesis 10 is at 45 of flexion. During articulation from 0 of flexion to 45 of flexion, the relative configurations of the condylar surfaces 18 and 20 and the respective articular surfaces 24 and 26, including the preferred essentially congruent profile contour configurations illustrated in FIG. 10, have enabled relative engagement at positions along arcuate tracks 40 and 44, and rotational movement between the femoral component 12 and the tibial component 22 about the longitudinal axis of rotation L, as illustrated in FIG. 8 by an angle B between the longitudinal plane LP and coronal plane CP, angle B representing a preferred rotational displacement between the femoral component 12 and the tibial component 22 of about 4 to 5 of rotation. At the same time, the relative configurations of the condylar surfaces 18 and 20 and the articular surfaces 24 and 26 have maintained the transverse axis of rotation T essentially within the longitudinal plane LP and have maintained the longitudinal plane LP in close proximity with coronal plane CP intermediate the lateral and medial compartments 25 and 27, with longitudinal plane LP preferably intersecting coronal plane CP very near to the intersection 48, which preferably is placed at the midpoint between the centers of curvature 42 and 46. With distance D serving as a radial distance DB and the longitudinal plane LP maintained essentially tangent to the arcuate path DP, the angular distance between intersection 48 and point of tangency PB, as represented by angle B, is very small and the distance between the intersection LCB of longitudinal plane LP with coronal plane CP and intersection 48 is even smaller. Longitudinal plane LP is otherwise maintained closely adjacent coronal plane CP.

[0025] As shown in FIGS. 11 through 14, viewed in conjunction with FIGS. 15 and 16, knee prosthesis 10 is at 60 of flexion. During flexion of knee prosthesis 10 from 45 of flexion, as illustrated in FIGS. 7 through 10, to 600 of flexion, the relative configurations of the condylar surfaces 18 and 20 and the respective articular surfaces 24 and 26 have enabled engagement and relative movement along arcuate tracks 40 and 44, and relative rotational displacement between the femoral component 12 and the tibial component 22 in response to relative rotational movement between the femoral component 12 and the tibial component 22 about the longitudinal axis of rotation L, as illustrated in FIG. 12 by an angle C between longitudinal plane LP and coronal plane CP. At the same time, the relative configurations of the condylar surfaces 18 and 20 and the articular surfaces 24 and 26, including the profile contour configurations shown in FIG. 14, have maintained the transverse axis of rotation T essentially within the longitudinal plane LP, and have maintained the longitudinal plane LP in close proximity with coronal plane CP, intermediate the lateral and medial compartments 25 and 27, with longitudinal plane LP preferably intersecting coronal plane CP at intersection LCC, very near to the intersection 48, which intersection 48 preferably is placed at the midpoint between the centers of curvature 42 and 46, and otherwise closely adjacent coronal plane CP. As before, longitudinal plane LP preferably is maintained essentially tangent to arcuate path DP, at a point of tangency PC spaced radially from longitudinal axis L a distance DC, preferably equivalent to distance D. The angular distance between the intersection 48 and point of tangency PC, as represented by angle C, is very small and the distance between the intersection LCC of the longitudinal plane LP with coronal plane CP and intersection 48 is even smaller.

[0026] In addition, during flexion from 45 of flexion to 60 of flexion, stabilizing mechanism 30 couples femoral component 12 with tibial component 22 to supply stability ordinarily provided in the natural knee by the posterior cruciate ligament, which cruciate ligament now is sacrificed. Thus, the post 34 projects upwardly, in the superior direction, into the stabilizing compartment 32 for coupling the femoral component 12 with the tibial component 22. The stabilizing mechanism 30 includes a cam surface 50 on the posterior aspect of the post 34, shown in the form of a posterior face 52 of post 34, and a follower surface 54 at an anterior aspect of a follower 56, shown in the form of an anterior face 58 of follower 56 extending transversely across the interior of the stabilizing compartment 32. The relative contour configurations of the cam surface 50 and the follower surface 54 enable relative rotation between the femoral component 12 and the tibial component 22 about longitudinal axis of rotation L to continue, to angle C between the longitudinal plane LP and coronal plane CP. At the same time, stabilizing mechanism 30 assists in maintaining the transverse axis of rotation T essentially in the longitudinal plane LP, and assists in maintaining longitudinal plane LP in close proximity with coronal plane CP, as flexion continues between 45 of flexion and 60 of flexion, as set forth above.

[0027] As described above, the transverse axis of rotation T is maintained essentially in the longitudinal plane LP during articulation through a prescribed range of flexion extending up to a predetermined degree of flexion. In the preferred embodiment illustrated in the form of knee prosthesis 10, the predetermined degree of flexion is about 60 of flexion and the prescribed range of flexion extends between about 0 of flexion and about 60 of flexion. In a portion of the prescribed range of flexion, shown as the portion between about 45 of flexion and about 60 of flexion, the stabilizing mechanism 30 couples femoral component 12 with tibial component 22 to supply stability ordinarily provided in the natural knee by the posterior cruciate ligament while, at the same time, assisting in maintaining the transverse axis of rotation T essentially in the longitudinal plane LP, and the longitudinal plane LP in close proximity with the coronal plane CP, intermediate the lateral and medial compartments 25 and 27, with longitudinal plane LP preferably intersecting the coronal plane CP, very near to intersection 48. To this end, the first and second centers of curvature 42 and 46 are located so that coronal plane CP is tangent to cam surface 50 at the intersection 48, and the intersection 48 is located at the midpoint between the centers of curvature 42 and 46. In addition, in the preferred construction the radius R1 of arcuate track 40 is equal to the radius R2 of arcuate track 44 so that the arcuate tracks 40 and 44 are essentially symmetrical about the intersection 48. Beyond 60 of flexion, engagement between the cam surface 50 and follower surface 54 induces rollback, and the transverse axis of rotation T is moved away from the coronal plane CP, in the posterior direction.

[0028] Further, as described above in connection with FIGS. 3 through 14 and 16, the tibial component 22 is displaced through angle C relative to femoral component 12, as the knee prosthesis 10 is articulated through the prescribed range of flexion, between about 0 of flexion and about 60 of flexion. The rotational displacement represented by angle C preferably is about 6 of rotation in an internal direction. As best seen in FIGS. 8 and 12, longitudinal plane LP is maintained essentially tangent to cam surface 50 along a segment 60, where the follower surface 54 engages the cam surface 50, throughout the portion of the prescribed range of flexion in which the stabilizing mechanism 30 couples the femoral component 12 with the tibial component 22, that is, between about 45 of flexion and about 60 of flexion. As set forth above, the centers of curvature 42 and 44 are located so that coronal plane CP is tangent to cam surface 50 at the intersection 48, preferably located at the midpoint between the centers of curvature 42 and 44. Segment 60 of cam surface 50 preferably is located along a cylindrical surface having a constant radius extending from the longitudinal axis L to cam surface 50, shown as DA, DB, and DC in FIG. 16, the radius preferably being essentially equal to the predetermined distance D, thereby maintaining an essentially constant distance between the longitudinal axis L and the longitudinal plane LP, that distance being equivalent to distance D. Longitudinal plane LP is maintained tangent to cam surface 50 at a respective point of tangency, shown in FIG. 16 as PA, PB and PC, located very near to the intersection 48 of sagittal plane SP and coronal plane CP, the angular distance between intersection 48 and the point of tangency PA, PB and PC at 0, 45 and 60 of flexion, respectively, as represented by angles A, B and C, respectively, being very small. The distance, as illustrated in FIG. 16 by PA, PB and PC, between the intersection of the longitudinal plane LP with the coronal plane CP, as shown by LCA, LCB and LCC in FIG. 16, and intersection 48 is even smaller. In this manner, articulation of knee prosthesis 10 mimics articulation of the natural knee within the prescribed range of flexion for better emulation of natural knee joint kinematics in knee prosthesis 10.

[0029] It will be appreciated that in view of the differences which exist in the physical characteristics and conditions encountered among the various recipients of knee implants, and the necessity for providing a finite number of sizes and configurations in femoral components and in tibial components to accommodate the needs of a particular recipient, as well as the nature and exigencies of surgery, the ideally precise relationships among the various engaged surfaces, axes and planes cannot always be realized fully in every recipient. Accordingly, the terms about, essentially and generally, as applied to the description of ranges of movement, the relationship between engaged condylar and articular surfaces, between axes and planes, and in the relative orientation of other elements of the described construction are meant to indicate that some departure from ideally precise relationships may be present without departing from the basic combination of elements which constitute the improvement of the present invention. Likewise, use of the terms close proximity, closely adjacent, very small and very near in referring to relationships between the longitudinal plane LP and the coronal plane CP is meant to encompass a combination of elements wherein some departure from an ideal relationship in which the position of the planes relative to one another is most effective is accommodated while still attaining the objects and advantages of the present invention.

[0030] It will be seen that the present invention attains the several objects and advantages summarized above, namely: Provides a knee prosthesis which better emulates movements of the natural knee for smooth knee flexion and extension; allows a recipient of a total knee prothesis to flex the knee easily and with less effort, while offering smooth prosthetic knee kinematics; enables the implant of a knee prosthesis utilizing current known surgical techniques while providing better prosthetic knee kinematics; provides a recipient of a total knee replacement with greater comfort and increased confidence in accommodating to the replacement; enables a more accurate emulation of the natural knee with a prosthetic knee having relatively few component parts, all of which are configured for simplified manufacture; provides an effective replacement for the natural knee, exhibiting exemplary performance over an extended service life.

[0031] It is to be understood that the above detailed description of preferred embodiments of the invention is provided by way of example only. Various details of design, construction and procedure may be modified without departing from the true spirit and scope of the invention, as set forth in the appended claims.