Emulating natural knee kinematics in a knee prosthesis

A knee prosthesis and method for emulating movements, during flexion, of a natural knee joint replaced by the knee prosthesis, by enabling engagement and relative movement between the femoral component and the tibial component of the knee prosthesis along arcuate tracks, including rotational movement about a longitudinal axis of rotation, during flexion about a transverse axis of rotation, and maintaining the transverse axis of rotation essentially in a generally medial-lateral longitudinal plane maintained in close proximity with a generally coronal plane passing through the centers of curvature of the arcuate tracks during flexion within at least a prescribed range of flexion extending from about 0° of flexion to a predetermined degree of flexion, preferably about 60° of flexion. A stabilizing mechanism couples the femoral component with the tibial component within a portion of the prescribed range of flexion, the portion being between about 45° of flexion and about 60° of flexion, for providing stability and for assisting in maintaining the transverse axis of rotation essentially in the longitudinal plane, and the longitudinal plane in close proximity with the coronal plane as the longitudinal plane is rotated about the longitudinal axis of rotation.

BACKGROUND OF THE INVENTION

1. Field of the Invention

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.

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°.

2. Description of the Related Prior Art

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.

BRIEF SUMMARY OF THE INVENTION

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.

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 during articulation about the transverse axis of rotation for flexion within at least a prescribed range of flexion extending up to a predetermined degree of flexion while enabling relative rotational movement between the femoral component and the tibial component to take place about a longitudinal axis of rotation, the generally arcuate track having a center of curvature placed in a generally coronal plane, and the longitudinal axis of rotation being located essentially in a generally sagittal plane intersecting the coronal plane at an intersection, and being spaced a predetermined distance from the intersection such that upon flexion within the prescribed range of flexion, the transverse axis of rotation will be maintained essentially 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 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 the prescribed range of flexion.

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 during articulation about the transverse axis of rotation for flexion within at least a prescribed range of flexion extending up to a predetermined degree of flexion while enabling relative rotational movement between the femoral component and the tibial component to take place about a longitudinal axis of rotation, the first and second centers of curvature being placed in a common generally coronal plane, and the longitudinal axis of rotation being located essentially in a generally sagittal plane intersecting the coronal plane at an intersection and being spaced a predetermined distance from the intersection such that upon if flexion within the pesecribed range of flexion the transverse axis of rotation will be maintained essentially in a generally medial-lateral longitudinal plane located in close proximity with the coronal plane, and 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 the prescribed range of flexion.

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 during articulation about the transverse axis of rotation for flexion within at least a prescribed range of flexion extending up to a predetermined degree of flexion while enabling relative rotational movement between the femoral component and the tibial component to take place about a, longitudinal axis of rotation located essentially in a generally sagittal plane intersecting the coronal plane at an intersection and spaced a predetermined distance from the intersection; 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 the prescribed range of flexion.

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 during articulation about the transverse axis of rotation for flexion within at least a prescribed range of flexion extending up to a predetermined degree of flexion while enabling relative rotational movement between the femoral component and the tibial component to take place about a longitudinal axis of rotation located essentially in a generally sagittal plane intersecting the coronal plane at an intersection and spaced a predetermined distance from the intersection; and maintaining the trans-verse 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 the prescribed range of flexion.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawing, and especially toFIGS. 1 and 2thereof, a knee prosthesis constructed in accordance with the present invention is shown at10and is seen to include a femoral component12having condylar elements illustrated in the form of condyles including a lateral condyle14and a medial condyle16. Each condyle14and16includes a condylar surface18and20, respectively, and the condylar surfaces18and20have a common axis of rotation T extending transversely across the femoral component12. In the preferred embodiment, axis of rotation T is located on the femoral component12such that upon implant of the knee prosthesis10, 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 component22has articular surfaces including a lateral articular surface24for engagement with lateral condylar surface18of lateral condyle14, within a lateral compartment25of the knee prosthesis10, and a medial articular surface26for engagement with medial condylar surface20of medial condyle16, within a medial compartment27of the knee prosthesis10.

Knee prosthesis10is to serve as a total replacement for a natural knee joint. In the total knee replacement provided by knee prosthesis10, both the anterior and the posterior cruciate ligaments are sacrificed, and knee prosthesis10includes a stabilizing mechanism30for stabilizing the engagement between the lateral condyle14and the lateral articular surface24, and between the medial condyle16and the medial articular surface26, during articulation of the knee prosthesis10within the range of articulation in which the posterior cruciate ligament ordinarily would provide stability in the natural knee. Stabilizing mechanism30includes a stabilizing compartment32on the femoral component12, between the condyles of the femoral component12, the compartment32preferably being located intermediate the lateral condyle14and the medial condyle16of the femoral component12, and a stabilizing post34on the tibial component22, between the articular surfaces of the tibial component22, the post34preferably being located intermediate the lateral articular surface24and the medial articular surface26of the tibial component22, for projecting in a superior direction into the stabilizing compartment32, in a manner known in posterior stabilized prosthetic knee implants.

Turning now toFIGS. 3 through 6, viewed in conjunction withFIGS. 15 and 16, knee prosthesis10is illustrated at 0° of flexion, and the condylar surfaces of the condyles of the femoral component12are engaged with the articular surfaces of the tibial component22. The condylar surfaces18and20and the articular surfaces24and26are configured such that upon implantation of the knee prosthesis10, relative rotational movement between the femoral component12and the tibial component22during articulation of the knee prosthesis10better emulates the relative rotation observed in the natural knee. Thus, engagement between lateral condylar surface18and lateral articular surface24is so complementary, and preferably essentially congruent, as illustrated by the profile configuration contours shown inFIG. 6, at positions along a first generally arcuate track40having a first center of curvature42located in the medial compartment27, and engagement between medial condylar surface20and medial articular surface26is so complementary, and preferably essentially congruent, as illustrated inFIG. 6, along a second arcuate track44having a second center of curvature46located in the lateral compartment25, that the relative configurations of the condylar surfaces18and20and the articular surfaces24and26will enable relative rotational movement between the femoral component12and the tibial component22in such a manner as to emulate the relative rotation observed in the natural knee during articulation about the transverse axis T.

In order to achieve such emulation, the first and second centers of curvature42and46are 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 intersection48. Longitudinal axis L is spaced from intersection48in 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 inFIG. 4oriented at an angle A to coronal plane CP, angle A representing a relative rotational displacement between the femoral component12and the tibial component22of approximately 2° to 3°, at 0° of flexion, in response to relative rotation between the femoral component12and the tibial component22. The longitudinal plane LP is shown in close proximity with coronal plane CP, intermediate the lateral and medial compartments25and27, the longitudinal plane LP being shown intersecting the coronal plane CP very near to the intersection48between the coronal plane CP and the sagittal plane SP. The intersection48is shown in a preferred location, at the midpoint between the first and second centers of curvature42and46, 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 intersection48, and illustrates the even smaller distance between the intersection48and the intersection LCA between the longitudinal plane LP and the coronal plane CP. Distance D is determined empirically, the predetermined distance D being a measure of the posterior spacing of the flexion axis from the longitudinal axis of rotation observed in the natural knee.

As shown inFIGS. 7 through 10, viewed in conjunction withFIGS. 15 and 16, knee prosthesis10is at 45° of flexion. During articulation from 0° of flexion to 45° of flexion, the relative configurations of the condylar surfaces18and20and the respective articular surfaces24and26, including the preferred essentially congruent profile contour configurations illustrated inFIG. 10, have enabled relative engagement at positions along arcuate tracks40and44, and rotational movement between the femoral component12and the tibial component22about the longitudinal axis of rotation L, as illustrated inFIG. 8by an angle B between the longitudinal plane LP and coronal plane CP, angle B representing a preferred rotational displacement between the femoral component12and the tibial component22of about 4° to 5° of rotation. At the same time, the relative configurations of the condylar surfaces18and20and the articular surfaces24and26have 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 compartments25and27, with longitudinal plane LP preferably intersecting coronal plane CP very near to the intersection48, which preferably is placed at the midpoint between the centers of curvature42and46. 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 intersection48and 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 intersection48is even smaller. Longitudinal plane LP is otherwise maintained closely adjacent coronal plane CP.

As shown inFIGS. 11 through 14, viewed in conjunction withFIGS. 15 and 16, knee prosthesis10is at 60° of flexion. During flexion of knee prosthesis10from 45° of flexion, as illustrated inFIGS. 7 through 10, to 60° of flexion, the relative configurations of the condylar surfaces18and20and the respective articular surfaces24and26have enabled engagement and relative movement along arcuate tracks40and44, and relative rotational displacement between the femoral component12and the tibial component22in response to relative rotational movement between the femoral component12and the tibial component22about the longitudinal axis of rotation L, as illustrated inFIG. 12by an angle C between longitudinal plane LP and coronal plane CP. At the same time, the relative configurations of the condylar surfaces18and20and the articular surfaces24and26, including the profile contour configurations shown inFIG. 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 compartments25and27, with longitudinal plane LP preferably intersecting coronal plane CP at intersection LCC, very near to the intersection48, which intersection48preferably is placed at the midpoint between the centers of curvature42and46, 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 intersection48and 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 intersection48is even smaller.

In addition, during flexion from 45° of flexion to 60° of flexion, stabilizing mechanism30couples femoral component12with tibial component22to supply stability ordinarily provided in the natural knee by the posterior cruciate ligament, which cruciate ligament now is sacrificed. Thus, the post34projects upwardly, in the superior direction, into the stabilizing compartment32for coupling the femoral component12with the tibial component22. The stabilizing mechanism30includes a cam surface50on the posterior aspect of the post34, shown in the form of a posterior face52of post34, and a follower surface54at an anterior aspect of a follower56, shown in the form of an anterior face58of follower56extending transversely across the interior of the stabilizing compartment32. The relative contour configurations of the cam surface50and the follower surface54enable relative rotation between the femoral component12and the tibial component22about longitudinal axis of rotation L to continue, to angle C between the longitudinal plane LP and coronal plane CP. At the same time, stabilizing mechanism30assists 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.

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 prosthesis10, 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 mechanism30couples femoral component12with tibial component22to 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 compartments25and27, with longitudinal plane LP preferably intersecting the coronal plane CP, very near to intersection48. To this end, the first and second centers of curvature42and46are located so that coronal plane CP is tangent to cam surface50at the intersection48, and the intersection48is located at the midpoint between the centers of curvature42and46. In addition, in the preferred construction the radius R1of arcuate track40is equal to the radius R2of arcuate track44so that the arcuate tracks40and44are essentially symmetrical about the intersection48. Beyond 60° of flexion, engagement between the cam surface50and follower surface54induces rollback, and the transverse axis of rotation T is moved away from the coronal plane CP, in the posterior direction.

Further, as described above in connection withFIGS. 3 through 14and16, the tibial component22is displaced through angle C relative to femoral component12, as the knee prosthesis10is 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 inFIGS. 8 and 12, longitudinal plane LP is maintained essentially tangent to cam surface50along a segment60, where the follower surface54engages the cam surface50, throughout the portion of the prescribed range of if flexion in which the stabilizing mechanism30couples the femoral component12with the tibial component22, that is, between about 45° of flexion and about 60° of flexion. As set forth above, the centers of curvature42and44are located so that coronal plane CP is tangent to cam surface50at the intersection48, preferably located at the midpoint between the centers of curvature42and44. Segment60of cam surface50preferably is located along a cylindrical surface having a constant radius extending from the longitudinal axis L to cam surface50, shown as DA, DB, and DC inFIG. 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 surface50at a respective point of tangency, shown inFIG. 16as PA, PB and PC, located very near to the intersection48of sagittal plane SP and coronal plane CP, the angular distance between intersection48and 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 between the intersection of the longitudinal plane LP with the coronal plane CP, as shown by LCA, LCB and LCC inFIG. 16, and intersection48is even smaller. In this manner, articulation of knee prosthesis10mimics articulation of the natural knee within the prescribed range of flexion for better emulation of natural knee joint kinematics in knee prosthesis10.

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.

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.

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.