CA2201799A1 - Deep flexion knee prosthesis - Google Patents

Description

~2UI 7q9 DEEP FLEXION KNEE PROSTHESIS

Field of the Invention The present invention relates to the field of ortho-paedics. In particular, the present invention provides a prosthetic knee kinematically and anatomically resembling a human knee, and providing a user thereof with enhanced flexion capability as compared to other currently available knee prosthesis.

Background of the Invention A knee is made up, essentially, of four parts. The most distal portion is the tibia, which is the larger of the two lower leg bones. The upper surface of the tibia is a generally horizontally oriented plateau. Lining the proximal tibial and distal femoral surfaces are the second important part of the knee, the cartilaginous bearing surfaces, upon which physically rests the distal surfaces of the third portion of the knee, the femoral condyles. Each condyle (the medial, or inward one, and the lateral, or outward one) is generally a toroidal projection on the end of the femur, that can rotate over the bearing surfaces on the tibia. To achieve a wide range of flexion, however, the condyles do not simply rotate on the tibia. They also slide in an anterior and posterior direction, and then revolve generally latero-medially about a centero-medial zone on the tibial plateau. The patella, the fourth part of the knee, in a muscle-tendon mechanism contacts the condyles anteriorly, acting as a pully for enhancing knee extension.
To prevent the femur from sliding off the tibia, the patella is positioned anteriorly of the condyles, between them and connected to the tibia and femur by the-patella tendons and quadriceps muscles respectively.

Prior art prosthetic knee designs have accounted reasonably well for rotational and posterior-anterior sliding.
For instance, in U.S. Patent No. 5,282,868 (Bahler) there is described a prosthetic knee that specifically addresses the need for the femoral prosthetic to slide anteriorly while it rotates. The femoral part may, moreover, revolve slightly about its central longitudinal axis. This feature, however, does not enhance flexion.

Similarly, in U.S. Patent No. 5,314,483 (Wehrli et al) a knee prosthesis is described which is capable of sliding and rotation, and which is capable of limited rotation about a centrally located axis. It is similarly limited in flexion.

It is the object of the present invention to provide a prosthetic knee that flexes over a range approximating ordinary knee flexion. In particular, an object of the present invention is to provide a knee capable of flexion in the area ~'20 ~ ;~S9 of 160~, which represents an enhanced flexibility of 15~ - 30~
over currently available prosthetic knees.

A further object of the present invention is to provide a femoral prosthesis that is adjustable in size.

5Twelve pages of drawings illustrating the present invention are attached hereto.

Detailed Description of the Drawings As the drawings illustrate, the present invention provides a prosthetic knee including a femoral condylar element, a 10tibial plateau element, a bearing element that may be in one or two pieces, and a patellar lining. In many instances, the patella does not require the lining of the present invention.

The femoral component has condylar bearing surfaces that extend posteriorly a far greater extent than those on prior art 15femoral prosthesis. This permits the rotation of the femur on the tibial plateau over a greater arc, for enhanced knee flexion. As can be observed from the drawings, however, in order for the femoral prosthesis to be fitted on the femur, the end of the femur is cut to a profile conforming to the interior 20of the prosthesis. Since the prosthesis must be slid over the end of the femur, however, the inner anterior and posterior surfaces of the prosthesis must be parallel or divergent.

7 q 9 Therefore, the implant is applied in an orientation of flexion at a specific angle with respect to the femoral shaft, thereby providing full posterior condylar coverage as well as coverage of the distal anterior parts. The femoral component also provides a deeply set anatomically curved anterior central and distally placed groove for appropriate articulation and accommodation of the patella throughout the flexion range.
This articulation begins as the patella enters the groove at the start of flexion with zonal contact and at terminal flexion with two regions of contact medially and laterally for the condylar separations beyond the intracondylar notch. (The notch anatomically separating the condyles and providing attachment of the cruciate ligaments to the femur.) The femoral component has suitably curved condylar surfaces, generally rounded posteriorly for matching interface with plastic bearings whose form and extent provides conforming contact with the femoral condyle over the entire postero-proximal articular regions (uncovered in current designs) and vitally needed to provide full flexion.

The tibial base plate is generally flat and more or less oval outline having a central stem for bone fixation and a postero-central cut out to preserve the posterior cruciate ligament (PCL) and as needed, the anterior cruicate ligament (ACL). Its articulating surface for a plastic bearing is f~ 9~

uniquely formed to match the asymmetric anatomic geometry of the medial and lateral femuro-tibial compartments.

Several specific embodiments of the present invention are presented in the drawings. Each maintain features separating medial to lateral parts by design difference to restore the specific separate functions that each compartment normally provides in axial rotation and flexion; the lateral mobility - antero-posteriorly thereby being greatly enhanced compared to the medial.

In both, the plastic bearing interface with the femoral condyles is conforming such that the posterior-distal convexity of condyles mates against concavities of each bearing with sufficient clearance to allow only a jog of displacement but ease of sliding rotation. In both, each medial and lateral plastic part articulates with the tibial base plate separately in such a way as to provide (A) MEDIALLY mainly loading and some limited sliding, rotation motion between convex distal plastic surfaces and matching concave proximal tibial surface, the radius of curvature of same bearing plastic-tibial interface being greater than the matched radii defined in curved surfaces of femur and bearing, and (B) LATERALLY
extended motion in which said bearing's distal surface is concave to match a gently rounded convex surface of the lateral tibial eminence whose radius is greater than the matched more 22~ 9~

proximal bearing femoral surface. The convex-concave medial (dished) interactive with the concave, convex lateral (compressed dumbbell form) bearings provides a means for rotation of the bearings on their tibial surfaces, and to each other and, by nature of conforming contact with the femur they thereby provide for femoral rotation about a natural anatomic axis located centero-medially.

The provision of axial rotation restores the requirement for normal knee motion seen in full extension as 'locking home' means for external tibial rotation, axial rotation in gait and of critical importance for these embodiments, internal tibial rotation during terminal parts of deep flexion.

In one embodiment, the bearings are made of plastic re shaped much as discs, medial being concave, convex (proximally - distally) and lateral concave-concave (dumbbell shape), having varying thickness of plastic and with the contour margins of bearings extending sufficiently anterior and pastorally combined with the large surface areas to provide for stability once inserted between elastically separated articulating surfaces of femur and tibia. Such elastic separation is the natural occurrence due to intact collateral, posterior cruciate and capsular structures. These are further supported by musculotendinous structures coopting joint surfaces and also providing motion. This design affords a 22~ 1 799 -'self-orienting' mechanism for bearing location between articulating metal parts.

This embodiment is highly reliant on carefully balanced soft tissues and will have most stability in circumstances when both Anterior cruciate ligament (ACL) as well as Posterior cruciate ligaments (PCL) may be preserved.

In another design embodiment the plastic bearings are shaped distally in the form of a dovetail whose form is curved in horizontal plane towards the centre of rotation in the midcondylar portion of the tibia and curved in a second dimension orthogonal to the first (mainly sagittal) to fit a matched female groove in medial and lateral tibial parts. Such captive mechanisms are designed to restrict sideways motion enough to prevent escape of the bearing. But, the tolerance of surfaces are such as to allow free sliding motion of the bearing in its curved dovetail groove, forwards and backwards and in part circular to radius centre. This embodiment is likely of greater application in arthritic cases that lack an ACL, but will maintain PCL. The sagittal curves have the same orientation of radius centres (opposite each other) as in unrestricted form.

In a third embodiment, a combination of bearing shapes is provided. Here a bearing, captive laterally by means upwardly _ ' ~0 t 7~9 and inward curved dovetail groove, is matched to an uncaptured medial bearing, convex-concave femoral surface and convex-concave tibial. Alternatively, a medial convex-concave femoral surface may be linked to a concave-convex tibial.

In the total knee replacement described the patella may or may not be resurfaced. In the latter event a resurfacing design is provided that will articulate in a zonal congruent pattern with the antero-distal circular femoral groove by a matching congruent surface plastic bearing. During maximal flexion the patella angle with respect to the femur changes and would induce 'lift-off' of said bearing if rigidly fixed to the cut bone posterior surface of the patella. Therefore, the design is such to allow a small element of motion, in three directions between the plastic bearing and a dish-shaped metal base plate attached to the patella. In one form the radius of curve is designed greater than the radius of the patella groove convex plastic to concave metal patella base plate. In another, a reversed configuration is described, concave plastic to convex metal base plate. These options may be chosen to best fit the nature of available bone stock. To prevent bearing dislodgement, a captive mechanism is designed as a central 'collar stud' of plastic that snaps into a centrally located recess in the metal base plate. Sufficient clearance is provided between the opening margin and neck of the plastic collar to permit motion.

It will be understood, moreover, that although a total knee replacement is described, it is feasible to utilize only a portion of the prosthesis. For instance, if only either the medial, or lateral condyle is damaged, it is not necessary to replace both. Any portion of the present invention may be used independently of the other. In particular, all medial components, or all lateral components may be implanted, without affecting the remainder of the knee.

It is to be understood that the examples described above are not meant to limit the scope of the present invention. It is expected that numerous variants will be obvious to the person skilled in the field of orthopaedic prosthesis design without any departure from the spirit of the invention. The appended claims, properly construed, form the only limitation upon the scope of the invention.

Claims