Complete knee joint prosthesis

Details Complete knee joint prosthesis Complete knee joint prosthesis

1999-09-01

2001-03-20

Philogene, Pedro (Department: 3732)

Prosthesis (i.e., artificial body members), parts thereof, or ai

Implantable prosthesis

Bone

C623S020180, C623S020260

Reexamination Certificate

active

06203576

ABSTRACT:

BACKGROUND OF INVENTION
The invention relates to a complete knee joint prosthesis intended to be implanted with ablation of the posterior cruciate ligament.
Such a prosthesis, known as a postero-stabilized prosthesis, generally consists of an element which is anchored in the end of the femur and has a shield which reproduces the femoral condyles and the trochlea of the natural joint, an element which is anchored in the end of the tibia and has an essentially plane upper wall perpendicular to the longitudinal axis of the tibia, and an intermediate plate which provides for the sliding of these two elements in relation to one another.
The intermediate plate comprises, on the side of the femoral element, two lateral glenoid cavities which receive the femoral condyles and, on the side of the tibial element, an essentially plane surface which comes to rest on the upper wall of the tibial element.
The plate also comprises a cam-shaped stud intended to be engaged in the trochlea of the femoral element, and the femoral element comprises a transverse intercondylar bar intended to come into abutment against the stud. This coming into abutment prevents posterior subluxation of the tibia, replacing the posterior cruciate ligament which has been removed.
The femoral and tibial elements are in general made of metal, while the intermediate plate is made of a material which favours sliding, such as high-density polyethylene.
During the movements of the joint, the femoral and tibial elements repeatedly exert great forces on the intermediate plate, which are applied in an unbalanced manner to a greater or lesser degree. In the long term, these forces cause wear and flow of the polyethylene plate, resulting in imbalance of the joint and abnormal stresses of the ligaments, and which may lead to loosening of the bone elements. This problem arises all the more because wear and flow increase as the femoral and tibial elements develop play in relation to the plate.
Furthermore, the contact between the stud of the plate and the intercondylar bar of the femoral element occurs relatively violently, in the middle of flexion, and takes place over a limited surface area, which contributes to wear of the plate and risks loosening of the elements.
These prostheses also suffer from problems of femoropatellar alignment, of limited mobility during flexion and of the existence of stresses during rotation which expose the prosthetic elements to loosening.
Prostheses exist, in which the plate is mounted pivotably in relation to the tibial element.
This mobility of the plate makes it possible to limit wear and flow of plastic plate, as well as limit the problems of femoropatellar alignment, limited mobility during flexion, and the existence of stresses during rotation without, however, eliminating them.
The aim of the present invention is to remedy this fundamental disadvantage of knee prostheses by providing a prosthesis which leads to minimal wear of the intermediate plate in the long term while retaining a mobility of the femoral and tibial elements which correspond to that of the natural joint.
SUMMARY OF THE INVENTION
The prosthesis to which the invention relates comprises, in a manner known per se, an element which is anchored in the end of the femur and reproduces the femoral condyles, an element which is anchored in the end of the tibia and has an essentially plane upper wall perpendicular to the longitudinal axis of the tibia, and an intermediate plate which provides for the sliding of these two elements in relation to one another, the plate being capable of pivoting in relation to the tibial element.
According to the invention, the following apply in combination:
the prosthetic condyles of the femoral element have a curvature in the shape of a circular arc in their rear part, and the femoral element has, between these condyles, a convex cylindrical wall which has an axis which coincides with the axis of the circular area in which the rear parts of the condyles lie;
the glenoid cavities of the intermediate plate have rear parts in the shape of a circular arc which have essentially the same radius, play excepted, as the rear parts of the condyles and an axis which coincides with that of the circle in which these same rear parts lie, while the intermediate plate comprises a central projecting rib in which a concave bearing surface in the shape of a circular arc is arranged, which has essentially the same radius, play excepted, as the convex cylindrical wall of the femoral element and has an axis which coincides with that of this wall, and
the intermediate plate or the tibial element has a cylindrical pin forming a pivot, while the tibial element or, respectively, the intermediate plate comprises a cavity of a section which is greater than the cross section of this pin, the cavity being intended to receive the pin with the possibility of multidirectional movement.
When the femoral element is positioned above the intermediate plate, the condyles engage closely in the glenoid cavities, and the convex cylindrical wall of the femoral element comes closely into contact with the concave intercondylar bearing surface of the intermediate plate.
The convex wall and the concave bearing surface actually constitute a “third condyle” which makes possible not only postero-stabilization of the prosthetic joint but also perfect guidance of the femoral element on the intermediate plate, about a distinct transverse axis and according to a smooth flexion movement, as well as the preservation of a large contact surface area whatever the degree of flexion of the joint.
The pin and cavity of the tibial element and intermediate plate make possible multidirectional movement of the plate in relation to the tibial element, so that the plate can follow the anteroposterior and lateral movements of the femur in relation to the tibia and remains permanently in close contact with the femoral element. In fact, the high degree of femorotibial congruence renders any rotation between the femoral element and the intermediate plate impossible. This rotation is permitted without limit, however, between the intermediate plate and the tibial element.
Thus, in the prosthesis according to the invention, the contact surfaces of the femoral element and of the intermediate plate perform no relative anteroposterior or lateral translation movement, nor any pivoting movement about a vertical axis. Rather the contact so only a pivoting movement about a distinct transverse axis.
The result is that the risks of wear and flow of the intermediate plate, as well as the risks of loosening of the bone elements, are greatly reduced in relation to existing prostheses.
Furthermore, the large contact surface area between the femoral element and the intermediate plate allows good distribution of stresses, which contributes to limiting of wear.
The coefficient of friction of the intermediate plate on the tibial element is reduced to the greatest possible extent by polishing the upper face of the tibial element. The contact surface areas are large, which also allows the wear of the plate to be reduced.
The multidirectional movement of the intermediate plate, and therefore the self-positioning or self-centring of this plate in relation to the tibial element, makes it possible to limit the anteroposterior or mediolateral stresses during rotation, to which the intermediate plate is subjected. It also has the advantages indicated above.
On a highly congruent prosthesis, the internal lateral ligament tightens during flexion and prevents complete flexion. The mobility during rotation and the self-centring of the plate make it possible release the lateral ligamentous tensions and therefore to increase the possibility of complete flexion of the joint.
The anteroposterior mobility makes it possible to restore a rolling-sliding movement to the prosthesis, that is to say a pure sliding movement at the femoral element/intermediate plate interface and a rolling movement of the femur in relation to the tibia, limited to a few millimeters. During flexion, the intermediate plate advance

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