GB2035090A

GB2035090A - Artificial knee joint

Info

Publication number: GB2035090A
Application number: GB7842357A
Authority: GB
Original assignee: Charnley Surgical Inventions Ltd
Current assignee: Charnley Surgical Inventions Ltd
Priority date: 1978-10-28
Filing date: 1978-10-28
Publication date: 1980-06-18

Abstract

An artificial knee joint comprises a ball-ended prosthesis (10) and a socket ended prosthesis (11) adapted to be mutually engaged, each prosthesis having a stem (12, 18) for insertion in and uniting (as by cementing) with a medullary cavity of one of the tibia and femur, the heads of the prostheses (10, 1) being so dimensioned that they can be implanted to leave substantial parts of the bearing surfaces of the condyles of the tibia and femur intact. <IMAGE>

Description

SPECIFICATION Artificial knee joint This invention relates to an artificial knee joint.

Known artificial knee joints are usually constructed so that the bearing surfaces of the condyles of the tibia and the femur are totally replaced by bearing surfaces of femoral and tibial prostheses. A ball and socket arrangement, having trunnions engaging lateral grooves for torsional stability, is known, but has to be of quite large size to bear the loads to which it is expected to be subjected. Further, large quantities of bone have to be removed from the femur and tibia. On other known artificial knee joints, the complete heads of the tibia and femur are replaced by prostheses which have a central locating and stabilizing ball and socket joint and lateral sliding and bearing surfaces for replacing the surfaces of the condyles.Both these types of prostheses are very large compared with the size of the knee joints and require considerable disturbance of the capsule of the joint for their insertion. Once one of these devices has been installed in a knee joint there is virtually no chance of the knee joint being returned to its former, albeit far from ideal, condition should the operation fail due to outside circumstances, such as incompatibility between the implanted prostheses and the tissues of the joint. In both types the ends of the femur and tibia are functionally irrevocably destroyed. Naturally, the relatively large size of these known prostheses means that they are expensive, because they have to be constructed mainly of costly metals, such as stainless steel a special alloy, which does not react with body fluids or tissue in any detrimental way.For these reasons, replacement of knee joints has not been very common, compared with the relative frequent replacement of malfunctioning hip joints.

An object of the present invention is to provide an artificial knee joint which is cheaper than known joints and whose installation requires relatively little destruction of the femur and tibia.

Accordingly the invention provides an artificial knee joint consisting solely of a femoral prosthesis and a tibial prosthesis, each prosthesis comprising a stem, for engagement with the medullary cavity of its associated bone, and a head, the head of the tibial prosthesis including a part-splenial ball and the head of the femoral prosthesis including a part-splenial socket, the heads being so dimensioned that each prosthesis can be attached to its respective bone whilst leaving a substantial part of the bearing surface of the condyle of that bone intact.

The knee joint of the invention is small compared with known joints, and thus is cheap because it needs less material. Further, it does not incorporate condyle replacement sliding surfaces which have hitherto been provided and which require expensive machining and polishing steps in their manufacture. As an additional advantage, the joint requires relatively little bone removal and the heads of the femur and tibia are not destroyed. Thus, if the prosthesis for any reason have to be removed, the joint can be restored to very close to its original state, that is the use of the prosthesis of the present invention is not an irretraceable step in the treatment of the limb.

Still further, the capsule of the joint is not disturbed by the insertion of the prosthesis to the same extent as has been necessary in the past with known total joint replacements.

In use, the prosthesis can be inserted to cause the contacting, diseased or worn-away, surfaces of the condyles of the femur and tibia to separate, placing the ends of the bones in their original positions, thus tensioning the capsule and ligaments of the joint to give stability. Under these conditions it is to be expected that the natural surfaces of the condyles will, relieved of the stess of contact, regenerate to some extent and eventually reassume their function as major lead bearers.

The ball can be of highly-polished metal (such as stainless steel) and the socket can be of high or ultra-high molecular weight polyethylene.

Advantageously a metal shell surrounds the socket and is integral with the stem. The socket can have a generally hemisperical portion and a part cylindrical portion standing therefrom in the axial direction away from the stem. This part-cylindrical portion can consist of an extension of the metal shell lined by an extension of the socket. This portion of the head is dispoed at the front of the knee joint in use and serves to guide tha ball back into the socket after it has partially or wholly left it after flexion of the knee past 90 . This particular design allows the knee to bend by more than 90 , which is a very important factor in giving a patient a natural knee function and gait after knee-joint replacement.

Preferably the axis of the ball is offset from the axis of the stem and the axis of the socket is offset from the axis of its stem.

The invention will be described further, by way of example, with reference to the accompanying drawings, wherein: Figure 1 is a perspective view, from the rear and one side, of ball and socket prosthesis of a preferred artificial knee joint of the present invention; Figure 2 is a rear view of the socket prosthesis in position in the lower end of a femur; Figure 3 is a perspective view, from the front and one side, of the ball prosthesis in position in the upper end of a tibia; Figure 4 is a perspective view from the front and one side showing the prosthesis and the bones in the configuration they take up when the knee is flexed to slightly more than 90 ; and Figure 5 is a view similar to that of Fig. 4, but when the knee joint is flexed still further.

A preferred embodiment of artificial knee joint conforming to the present invention comprises a ball or tibial prosthesis 10 and a socket femoral prosthesis 11. The tibial prosthesis 10 is of stainless steel or other inert alloy and comprises a stem 12, suitable for insertion into the medullary cavity of a tibia 13, integral with a neck 14 mounting a partspherical ball 15. The axis of the neck 14 and ball 15 is offset to lie to the rear of the median longitudinal axis of the stem 12 so as to produce a plateau 16 on the top of the stem 12 in front of the ball 15. On each side the stem 12 has a rib 17 for engaging a slot (not shown), cut in the bone of the tibia 13 and preventing rotation of the prosthesis 10.

The femoral prosthesis 11 is mainly of stainless steel and comprises a stem 18 and an integral shell or cup 19 at a wider end thereof. The axis of the cup 19 is offset to lie to the rear of the longitudinal median axis of the stem 18. The cup has a generally U shaped slot 20 in its rear side to leave it with a basically blind cylindrical upper portion 21 and a part cylindrical extension 22. The cup accommodates, by a friction-fit, snap-fit, or by the use of adhesive or fasteners, a socket 23 of high (or ultra-high) molecular weight polyethylene. The socket 23 has an upper part having a part-spherical cavity 24 complementary to the ball 15 and a part-cylindrical small portion 25 lining the extension 22. The cup 19 has two ribs 26 for engaging complementary slots 27 cut in the bone of a femur 28.

In use, (Figs. 2 to 5) a central hole is cut in the centre of the relevant bone 13, 28, leaving the bearing surfaces 29, 30 of the condye 31, 32 substantially intact, cement is placed into the medullary cavities and the prostheses 10 and 11 inserted into the tibia 13 and femur 28. The ribs 17 engage slots cut in the bone of the tibia 13 and the ribs 26 engage slots 27 in the bone of the femur 28.

This prevents rotation of the prosthesis 10, 11 relative to the tibia 13 and femur 28. The positioning of the prostheses 10 and 11 must be central of the condyles 31, 32 and the depths of penetration of their stems 12, 18 into the relevant medullary cavity are carefully selected so that the existing bearing surfaces 29, 30 of the condyles 31, 32 are spacedapart a distance corresponding to the distance by which the surfaces 29, 30 have moved from their original healthy condition. Thus immediately after insertion of the prostheses the surfaces 29, 30 are spaced apart as indicated at 33. This has the effect that the capsule and ligaments surrounding the knee are tightened and thus apply increased stabilizing and locating forces to the components of the knee joint.

Further, it is expected that the diseased or worn bearing surfaces 29, 30 of the condyles 31, 32, relieved of load bearing by the implanted prostheses 10 and 11 will eventually regenerate and re-assume some of their loadbearing and stabilizing functions. The aforementioned relative smallness of the prostheses 10 and 11 means that the capsule of the joint does not have to be too severely disturbed during insertion thereof. Further, the ends of the tibia 13 and femur 28 are not irrevocably destroyed, and should it ever be necessary to remove the prostheses 10, 11 for any reason, the knee can be returned to substantially its pre-implantation condition. The size of the prostheses 10, 11 compared to existing prostheses means that they are relatively cheap.

The particular shape of the socket 23 gives the installed knee joint (Figs. 4 and 5) a unique function in flexion of the knee beyond 90 (which many known prostheses do not permit at all). When such flexion occurs the ball 15 and the part spherical portion 24 of socket 23 separate, and relative movement of the ball is away from the socket 23 continues until it is clear completely of the portion 25 of the socket 23. Upon extension of the knee from this position the ball 15 is guided back into the socket by the particular shape of the head 10 of the femoral prosthesis 11 and the adjacent condyle of the femur. As will be seen from Fig. 4, the slot so accommodates the neck 14 during initial separation at or just greater than 90 . This feature of the implanted joint provides a particularly simple means of ensuring flexion to the full range of the natural knee without complicated and expensive machined and polished bearing surfaces.

The invention is not limited to the precise details of the foregoing and variations can be made thereto within the scope of the follow

Claims

ing claims.

1. An artificial knee joint consisting solely of a femoral prosthesis and a tibial prosthesis, each prosthesis comprising a stem, for engagement with the medullary cavity of its associated bone, and a head, the head of the tibal prosthesis including a part-spherical ball and the head of the femoral prosthesis including a part-spherical socket, and heads being so dimensioned that each prosthesis can be attached to its respective bone whilst leaving a substantial part of the bearing surfaces of the condyle of that bone intact.

2. A knee joint as claimed in claim 1, wherein the ball is of metal and polished and the socket of high (or ultra-high) molecular weight polyethylene.

3. A knee joint as claimed in claim 1 and 2, wherein a metal shell surrounds the socket and is integral with the stem.

4. A knee joint as claimed in claim 1, 2 or 3, wherein the socket has a portion bounding and defining a part-spherical cavity and a partcylindrical portion extending therefrom in the axial direction away from the stem.

5. A knee joint as claimed in claim 4 when appendant to claim 3, the part-cylindrical portion lining a part-cylindrical extension of the shell.

6. A knee joint as claimed in any preceding claim, wherein one or each prosthesis has one or more ribs for engaging complementary slots in the bone(s) to prevent rotation.

7. A knee joint as claimed in any preceding claim, wherein the axis of the head of the femoral prosthesis is offset to lie behind the median longitudinal axis of the stem thereof.

8. A knee joint as claimed in any preceding claim wherein the ball is connected to the stem by a neck.

9. A knee joint as claimed in claim 8, wherein the axis of the head of the tibial prosthesis is offset rearwardly of the medium longitudinal axis of the stem thereof.

10. A knee joint as claimed in any preceding claim, wherein the prostheses are dimensional to allow implantation thereof whilst leaving at least 60% of the surfaces of the condyles intact.

11. An artificial knee joint substantially as herein before described with reference to and as illustrated in the accompanying drawings.

13. Component parts for, and capable of being united to form, an artificial knee joint as claimed in any preceding claim and consisting of a ball prosthesis and a socket prosthesis.