IE42356B1 - Endoprosthetic knee joint devices - Google Patents

Abstract

1534263 Endoprosthetic knee joints NATIONAL RESEARCH DEVELOPMENT CORP 17 Nov 1975 [18 Nov 1974] 49795/74 Heading A5R An endoprosthetic knee joint device comprises: a femoral component 10 having a bearing body 11 in the form of a curved strip having a convex face which defines a part-spherical bearing surface 12 and a grooved concave face 13 formed with intracancellous stems 14 for securement to the femoral condyle; a tibial component 20 having a bearing body 21 in the form of a D-shaped platform having a planar bearing surface 22 and a fixation surface 23 formed with intra cancellous stems 24 for securement to the tibia; and a meniscal component 30 comprising a bearing body 31 in the form of a circular disc having one bearing surface 32 concavely part-spherically shaped to the same radius as surface 12 and a second bearing surface 33 planar. In use, one or two sets of the above components may be used in one leg.

Description

This invention concerns endoprosthetic knee joint devices.

Work on the hip joint has revealed a close association between the geometry of the articulatory surfaces, the load distribution across these surfaces, and the patterns of degeneration observed in the population. A fundamental feature of the hip joint is that the surfaces exhibit a fine incongruity which enables the contact areas to grow with increasing loads, and at large loads, when all the available cartilage on the acetabulum is in contact, the distribution of cartilage thickness appears to be such that a state of uniform pressure is achieved (A.S. Greenwald and J.J. O'Connor, The transmission of load through the human hip joint, J. Biomechanics, 1971, 4., 507 - 528).

Similar geometrical design has been discussed in the elbow joint and appears to be an important feature of joint design in man and other animals (J.W. Goodfellow and P.G. Bullough, The pattern of ageing in the articular cartilage of the elbow joint, and J.J. O'Connor, The relatiohship between degenerative changes and load bearing in the human hip, J. Bone and Joint Surgery, 1973, 55B, 746).

However, the knee joint appears to be an exception to this more general rule, unless it can be shown that the menisci play a significant role in the transmission of load. Work leading to conception of the present invention has demonstrated that this is, in fact, the case and that in the loaded situation the menisci do transmit, a considerable proportion of the load. Any resultant tendency for (he femoral condyles to push the menisci radially outwardly is resisted by circumferential tension in the menisci.

Now it is reasonable to presume that, among other things, an ideal prosthesis would reproduce the physiological range, modes and axes of movement, and also the normal pattern of load bearing of the natural joint. The mode and axis of movement in the human knee is complex and changing, and in any one position depends both upon the geometry of the articulatory surfaces and the direction and magnitude of the tensile forces developed in the associated muscles and ligaments. Accordingly, the ideal prosthesis can be regarded as reproducing as accurately as possible the geometry of the relevant surfaces, while interfering as little as possible with the continued function of the muscles and ligaments and, at the same time, maintaining a reasonably uniform distribution of load.

A review of previously proposed knee joint prostheses employed in clinical practice indicates that this ideal is not attained. Earlier proposals involve hinge devices which constrain the motion to simple pivotal rotation about a single axis. Such hinge devices have, in any case, led to an unacceptable failure rate. More recent proposals seek to remedy the failures of hinge devices, and involve two components which are respectively connected with the femur and tibia and held in engagement by the muscles and ligaments to produce a more life-like situation.

However, these more recent proposals face a contradictory 23 56 requirement in that matching of the engaged surfaces of the components afford uniform load distribution allows little freedom for variation in the mode and axis of movement, while incongruence between the surfaces to suit the requirements of movement necessarily 5 reduces and/or varies the surface contact area with consequently disadvantageous load distribution.

An object of the present invention is to reduce the difficulties of this situation by providing a device which allows a closer approach to attainment of the ideal.

To this end, the invention provides an endoprosthetic knee joint device, comprising a femoral condylar component for securement to the femur and having a convexly curved articulatory bearing surface, a tibial condylar component for securement to the tibia and having a relatively flattened articulatory bearing surface compared to that of said femoral component, and a meniseal component having two articulatory bearing surfaces in opposed disposition and of individual forms respectively substantially complementary to said femoral and tibial r omponent art i cu1at o ry surface s· In use of the invention the femoral and tibial components are to be secured respectively with the femur and tibia with the articulatory surfaces of these components in facing disposition, and the meniscal component will be located therebetween with its articulatory bearing surfaces in respective engagement with those of the former components. h The consequences of this arrangement are that: (1) The femoral and raeniscal components, and the meniscal and tibial components, are capable of independent relative movement by virtue of the complementary nature of the two pairs of engaged surfaces. More particularly, the femoral and meniscal components are capable of mutal rotation about three orthogonal axes, and the meniscal and tibial components are capable of mutual sliding in two of the relevant axial directions and mutual rotation about the third of such axial directions. The resultant capability for relative movement between the femoral and tibial components is accordingly extensive and can embrace rolling, gliding, twisting, and combinations thereof, such as are found in the knee joint. (2) The convex and relatively flattened shapings of the femoral and tibial component bearing surfaces can reproduce the natural shapings of the natural femoral and tibial condyles sufficiently closely that the complete interactions between the surface shapes and the forces in the surrounding muscles and ligaments, which control the stability of the joint, will be life-like. ' (3) The complementary nature of the engaged surfaces of the components in such that a uniform distribution of surface pressure can be. achieved in all positions of these components, (4) The meniscal component is entrapped between the femoral and tibial components by virtue of the complementary convex-concave shaping of the engaged femoral-meniscal component surfaces and the different, relatively flattened, complementary shaping of the engaged meniscal-tibial component surfaces. 423 50 Development of the invention since its initial conception has shown that, while a variety of prospectively advantageous forms are possible within the scope of the invention, as claimed hereinafter, the above consequences can result from a Simple form of the invention in which the engaged femoral and meniscal bearing surfaces are part-spherically shaped, and the engaged meniscal and tibial component bearing surfaces are planar.

In order that the invention may be fully and clearly understood, the same will now be described by way of example with reference to the accompanying drawings, in which:Figures 1 and 3 are respective mutually-perpendicular, schematic cross-sectional views of one embodiment of the above simple form of the invention, and Figure 3 schematically illustrates a modification of such embodiment.

The embodiment of Figures 1 and 2 is a bicondylar device comprising two sets of components parts for use in respective substitution for the lateral and medial condyles and meniscus of the natural joint. The two sets of components can be identical and so, for convenience, the structural form of only one set will be described.

In the relevant set, the femoral, tibial and meniscal components are respectively denoted by reference numerals 10, 20 and 30. Also shown in Figures 1 and 2 in broken outline are the associated distal ehd portions of the femur and the proximal end portion of the tibia at 40 and 5θ, respectively.

The femoral component 10 comprises a bearing body 11 in the •jei.rr.ii form of a longitudinally curved strip of which the convex face defines a part-spherical 1 y shaped surface 12 to serve as an articulatory bearing surface. The concave face of the body 11 serves as a fixation surface 13 adapted for securement to the femur in accordance with existing techniques so that the surface 12 serves as a femoral condylar surface replacement. In the present instance this securement is to be effected with the use of acrylic bone cement and for this purpose the surface is formed with one or 10 more intrac.ancellous stems 14 integrally projecting therefrom, and the surface l'J is also grooved.

The tibial component 20 comprises a bearing body 21 in the form of a D-shaped platform of generally uniform thickness. One face of the body 21 defines a planar surface 22 to serve as an articulatory bearing surface. The other face of the body 21 serves as a lixation surface 23 adapted for securement to the tibia, in accordance with similar techniques to those for the femoral component, so that the surface 22 serves as a tibial condylar surface replacement. For this purpose the surface 23 is formed 20 with one or more intracancellous stems 24 integrally projecting therefrom.

The meniscal component 30 comprises a bearing body 31 in the form of a circular disc. One face of this body is concavely part-spherically shaped to the same radius as the surface 12 of the femoral component to serve as an articulatory bearing surface 32 engaged with the surface 12. The other face of the body 31 is planar to si:rve as an articulatory bearing surface 33 engaged With the surface 22 of the tibial component.

In use of these components the femoral and tibial components are secured to suitably prepared.sites in the femur and tibia, and the meniscal component is then engaged therebetween. The overall surgical procedure need involve no special requirements since this procedure can be similar to those already developed for existing bicondylar knee joint devices such as the so-called 'Polycentric' device devised by Gunston.

A primary advantage of the illustrated device results from the first three consequences noted above, namely, that the movements of the natural joint can be closely simulated without significant distortion of the natural controlling and stabilising mechanisms while, at the Same time, uniform distribution of surface pressure is maintained through the device. In so far as the shaping of the component bearing surfaces do not precisely reproduce the natural condylar shapings, there can be some differences in the respective overall positional relationships between these shapings for a given operating condition of the muscles and ligaments during the flexion20 extension cycle. However, these differences will be very minor and this has been confirmed by a cadaveric trial in which the known characteristics of the natural joint movement were clearly exhibited.

Thus: the femoral component rotated with little translational movement relative to the tibial component during initial extension the femoral component continued to rotate with increasing forward translation of the meniscal component during further extension so 23 56 that the axis of femoral-tibial rotation moved rearwardly in the femur; the latter translation was accompanied by a transverse rotation near full extension so that the tibia twisted relative to the femur about the longitudinal axis of the leg aa in the so-called 'screw home' action; and a reverse sequence of events occurred during a flexion.

It is to be noted that, while this sequence of events entails relative rolling, sliding, and twisting between the femur and tibia, the components of the device are only subjected to sliding movements under uniform pressure distribution. Accordingly, as a further advantage, there will be no undue dynamic stresses leading to accelerated wear in the components.

Other advantages stem from the meniscal component. This component is readily replaceable by way of simple surgery to take account of wear. Further advantage can be taken of this fact by making the meniscal component of plastics material and the other components of metal so that the interfaced materials can provide known low friction properties, the secured components will be inherently stable, and the more likely long-term wear will occur In the more readily replaceable component.

A further advantage stemming from the meniscal component is that this component can be made available in a range of different thicknesses and the surgeon can select from this range to tension the capsule appropriately, particularly to avoid undue laxity, and also to correct any varus or valgus deformity.

Yet another advantage of the illustrated device is that the surgeon is relieved of the requirement to locate the femoral and meniscal components, in a closely prescribed positional relationship when securing the same. This requirement normally arises with existing devices, but the meniscal component of the present device serves as a self-adjusting intermediary to accommodate different positional relationships between the other two components.

While the invention has been described with more particular reference to the illustrated embodiment, it is not intended that the invention be limited thereby. The provision of the invention in a bicondylar device form is presently preferred since this form offers special advantages in the ability to retain the cruciate ligaments and the absence of an inherent requirement for patella detachment for the purposes of component securement. However, even in this form, the device can be varied from the illustrated embodiment.

One such variation is illustrated by Figure 3 in which modified tibial and meniscal components are shown. The modification involves the provision of a generally mushroom-shaped projection 2r> from the central zone of the tibial component bearing surface 22. This projection is engaged in an undercut recess 34 in the meniscal component. The mouth of the recess has a diameter significantly larger than that of the projection stem, but is slightly smaller than that of the projection head, so that engagement of the projection and recess involves a snap action whereafter relative sliding movement can occur in all directions of the surfaces 22 and 33 up to the limits imposed by abutment of the projection with the recess side wall* Preferably the projection and recess are of equal depth with planar surfaces at their respective free end and base, which surfaces are parallel to the surfaces 22 and 33 so that the former surfaces are slidably engaged when the latter surfaces are similarly engaged.

Although it is considered that the meniscal component of an embodiment such as that of Figures 1 and 2 can be adequately captively retained between the other two components, the modification of Figure 3 can provide enhanced stability without compromising the desired motion capabilities. The preferred sliding engagement between the projection and recess maintains the area of contact between the tibial and meniscal components relative to Figures 1 and 2, and in fact slightly increases this area.

A similar stabilising arrangement can be effected between the meniscal and femoral components by use of a projection from surface 32 and a slotted recess in surface 12. stability of the meniscal components can also be enhanced by elongating this component to an oval form in plan view so thut the curved area of contact with the femoral component, which is a basic cause of meniscal component entrapment, is increased. In use, this elongation is preferably maintained in a generally anteroposterior attitude and, for this purpose, it may be appropriate to provide the tibial component with a raised side wall, as’indicated at 26 in Figure 3, to limit the extent to which the meniscal component can spin. 23 5(: Enhanced stability in a lateral sense may alsb be provided in an implanted bicondylar device by mutually laterally inclining the interfaces between the tibial and meniscal component surfaces 22 and 33- While this can he effected by the provision of appropriately inclined sites in the tibia for tibial components of uniform thickness, generally wedge-shaped tibial components can be provided for use in more conventionally disposed sites.

Also, it will be appreciated that the proposed form of bicondylar device is riot limited to use of particular securement techniques or specific materials.

The invention is also applicable to other than bicondylar devices. In a simple fom the ittvehtion can be applied to a device having a singular set of components for the whole knee (as in the so-called Freeman-Swanson device); a bicondylar form can be modified by integrating some Or all of the corresponding components while providing slotted component structures for retention of the cruciate ligaments (as in the so-called 'Geomedic' device); the bearing Surface of the femoral component can be of varying curvature and differ in part from that with which it articulates (as in the Freeman-Swanson device); and the engaged surfaces of the tibial and meniscal components need not necessarily be planar, but these surfaces will be relatively flat compared to the other engaged surfaces. These variations will not necessarily afford all of the advantages discussed- above, but it is clearly 235 0 possible to obtain advantage relative to the nearest equivalent device among those previously proposed* Also, modification can be made by the use of resilient plastics material for the meniscal component ao that this 5 component can comply with changing geometry, particularly in an associated femoral surface, during articulation. Preferably, any such resilience should reside predominantly in the longitudinal axial direction relative to the leg, that is, generally perpendicularly relative to the meniscal component bearing surfaces, 10 while the component is relatively stiff circumferentially to restrain radially outward components of load as in the natural menisci. Such differential resilience may be achieved with a circumferentially fibre-reinforced plastics construction, such as silicone rubber within an annular sock of nylon or other synthetic

Claims (11)

1. An endoprosthetic knee joint device comprising a femoral condylar component for securement to the femur and having a convexly curved articulatory bearing surface, a tibial 5 Condylar component for securement to the tibia and having a relatively flattened articulatory bearing surface compared to that Of Said femoral component, and a meniscal component having two articulatory bearing surfaces in opposed disposition and of individual forms respectively substantially complementary to said 10 femoral and tibial component articulatory surfaces.

2. A'device according to Claim 1 wherein said femoral component surface and said meniscal component surface complementary thereto are each part-spherically shaped to equal radii of curvature.

3. A device according to Claim i or 2 wherein said tibial 15 component surface and said meniscal component surface complementary thereto are each planar.

4. A device according to Claim 1, 2 or 3 wherein each of said femoral and tibial component is of metal construction, and said meniscal component is of plastics material 20 construction.

5. A device according to any preceding claim wherein Said components each have two of the respective ones of said surfaces for respective location in the lateral and medial compartments of the knee joint.

6. A device according to Claim 5 wherein at least said meniscal component is subdivided into two discrete parts for respective location in the lateral and medial compartments of the knee joint.

7. A device according to any preceding claim wherein a part of said 5 components constituted by said meniscal component and one of said femoral and tibial components are linked by an integral projection from the respective bearing surface of one of said pair of components and a recess in the respective bearing surface of the other of said pair of components, said projection extending into said recess and being laterally movable 10 therein.

8. A device according to Claim 7 wherein said projection extends from said tibial components, and said recess is formed in said meniscal component.

9. A device according to Claim 8 wherein said projection is of 15 generally mushroom shape, and said recess is undercut to receive said projection by a snap action.

10. A device according to Claim 1 wherein said meniscal component comprises a disc of which the opposed faces define the respective bearing surfaces, the disc being made of resilient plastics material and 20 being reinforced around the peripheral portion thereof between said faces.

11. An endophosthetic knee joint device substantially as herein described with reference to Figures 1 and 2, or Figures 1 and 2 and modified by Figure 3, of the accompanying drawings.