IE52486B1 - Endoprosthetic bone joint devices - Google Patents

Abstract

An endoprosthetic bone joint, for example a knee joint, has a first and second component (1, 2) with convexly and concavely curved interengaging surfaces, the components being interconnected by a linkage member (5) which has an enlarged head (32) captively located in a cavity (23) in the first component. The cavity (23), which is preferably formed in a plastics insert (20) in the first component (1) has an elongate shape, straight or curved, which through a camming action causes flexion of the joint to be accompanied by a rearward sliding movement of the first component on the second component, increasing the range of permitted flexion of the joint.

Description

KHDOPKOSTHETIC BONE JOINT DEVICES This invention concerns endopror letic bone joint devices and is particularly, but not exclusively, applicable to endoprosthetic knee joint devices.

A known type of knee joint device which has been 5 in routine surgical use now for some years and which \has proved generally successful is described in U.K. Patent specification GB-A-1413477. This known device comprises, in broad terms: a first component having a convex bearing surface and a slot in such surface, which slot terminates in an enlarged cavity within said first component remote from said surface; a second component having a concave bearing surface in mutual articulatory engagement with said convex surface; a linkage member projecting from said concave surface into said slot to limit the range of said engagement, said member having an enlargement at its end remote from said concave surface, which enlargement is housed in said cavity to hold said linkage member captively located in said slot over at least a major portion of said range of engagement; and said components and linkage member being adapted for separation of the latter from one of the former in at least one relative disposition of said components outside said major portion of said range of engagement. When such a device is used as an endoprosthetic knee joint the first component is a femoral component adapted for securement to the femur, and the second component is a tibial component adapted for securement to the tibia. - 2 The basic benefits of such known joint devices include relatively unconstrained mobility for the purposes of flexion-extension and rotational movements of the jointdevices, while the two joint components are held in engagement by the ligaments and other natural elements. The linkage member affords lateral stability between the two components as in mechanically constrained 'hinged devices.

A difficulty which arises from the use of the aforesaid known joint devices is the fact that, in practice, the cavity in the first component is spherically shaped and the linkage member enlargement has a complementary spherical ball shape. This means that there is nt possibility for relative translational movement between the two components in any direction transverse to the longitudinal axis of the linkage member. It is now found that this missing possibility can be important, particularly in a knee joint, where relative movement in the anteroposterior direction has an effect on the range of flexion-extension of the joint. The reason for this is that, although in the natural knee joint and endoprosthetic joint devices of the type referred to the limit of flexion is determined by abutment of the femur and tibia, or entrapment of tissue therebetween, the attainment of this limiting condition is delayed upon flexion of the natural joint by sliding movement of femur on the tibia in a posterior direction. 5248G - 3 An object of the present invention is to provide for a similar relative sliding movement in joint devices of the type referred to.

According to the present invention there is provided 5 an endoprosthetic bone joint device comprising: a first component having a first bearing surface provided with a slot terminating within the component in 'an enlarged cavity; a second component having a second bearing surface 10 in articulatory contact with the first- bearing surface, and a linkage member projecting from the second surface into the said slot, said member having an enlarged head which is housed in said cavity to render the linkage member captive in the slot over a range of articulatory movement of the two components, the said cavity having an elongation such as to permit limited movement of the said enlarged head along the cavity, while remaining captive therein, to engender a predetermined relative sliding movement between the two components upon flexion of the joint device.

The cavity is preferably elongate in the plane of the slot,whereby an initial tendency towards a condition limiting flexion movement will naturally involve forces tending to cause translational movement of the first component relative to the second component, and the enlarged head of the linkage member will be free to undergo consequential relative movement within the cavity. 3486 - 4 The elongation of the cavity may be lineal. In an alternative arrangement the cavity is enlarged to allow the desired movement, but is shaped and . disposed such that, during flexion, the first component necessarily reacts against the linkage member to cause the desired movement. In such an arrangement the cavity is suitably of elongated curved form. In a device for < use as a prosthetic knee joint the cavity may have a curved elongate form with its convex and concave interior bounding surfaces respectively facing generally forwardly and rearwardly of the femoral component.

In preferred embodiments of the invention metal-toplastic surface contact occurs between the first and second components and between the enlarged head of the linkage member and the bounding surfaces defining the cavity.

Thus the cavity may be formed directly in the first component, but preferably the first component and the linkage member are made of biocompatible metal, and the cavity is formed in an insert of biocompatible plastics material located captively within the first component. The insert may be provided with key formations which enable it to be snap-engaged in a complementary recessed portion of the first component.

The invention will now be further described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic longitudinal sectional view of an assembled prosthetic knee joint device illustrating one embodiment of the invention; - 5 Figures 2 to 4 are, respectively, schematic side, front and longitudinal sectional views of the femoral component of the joint device shown in Figure 1; Figure 5 is a plan view of the femoral component 5 in the direction of arrow V in Figure 3; Figures 6 to 8 are, respectively, schematic side, longitudinal sectional and plan views of a femoral component insert used in the device of Figures 1 to 5; Figure 9 is a side elevational view of a linkage 10 member forming part of the said linkage device; Figures'10 to 12 are respective schematic side, front/rear, and longitudinal sectional views of the tibial component of the joint device shown in Figure 1; Figure 13 is an underneath plan view of the tibial 15 component in the direction of arrow XIII in Figure 10; Figure 14 is a schematic longitudinal sectional view of a modified femoral component, and Figure 15 is a; schematic longitudinal sectional view of a modified insert for use with the component of Figure 14.

The device of Figures 1 to 13 is an endoprosthetic knee joint comprising a femoral component 1 and a tibial component 2 which are secured respectively to the femur 3 and tibia 4 of a patient. The two components 1 and 2 are interconnected by a linkage member 5 as described later.

The illustrated femoral component 1 is made of a biocompatible metal such as a chrome-cobalt-molybdenum alloy. The component 1 comprises a tapered intramedullary 53486 - 6 stem 11 leading at its wider end into a bearing portion 12 defining a bearing surface area 13.

The area 13 includes two alike condyle-simulating convexly curved first bearing surfaces comprising ribs 14 transversely joined over corresponding parts of their lengths towards one end thereof by a patella-accommodating groove 15. It is to be noted that the ribs 14 have varying longitudinal ^curvature which is least over intermediate portions thereof and increased towards the ends as seen in Figures 2 and 4. The transverse curvatures are uniform.

The component 1 is formed with a generally sectorally shaped slot 16 of uniform width opening between the ribs 14, this slot extending angularly away from the inner end of groove 15 towards a point beyond the ends of the ribs 14.

This slot terminates within the component in an enlarged cavity which is itself the inner end of a substantially cylindrical recess 17 extending outwardly through the slot.

The recess 17 has a circumferential keyway 18 formed therearound partway along the length of the passageway, and also has a longitudinal keyway 19 on the side of the passageway remote from the groove 15. The keyways 18,19 are, of course, only present where the recess 17 enlarges the slot 16. The recess 17 is designed to receive an insert 20.

The insert 20 (Figures 6 to 8) is made of a biocompatible plastics material such as ultra-high molecular weight polyethylene. This insert has the general form of a hollowed plug engageable by a snap fit in the cylindrical recess 17. The exterior of the insert 20 is complementary 53486 - 7 with the recess 17, such exterior including a circumferential key 21 which snap-engages in the keyway 18, and a longitudinal key 22 to engage in the keyway 19 and orientate the insert 20 in a predetermined manner relative to the recess 17.

The hollow insert 20 has an internal cavity 23 . of generally elongate and curved form bounded by forwardly and rearwardly facing convexly and concavely curved interior surface portions 24 and 25 respectively adjacent to and remote from the longitidnal key 22.

The shape of the cavity 23 is best viewed as defined by the space followed by a notional spherical ball moving with its centre tracking an arcuate locus 26 indicated in chain line in Figure 7. The ball in question is to have a diameter greater than the width of the slot 16 in the femoral component. The cavity 23 is formed adjacent its lower end with a part-spherical seat 27 facing into the cavity. Also, the insert 20 is : ormed with a longitudinal slot 28 extending partway therealong from its lower end and opposite the key 22, this slot 28 intersecting the cavity 23 and forming a smooth inter-communication between the cavity 23 and the femoral component slot 16 when the insert 20 is fitted in the recess 17.

The linkage member 5 (Figure 9) is made of the same metal as the femoral component 1. This member 5 comprises a circular cylindrical rod 31 terminating at one end - 8 in a spherical ball shaped head 32, and having a relatively narrow flat 33 almost wholly along its length from the other end thereof. The rod 31 has a diameter slightly smaller than the width of the femoral component slot 16, and the head 32 has a diameter slightly less than that of the notional ball referred to above.

The overall forms and dimensioning of the insert \ 20 and the linkage member 5 are such that the head 32 of the latter can be engaged by a snap fit in the cavity 23 of the former, and that the insert 20 can then be engaged by the previously described snap engagement in the femoral component 1. The head 32 is then captively held in the insert cavity 23, but is movable between varying positions within the cavity generally along the locus 26 therein, while the linkage member 5 is rotatable about the centre of the head 32 in any such position subject only to the limits that the mouth of the slot 28 of the insert, together with the femoral component slot 16,impose on the related movement of the linkage member rod 31.

Turning now to the tibial component 2 of Figures 9 to 12, this is made of the same plastics material as the insert 20. The component 2 comprises a tapered intramedullary stem 41 leading at its wider end into a bearing portion 42 defining a bearing surface area 43. This area includes two similar condyle-simulating second bearing surfaces in the form of shallow troughs 4 4 transversely joined through a ridge 45. It is to be noted that in longitudinal section each trough 44 has - 9 a flat central portion, as shown in broken outline in Figure 10, curving upwards at the ends. The transverse curvatures of the troughs 44 are uniform and complementary with those of ribs 14, as shown in broken outline in Figure 11 The tibial component 2 is formed with a primary blind bore 46 which passes coaxially through part of the length ;of the stem 41 and opens into the ridge 45. This bore is of a diameter slightly greater than the linkage member rod 31. In addition, two secondary bores 47 of much smaller diameter enter respective ones of the troughs 44 adjacent to the bore 46 and pass through the component to communicate with the bore 46 adjacent its closed end.

The surgical fitting of the joint device involves respective securement of the femoral and tibial components 1, 2 to the femur 3 and tibia 4, after suitable preparation of these bones, using bone cement in established manner. This securement will be such that the ribs 14 and troughs 44 are located in substitution for the respective condylar surfaces, with the groove 15 being directed generally anteriorly and the longer freely projecting ends of the ribs 14 being directed generally posteriorly. The head 32 of the linkage member 30 is connected in the insert, and the insert 20 then snap-engages in the femoral component 1 as described above. The ribs 14 and the troughs 44 are then brought into bearing engagement, while at the same time the projecting rod 31 of the linkage member 5 is slidably received in the primary bore 46. - 10 This securement and engagement is to be such that at maximum extension the linkage member head 32 is located in the insert cavity 23withits centre towards the upper end of the locus 26, as shown in Figure 1, and the free end of the linkage member rod 31 is located towards the bottom of the primary bore 46.

Flexion of the fitted joint device from the extended 'position shown in Figure 1 involves, initially, mutual rotation between the ribs 14 and troughs 44 with associated rotation of the rod 31 in the slot 16, and, because of the varying longitudinal curvature of the areas 14, outward sliding of the rod within the primary bore 46. These movements continue until the rod 31 abuts the convex surface portion 24 of the insert cavity 23, whereupon further flexion causes posterior sliding of the rihs 14 along the flat portions of the troughs 44 and associated movement of the head 32 through the insert cavity 23. During this further phase of flexion the rod 31 will continue to move outwardly from the bore, but over the terminal flexion phase the rod may move inwardly of the bore, these movements being determined by the shaping of the insert cavity 23.

It will be appreciated that the overall flexion action involves mutual femoral-tibial rotation plus, in a latter portion thereof, rearward femoral translation. This closely simulates the natural joint movement and leads to a situation in which flexion is terminated at a later stage than may otherwise be the case and under conditions more like those in the natural joint. - 11 In other words, the range of permissible flexion of the.joint is increased compared with that permitted by known joint devices in which such translational movement does not occur.

Extension of the device reverses the above sequence of events with the concave profile of the insert cavity 23 reacting with the linkage member ball to translate the femoral component forwardly. This reversal also involves inward movement of the rod 31 in the primary bore 46, so that flexion-extension causes a reciprocation of the rod in the bore. The passageway created between the rod flat and the bore allows synovial fluid to enter the space at the bottom of the bore 46 as the rod 31 rises, and this fluid is then pumped through the secondary bores 47 to lubricate the rib and groove bearing surfaces 14 and 44 as the rod returns, these secondary bores having greater overall flow cross section than the fluid inlet path between the rod 31 and the wall of the primary bore 46.

Considering modification of the above-described embodiment to an alternative form, Figures 14 and 15 respectively show changes in the femoral component and its insert, other features being unchanged. Corresponding reference numerals are used as before, but increased by 100.

In the femoral component 101 the slot 116 is a little different in extent, basically to accommodate change in the disposition of the recess 117 and the form of the insert 120. The recess 117 has less forward inclination, - 12 being substantially parallel with the stem 111, and the keyway 119 is located adjacent the groove 115. In the insert 120, the cavity 123 differs from the cavity 23 by being wider and straight,rather than curved,and the slot 128 is of a lesser extent.

The modified form of device is used as before, Articulation is such that, as before,during flexion, the femoral component 101 rotates relative to the tibial component and the linkage member lifts to accommodate the varying curvature of the femoral component bearing surface areas. However, if, as flexion progresses towards its limit, natural conditions arise to terminate the rotation before abutment of the linkage member rod at the end of the slot 128 these conditions cause the femoral component 101 to move rearwardly across the tibial component. This last movement is made possible by the fact that the insert will have rotated about the linkage member ball and thereby allow translation of the ball generally axially within the insert hollow.

This translation can be accompanied by sliding of the linkage member in the tibial component bore in the event that the translation capability of the ball cannot match that of femoral component.

It will be understood that further variation is possible within the scope of the invention, as discussed more generally in the introduction. For example, in the first embodiment the insert so reacts positively against the linkage member 5 to cause translation in respectively - 13 different senses during flexion and extension; in a modification only one such reaction may be provided for, with the other translation occuring by natural action as in rhe second embodiment.

Another possibility is that a common femoral component can be made for receipt of alternative forms of insert, generally as illustrated.

Yet another possibility is that, as in some other ' endoprosthetic knee joint devices, the bearing surfaces of the tibial component may be planar, and the cooperating surfaces of the femoral component may have little or substantially no transverse curvature.

Claims (11)

1. An endoprosthetic bone joint device comprising: a first component having a first bearing surface provided with a slot terminating within the component in an enlarged cavity; a second component having a second bearing surface in articulatory contact with the first bearing Surface, and a linkage member projecting from the second surface into the said slot, said member having an enlarged head which is housed in said cavity to render the linkage member captive in the slot over a range of articulatory movement of the two components, the said cavity having an elongation such as to permit limited movement of the said enlarged head along the cavity, while remaining captive therein, to engender a predetermined relative sliding movement between the two components upon flexion of the joint device.

2. A device according to Claim 1, in which the cavity is elongate in the plane of the slot.

3. A device according to Claim 1 or Claim 2, in which the elongation of the cavity follows a curved path.

4. A device according to Claim 1 or Claim 2, in which the elongation of the cavity is linear. 53486 - 15

5. A device according to Claim 3, for use as a prosthetic knee joint, in which the first component is a femoral component and the second component a tibial component, the cavity having a curved elongate form with its convex and concave interior bounding surfaces respectively facing generally forwardly and rearwardly of the femoral component. \

6. A device according to any one of the preceding claims, in which metal-to-plastic surface contact occurs between the first and second components and between the enlarged head of the linkage member and the bounding surfaces defining the cavity.

7. A device according to Claim 6, in which the first component and the linkage member are made Of biocompatible metal, and the cavity is formed in an insert of biocompatible plastics material located captively within the first component.

8. A device according to Claim 7, in which the insert is provided with key formations which enable it to be snap-engaged in a complementary recessed portion of the first component.

9. A device according to any one of the preceding claims, in which the linkage member includes a rod which is slidable in' an axial blind primary bore in the second component, and in that at least one secondary bore in the said second component provides a flow passage interconnecting the blind end of said main bore and the bearing surface of the second component.

10. A device according to Claim 9, in which the secondary bore or bores have a greater overall flow cross section than that of the fluid path between the rod and the wall of the primary bore.

11. An endoprosthetic bone joint device substantially as herein described with reference to and as shown in Figures 1 to 13, or Figures 14 and 15 of the accompanying drawings.