IE46519B1 - Endoprosthetic joints - Google Patents

Abstract

The endoprosthesis for metacarpal joints or similar hinge joints comprises two components (11, 21) one of which (21) possesses a hook-shaped end which engages a bearing part held in a fork-shaped head of the other component (11). The two components are also provided with complementary elements, for example cam-like projections and shoulders, interaction of which progressively delimits the lateral movement of the joint components when the joint is folded. The artificial joint is expediently made of physiologically inert plastic materials. In a preferred embodiment the fork-shaped end of the one component is designed such that it can be connected with the bearing part of the other component by snap-on engagement.

Description

This invention relates to endoprosthetic joints and. more particularly, to endoprosthetic joints for ginglymus joints, especially metacarpophalangeal joints.

Ginglymus joints with which the invention is concerned are joints of a hinge-like character and in man include in particular the interphalangeal (finger and toe) and elbow joints, in addition to the metacarpophalangeal (knuckle) joints. It is to be understood that although the invention has been particularly developed for, and will be described with respect to the metacarpophalangeal joint, it may also be suitable for application to other ginglymus joints.

Progressive degeneration of the joints is a particularly serious problem in the disease states of rheumatoidand osteo-arthritis. The effects of rheumatoid-arthritis in particular are often especially marked on the metacarpo- and inter-phalangeal joints of the hands with consequent serious impairment of manipulative capacity.

Surgically implanted artificial replacement joints (endoprosthetic joints) are now commonly used to replace such defective natural joints. However the currently available endoprosthetic metacarpophalangeal joints (hereinafter referrred to as MCP-joints) are relatively few in number and not without disadvantages. The majority of these disadvantages stem from the inherent difficulty of providing the MCF-joint with the articulation capability of the natural joint, which, although primarily capable of flexion-extension (longitudinal bending), is also capable of limited abduction-adduction (lateral bending) such that it is at a maximum when the joint is in the extended position and at a minimum when the joint is in the flexed position.

Several of the currently used MCP-joints have component parts which are designed to be linked and held together wholly by the natural joint capsule and associated ligaments. Such MCP-joints suffer from the disadfantage that the natural joint capsule and ligaments especially in arthritic 10 joints, are frequently seriously distorted from their normal condition. In addition because of their reliance on the natural joint capsule and ligaments to link them together, such joints cannot be fully used until recovery is complete from the surgery associated with their implantation in the body. Also because of their indirect mechanical .linkage, such joints cannot be fully used until recovery is complete from the surgery associated with their inplantation in the body. Also because directly mechanically linked type for example in the form of a hinge 20 and pin. However such joints are also not without disadvantage in that they do not dislocate even under excessive load, because of their direct mechanical linkage. As a consequence, the bone ends adjacent to the joint are often broken, or the seating of the endoprosthesis in the bone ends is loosened.

Such joints are also normally effectively constrained to permit pivotal motion about one axis and thus cannot simulate the abduction-adduction capability of the natural joint.

A further currently employed variant of the conventional hinge - 3 46519 joint is of one piece construction from resilient plastics material, such as a silicone elastomer. These particular joints are relatively weak mechanically, so that in general only light tasks can be performed. They are capable of virtually unrestricted longitudinal and lateral flexing and often fracture along the line of repeated flexing. In addition, because of their resilience, they are relatively difficult to locate positively in the bone-ends.

Yet further currently used MCP-joints are of the 10 ball-and-socket general type, for example, in which a protruberant ball on one component is in snap engagement in a socket of the other component. Such joints are normally . capable of smooth pivotal motion about more than one axis without excessive wear occurring to the bearing surfaces even under high loading but are generally relatively easy to dislocate in normal use. However, such joints have so farbeen of relatively complex construction in order to simulate the limited abduction-adduction capability of the natural joint when in the extended position, whilst also providing bearing surfaces of sufficient smoothness and sufficient structural tolerance to minimise joint wear in use, and consequently have not been particularly adapted to ease or flexibility of manufacture.

It is an object of the present invention, therefore, ' to provide an endoprosthetic ginglymus joint which is capable of approximating to the articulation characteristics of the normal natural joint without ready dislocation during normal use, and which is adapted to relative ease and flexibility of manufacture. ry - if 46519 According to the invention therefore there is provided an endoprosthetic ginglymus joint comprising mutually cooperating first and second components adapted to be attached to the respective bones to be inter-connected by tl· joint; the first component including a bearing support member having a bifurcated head, attached to and bridging the furcations of which is a bearing having a first bearing surface of convex circumferential form symmetrically disposed about an axis passing through the furcations; the second component including a hooked member, having a hook adapted on its radially inner surface to define a second bearing surface of such generally complementary shape to said first bearing surface as to be capable of engagement therewith for cooperative pivotal motion between said first and second components about the bearing; and said furcations and said hooked member being formed with structures which are capable of mutually interacting to restrict the pivotal motion of the joint to pivotal motion about said axis passing through aaid furcations.

The first bearing surface may be of convex circumferential and convex axial form,for example spheroidal form, which is at present preferred, and in which case, the second bearing surface is provided by an arcuate, bngitudinally concave bearing groove, recessed on the radially inner surface of the hook. Alternatively, the first bearing surface may be of convex circumferential and concave axial form, for - 5 46519 example in the form of an axially concave bobbin, and in which case the second bearing surface is provided by the radially inner surface of the hook having a concave longitudinal, convex transverse form with a radius of convex curvature less than the radius of concave curvature of the bobbin.

It will be understood that the overall dimensions of the hook are necessarily limited by the overall dimensions of the bifurcated head to allow 3uch cooperative pivotal movement of the first and second components about the bearing as approximate to the flexion-extension and limited abductionadduction capabilities of the natural ginglymus joint.

In a preferred modification of the invention the hook is sufficiently resilient, for example by construction from physiologically inert resilient plastics material, for example high molecular weight polyethylene, and the second bearing surface is of such an angular extent, for example it subtends an angle between l8o° and 200°, that the hook is capable of snap engagement over the first bearing surface so as to be positively retained thereover and yet permit cooperative pivotal motion between the first and second components as stated hereinabove. This modification is particularly advantageous in that it facilitates connection of the joint components during the surgical implantation of the endoprosthetic joint, and generally results in pivotal movement of the joint being possible at an earlier time after implantation than with a joint relying primarily on the natural joint capsule, and associated tendons to hold it together. - 6 46518 It is also to be understood that the term « «I spheroidal includes surfaces which are perfectly spherical in addition to surfaces which are generated by revolution of an ellipse about either its major or ικ^,,ογ axis.

The particular structures on the furcations and hooked member which are capable of mutually interacting to r strict the pivotal motion of the joint to pivotal motion about the axis passing through the furcations, and which are at present preferred, are a matching cam-like extension, eccentrically disposed on the end of each furcation and,on the hooked member, a flanged shoulder which extends outwards laterally from the hooked member; each of ouch a size and in such a position as to be capable of mutual interaction on pivotal motion of the joint about the axis passing through the furcations.

The first and second components of the joint may be adapted to be attached to respective bones to be connected by the joint by any conventional means known in the art, which results in a positive, stable location of the joint. Thus for example they may be provided with a stem, for example an elongate stem, which tapers towards a relatively narrow boneproximate end and which is longitudinally grooved along its periphery to facilitate retention, and to minimise rotation, within the intramedullary canal of the particular bone to be connected by the joint. Alternatively they may be provided with a stem having,for example,a screw-thread or laterally disposed peripheral ridges about part of its length to facilitate engagement within the intramedullary canal. As a further - 7 46519 alternative they may be provided with a stem having a semi-porous surface into which the natural bone-cells can grow following implantation of the joint, to facilitate positive location in the bone. Although elongate stems are generally preferred other generally known types of stem may also be suitable and, in addition, the above mentioned alternatives may be used -in combination one with another.

In order that the invention may be more clearly understood, two particular embodiments of an MCP-joint according to the invention will now be described, hy way of example only and with reference to the accompanying drawings in Which:Figure 1 is an underside plan view of a first embodiment of the MCP-joint; Figure 2 is a longitudinal sectional view along the line 2-2 in Figure 1; Figure 3 is an underside, partial plan view of the metacarpal component of the first embodiment; Figure ή is a transverse sectional view along the line ή-4 in Figure 3; Figure 5 is a partial side elevation of the proximal phalangeal component of the first embodiment; Figure 6 is a transverse sectional view along the line 6-6 in Figure 5; Figures 7 and 8 are partial fragmentary views of the first embodiment shown in the extended, and flexed position, respectively; and Figure 9 is an underside plan view of an alternative - 8 46519 embodiment of the MCP-joint.

The illustrated metacarpal component of the first embodiment comprises a bearing support member (11) of integral construction, for example from physiolvgically inert plastics material, having a bifurcated head of generally U-shaped aspect, the base (12) of which forms a bone-end stop and from which extends an elongate stem (13). The stem '-.'.lich is of smaller cross-sectional area than the head and tapex's towards a narrow rounded end (1*0,is provided with two diametrically disposed longitudinal grooves (15) to facilitate its insertion and location within the intramedullary canal of the metacarpal bone to be connected by the joint. The groove (15) may be of generally curved cross-section, for example as depicted in Figure ll.

The bifurcated head has two, substantially parallel plate-like furcations (16) (which constitute the side-arms of the U-shape) formed in mirror-image relationship, attached to and bridging which is a bearing (17) having a generally spherical bearing surface. The bearing is provided with two diametrically disposed cylindrical axles (18), each having a keyed free end (19) adapted to locate positively in a complementary hole near the centre of the face of each furcation, and shaped to prevent rotation of the bearing in use. The axles may alternatively be of tapering cylindrical form such that their free ends are wider than the ends adjoining the bearing surface, in order to provide for a more secure attachment of the bearing to the furcations. rr Each of the plate-like furcations has a partly curved outer end, generally centred on the point at which an a'xle of the bearing is attached, and of substantially constant radius of curvature for part of its curved extent and, for the remainder, of increasing radius of curvature so that each furcation is provided at its end with a matching , eccentrically disposed cam-like extension (20).

The illustrated proximal phalangeal component of the first embodiment is conveniently of integral construction, for example, from physiologically inert resilient plastics material, and comprises a hooked member (21) having a resilient hook (22) at one end and an elongate stem (23) of narrower cross-sectional area than the hook, adapted for retention within the intramedullary canal of the proximal phalangeal component to be connected by the joint, in a similar manner to that described above for the metacarpal component. The hook (22) and stem (23) are joined through a flanged shoulder (24) which extends outwards from the base of the hook to provide two symmetrically disposed curved ridges (25) on either side of, and generally curved in the same direction as the hook (22), The hook (22) itself is recessed on its radially inner surface to define an arcuate, longitudinally concave bearing groove (26), which has a curved surface generally complementary to that of the spherical bearing surface of the bearing (17) in the metacarpal component. The bearing groove (26) has an angular extent which is slightly greater than 180°, and the hook (22) ’TΙΟ 46519 • is of such resilience, so that it is capable of snap engagement over the bearing (17) and is positively located thereover, such that the metacarpal and proximal phalangeal components are capable of mutual pivotal motion thereabout.

The curved ridges (25) have essentially the same centre of curvature as the bearing groove (26) and a radius of curvature which is slightly less than the maximum extent of the cam-like extension (20) on each furcation (16) from the point of attachment of an axle (18) of the bearing (17), so that when the two components are assembled to form the MCPjoint, the cam-like extensions (20) on the metacarpal component and the ridges (25) on the proximal phalangeal component are capable of mutually interacting to bear against each other when the joint is flexed to the 90° position, as shown in Figure 8. The: pivotal motion of the joint is thereby restricted to pivotal motion about a first single axis passing through the axles (18) of the bearing.

Equally, when the assembled MCP-joint is in the extended position, as shown in Figure 1 and 7, the ridges (25) and the cam-like extensions(20) do not necessarily interact, and thus some pivotal motion Is also possible about a second axis at 90°to the aforementioned first axis. The angular extent of the pivotal motion about the second axis is limited by mutual abutment of the ridges (25) on the hooked member (21) with the curved outer ends of the furcations (16) and/or by mutual abutment of the outer open end (27) of the hook with the inside face of each furcation (16), depending on the relative dimensions and separation of the hook (22), ridges (25) 4 6 519 and furcations (16). In the illustrated embodiment, the maximum angular extent of pivotal motion is limited to an arc of approximately 10-15° by mutual abutment of the ridges (25) on the hooked member (21) with the curved outer end of each furcation (16).

Thus the mutual interactions of the ridges (25) and curved outer end of each . furcation (16) on the proximal phalangeal (21) and metacarpal (11) components respectively, on flexion-extension enable the embodiment to approximate to the abduction-adduction capabilities of the natural metacarpophalangeal joint, as stated herein above.

In a second embodiment of an MCP-joint according to, the invention the metacarpal (31) and proximal phalangeal (32) components differ from those in the first embodiment only in respect of the shape of the bearing and hook. Thus in a second embodiment as shown in Figure 9 the bearing is in the form of an axially concave bobbin (33) axially attached to the furcations of the bifurcated head of the metacarpal component (31) by axles (31)) modified as described for the first embodiment, and the hook (35) has a concave longitudinal, convex transverse form with a radius of convex curvature less than thenradius of cohcave curvature of the bobbin (33), and as described above for the first embodiment, is of such an angular extent and resilience that it is capable of snap engagement over the bearing to permit pivotal movement thereabout. Lateral movement appoximating to th abduction-adduction of the natural joint is possible by rocking or rolling of the hook (35) about the bobbin (33) because of their difference in curvature. This abduction-adduction *T is limited in exactly the same manner as described above for the first embodiment by mutual interaction of the cam-like extensions (20) on the furcations (16) with the ridges (25) on the respective components (31) and (32).

It will be appreciated, that an MCP-joint of the invention may be readily modified, for example, by decreasing the size of the cam-like extensions (20) and correspondingly decreasing the distance of the ridges (25) from the centre of curvature of the bearing part of the hook (22 or 35) in the respective components, so that the cam-like extensions'(20) and ridges (25) may be arranged to bear against each other for progressively more of the flexion-extension range of the joint, and thus the abduction-adduction capability reduced to a minimal extent.

Such a modified joint according to the invention is suitable for replacement of ginglymus joints which have essentially no abduction-adduction capability, for example the proximal interphalangeal finger joint.

It will be observed that hyperextension of an MCPjoint according to the invention is limited by abutment of the back, non-bearing part of the hook (22 or 35) with the base (12) of the bifurcated head on the respective components. Equally, flexion through an angle greater than 90’ is limited by progressively increased mutual interaction between the ridges (25) on the hooked member and the cam-like extensions (20) on the furcations (16) as the 90° flexion position is exceeded.

Initially the increased interaction is accommodated by the resilience of the hook (22 or 35) but on excessive flexion the Interaction is sufficient to dislocate the hook (22 or 35) from over the bearing (17 or 33). - 13 46519 This property of dislocation on excessive flexion is particularly advantageous when an MCP-joint according to the invention is inserted in place of an arthritic natural joint. The bones of such joints are frequently so weakened by the arthritis that excessive pressure5such as that arising on falling, or from using the hands with the fingers in the flexed position to raise the body from a sitting position, would cause fracture of the bones,if transmitted by the MCP-joint. Furthermore, relocation of the hook (22 or 35) over the bearing (17 or 33) may be readily accomplished, generally without recourse to surgery.

The metacarpal component including the bearing and the proximal phalangeal component of the illustrated embodiments are conveniently each of integral construction, for example, from physiologically inert resilient plastics material, for example high molecular weight polyethylene, and as such are particularly adapted to manufacture by conventional moulding techniques known for such materials. Thus the necessity for complex machining operations, so far often present in the manufacture of MCP-joints, is avoided, Because of the overall structural requirements of an MCP-joint according to the invention it is also possible simply to incorporate a bearing of different material from the rest of the MCP-joint, for example a metal bearing, for example by adding the bearing as an insert prior to moulding of the remainder of the metacarpal component around it. This is particularly advantageous since the exact size, shape and surface condition of the bearing surfaces in endoprosthetic - 14 46519 joints ia critical, especially where considerable loads will be placed on the joint, if surface wear is to be minimised. Particularly suitable materials from which the bearing for a joint according to the invention may be constructed are, in addition to high molecular weight polythene, any conventionally employed physiologically inert bearing material, for example a chromium cobalt alloy, a stainless steel or titanium.

In addition, the bearing portion of the hook, for example the bearing groove (26) of the first embodiment, may be made separately from a suitable physiologically inert bearing material as defined above and incorporated, for example, as an insert prior to the moulding of the remainder of the proximal phalangeal component around it. In this case, and also in the case when the bearing is incorporated as an insert in the moulding of the remainder of the proximal phalangeal component, the material used for the moulding may be a physiologically inert resilient plastics material unsuited for use in bearings, for example a polyester or polypropylene.

Insertion of an MCP-joint according to the invention may be carried out using known conventional surgical techniques, for example as described by M. Devas and V. Shah in the Journal of Bone and Joint Surgery, 1975, 57, 72-77.

Only a relatively small incision is necessary, since the components are first inserted and located individually in their respective bone-ends and then the hook is located over the bearing by the surgeon. The components of the MCP-joint may either be positively located in the medullary-canal of each bone end either solely by mechanical means, or with the aid of a suitable adhesive or cement, for example. - 15 46519 Although the embodiments described herein have been particularly developed with respect to replacement of the natural metacarpophalangeal joint, it is to be understood that the invention may also be applied to other natural ginglymus joints having a limited abduction-adduction capability which varies with the degree of flexion-extension. - '

Claims (12)

1. CLAIMS:1. An endoprosthetic ginglymus joint comprising mutually cooperating first and second components adapted to be attached to the respective bones to be inter-connected by the joint; the first component including a bearing support member having a bifurcated head, attached to and bridging the furcations of which is a bearing having a first bearing surface of convex circumferential form symmetrically disposed about an axis passing through the furcations; the second component including a hooked member, having a hook adapted on its radially inner surface to define a second bearing surface of such generally complementary shape to said first bearing surface as to be capable of engagement therewith for cooperative pivotal motion between said first and second components about the bearing; and said furcations and said hooked member being formed with structures which are capable of mutually interacting to restrict the pivotal motion of the joint to pivotal motion about said axis passing through said furcations.

2. A joint as claimed in claim 1 wherein the first bearing surface is of convex circumferential and convex axial form, and the second bearing surface is in the form of an arcuate, longitudinally concave, bearing groove recessed on the radially inner surface of the hook.

3. A joint as claimed in claim 2 wherein the first bearing surface is in spheroidal form. 17 46519

4. A joint as claimed in claim 1 wherein the first bearing surface is of convex circumferential and concave axial form, and the second bearing surface is provided by the radially inner surface of the hook having a concave longitudinal, convex transverse form with a radius of convex curvature less than the radius of concave curvature of the first bearing surface.

5. A joint as claimed in claim 4 wherein the first bearing surface is in the form of an axially concave bobbin.

6. A joint as claimed in any preceding claim wherein the hook of the seeond component is sufficiently resilient and the seeond bearing surface is of such an angular extent that the hook is capable of snap engagement over the first bearing surface so as to be positively retained thereover and yet permit cooperative pivotal m'otion between the first and second components.

7. A joint as claimed in any preceding claim wherein' the structures on the furcatiorh and hooked member which are capable of mutually interacting to restrict the pivotal motion of the joint to pivotal motion about the axis passing through the furcations are a matching cam-like extension eccentrically disposed at the end of each furcation and a flanged shoulder which extends outwards laterally from the hooked member.

8. A joint as claimed in any preceding claim wherein the first and second components are adapted to be attached to the respective bones to be inter-connected, by being provided with an elongate stem which tapers towards a narrow bone-proximate end and which is longitudinally grooved - 18 46519 along its periphery.

9. A joint as claimed in any preceding claim where in the first and second components are each of integral construction from physiologically inert resilient plastics 5 material.

10. A joint as claimed in any one of claims 1-8 • wherein the bearing is of metal construction and wht?'..in the bearing support member and the second component are each of integral construction from physiologically inert resilient 10 plastics material.

11. An endoprosthetic ginglymus joint substantially as described herein with reference to Figures 1-8 of the accompanying drawings.

12. An endoprosthetic ginglymus joint substantially 15 as described herein with reference to Figure 9 of the accompanying drawings.