GB2210793A - Prosthetic total shoulder joint devices - Google Patents

Abstract

A prosthetic total shoulder joint comprises a scapular component (10) and a humeral component (20) respectively defining a socket and a ball (21), the scapular component being grossly of hemispherical form without any significant projections. The scapular component is suitably formed for fixation in bone when fully seated, the intention being that fixation is effected in the humeral head (31) with such head being divided from the humerus and fixed by arthrodesis with the scapula (33). The humeral component suitably has its ball connected by way of a necked portion (22) with a stem (23) for intramedullary fixation in the shaft of the humerus (30). <IMAGE>

Description

PROSTHETIC TOTAL SHOULDER JOINT DEVICES Prosthetic total shoulder joint devices commonly involve two components respectively defining a socket and a ball, which components are deployed in cooperation to form a ball and socket joint. One component, usually that defining the socket, is fixed proximally to the scapula and the other to the humeral shaft, with the two components being held in articulatory engagement by the natural joint capsule to replace the function of the natural joint surfaces.

A variety of proposals have been made in the past for such devices, but these proposals have differed largely in respect of the form of the scapular component. This reflects that fact that the provision of a satisfactory design for the scapular components is problematical. This arises in turn from the fact that the natural scapular cavity, the glenoid, is of small angular extent compared to a hemisphere, say, and the scapula is largely of a webbed form with little in the way of bulk. The lack of bulk renders fixation of a component difficult by the use of pegs, screws or other projecting elements and so some proposals involve a socket of small extent, similar to the glenoid, with relatively modest intrusion into the bone for fixation.However, this approach must be associated with a risk of dislocation in the resultant joint replacement if, as is possible, the capsule is not of fully normal integrity, and also a risk of scapular component loosening due to the effect of shear stresses and other forces transmitted, in use, through the component fixation. Other proposals have involved a socket of larger extent, and in some cases the ball in a reversal of the normal anatomical configuration, in association with fixation involving a greater degree of intrusion into the limited regions of the bone where this seems possible. In this case the articulatory part of the component, be it the socket or ball, will inevitably project from the scapula and be subject to increased stresses so that loosening is still a serious risk.

An object of the present invention is improvement of this situation and attainment of this objective centres on the use of a different surgical procedure involving separation of part of the humerus in the region of the head, which part is joined with the scapula and excavated to receive a socket component. The benefits of this procedure are that a more extensive socket can be used, this socket can be seated fully in the amplified scapula, and this socket can be fixed without the provision of projecting elements.

Accordingly, the present invention provides a prosthetic total shoulder joint device comprising a scapular component and a humeral component respectively defining a socket and a ball, the scapular component being grossly of hemispherical form without any significant projections. In this context, the term "significant" is intended to mean of the same order of projecting dimensions as those for the socket.

The scapular component is to be adapted for fixation with bone by the use of conventional techniques which involve structures of relatively small dimensions. For this purpose the convex surface of the component typically has a ribbed or grooved structure for cooperation with bone cement.

The humeral component will suitably be of a one piece construction including a ball connected with an intramedullary stem by way of a neck. This necked form is appropriate to provide a wide range of articulation in association with the more extensive socket and its full siting in bone.

In order to further clarify the invention, an embodiment of the same is described below by way of example with reference to the accompanying drawings,in which: Figures 1 and 2 respectively illustrate the embodiment with its components separated before use and engaged after fixation.

The illustrated embodiment comprises scapular and humeral components 10 and 20 each of one piece construction and respectively made of plastic material and metal. The scapular component 10 is essentially in the form of a hemispheral cup having its outer convex surface 11 formed with ribs and grooves and a radial flange 12 at the outer edge of its mouth. The humeral component 20 includes a ball 21 dimensioned for articulatory engagement in the scapular component cup and connected by way of a necked portion 22 to the wider end of a tapered stem 23.

The necked portion has greater transverse dimensions than the stem at their junction to define a shoulder 24 facing along the stem.

In operative use of the embodiment the humerus 30 is divided at its neck to separate the head 31 and the head is excavated from its excised face to create a cavity in which the scapular component 10 is secured with bone cement 32. The head is then located relative to the scapula 33 to achieve the correct orientation for the component fixed in the head, and the articular surface of the head and that of the scapula at the glenoid are excised by parallel cuts. Thereafter the head is fixed with the scapula by arthrodesis to fuse the last cut faces, the faces being held engaged for this purpose by suitable means such as pins 34.The humeral component 20 is secured in the shaft of the humerus by engagement of the stem 23 in the medullary canal following suitable preparation of the latter and the application of further bone cement, to seat the shoulder 24 on the cut end face of the humerus and correctly orientate the ball 21 for articulatory engagement in the scapular component.

While the invention has been described with particular reference to the illustrated embodiment, this is indicated as an example and variations are possible within the scope of the appended claims. For example, the scapular component may be made for fixation without the use of bone cement by the provision of a thread formation around its rear for screw connection in the associated bone. For this purpose the component is suitably of a two-part construction involving an outer socket part of metal, say, and a replaceable liner of plastics material. Also in order to obviate problems of dislocation in the early post-operative period the two components can be formed for semi-captive connection such as by snap fitting. Again, the range of mutual articulation between the two components can be increased compared to that which is otherwise possible by forming the scapular component with indentations in its rim at 3-5 o'clock and 7-9 o'clock to simulate more closely the related range in the natural joint.

A final comment is appropriate in connection with the fact that the proposed device is superficially similar to endoprosthetic devices of well-known form for total hip joint replacement. The present device will of course differ critically from the known ones in terms of the dimensioning and/or shaping of the respective components by virtue of the different fundamental anatomical structures of the joints for which application is intended.

Claims (8)

1. A prosthetic total shoulder joint device comprising a scapular component and a humeral component respectively defining a socket and a ball, the scapular component being grossly of hemispherical form without any significant projections.

2. A device according to Claim 1 wherein the convex surface of said socket is formed with ribs and/or grooves.

3. A device according to Claim 1 or 2 wherein said socket is formed with a radially extending flange at the junction of its convex surface and rim.

4. A device according to Claim 1, 2 or 3 wherein said ball and socket are engageable in snap-fitting or other semi-captive manner.

5. A device according to any preceding claim wherein said socket is formed with indentations in its rim at 3-5 o'clock and 7-9 o'clock.

6. A device according to any preceding claim wherein said humeral component comprises a ball connected by way of a necked portion to the wider end of a tapered stem.

7. A device according to Claim 6 wherein said necked portion has greater transverse dimensions than said stem at their junction to define a shoulder facing along the stem.

8. A prosthetic total shoulder joint device substantially as herein described with reference to the accompanying drawings.

8. A prosthetic total shoulder joint device substantially as herein described with reference to the accompanying drawings.

Amendments to the claims have been filed as follows 1. A prosthetic total shoulder joint device comprising a scapular component and a humeral component respectively defining a socket and a ball, the scapular component being grossly of hemispherical form without any significant projections and being adapted for securement with bone when substantially fully seated therein.

2. A device according to Claim 1 wherein the convex surface of said socket is formed with ribs and/or grooves.

3. A device according to Claim 1 or 2 wherein said socket is formed with a radially extending flange at the junction of its convex surface and rim.

4. A device according to Claim 1,2 or 3 wherein said ball and socket are engaaeable in snap-fitting or other semi-captive manner.

5. A device according to any preceding claim wherein said socket is formed with indentations in its rim at 3-5 o'clock and 7-9 or clock.

6. A device according to any preceding claim wherein said humeral component comprises a ball connected by way of a necked portion to the wider end of a tapered stem.

7. A device according to Claim 6 wherein said necked portion has greater transverse dimensions than said stem at their junction to define a shoulder facing along the stem.