HUMERAL COMPONENT OF A SHOULDER JOINT PROSTHESIS

Abstract

A humeral component of a reverse shoulder prosthesis includes a stem part configured for location within the intramedullary cavity of the humerus, the stem part having a stem axis, an epiphyseal part connected to the elongate stem part and having a concave bearing surface defining an epiphyseal axis, and wherein the epiphyseal axis is offset posteriorly relative to the stem axis.

Claims

1-13. (canceled)

14. A humeral component of a reverse shoulder prosthesis, comprising: a. an elongate stem part for location within the intramedullary cavity of the humerus, the stem part having a proximal face; and b. an epiphyseal part having (i) a concave bearing surface for articulation with the convex bearing surface of a glenoid component, (ii) an opposite distal face which is directed towards the proximal face of the stem part when the stem part and the epiphyseal parts are assembled, and (iii) a surface extending between the distal face and the concave bearing surface; wherein the elongate stem part defines a stem axis and the concave bearing surface of the epiphyseal part defines an epiphyseal axis, and in which one of the distal face of the epiphyseal part and the proximal face of the stem part carries a spigot connection feature and the other of the distal face of the epiphyseal part and the proximal face of the stem part carries a socket connection feature in which the spigot connection feature can be received so that the epiphyseal part and the stem part can be assembled with one another, the connection feature on the epiphyseal part being offset relative to the epiphyseal axis so that the epiphyseal axis in the assembled humeral component is offset posteriorly relative to the stem axis; wherein the surface of the epiphyseal part which extends between the distal face and the concave bearing surface is adapted to contact bone tissue and to be positioned within the cortical tissue of the humerus.

15. The humeral component as claimed in claim 14, wherein the distance between the stem axis and the epiphyseal axis is at least about 1.5 mm.

16. The humeral component as claimed in claim 14, wherein the distance between the stem axis and the epiphyseal axis is not more than about 7 mm.

17. The humeral component as claimed in claim 14, wherein the epiphyseal part comprises a shell part which defines a recess, and a bearing part which can be received in the shell part and which provides the bearing surface for articulation with a glenoid component.

18. The humeral component as claimed in claim 17, wherein the rim of the shell part is approximately circular.

19. The humeral component as claimed in claim 17, wherein the rim of the shell part is approximately planar.

20. The humeral component as claimed in claim 19, wherein the plane defined by the rim of the shell part is approximately perpendicular to the epiphyseal axis.

21. The humeral component as claimed in claim 14, wherein the angle between the stem axis and the epiphyseal axis, when viewed along the anterior-posterior axis, is at least about 20.

22. The humeral component as claimed in claim 14, wherein the angle between the stem axis and the epiphyseal axis, when viewed along the anterior-posterior axis, is not more than about 35.

Description

[0021] Embodiments of the invention will now be described by way of example with reference to the accompanying drawings, in which:

[0022] FIG. 1 is a side elevation view of a humeral component of a shoulder joint prosthesis according to the invention.

[0023] FIG. 2 is an exploded isometric view of the stem and epiphyseal parts of a humeral component according to the invention.

[0024] FIG. 3 is another isometric view of the stem and epiphyseal parts of a humeral component shown in FIG. 2.

[0025] FIG. 4 is a side elevation of a humeral component according to the invention.

[0026] FIG. 5 is a sectional elevation of the humeral component shown in FIG. 4.

[0027] FIG. 6 is a side elevation view, partially cut away, of a humeral component according to the invention implanted in a patient's humerus.

[0028] FIG. 7 is a view generally along the medial lateral axis of a shoulder joint, showing the scapula and the humerus, after implantation of a shoulder joint prosthesis according to the invention.

[0029] Referring to the drawings, FIGS. 1 to 6 show a humeral component 2 of a reverse shoulder prosthesis. It comprises an elongate stem part 4 for location within the intramedullary cavity of the humerus 6. An epiphyseal part 8 defines a recess for a bearing part 10. The bearing part has a concave bearing surface 12 for articulation with the convex bearing surface of a glenoid component (not shown).

[0030] The stem part 4, the epiphyseal part 8 and the bearing part 10 are provided as separate modular parts which are assembled to form the humeral component prior to implantation. Features which are incorporated into modular orthopaedic joint prostheses, especially shoulder joint prostheses, which are suitable for use in the prosthesis of the present invention are known, for example from the DELTA and GLOBAL prostheses which are manufactured and sold by DePuy Products Inc, and from the AEQUALIS prosthesis which is manufactured and sold by Tornier S A. Details of one embodiment of humeral component are shown in FIGS. 2 to 5.

[0031] In the embodiment of the humeral component shown in FIGS. 2 to 5, the stem part 4 has a planar proximal face 102, with an axial bore 104 formed in it, extending normal to the proximal face and generally along the axis of the part. An upstanding lug 106 is provided on the proximal face. The stem part has two shoulders 108 at its proximal end which are to provide rotational stability of the stem part in the humerus after implantation.

[0032] The axial bore 104 is formed in two parts, with a proximal large diameter plain bore portion 110 and a distal smaller diameter threaded bore portion 112 which is configured for threaded engagement with a fixation screw 113 for the epiphyseal part 8.

[0033] The epiphyseal part 8 has a planar distal face 114 with a short spigot 116 extending from it, normal to the plane of the distal face 114. The spigot has a bore 117 extending through it which is configured so that the shank of the fixation screw 113 can extend through it. The distal face 114 of the epiphyseal part 8 has seven blind recesses 116 formed in it, in which the lug 106 on the proximal face of the humeral part can be received.

[0034] The lug and the series of blind recesses allow the angular orientation of the epiphyseal part relative to the stem part to be adjusted. This is accomplished by locating the spigot on the epiphyseal part loosely in the bore on the stem part. The epiphyseal part can be turned relative to the stem part around the axis of the stem part. The lug is located in an appropriately positioned one of the recesses 116 to lock the epiphyseal part against rotation. The epiphyseal part is then locked to the stem part by means of the fixation screw 113 engaging the threads in the small diameter portion 112 of the bore in the stem part.

[0035] FIGS. 4 and 5 show in addition the bearing part 10 of the humeral component. This and the epiphyseal part have circumferential grooves 118, 120 formed in them which are aligned with one another when the parts are properly assembled. A resilient spring in the form of a zigzag wire formed into an approximately circular loop can be can be used to fasten the polymeric bearing part into the stem part of the glenoid component. The spring can be located in one of the grooves prior to assembly, which is deformed during assembly, and allowed to relax into the other of the grooves when the parts are assembled.

[0036] The use of such a spring to fasten a polymeric bearing part into the stem part of an orthopaedic joint prosthesis is well known.

[0037] The stem part 4 is formed from an alloy which is suitable for an orthopaedic joint prosthesis component such as a titanium alloy or a stainless steel. Suitable materials are known. The configuration of the stem part is as in known shoulder joint prostheses. Features of the stem part can include (a) a taper towards the distal tip, (b) axial flutes in the side walls in the portion towards the distal tip, (c) a coating over at least part of the bone engaging surface (particularly in the proximal region) of a material which promotes fixation of the implant in the intramedullary cavity, and so on. The configuration of the stem component should be optimised to ensure that it can be fixed securely in a patient's humeral cavity, as is known.

[0038] The epiphyseal part 8 of the component will generally be formed from the same material as the stem part, or at least a similar material (for example the stem part and the epiphyseal part might both be formed from metals, especially the same metal).

[0039] The bearing part of the component will frequently be formed from a polymeric material such as an ultrahigh molecular weight polyethylene (UHMWPE). Such materials are known for use in orthopaedic joint prostheses for articulation with bearing surfaces of other components provided by hard materials including metallic and ceramic materials. The bearing surface of the humeral component can be provided by materials other than polymeric materials, for example by metallic materials or ceramic materials. A bearing part need not be included in the humeral component, for example when the epiphyseal part provides the bearing surface. This can be particularly appropriate when the bearing surface of the humeral component is provided by a hard material such as a metallic material or a ceramic material.

[0040] The stem part 4 of the component defines a stem axis 13, which is intended to be aligned with the patient's humeral axis when the component is implanted.

[0041] The bearing surface of the humeral component should be shaped according to the shape of the convex bearing surface of the glenoid component with which it articulates. The bearing surface of the humeral component will generally be approximately rotationally symmetrical, for example in the form of a part of a sphere or part of an ovoid. (However, when viewed along the axis of symmetry, the bearing surface need not be circular.) The axis of symmetry of the bearing surface defines the epiphyseal axis of the humeral component.

[0042] In the humeral component of the present invention, the stem axis and the epiphyseal axis are not coincident. Preferably, they are approximately parallel. However, as shown in FIG. 1, the epiphyseal axis is offset posteriorly relative to the stem axis by about 2.5 mm.

[0043] FIG. 6 shows the humeral component described above with reference to FIG. 1 after implantation in a patient's humerus 6. As shown in FIG. 6, the humerus is prepared for implantation of the joint prosthesis by resection, as generally known. The bone is resected along a resection plane 14 which is inclined to the axis of the humerus.

[0044] It is an advantage of the present invention that the epiphyseal part 8 of the humeral component, when implanted in the humerus, can be arranged to lie wholly within the cortical tissue of the humerus on the resection plane 14. This is possible without having to compromise the size of the humeral component (which is an advantage in order to maintain optimum load transfer area on the articulating surfaces of the prosthesis). The ability to implant the humeral component without disrupting the cortical tissue of the humerus has the advantage of minimising weaknesses introduced to the bone tissue as a result of implantation of the humeral component.

[0045] FIG. 7 shows a shoulder joint, with a shoulder joint prosthesis according to the invention implanted therein. A glenoid component 20 is implanted in the scapula 22, the glenoid component having a convex articulating surface 24. The glenoid component can have the features of the corresponding component of the DELTA shoulder joint prosthesis which is manufactured and sold by DePuy Products Inc. A humeral component 26, having the features of the humeral component described above with reference to FIG. 1, is implanted in the humerus 28.

[0046] Control over international rotation of the shoulder joint is provided in part by the subscapularis tendon (shown schematically by a dotted line 30). The tendon is attached to the greater tuberosity 32 on the humerus, located anteriorly relative to the humeral axis.

[0047] The mechanical advantage associated with the action of the subscapularis muscle on the shoulder joint depends on the distance between the point of attachment of the subscapularis tendon to the humerus and the centre of rotation of the joint. It is an advantage of the humeral component of the present invention that the mechanical advantage is increased as a result of the posterior offset of the centre of rotation of the joint relative to the axis of the humerus.