Alignment guide

1. An orthopaedic joint prosthesis assembly which comprises:

a. a head part of an orthopaedic joint prosthesis component, which has a spherical bearing surface for articulation with a corresponding joint surface, and an assembly surface having a first bore formed in it for receiving a spigot on another part of the orthopaedic joint prosthesis, said bore having a first axis that extends perpendicular to the assembly surface, and in which there is a discontinuity at an interface between the bearing surface and the assembly surface, the said assembly surface being arranged on a plane which is parallel to, or contains, a plane which is defined by the opening to the bore in the head part when the head part is viewed from one side in cross-section, and

b. an alignment guide comprising an axial portion and an arm extending from the axial portion, the arm including a distal portion and a proximal portion, the distal portion being configured to engage the assembly surface of the head part, the axial portion defining a second axis and being arranged to engage the bearing surface of the head part, directly or indirectly, when the head part is mounted within the guide, with the second axis coincident with the first axis, such that when an impaction force is applied along the second axis the force is directed along the first axis;

wherein the alignment guide includes at least two arms and in which the distal portion of each arm includes an in-turned lip; and

wherein at least one of the arms is capable of being pivoted relative to the other arm to provide access for mounting the head part within the alignment guide.

2. The alignment guide as claimed in claim 1 , in which the guide further comprises a bearing surface seating member.

3. The alignment guide as claimed in claim 2 , in which the bearing surface seating member has a contact surface which is concave so that it is configured to contact the bearing surface of a prosthesis component head part which is convex.

4. The alignment guide as claimed in claim 1 , in which alignment guide includes a shaft through which an impaction force can be applied through the hearing surface seating member to a prosthesis component head part mounted within the alignment guide, and in which the shaft is mounted in the axial portion so that its axis is coincident with the second axis defined by the axial portion.

5. The alignment guide as claimed in claim 4 , in which the axial portion includes a sleeve and the shaft is able to translate within the sleeve along the axis defined by the sleeve.

6. The alignment guide as claimed in claim 5 , in which the sleeve and the shaft have cooperating threads so that the shaft can be advanced through the sleeve by rotating it about its axis.

7. The alignment guide as claimed in claim 4 , in which the axial portion includes a hub spar having a socket and the shaft is able to translate within the socket along the axis defined by the socket.

8. The alignment guide as claimed in claim 7 , in which the hub spar includes a feature defining the location of the hearing surface seating member, the bub spar having a concave inner surface defining a space in which a head part can be received.

9. The alignment guide as claimed in claim 8 , in which the huh spar includes first and second arms which are connected to the hub spar at its widest point so that they can be pivoted relative to the huh spar between a retracted position which allows a head part to be mounted within the said space and a deployed position in which ahead part position in the said space is retained therein.

10. The assembly as claimed in claim 1 , which includes a part of the orthopaedic joint prosthesis having a spigot which can be received in the bore in the assembly surface of the head part.

11. The assembly as claimed in claim 1 , which the assembly surface includes a chamber portion extending around the head part, inclined to the plane which is defined by the opening to the bore in the head part.