There is provided an orthopaedic joint prosthesis assembly which comprises a head part (500) of an orthopaedic joint prosthesis component and an alignment guide (510). The head part has a spherical bearing surface (504) for articulation with a corresponding joint surface, and an assembly surface (508) having a bore (506) formed in it for receiving a spigot on another part of the orthopaedic joint prosthesis. The bore has a first axis that extends perpendicular to the assembly surface. There is a discontinuity at an interface between the bearing surface and the assembly surface. The assembly surface is 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. The alignment guide comprises an axial portion (511) defining a second axis coincident with the first axis, such when an impaction force is applied along the second axis the force is directed along the first axis. The axial portion comprises a plain bore sleeve (518) and a pair of arms (512, 514) that extend from a distal portion of the sleeve such that a space is formed between each arm. Each arm comprises a proximal end (516), a distal end (520) and an inner surface that has a contact surface shaped to substantially correspond to the bearing surface of the head part of the orthopaedic joint prosthesis, such that when the head part is mounted within the guide a nesting fit is formed between the bearing surface of the head part and the contact surface of each arm. The distal end of each arm joins to form a U-shaped bracket (522) which carries a U-shaped assembly surface seating member configured to engage the assembly surface of the head part of the orthopaedic joint prosthesis when the head part is mounted within the guide.
