An assembly of a cup prosthesis (10) and instrument head is described. A cup prosthesis has a circumference and a central axis which defines axial and radial directions. An attachment member (8) is applied around the circumference of the cup. The attachment member comprises: a circumferential band (46)； and at least two tabs (50) extending in an axial direction from the circumferential band and having a circumferential region of weakness (60). A surgical instrument head is attached to the attachment member and engaged with the cup prosthesis, and has a radial retaining element (38) which engages an outer radial face of the at least two tabs.
