A surgical instrument head for use with a cup prosthesis (10) is described. The cup prosthesis has an attachment member (8) applied around the circumference of the cup. The surgical instrument head is configured to engage the cup prosthesis and attach to the attachment member. The surgical instrument head includes a tab separation member (6) rotatable about a central axis of the instrument head relative to the instrument head and having a formation for engaging a rotation instrument (310). The tab separation member defines at least one radial cam surface (40) for engaging at least part of the attachment member in use. An assembly of the surgical instrument head and cup prosthesis and a tool (300) for operating the assembly are also described.
