A prosthesis implantation inserter is used with a flexible acetabular cup which has an opening or openings in its peripheral rim. The inserter has engagement elements which in an operational position frictionally engage the opening or openings in the flexible cup rim. A release is provided which can be operated to withdraw the engagement element from the opening or openings. When the cup is held .sctn.in position it can be guided and inserted into the acetabulum and impacted. Once the position of the prosthesis satisfies the surgeon the engagement element can be removed by operating the release and the cup inserter is removed without any risk of compromising the position or the stability of the implant.