A cap used during the insertion of an acetabular cup prosthesis. The cap comprises an impaction plate having an upperside and an underside. The impaction plate comprises a plurality of separate sectors mutually connected at a connection point on the impaction plate by means of flanges. Each sector includes interlocking means which in use enable the impaction plate to be connected to an introduction tool. The cap also comprises at least one lug extending downwardly from the underside of each sector. The lug is located at the portion of the underside of the impaction plate which will in use enable the lug to be interconnected with a corresponding recess on an internal wall, an external wall or a rim of a prosthesis.