An insert (1) designed for insertion into a central cavity (10) within the upper part of the prosthetic stem (13) of a shoulder prosthesis. The insert (1) is provided with at least one transverse hole (6) that is threaded or provided with an inner retaining rib or any other screw-locking means. The insert (1) is designed for locking of transverse tuberosity bone screws (4) in the same way as a cross-dowel-nut. The outer surface of the prosthetic stem (13) is provided with at least one transverse unthreaded hole (7) communicating with the central cavity (10) in order to allow a sliding passage for the bone screw (4).The insert for a prosthetic humeral head implant of an anatomical shoulder prosthesis is interchangeable with an insert for the cup-shaped humeral implant (18) of a reverse shoulder prosthesis. The insert (1) is provided with a fixation site (8) for a targeting arm.