The head replaces the natural head of the upper arm bone (humerus head) and is usually made of ceramic or metal. In most models, it is placed onto the shoulder stem. Secure fixation on the stem is ensured by a conical connection.
The inverse prosthesis is based on a different concept: the head is screwed directly into the shoulder blade rather than the joint socket, and a concave joint surface is anchored in the upper arm bone. This means that compared to the other prostheses, the joint partners are reversed. In this model, the head is made of polyethylene.