Abstract: The prosthetic acetabulum is formed of a metal shell structure of metal mesh and a plastic cup disposed in the shell structure. Both the shell structure and plastic cup have coincident meridian-disposed slots to permit the shell structure and cup to adapt to movements of the pelvis. The cup is also provided with a cavity for the femoral head which has spherical surface areas of different radii. The larger spherical surface area contains the slot and avoids locking of the femoral head during closing of the slot. A fixing tab is provided diametrically opposite the slot in the shell structure and is secured by bone screws to the edge of an undersurface of the pelvis.