A modular prosthetic acetabulum apparatus for use in reconstruction of a hip joint by placement in an acetabulum cavity has a supporting plate with a shape suitable for placement in the acetabulum cavity, an anchor cooperative with the supporting plate for adjustably positioning the supporting plate in the acetabulum cavity, a spacer locked onto the supporting plate and selectively rotatable with respect to the supporting plate, and a cup affixed within the spacer. The supporting plate has an annular internal surface cooperative and complementary to a tapered annular exterior surface of the spacer. The cup has a tapered exterior surface complementary to and cooperative with an annular tapered interior in the spacer.