A reverse hip prosthesis comprising an acetabular cup for implanting in a previously implanted acetabular cup. The acetabular cup is secured to the previously implanted acetabular cup using an o-ring. The acetabular cup has a stem extending from the center of a concave portion thereof and an acetabular ball is attached to the stem, preferably by means of a Morse taper. A femoral implant is provided with a femoral cup attached to the proximal end thereof. The femoral cup has a stem which can be constructed in a modular fashion with several stem lengths in order to accommodate the various sizing requirements of different patients. After implantation of the acetabular cup and ball and the femoral cup, the members are assembled together so that the femoral cup can articulate on the acetabular ball. As the femoral cup is articulated, the edge of the femoral cup moves into a space located between the acetabular cup and the acetabular ball and thereby becomes constrained, thus reducing the likelihood of dislocation during extreme ranges of motion.