Illustration of the components of a prosthetic hip joint. The prosthesis consists of a shaft and ball that is implanted into the top of the femur (thigh bone) and a socket that is inserted into the pelvis. The majority of hip replacements are a result of osteoarthritis, a degenerative joint disease that results in the loss of cartilage between the joint and the growth of bone in place of the cartilage. This causes pain, stiffness and loss of mobility.