Loose replacement hip joint. Image 1 of 2. X-ray of a loose replacement hip joint in the leg of a 77 year old woman. The joint consists of a socket fixed into the pelvis (wire loop at upper centre) which articulates with a ball attached to a shaft which is cemented into a cavity in the thigh bone (femur). This prosthesis is used when the original joint has been damaged, either by a disease such as arthritis, or by an injury. Either part of the joint can become loosened by wear (as here) or sometimes by the effects of an infection or inflammation. In these cases, a new ball and socket are fitted into the remaining bone. For an X-ray of the new hip joint, see image M600/171.