Hemiarthroplasty is a surgical proceure which replaces one half of the joint with an artificial surface and leaves the other part in its natural (pre-operative) state. This class of procedure is most commonly performed on the hip after fracturing the neck of the femur (a hip fracture). The procedure is performed by removing the fractured head of the femur and replacing it with a metal or composite prosthesis. The most commonly used prosthesis designs are the Austin Moore prosthesis and the Thompson Prosthesis. More recently a composite of metal and HDPE which forms two interphases (Bipolar Prosthesis) has also been used. The procedure is recommended only for elderly and frail patients, due to their lower life expectancy and activity level. This is because with the passage of time the prosthesis tends to loosen or to erode the acetabulum.