It is performed when the original primary total hip replacement has worn out or loosened in the bone. Revisions are also carried out if the primary hip replacement fails due to recurrent dislocation, infection, fracture or very rarely, ongoing pain and significant leg length discrepancy. The revision total hip replacement is a more complex procedure, often because there is a reduced amount of bone to place the new total hip into. Extra bone may be required and this is usually received from a bone bank. Bone bank (allograft bone) is safe and has been irradiated to eliminate any chance of disease transmission. There are also artificial bone substitutes that may be used.