Cup positioning in total hip arthroplasty.

The "optimal" positioning of the cup in total hip arthroplasty can improve hip function and reduce wear, impingement and dislocation. The cup position is described as the spatial relation between the hip rotation centre and the pelvis and, as the cup orientation around the rotation centre. The first parameter affects hip balance and, if not managed properly, might result in poor function and leg length discrepancy. The second parameter is often "silent", unless impingement or dislocation occurs. However, inappropriate cup orientation can accelerate wear and cause early failure. As such, it is mandatory to get both right, taking into account multiple parameters: the stem position, the approach, the bearing surface, the cup coverage and the pelvic orientation during loading. In most cases a "standard" cup position is adequate. However, specific anatomic features might require an individualized approach. This paper aims at reviewing the parameters that impact on the optimal cup position. This should allow for more judicious choices in those particular cases.