Failure mechanism of a constrained liner: a case report.

Instability after primary and revision total hip arthroplasty continues to be a problem. The use of a constrained system helps manage this problem. A new constrained total hip arthroplasty, Trilogy constrained liner (Zimmer, Warsaw, IN), is currently in use. We report a case showing dislocation following a Trilogy constrained total hip arthroplasty. In this case, when an impingement between the femoral neck and the anterior part of the polyethylene liner occurred, the hip dislocated easily although both polyethylene and reinforcing ring were properly positioned. The lever-out test showed that the Trilogy constrained liner is safe and compares favourably with other implants. Surgeons should be aware that constrained acetabular systems are not infallible and they should pay attention to place implants in good position even when constrained THA is performed.