The use of constrained components in total hip arthroplasty.

The use of a constrained component may be appropriate for the surgical treatment of recurrent dislocation due to soft-tissue insufficiency following a total hip arthroplasty. Constrained components usually include a locking mechanism incorporated into the polyethylene acetabular liner to keep the prosthetic femoral head in place. Two different prosthetic designs are available and have been approved by the U.S. Food and Drug Administration. The S-ROM constrained component uses additional polyethylene in the rim, which deforms to more fully capture the femoral head and then is held in place by a metal locking ring. The Howmedica Osteonics constrained component is a tripolar device; its bipolar component articulates with another polyethylene liner. These constrained components transfer hip forces that would otherwise lead to dislocation to the locking mechanism, the liner-shell interface, or the bone-prosthesis interface. These forces may eventually contribute to failure of the component due to loosening, dissociation, breakage, or recurrent dislocation. Studies of these components show a failure rate of 4% to 29% at relatively short-term follow-up.