The unstable total hip replacement.

Instability after total hip arthroplasty (THA) is not a rare occurrence. Numerous factors have been associated with dislocation including surgical approach, implant design, failure to restore proper hip mechanics and soft tissue restraints, and patient variables such as early postoperative compliance, soft tissue integrity, and neurologic conditions such as poor proprioception. A thorough understanding of the mechanism of dislocation, timing, and direction of dislocation is mandatory in formulating an approach toward treatment. The radiographic evaluation should evaluate hip mechanics including component orientation, adequacy of leg lengths, and restoration of offset in the frontal and sagittal planes. The treatment of the unstable total hip replacement is based on numerous variables including the timing of instability, direction, and mechanism. Instability in the early postoperative period in the hip with proper orientation and restored mechanics often is treated successfully with patient reeducation and use of adjunctive bracing. The treatment of the recurrent dislocator can be more difficult. Although reestablishing proper mechanics and orientation can be successful in many instances, some patients continue to have dislocation. Constrained acetabular liners have significantly improved the success rate of reducing the incidence of dislocation, but problems related to premature wear and dislodgement are a major concern. The use of larger articulating bearing surfaces used with the newer cross-linked polyethylene may provide the solution to instability minimizing the concerns related to constrained components.