Classification and management of the unstable total hip arthroplasty.

Seventy-five total hip arthroplasty revisions for instability were classified into 6 primary etiologies: I, acetabular component malposition; II, femoral component malposition; III, abductor deficiency; IV, impingement; V, late wear; or VI, unresolved etiology. The most common etiologies were cup malposition (type I; 33%) and abductor deficiency (type III; 36%). At a mean of 35.3 months, 11 redislocations occurred (14.6%). Acetabular revisions were protective against redislocation (P < .015). The number of previous operations (P = .0379) and previously failed constrained liners (P < .02) were risk factors for failure. Tripolar constrained liners demonstrated improved survivorship vs locking ring types (P < .02); cemented constrained liners failed more often than modular constrained liners (P < .0018). The highest risk of failure was in patients with abductor insufficiency with revisions for other etiologies having a success rate of 90%.