Redefining the Acetabular Component Safe Zone for Posterior Approach Total Hip Arthroplasty.

BACKGROUND Acetabular component orientation influences joint stability in total hip arthroplasty (THA). The purpose of this study was to evaluate the effect of cup orientation and other variables on hip dislocation risk and to define a posterior approach specific safe zone. METHODS A cohort of 1289 posterior approach primary THA cases was prospectively followed and component position measured radiographically. RESULTS Cup malposition, with respect to the Lewinnek safe zone, was an independent risk factor for dislocation (OR1.88). Modifying the anteversion safe zone limits to 10-25° strongly predicted increased dislocation risk (OR2.69). No dislocations occurred within a zone defined by a circle centered at 41.4° abduction and 17.1° anteversion, radius 4.3°. CONCLUSION Utilizing a posterior approach specific safe zone of 10-25° anteversion and 30-50° abduction may minimize THA dislocations. LEVEL OF EVIDENCE Level III.