Is Restoration of Hip Center Mandatory for Total Hip Arthroplasty of Protrusio Acetabuli?

Background: While initial fixation by a press-fit of the acetabular cup is essential for the durability of the component, restoration of the hip center has been known as an attributable factor for implant survival and successful outcome. In protrusio acetabuli (PA), it might be difficult to obtain both restoration of the hip center and the press-fit of the acetabular cup simultaneously during total hip arthroplasty (THA). We tested a hypothesis that medialized cup, if press-fitted, does not compromise the implant stability and outcome after cementless THA of PA. Methods: We reviewed 27 cementless THAs of 23 patients with PA. During THA, we prioritized press-fit of the cup than the hip center restoration. A press-fit was obtained in 24 hips. In the remaining 3 hips, a press-fit could not be obtained, and reinforcement acetabular components were used. The hip center was restored in 18 cups; 15 primary cups and 3 reinforcement components, while it was medialized in 9 cups. We compared implant stability and modified Harris hip score (mHHS) between the 2 groups at a mean of 5.2 (2-16) year follow-up. Results: One restored reinforcement cup was loose. The remaining 26 cups; 17 restored cups and 9 medialized press-fitted cups, remained stable. The final mHHS was similar between the restored group and the medialized group (81.8 ± 10.8 vs 83.6 ± 12.1, p = 0.498). Conclusions: Press-fitted cups, irrespective of hip center restoration, rendered implant stability and favorable results. Initial fixation of the cup is more important than the restoration of hip center.