Center of rotation restoration and acetabular cup sizing in total hip arthroplasty: the predictive role of radiographic femoral head diameter. A single-center study from the FP-UCBM hip study group.

PURPOSE: Restoring the center of rotation (COR) in total hip arthroplasty (THA) is essential. We hypothesized that relatively smaller cups are associated with COR displacement. Our aim is to evaluate how acetabular cup size (ACS), relative to the radiographically measured femoral head diameter (FHD), correlates with COR positioning in THA. METHODS: We retrospectively analyzed 251 patients undergoing unilateral THA using a posterolateral approach with a non-arthritic contralateral hip that was used to assess FHD on preoperative x-ray. From postoperative radiographs, cranial cup displacement (cCD) and medial cup displacement (mCD) relative to contralateral hip COR were measured. Patients were divided into three groups based on the difference between ACS and FHD (Δ): Group A (Δ≤2.5 mm), Group B (2.5 mm<Δ<8.5 mm), and Group C (Δ≥8.5 mm). Patients with a cCD <3 mm and mCD <5 mm were included in the "Restored COR" group; the others in the "Non-Restored COR" group. RESULTS: Mean cCD and mCD values were: 3.93 ± 4.29 mm and 5.13 ± 4.13 mm (Group A), 2.14 ± 3.25 mm and 2.56 ± 3.28 mm (Group B), 2.41 ± 2.75 mm and 2.06 ± 3.09 mm (Group C); Group A showed significantly higher cCD and mCD than Groups B and C (p < 0.001). In Group A, the relative risk (RR) of non-restored COR was 1.55 (95% CI 1.26-1.91; p < 0.001) compared to group B + C. Patients with Restored COR (46.6%) had lower FHD than patients with Non-Restored COR (53.4%). CONCLUSION: COR restoration in THA is influenced by cup size. The implant of cups with ACS ≤ FHD+2.5 mm is associated with a significantly increased risk of COR displacement, especially in patients with larger FHD. LEVEL OF EVIDENCE: III (retrospective comparative cohort study).