Factors influencing cup orientation in 500 consecutive total hip replacements

We sought to establish the percentage of acetabular components used in total hip arthroplasties that were located outside a presumed safe range of cup orientation. Data were analyzed to assess whether dislocation in this series was different inside and outside that presumed safe zone. We also asked whether acetabular cup orientation depended on patient body mass index, the amount of preoperative acetabular head coverage, the surgeon, or the use of minimally invasive technique. We assessed cup orientation in 500 total hip arthroplasties performed at one institution. Of these 500 total hip arthroplasties, 400 were done using conventional approaches whereas mini-incisions were used in 100. We found 19.8% of cups were oriented outside the presumed safe range for inclination, and 11.2% of cups were oriented outside the presumed safe range for anteversion. Dislocation was not greater in the group with inclination and anteversion outside the presumed safe. Cup orientation was influenced by preoperative acetabular head coverage, the surgeon, and minimally invasive technique, but not body mass index. Cup variability was greater than expected. It was not confined to one surgeon, but to the entire group of surgeons experienced in doing total hip replacements. Variability points toward continuous refinement in surgical technique and instrumentation to promote consistency in cup placement.