Trends in cup position utilizing computer-assisted navigation during total hip arthroplasty: A retrospective observational study

Background: Proper positioning of the acetabular cup deters dislocation after total hip arthroplasty (THA) and is therefore a key focus for orthopaedic surgeons. The concept of a safe zone for acetabular component placement remains widely utilized in contemporary THA practice; however, components positioned in this safe zone still dislocate. The purpose of this study was to characterize trends in acetabular cup positioning and to determine if the surgical approaches used affected the rate at which the components were placed in the safe zones. Methods: This retrospective, observational study reviewed acetabular cup position in 1236 patients who underwent THA using computer-assisted navigation. Outcomes included acetabular cup position measurements derived from a surgical navigation device and surgical approach. Results: The overall mean cup position of all recorded cases was 21.8 degrees (+/- 7.7 degrees, 95% CI=6.7 degrees, 36.9 degrees) of anteversion and 40.9 degrees (+/- 6.5 degrees, 95% CI=28.1 degrees, 53.7 degrees) of inclination. For anteversion and inclination, 65.5% (809/1236) of acetabular cup components were within the Lewinnek safe zone, and 58.4% (722/1236) were within the Callanan safe zone. Acetabular cups were placed at a mean of 6.8 degrees of anteversion (posterior/lateral approach: 7.0 degrees, anterior approach: 5.6 degrees) higher than the Lewinnek and Callanan safe zones, whereas inclination was positioned 0.9 degrees higher than the reported Lewinnek safe zone and 3.4 degrees higher than the Callanan safe zone. Conclusions: Our data show that while most acetabular cups were placed within the traditional safe zones, the mean anteversion orientation is considerably higher than those suggested by the Lewinnek and Callanan safe zones. The implications of this observation warrant further investigation.