Association between functional combined anteversion and dislocation after revision total hip arthroplasty

BACKGROUND: Dislocation is a serious complication that should be avoided in total hip arthroplasty (THA). Combined anteversion (CA) of the cup and stem is a concept for appropriate implant positioning; however, the effect of functional changes in femoral rotation has not been well investigated. In this study, we investigated whether functional CA, considering femoral rotation, is associated with dislocation in patients who underwent revision THA. METHODS: Overall, 82 patients who underwent revision THA and had at least one year of follow-up with pre-operative and post-operative supine computed tomography imaging were included. The cup and stem were placed with a target combined angle of 37.3° using Widmer's formula. Anatomical and functional CAs were calculated post-operatively. Functional CA was defined as the sum of cup anteversion and stem anteversion, with femoral external rotation. Patient demographics, component alignment parameters, CA, and their association with post-operative dislocation were statistically evaluated. RESULTS: Dislocation was observed in 12 patients. In these dislocated cases, there were no significant differences in cup angle, stem angle, and anatomical CA compared to non-dislocated cases. However, dislocated cases showed significantly higher values of functional CA (50.0 ± 17.4° [range, 5.5-67.6] vs. 35.6 ± 13.0° [range, 4.0-68.8], p = 0.022) and significant deviation from identical CA [15.0 ± 8.9° [range, 3.1-31.8] vs. 7.5 ± 8.1° [range, 0.1-33.3], p = 0.014). CONCLUSIONS: Functional CA, considering femoral rotation, was associated with post-operative dislocation after revision THA. Therefore, consideration of femoral rotation may be important for implant positioning in revision THA.