Obturator Dislocations Following Direct Anterior Approach Total Hip Arthroplasty: Two Case Reports With Analysis of Implant and Spinopelvic Factors

Dislocation remains a potential complication after total hip arthroplasty (THA), even with the anterior approach, which is generally associated with lower instability rates. Obturator dislocation, although well-documented in native hips following high-energy trauma, is extremely rare in the context of THA and has not been previously described in patients with preoperative spinopelvic assessment. We report two cases of obturator dislocation following direct anterior approach THA in young female patients without medical comorbidities. Both dislocations occurred during extreme hip flexion combined with abduction and external rotation. In one case, closed reduction attempts initially failed, requiring reduction on a traction table. Radiographic and 3D analyses revealed shared features, including significant medialization of the acetabular component (10 mm), low combined anteversion (16° and 24°), and lumbopelvic mobility patterns dominated by hip flexion. Preoperative functional imaging showed no signs of spinopelvic imbalance or stiffness. Both patients exhibited a "hip user" profile with low pelvic incidence and distal lumbar lordosis apex. These cases highlight multiple converging factors contributing to instability: loss of offset due to cup medialization, stem retroversion resulting in low combined anteversion, and postoperative increase in compensatory hip flexion. Notably, no classic spinopelvic risk factors were present. These findings underscore the need for precise implant positioning, especially for acetabular positioning and femoral version.