Background: It is still unclear whether morphological changes in hip disorders is a pathogenic or independent factor for the variations in leg alignment. The purpose of this study was to elucidate the characteristics of the change in leg alignment after total hip arthroplasty (THA) and the morphological factors affecting the ipsilateral and contralateral leg alignment. Methods: Both pre-operative and post-operative bilateral whole-leg radiographs in the standing position were taken in 100 patients who underwent THA. Hip-knee-ankle angle (HKAA), joint line convergence angle (JLCA), height of the hip center, lateral width to the hip center, femoral offset, and leg length discrepancy were measured. After the pre-operative legs were divided into the varus, neutral, or valgus groups, correlations between the change in HKAA and each hip morphological parameter were assessed. Results: The mean change in HKAA on the THA side was 0.8° in the varus direction, which was significantly correlated with JLCA change. On the leg ipsilateral to THA, if the pre-operative alignment was valgus, the medial shift of the hip center was significantly correlated with the varus change in HKAA. On the side contralateral to THA, the change in leg length discrepancy was a significant correlative factor to the varus change in HKAA, if the pre-operative alignment was valgus or neutral. Conclusion: The significant morphological parameters affecting the ipsilateral and contralateral leg alignment after THA were medial shifting of the hip center and the change in leg length discrepancy, respectively. Level of evidence: Level Ⅳ, Retrospective cohort study.
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