Impact of Hip Rotation Angle Following Total Hip Arthroplasty with Leg Lengthening

<b>Background/Objectives</b>: Few studies report on hip rotation after total hip arthroplasty (THA); however, details of the factors affecting the hip rotation angle are unknown. We aimed to investigate the factors related to hip rotation after THA. <b>Methods</b>: This study included 124 consecutive patients who underwent THA. We retrospectively analyzed the correlation between changes in the rotation angle of the femur relative to the pelvis, global femoral offset, and femoral version and leg lengthening. Moreover, we performed a multivariate regression analysis of these parameters to calculate the efficacy of the change in the rotation angle of the femur relative to the pelvis. <b>Results</b>: Leg lengthening and femoral version change were negatively correlated, whereas change in global femoral offset was positively correlated with leg lengthening, with correlation coefficients of 0.376, 0354, and 0.334, respectively. Regarding the multiple regression analysis, only leg lengthening was correlated with the change in rotation angle of the femur relative to the pelvis, with a coefficient of -0.336. <b>Conclusions</b>: The change in the rotation angle of the femur relative to the pelvis is only associated with leg lengthening in multivariate analysis. In actual planning, in cases where the hip is internally rotated, it may be better not to excessively increase leg length, decrease anterior stem anteversion, or increase global femoral offset. Thus, physicians should avoid large leg lengthening for patients with highly external rotation in their hip joint as it may lead to increased internal rotation of the hip, consequently resulting in relative malpositioning and subsequent implant impingement and/or dislocation following THA.