Evaluation of femoral anteversion, hip rotation, and lateral patellar tilt after total hip arthroplasty using a changeable neck system

In patients with secondary osteoarthritis due to acetabular dysplasia, femoral anteversion has many variations. A changeable neck system is one useful option to adjust the femoral anteversion. Retroverted necks can effectively adjust anatomical anteversion (AA), femoral rotational angle (FRA), and functional anteversion (FA); however, effectiveness of anteverted necks for these adjustments has not been investigated. Moreover, although the lateral patellar tilt after total hip arthroplasty (THA) has been reported to externally rotate, the influence on lateral patellar tilt using a changeable neck system remains unknown. To clarify the effectiveness of anteverted necks in THA, 96 consecutive patients (111 hips) who underwent THA using anatomical short stem with a changeable neck system were retrospectively investigated using pre- and post-operative computed tomography. Patients were divided into the straight (ST) group using straight and 4-mm-high-offset neck (N = 34) and the anteverted (AV) group using 15°-anteverted and 15°-anteverted/3-mm-high-offset neck (N = 34) after age, body mass index, and surgical approach were matched using propensity scores. AA did not change in the ST group, while it increased by 14.0° in the AV group. FRA decreased after surgery in both groups. FA decreased after surgery in the ST group, while it did not change in the AV group. Lateral patella tilt did not significantly change in both groups between pre- and post-operative position. In conclusion, with a changeable neck system, straight and anteverted necks can adjust AA to achieve pre-operative planning while not influencing lateral patellar tilt.