Little has been reported on knee pain after total hip arthroplasty (THA). The purpose of this study was to investigate the incidence and mechanism of knee pain after THA. Two hundred fifty-two patients with hip dysplasia were clinically and radiographically assessed for knee pain before and after THA. Incidences of knee pain and patellofemoral alignment were analyzed with reference to postoperative change in leg length, femoral anteversion, and the femoral offset. Anterior knee pain was present in 16 patients (7.3%). Lateral patellar tilt was increased in all patients with knee pain and significantly larger compared to that seen in patients without knee pain. The increased patellar tilt disappeared within 3 months, but symptoms in 4 patients persisted for more than 3 months. The patellar tilt was significantly related to the amount of leg lengthening. This study demonstrates that THA influences the patellofemoral joint via leg lengthening and causes anterior knee pain.
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