Femoral shaft bowing influences the correction angle for high tibial osteotomy

Background The optimal femorotibial angle (FTA) after high tibial osteotomy (HTO) is still controversial. Our hypothesis was that FTA itself may not be reliable because FTA cannot represent the accurate alignment of the whole lower extremity. Methods Non-weight-bearing radiographs of the lower extremities were taken in 100 Japanese subjects with medial osteoarthritic knees, and seven anatomic parameters were assessed. The correction angle by FTA was calculated so that the postoperative FTA was set at 166 degrees (14 degrees valgus). Another correction angle was calculated so that the mechanical axis passed through the lateral one-fourth of the tibial articular surface after HTO. After the correlation between two correction angles was assessed, influences of anatomic parameters on the discrepancy between two correction angles were assessed. Results There was a high correlation between two correction angles (R-2)= 0.777, P < 0.001). The mechanical axis passed through the lateral one-fourth of the tibial articular surface when the postoperative FTA was set at 166 degrees in 80% of subjects. However, discrepancy between the two correction angles was 3 degrees or larger in 20% of subjects. Femoral shaft bowing and tibial shaft bowing significantly influenced the correction angles. Even though FTA was the same, the femoral head shifted medially in cases with lateral bowing of the femoral shaft, and the correction angle by FTA should be set larger. On the other hand, the correction angle by FTA can be set smaller in knees with medial bowing of the femoral shaft. Tibial shaft bowing also influences the correction angle by FTA. Conclusions The correction angle by FTA for HTO should be calculated taking femoral and/or tibial shaft bowing in the frontal plane into account.