The alignment of the lower extremities after high tibial osteotomy (HTO) varies widely on the radiographs. It is difficult to assess the rotation change on the radiographs. We developed computer software to calculate the alignment of the lower extremities and to simulate HTO. The purpose of this study is to compare and evaluate the variance in the femoro-tibial angle (FTA) and hip-knee-ankle angle (HKA) in relation to rotational shift of the lower extremity on radiographs and to clarify how the rotational shift affects the FTA and HKA by using three-dimensional (3D) CT simulation software for HTO. The mean absolute rotation angle of the lower extremity position on radiographs was 7.8 degrees, ranging from an external rotation of 8 degrees to an internal rotation of 14 degrees. Within the same range of rotation, the mean change in the FTA was 3.7 degrees, the mean change in the HKA was 1.5 degrees.
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