Results
In the shoulder model, unexpected external rotation of the arm occurred after two-plane motion. The rotation angle of the shoulder after motion increased as the forward flexion or abduction angle increased ( Table 1 ).
In the varization and derotation model, closed-wedge osteotomy with a proximal cutting plane perpendicular to the axis of the long bone did not lead to unexpected angular deformity after rotating the proximal fragment. In this situation, the angle of inclination of the cutting plane of the proximal fragment was 0 degrees. Conversely, closed-wedge osteotomy with a distal cutting plane perpendicular to the axis of the long bone resulted in unexpected angular deformity after rotating the proximal fragment. As the proximal fragment was rotated internally, extension on the sagittal plane and valgus on the coronal plane occurred. The valgus on the coronal plane and the 3D angle increased as the internal rotation angle increased, and extension on the sagittal plane increased as the internal rotation angle increased at an internal rotation of less than 60 degrees. As the angle at the hinge point of the closed wedge (angle α) increased, unexpected angular deformity to the axis of the distal fragment also increased ( Table 2 ).
In a femoral 3D reconstruction model of the right leg, the femoral neck shaft angle was 128.8 degrees, and the degree of femoral anteversion was 18.8 degrees. After FDO at the intertrochanteric area, unexpected varus on the coronal plane and extension on the sagittal plane were observed ( Table 3 ). The angulation on the coronal plane was more affected than that on the sagittal plane. The 3D angle of the deviated mechanical axis was greater than 5 degrees when the derotation angle was larger than 33 degrees.