Fig. 2

Coordinate systems. In red the baseline osteotomy planes. A coordinate system in the middle of the femoral shaft on the level of the osteotomy-plane was defined. According to the y-axis (blue) internal- respectively external rotation of the distal fragment was performed. On the left side the z-axis (green) is a projection of the femoral neck axis on the osteotomy plane. On the right side the z-axis (green) is a projection of a tangent to the posterior femoral condyles on the osteotomy plane. The x-axis (red) in both figures is the cross product of the y- and z-axis