Fig. 2

Coordinate systems of each bone. A Coordinate system of the tibia was defined as reported by Sato et al. and the International Society of Biomechanics. The rotations around X, Y, and Z were defined as the varus/valgus, external/internal rotation, and flexion/extension, respectively. B After three-dimensional (3D)–3D surface registration of the femur, the relative movement of the tibia due to weight-bearing was evaluated in the sagittal, coronal, and axial planes