Geometric model to represent the orientation of the stem implant. Left: coronal view, right: sagittal view. First, a given neck-shaft angle (NSA) angle was applied. Then, the orientation of the stem implant was achieved by the following rotations: rotation around the cranial-caudal axis by the initial antetorsion (iAT) angle, rotation around the medial-lateral direction by the initial femoral tilt (iFT) angle, and rotation around the anterior-posterior axis by the varus-valgus (VV) angle. The cranial-caudal direction was defined by the mechanical axis and the coronal plane by the plane spanned by the mechanical axis (MA) and the direction of the posterior condyle axis. Further abbrevations: neck axis (NA), proximal shaft / stem axis (SA).