(A) Reference level, intersection between the widest intercondylar diameter and perpendicular line at the middle of both lines. The intersection point is Cartesian or coordinate point (0, 0), (B) center of the medullary canal is represented by the center of the red circle/ellipse best fitting the internal diameter of the medullary canal, (C)–(F) shows the movement of the instantaneous center of the medullary canal along the Y-axis (sagittal plane) and X-axis (coronal plane).