Fig. 2

Femoral and tibial coordinate systems. a Shows the femoral coordinate system. The femoral reference point is located at the most distal point on the mid-plane of the intercondylar notch. The Z-axis points proximally and passes through the centre of the femoral head. The X-axis passes the femoral reference point and is parallel to the line connecting the most posterior points of the two femoral condyles. The Y-axis is mutually perpendicular to the X- and Z-axes. b Shows the tibial coordinate system. The tibial reference point is located at the centre of the apex of the medial–lateral intercondylar ridge and was the lowest point on the bone surface. The Z-axis points proximally and passes from the centre of the ankle joint and through the tibial reference point. The X-axis passes the tibial reference point and is parallel to the line connecting the midpoints of the medial and lateral tibial condyles. The Y-axis is mutually perpendicular to the X- and Z-axes