Fig. 2

Coordinate system of the tibia. The tangent is set behind the tibial condyle (line 1) at the top level of the head of the fibula, and it is fitted onto the medial and lateral tangents perpendicular to the posterior tangent (lines 2 and 3), and the anterior tangent is set to create a rectangle (line 4). Z-axis: transverse bisector line of the rectangle. X-axis: anteroposterior bisector line of the rectangle. Y-axis: vertical line to the X-Z plane