Fig. 4.

Boot coordinate system is found by identifying locator points (shown circled). The position of the peg is first adjusted so that it is collinear with the mechanical axis of the tibia. The vector connecting the two points on the boot peg forms the superior-inferior axis or the z-axis, corresponding to the tibial mechanical axis. The cross product of a vector connecting the proximal boot peg point and the anterior point with the z-axis forms the medial-lateral axis or x-axis. Finally, the cross product of the z-axis with the x-axis forms the y-axis. The origin of the boot coordinate system is unimportant for later calculations but is arbitrarily set as the proximal boot peg point.