Fig. 2

a The centers of medullary cavity from all layers are marked and all center points are automatically fitted into an arc, as shown the blue circle in a and the green arc-part of the circle in c and d. Because of its small radius, it can be approximately fitted into a straight line, as shown the red line in a, c and d representing the alignment of the medullary cavity. b Establish the coronal plane which simulates the coronal field of vision shown during surgery. c and d The blue line is the alignment line which can reflect the coronal extramedullary system alignment, and the osteotomy plane as shown the blue section plane in c; the red line in a, c and d representing the alignment of the medullary cavity, and the osteotomy plane perpendicular to the alignment as shown the red section plane in c