Fig. 1

Quantification of the torsional angle of the first metatarsal head. a The surface model of the first metatarsal (1MT) is generated, and two characteristic points are chosen from the most dorsal and plantar edges of the proximal metatarsal surface to define the orientation of the base of the 1MT (vector A). b The articular surface of the metatarsal head is approximated by a cylindrical shape with a least-square method, and the orientation of the cylindrical axis in the direction of the fifth toe toward the hallux is calculated (vector B). c The angle between the vectors A and B (θ) minus 90° is defined as the torsional angle of the 1MT. The angle is positive for eversion