Radiographic features that enable assessment of first metatarsal rotation: the role of pronation in hallux valgus

Abstract

This study describes a method of detecting first metatarsal pronation on the basis of the movement of the inferior tuberosity of the base of 20 cadaveric first metatarsals at 0°, 10°, 20° and 30° pronation. On pronation, the inferior tuberosity of the base of the first metatarsal moved lateral to the mid-line axis. At 10°, the tuberosity pointed to the junction of the inner third and outer two-thirds of a line between the midpoint and lateral tubercle of the base. At 20°, it pointed to the junction of the inner two-thirds and outer third of that line. At 30°, it pointed to the outer margin of the lateral third. Using these features, the amount of first metatarsal pronation in 100 consecutive weight-bearing views of feet was recorded and plotted against the corresponding intermetatarsal angles in those feet. Four of 43 patients with an intermetatarsal angle of less than 9° had pronation greater than 10°, 48 of 57 patients with an intermetatarsal angle greater than 9° had pronation greater than 10° (P<0.001). As intermetatarsal angles increase, the amount of first metatarsal pronation increases (r= 0.69). Pronation and varus deviation of the first metatarsal are linked; both alter the tendon balance maintaining proximal phalanx alignment and lead to the development of hallux valgus.