Ball and socket ankle joint in metatarsus adductus varus. (S-shaped or serpentine foot).

Metatarsus adductus varus is characterised by fixed adduction and supination of the forefoot and valgus deformity of the heel. On weight-bearing this valgus becomes more pronounced, thus enabling the forefoot to pronate and increase its area of contact with the ground (Figs. 1 and 2). This rare and unyielding deformity of the forefoot and hindfoot must be clearly distinguished from the common metatarsus varus ofinfancy which is often passively correctable from the first. Resolution is spontaneous in about 88 per cent of such feet (Ponseti and Becker 1966). Metatarsus adductus varus on the contrary is an obstinate deformity which resists correction : although the forefoot deformity may yield, the valgus heel defies us. We believe that this difficulty may be due to a ball and socket anklejoint (Figs. 3 and 6). This component in metatarsus adductus varus has apparently not been previously recognised and is of some significance because increased lateral mobility of the ankle will frustrate any attempt to correct the valgus deformity of the heel at the subtalar joint.