9. The Cavus Forefoot

The metatarsal formula is usually of square, occasionally of Greek or Egyptian configuration. There is longitudinal balanced or varus pes cavus and the condition is characterised by the appearance of a transverse concavity under the ball of the foot associated with typical callus formation under the first and fifth metatarsal heads, displacements at the tarsometatarsal joint line and displacements of the metatarsophalangeal joints with contractures of the integument and capsulotendinous structures. Initially the foot may be supple with a precavus shape, but the longitudinal cavus progressively increases and deviates into valgus or varus. Rarely, the condition may be associated with a flat contracted foot. As well as the transverse concavity at the metatarsal heads, there may be increased cavus at the level of the talonavicular joint line or at the midtarsus corresponding to the cuneiforms and the cuboid. Increased cavus at the midtarsal level results in a reduction in frontal convexity at the Lisfranc joint line; there follows an associated increase in equinus of the antetarsus generally and an absence of divergence of the metatarsals, so that the antetarsus is maintained compressed as well as in equinus. The metatarsals are parallel, and in the majority of cases, they are of equal length; the central three exhibit a relatively reduced equinus. In these circumstances the cavus of the foot is balanced. In some instances, there is congenital shortening of the first metatarsal, increasing the degree of equinus necessary for the head to reach the ground and achieve collateral stability, in which case the cavus becomes accompanied by a varus posture.