An os intermetatarseum is found between the bases of the first and second metatarsals. It varies in shape and size from a small rounded ossicle to a fusiform one half as long as the adjacent metatarsal. It may be a distinct ossicle or attached to the first or second metatarsal or to the medial cuneiform. It is found in about 1.5% of the population (the case shown here was found after going through about 160 foot x-rays).
Os intermetatarseum is usually asymptomatic, but it can fracture or be associated with a varus deformity of the first metatarsal and with hallux valgus.
For such a tiny ossicle, the origin of the os intermetatarseum is controversial. It has been variously described as a vestigial sixth digit, a sesamoid, or a developmental variant of the medial cuneiform. In support of the first two, it can be found in an accessory tendon of the first dorsal interosseous muscle or in the tendon of a vestigial plantar interosseous tendon.
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