The os intermetatarseum is an accessory ossicle located at the base of the first or second metatarsal, between the first and second metatarsal bases or in the cuneonavicular region. Present in approximately 1-3% of individuals, it is usually asymptomatic but can contribute to Lisfranc joint complex crowding and may impinge on the deep peroneal nerve and extensor hallucis longus tendon at the first web space.
The os intermetatarseum is a diagnostic pitfall in Lisfranc injury assessment: it can be mistaken for a first cuneiform avulsion fracture. Its smooth cortical margins and bilateral presence distinguish it from acute fracture. When symptomatic (dorsal first web space pain and deep peroneal nerve irritation), excision through a longitudinal first web space incision provides relief. Pre-operative identification prevents unnecessary investigation for Lisfranc instability.
An os intermetatarseum at the first-second metatarsal base interval is identified on foot radiograph after dorsal midfoot trauma and mistaken for a Lisfranc ligament avulsion fracture; smooth cortical margins and absence of first-second metatarsal diastasis on weight-bearing radiograph confirm the normal variant.
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