The os vesalianum is a small accessory ossicle at the proximal lateral foot, located at or just proximal to the base of the fifth metatarsal styloid in the peroneus brevis tendon. It represents an accessory ossification centre that failed to fuse with the fifth metatarsal styloid. It must be distinguished from a Jones fracture (zone 2, metaphyseal-diaphyseal junction, more distal) and a tuberosity avulsion (zone 1, at the styloid base).
The os vesalianum is a diagnostic pitfall in lateral foot pain after ankle inversion injury: it can be confused with a fifth metatarsal avulsion fracture on radiograph. The distinguishing features are smooth rounded cortical margins (vs irregular fracture margins), bilateral presence on comparison views, and its location within the peroneus brevis tendon proximal to the actual styloid process. MRI demonstrates the ossicle within the tendon substance without adjacent bone marrow oedema unless the ossicle is symptomatic.
An os vesalianum at the fifth metatarsal base is identified on ankle inversion radiograph and misdiagnosed as an acute avulsion fracture; comparison radiograph of the contralateral foot demonstrating the same ossicle, smooth cortical margins, and location within the peroneus brevis tendon confirm the normal variant and avoid unnecessary immobilisation.
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