The os peroneum is a sesamoid ossicle embedded within the peroneus longus tendon as it curves around the lateral foot in the peroneal groove at the cuboid (cuboid notch). Present as a bony ossicle in approximately 20% of individuals (the remainder have fibrocartilaginous versions), it is located at the lateral plantar aspect of the cuboid between the calcaneus and the fifth metatarsal base.
The os peroneum is clinically relevant as a site of pain and instability in peroneus longus tendon disease. Painful os peroneum syndrome (POPS) occurs from os peroneum fracture, diastasis of a bipartite os peroneum, entrapment in the cuboid tunnel, or adjacent peroneus longus tendon tear. MRI and CT differentiate these from the normal variant appearance. A fractured os peroneum has irregular cortical margins and marrow oedema on MRI, distinguishing it from a bipartite normal variant.
The os peroneum becoming entrapped or fractured in the cuboid tunnel produces lateral plantar pain at the os peroneum level reproduced by resisted plantarflexion and eversion; MRI demonstrates peroneus longus tendon discontinuity adjacent to the ossicle and marrow oedema within the os, guiding surgical treatment by os peroneum excision with peroneus longus repair.
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