Os Peroneum

Os peroneum is one of several accessory ossicles of the foot and ankle, located lateral to the cuboid within the distal peroneus longus tendon. Like most accessory ossicles, os peroneum is generally asymptomatic and detected incidentally on imaging, though its intratendinous location can result in its involvement in pathologies affecting the peroneus longus tendon. Painful os peroneum syndrome is a general term referring to conditions presenting as lateral midfoot pain associated with the os peroneum, such as acute fracture, stress fracture related to chronic repetitive trauma, contusion, avascular necrosis, or tendinosis, tenosynovitis, tendon tears, and subluxation involving the peroneal tendons. Other causes of lateral midfoot pain also include ligamentous injury (talofibular or calcaneofibular), fractures of the base of the fourth or fifth metatarsal bone, anterior calcaneus, or cuboid.  Routine foot radiographs are a common first step in assessing lateral midfoot pain. Abnormalities in the morphology and location of the os peroneum can easily be identified on radiographs, specifically the oblique view, thereby elevating clinical suspicion for a peroneus longus tendon injury.