This ossicle may occasionally be symptomatic, presenting as dorsal foot pain or paraesthesia with numbness over the first intermetatarsal space, probably due to compression of the branches of the deep peroneal nerve.( 12, 13) The onset usually follows trauma, with other reported inciting factors such as tight footwear and cavus feet.( 12) The os intermetatarseum may also be mistaken for a fracture at the base of the second metatarsal, which occurs in Lisfranc injuries. This is in keeping with a fractured os intermetatarseum. (c) CT image shows that the bony opacity is well corticated, apart from a fracture at its distal aspect (arrow). (a) Foot radiograph and (b) coronal CT image show a bony opacity (circle in a & arrow in b) between the bases of the first and second metatarsals. 10) may show abnormal T2 signal within the ossicle and tendinosis of the peroneus longus tendon.( 2) Ultrasonography may also be used to demonstrate inflammatory change in the soft tissue, and tendon tears or tenosynovitis of the peroneus longus tendon.( 10) 10) or degeneration.( 8) It can result in a tear of the peroneus longus tendon that is evidenced by a displaced os peroneum on imaging.( 10) MR imaging ( Fig. This can be due to fracture of the ossicle ( Fig. The os peroneum may become symptomatic, presenting as lateral foot pain( 9) and tenderness, which is also known as painful os peroneum syndrome. (b) This is similarly witnessed on the oblique view (arrow), raising the possibility of a fracture of the ossicle. This represents the os peroneum which, in this case, appears fragmented. (a) Lateral ankle radiograph shows a few bony opacities (arrow) projected over the calcaneocuboid joint in an 81-year-old patient who presented with ankle pain. Its overall prevalence is about 2%–21%, making it the second most common accessory ossicle.( 2) The accessory navicular is bilateral in about 50% of cases( 2) and is more common in women. 3), which makes up the remaining 20% of cases, is a fused ossification centre, resulting in a prominent median eminence,( 1) and there is no synchondrosis. 2), seen in about 50% of cases, is located close to the median eminence of the navicular bone, forming a flat facet with an intervening synchondrosis, which is a cartilaginous interface between the accessory navicular and the navicular bone. It is typically small, measuring about 2–3 mm in size, does not have a cartilaginous interface with the navicular and is considered to be a sesamoid bone of the posterior tibial tendon.( 1) Type 2 accessory navicular ( Fig. 1) is found within the posterior tibial tendon, close to its attachment to the navicular, and makes up about 30% of cases. Three types of accessory navicular have been described. This paper aimed to provide a pictorial representation of the more commonly encountered accessory ossicles of the foot and ankle, and showcase their anatomic features, resulting pathology and potential pitfalls in diagnosis. Higher modalities, such as computed tomography (CT), magnetic resonance (MR) imaging and even scintigraphy, are used in cases with diagnostic difficulty or when resulting pathology is suspected. They may also contribute to or exacerbate underlying pathology, giving rise to symptoms. At times, they mimic fractures or loose bodies, proving to be a diagnostic conundrum, thus leading to misdiagnoses. Accessory ossicles can be found adjacent to various joints, such as the wrist, shoulder, hip or knee, but the foot and ankle are relatively common locations.Īccessory ossicles are usually an incidental finding on radiographs and often overlooked. They may, however, also be the result of prior trauma. They result from unfused ossification centres and are frequently congenital. Accessory ossicles are well-corticated bony structures found close to bones or a joint.
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