Bilateral os cuneo-I metatarsale-I dorsale: a rare case in which serial imaging suggested a localized ossification disturbance.

Os cuneo-I metatarsale-I dorsale is an exceptionally rare accessory bone located on the dorsal aspect of the medial cuneiform. This report describes a case of symptomatic bilateral os cuneo-I metatarsale-I dorsale in a 9-year-old boy who was a competitive kendo practitioner. Longitudinal imaging revealed development of a superficial subchondral irregularity into a distinct ossified fragment over a 1-year interval, providing novel evidence that an ossification disorder contributes to the etiology of os cuneo-I metatarsale-I dorsale. Unlike the cutaneous nerve entrapment typically associated with an accessory bone around the medial cuneiform, the clinical, intraoperative, and magnetic resonance imaging findings in this case showed that mechanical impingement of the extensor hallucis longus tendon was the primary source of pain. Surgical excision was effective, resulting in complete resolution of symptoms and a full return to sports. This report affords a rare longitudinal insight into the pathogenesis of this accessory bone and underscores the importance of accurately determining the source of pain by physical examination.