From the case:
Os intermetatarseum
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A well-corticated osseous density is noted between the first and second metatarsals proximally, as well as lateral to the cuboid.
Case Discussion
Accessory ossicles are commonly noted in the foot in characteristic locations, and are important not to mistake for fractures, heterotopic ossification, or other pathology. Os intermetatarseum is a rare ossicle of the foot, but is none-the-less typical in this case, between the proximal aspect of the 1st and 2nd metatarsals. The ossicle lateral to the cuboid represents an os peroneum, which is quite commonly observed.