Os subcalcaneum.
An 11-year-old female presented with a history of pain in the left heel. Radiologic studies of both feet displayed a bony mass unilaterally. This bony mass had been identified radiologically as being separate from the posterior portion of the calcaneus. Clinically, the os subcalcaneum displayed a syndesmotic attachment to the calcaneus. Removal of the ossicle rendered the patient aysmptomatic.
Calcaneus↗