The differential diagnosis for this bony fragment at the medial malleolus includes:

• fracture - unlikely as non-tender and well corticated
• prior trauma - if prior history of trauma
• os subtibiale - lies in association with the posterior colliculus; this ossicle lies in relation to the anterior colliculus
• unfused malleolar accessory ossification center - smaller than os subtibiale, more rounded and lie anteriorly

Given appearances and history an unfused malleolar accessory ossification or old avulsion fracture are favored.