Abstract
Myositis ossificans circumscripta is a benign, self-limiting heterotopic ossification usually occurring after a traumatic event. Our case describes a rare etiology given the occurrence after vaccination in a child. Indeed, to date, only two cases of myositis ossificans circumscripta after COVID-19 vaccination have been reported, both in adults. In this first pediatric case, radiographs performed at appropriate time allowed for a conclusive tissue characterization and hence diagnosis. CT and MR scans performed early after clinical manifestation showed only progressive, severe edema of the soft tissue of the shoulder while the later acquired radiograph and radiographic follow-up demonstrated the heterotopic ossification in the deltoid. Rapid soft tissue swelling and edema, in correlation with a potential local traumatic event, even if atypical, should raise suspicion of myositis ossificans circumscripta. With proper timing (at least 10 days after the trigger), radiographs can be crucial in showing the typical ossification pattern.