Subperiosteal hematoma of the ilium in adolescent athletes with acute groin pain: clinical and MRI features of a radiograph-occult injury pattern.

Subperiosteal hematoma of the ilium is an uncommon MRI finding in young athletes with acute groin pain and is frequently occult on plain radiographs. This study aimed to characterize its clinical presentation, MRI findings, associated injuries, and return-to-sport outcomes. This retrospective single-center case series included athletes aged ≤22 years who underwent MRI for acute sports-related groin pain with negative or inconclusive radiographs between 2017 and 2025. Patients meeting predefined MRI criteria for subperiosteal hematoma of the ilium were identified, and clinical, imaging, and outcome data were analyzed descriptively. Twenty of 91 patients (22.0%) met the MRI criteria. Their mean age was 13.9 ± 1.5 years, 19 (95%) were male, and 18 (90%) had open physes. Periosteal elevation was present in all patients, and 19 (95%) demonstrated a predominantly subperiosteal fluid collection, most commonly with a lentiform configuration. Associated anterior inferior iliac spine avulsion injury or intramuscular iliacus injury was identified in 10 patients (50%). All patients were managed nonoperatively and returned to unrestricted sports participation after progressive rehabilitation at a mean of 4.5 ± 2.2 weeks. Subperiosteal hematoma of the ilium is a reproducible MRI finding in predominantly skeletally immature athletes with acute sports-related groin pain and negative or inconclusive radiographs. Recognition of periosteal elevation with a predominantly subperiosteal fluid collection may facilitate diagnosis of this radiograph-occult injury pattern. Although the underlying injury mechanism remains hypothetical, prospective studies are needed to clarify its clinical significance.