MRI prevalence and characteristics of supraacetabular fossae in patients with hip pain

Background: Supraacetabular fossae (SAF) are normal anatomic variants found at the 12 o’clock position of the acetabulum and can be mistaken for osteochondral defects. Purpose: Determine SAF prevalence on MRI of patients with hip pain. Characterise SAF by type, size, and location. Determine sensitivity of radiology reports and sensitivity of radiographs in SAF detection. Methods: MRIs performed over 1 year in patients with hip/groin pain were analysed for SAF. Measurements of SAF in the coronal and sagittal planes and location using clockface notation were recorded. Radiology reports were reviewed to determine if SAF were identified and radiographs were assessed for presence of SAF. Mean characteristics of type 1 and 2 SAF were compared using Student’s t-test. Results: 214 hips (mean age 35.9 ± 14.2 years; 66.8% female) were analysed. Twenty-seven hips (12.6%) had SAF. There were five type 1 SAF (mean age 16.8 ± 2.2 years) and 23 type 2 SAF (mean age 33.0 ± 16.3 years). Mean dimensions of type 1 and 2 SAF (coronal width × sagittal width × depth, in millimetres) were 7.1 × 6.5 × 3.4 and 6.1 × 5.9 × 2.8, respectively. Mean clockwise location in the coronal and sagittal planes (in minutes) was 1236 and 1212 for type 1 SAF and 1213 and 1207 for type 2 SAF. Radiology reports identified 7.1% of SAF identified on MRI. Plain radiographs demonstrated 67.9% of SAF identified on MRI. Conclusion: This study found a 12.6% prevalence of SAF in hips. Radiology reports had a 7.1% sensitivity in correctly identifying SAF on MRI. IRB: Pro00016584.