Supraacetabular fossa is more prevalent in acetabular protrusion than in dysplastic hips

The supraacetabular fossa (SAF) is a bony fossa in the acetabular roof's load-bearing region. Its role in acetabular development and hip morphology remains unclear, with potential implications for hip preservation procedures. We aimed to (i) determine the prevalence of SAF by age, (ii) measure its dimensions in patients with hip pain undergoing magnetic resonance (MR) arthrogram, and (iii) assess associations between SAF and acetabular or femoral morphologies. We performed a retrospective analysis of patients with hip pain who underwent MR arthrograms. SAF were classified as type 1 (unfilled) or type 2 (cartilage-filled), and its dimensions measured. Acetabular and femoral morphology were assessed on radiographs, and hips were categorized into 13 morphology groups. 697 hips from 615 patients were included. SAF was present in 10.9% (76/697) of hips, with affected patients being younger (24.3 ± 7.6 years) than those without SAF (31.8 ± 11.2 years, P < 0.001). Type 1 SAF had a mean depth of 3.1 mm versus 4.8 mm for type 2 (P < 0.001). SAF prevalence was 30.8% in acetabular protrusion and 5.2% in dysplasia (P = 0.0135). Acetabular protrusion (OR 3.78, P = 0.03) were risk factors for SAF, but no associations were found with femoral morphology. SAF occurs predominantly in patients under 25 years and is nearly six times more common in acetabular protrusion than dysplasia. Future studies should investigate the implications of SAF on acetabular rim trimming and reorientation procedures, particularly focusing on their effect on residual joint contact forces.