EP1.65 The Sourcil Index identifies cam-hips with occult dysplasia: a novel mixed morphology requiring careful surgical strategy

Abstract Introduction Femoroacetabular impingement (FAI) and hip dysplasia are typically managed as distinct pathologies. However, patients with overlapping mixed morphologies often experience accelerated osteoarthritis. Traditional 2D radiographic metrics fail to fully capture the biomechanical risk of this overlap. This study aimed to characterise structural variation across isolated and mixed morphologies using automated 3D-CT metrics, and to identify occult structural vulnerabilities that influence surgical decision-making. Method 235 symptomatic hips were retrospectively analysed using CT-based morphometrics. Patients were classified into normal, isolated cam, isolated dysplasia, and mixed (cam-dysplasia) groups based on alpha angles and lateral centre-edge angle (LCEA). Secondary measurements included the sourcil index (SI) and femoral head coverage. A sub-analysis examined a novel "short weight-bearing cam" (SWB-Cam) subgroup (n=14), defined as hips with a normal LCEA but a deficient SI (<56°). Results Cam-dysplasia hips demonstrated significantly reduced anterior and posterior femoral head coverage and a lower SI compared to isolated morphologies (p<0.05). Strikingly, SWB-Cam hips showed significantly larger anterior cam deformities at 2-o'clock (66.3° vs. 59.8°; p=0.045) and 3-o'clock (58.7° vs. 52.9°; p=0.021) than isolated cam hips. Furthermore, larger cam lesions inversely correlated with smaller sourcil indexes (p=0.03). Discussion Mixed cam-dysplasia and SWB-Cam hips exhibit severe structural vulnerabilities missed by traditional 2D metrics like the LCEA. Furthermore, we identified a high-risk subgroup with a normal LCEA but deficient sourcil, which paradoxically presented with the largest anterior cam deformities. These ‘occult dysplastic’ hips may be at high risk for iatrogenic instability and surgical failure if managed with isolated arthroscopic osteochondroplasty without addressing the underlying functional undercoverage.