Background: Within the hip joint, the anatomy of the acetabulum and cotyloid fossa is well established. There is little literature describing the association between the size of the cotyloid fossa relative to the acetabulum and characteristics of patients with femoroacetabular impingement (FAI). Purpose/Hypothesis: The purpose was to calculate the cotyloid fossa coverage percentage in the acetabulum and determine its association with patient characteristics, radiographic parameters, intra-articular findings, and preoperative patient-reported outcomes in patients with FAI. We hypothesized there is an association between the cotyloid fossa coverage percentage of the acetabulum and characteristics of patients with FAI. Study Design: Cross-sectional study; Level of evidence, 3. Methods: Patients were included who underwent standard clinical 3-T magnetic resonance imaging of the hip and primary arthroscopic FAI correction surgery during 2015 and 2016. Exclusion criteria were age 40 years, osteoarthritis, labral reconstruction, previous ipsilateral hip surgery, and hip dysplasia. Measurements of the cotyloid fossa and surrounding lunate cartilage were performed to calculate cotyloid fossa width (CFW) and cotyloid fossa height (CFH) coverage percentages. The relationships between coverage percentages and patient characteristics and intraoperative findings were assessed using independent t tests or Pearson correlations. Results: An overall 146 patients were included. Alpha angle negatively correlated with CFH coverage percentage (r = -0.19; P = .03) and positively correlated with labral tear size (r = 0.28; P < .01). CFH coverage percentage was negatively correlated with labral tear size (r = -0.24; P < .01). Among patients with degenerative tears, CFH was negatively correlated with labral tear size (r = -0.31; P < .01). However, this association was no longer significant after adjusting for sex (partial r = -0.10; P = .39). Cotyloid fossa coverage was not associated with the condition of the cotyloid fossa synovium (synovitis vs no synovitis). CFW coverage percentage was negatively correlated with the 12-Item Short Form Health Survey (SF-12) physical component summary score (r = -0.23; P < .01). Conclusion: The CFW and CFH coverage percentages may be associated with alpha angle, labral tear size, and SF-12 physical component summary score in patients with FAI. We may be able to predict the labral condition based on preoperative measurements of CFH and CFW coverage percentages.
