Establishing Normative Cartilaginous Acetabular Index Values in North American Children for Evaluation of Developmental Dysplasia of the Hip

BACKGROUND: Cartilaginous acetabular index (CAI) factors in the cartilaginous component of acetabular morphology and may enhance evaluation of developmental dysplasia of the hip (DDH). No studies have established normal CAI values in North American children to date. The purpose of this study was to describe CAI in young patients and to determine the inter- and intrarater reliability of this measurement. METHODS: Patients aged 6 months to 5 years who underwent pelvic MRI between 2010 and 2025 were included. We excluded patients with bilateral DDH, prior hip surgery, neurological disease, or other congenital abnormalities. Measurements were performed by 6 observers of varying clinical experience and backgrounds. Age-specific CAI values were compared with previously established age-specific acetabular index (AI) values on X-ray in the same cohort using Pearson correlation. RESULTS: Forty-eight normal hips (24 girls and 24 boys; 4 of each sex per age group) were analyzed. The mean CAI decreased from 17.3 degrees (±2.6 degrees) at 6 months of age to 9.4 degrees (±1.4 degrees) at 5 years of age. Intrarater reliability was fair to excellent across raters, and inter-rater reliability was good. There was a strong, positive correlation between AI and CAI across age groups (r=0.90, 95% CI: 0.34-0.99, P=0.014). CONCLUSIONS: In this single-institution study, we provide preliminary age-specific reference CAI values for North Americans aged 6 months to 5 years. This measurement demonstrated fair to good to excellent intra- and inter-rater reliability, and there is a strong, positive correlation between CAI on MRI and AI on X-ray. These findings support the use of CAI as a reliable measurement in the assessment of DDH and suggest its potential role as an adjunct to existing radiographic parameters. LEVEL OF EVIDENCE: Level IV.