Knee Bone Age Estimation Using Capping Interval Index on MRI: Development and Validation of a Quantitative Method

Knee bone age can differ from hand bone age and is more relevant to lower-extremity pediatric conditions. This study developed and validated a quantitative method for estimating skeletal maturity using knee MRI images.

We identified 695 participants aged 18 years or younger without conditions affecting knee anatomy or maturation who underwent knee MRI. Participants were randomly allocated to the modeling (n=417) and validation (n=278) groups. Twenty-six maturation-related parameters were measured on coronal MRI, including the articular and peripheral cartilaginous thicknesses, physeal thicknesses, ossified epiphyseal width, and capping interval. The capping interval was defined as the distance from the medial or lateral metaphyseal end to the nearest ossified epiphysis. Each parameter was normalized to metaphyseal width and assessed by univariable linear regression against log-transformed chronological age. Top-ranked variables were entered into sex-specific multivariable models.

The sums of the medial and lateral capping intervals from the femur and tibia were the only parameters with R2>0.9 in both sexes and were defined as the femoral and tibial capping interval indices (CIIs), respectively. Final models were age=exp(-1.918[femoral CII]-1.797[tibial CII]+2.813) for males and age=exp(-2.056[femoral CII]-2.487[tibial CII]+2.738) for females. In the validation group, estimated knee bone age showed minimal difference from chronological age (0.1±1.1 y, P=0.534) and very strong correlation (rs=0.960).

Knee bone age estimated from the femoral and tibial CII showed close agreement with chronological age in the independent validation cohort. This study introduces a novel, simple quantitative method that enables estimation of bone age on knee MRI using a minimal number of parameters.

Level III-diagnostic.