This study aimed to assess the validity and reliability of spheno-occipital synchondrosis fusion stages as a skeletal maturity indicator by comparing it with the established Björk hand-wrist maturation (HWM) index and Baccetti cervical vertebral maturation (CVM) stages.

CBCT scans, hand-wrist radiographs, and lateral cephalograms of 177 patients (78 males, 99 females; aged 7.3–16.9 years) were evaluated to determine the stages of SOS fusion, HWM, and CVM, respectively. Spearman correlation coefficients and weighted kappa coefficients were used to assess the pairwise correlations and the degree of agreement. The diagnostic performance of each SOS stage for identifying its corresponding HWM or CVM stage was evaluated using ROC curve analysis.

A strong positive correlation and substantial agreement were observed between all pairs of skeletal maturity indicators in both sexes. Regarding agreement, SOS and HWM showed the highest concordance (males: κ = 0.781; females: κ = 0.788), followed by CVM and HWM (males: κ = 0.752; females: κ = 0.717), and then SOS and CVM (males: κ = 0.722; females: κ = 0.692). Among females, SOS and HWM showed the strongest correlation (r = 0.891), followed by CVM and HWM (r = 0.861), and SOS and CVM (r = 0.830). Among males, CVM and HWM had the highest correlation (r = 0.883), followed by SOS and HWM (r = 0.877), and SOS and CVM (r = 0.838).

SOS fusion demonstrated strong correlations and overall substantial agreement with both HWM and CVM, supporting its use as a valid and reliable indicator of skeletal maturity.