Validation Study of the Thumb Ossification Composite Index (TOCI) in Idiopathic Scoliosis: A Stage-to-Stage Correlation with Classic Tanner-Whitehouse and Sanders Simplified Skeletal Maturity Systems

BACKGROUND: The new simplified thumb ossification composite index (TOCI) based on ossification of the thumb epiphyses and adductor sesamoid has demonstrated simplicity, excellent reliability, and high accuracy for predicting skeletal maturity, comparable with the Sanders simplified skeletal maturity system (SSMS). It was our belief that, because the terminology of the SSMS system has been commonly used for skeletal maturity prediction in idiopathic scoliosis in publications over the past decade, the clinical applicability of the TOCI system would increase if the stages in the 2 systems were found to be interchangeable and highly correlated. METHODS: Hand radiographs of 125 premenarchal girls with newly diagnosed adolescent idiopathic scoliosis who had been followed longitudinally until skeletal maturity were all scored with use of the Tanner-Whitehouse III (TW3) system (stages E through I), the TOCI, and the SSMS. The scores for the epiphyses of the ulnar 4 digits were compared with those for the thumb and correlated with the timing of peak height velocity. Correlations were analyzed with the chi-square test and Cramer V and Somers delta correlations. RESULTS: Six hundred and forty-five hand radiographs (an average of 5 for each girl with idiopathic scoliosis) and 11,517 epiphyses were scored. The rate of concordance between TW3 stages F, G, and I for the thumb proximal phalangeal epiphysis and those for all of the epiphyses of the ulnar 4 digits were 72.5%, 72.5%, and 89.9%, respectively. The overall concordance rate (including all epiphyses) was 71.3%, with a very high Cramer V correlation and significance (p < 0.01). High interchangeability was demonstrated for the TOCI and SSMS stages, supported by a high Somers delta correlation (>0.8) with significance (p < 0.05). CONCLUSIONS: The TOCI is highly practical for clinical use, and its stages are highly interchangeable with those of the SSMS. CLINICAL RELEVANCE: The TOCI could serve as a simplified “marker” of skeletal maturity on hand radiographs and minimize the learning-curve problems associated with the SSMS in a busy clinical practice.