Development of a New Simplified Thumb Ossification Composite Index (TOCI) and Its Application to Predict Skeletal Maturity and Curve Progression in Idiopathic Scoliosis and Normal Subjects

Introduction

Accurate skeletal maturity assessment is important for prediction of curve progression and clinical management of adolescent idiopathic scoliosis (AIS) including bracing decision and counseling for prognosis. Determination of the timing of peak growth height velocity and growth remaining are paramount important.1,2 Commonly used clinical or radiological methods are still inadequate or too complex for rapid clinical use in the outpatient setting.3-5 Risser sign had disadvantages of low visibility in PA spinal radiograph, wide variability with maturity level and imprecise representation of PHV timing.6 Greulich and Pyle atlas (GP atlas) and Tanner-Whitehouse-III (TWIII) method are more reliable and comprehensive classifications to predict maturity, but they are cumbersome and time consuming to be used clinically.7 Both methods require the usage of an atlas, a learning curve required for exact matching of atlas plate or assignment of scores to bones.8

Some latest skeletal maturity bone models were evolved trying to solve these problems but were still imperfect.9 Recently, Sanders made modifications to TWIII method to form Skeletal Maturity Scoring System (SMSS) which focused on epiphysis of small hand bones from all 5 digits. SMSS was proved to have excellent correlation with the curve acceleration phase.10 Digital skeletal age scores between 400 and 425 are associated with the beginning of the curve acceleration phase in adolescent idiopathic scoliosis.11 Excellent intra-observer reliability and substantial inter-observer reliability among senior surgeons were demonstrated in SMSS.12

A new skeletal age classification has yet to be developed, although SMSS has considered a promising method. A recent study to test SMSS's reliability in less experienced staffs demonstrated average inter-observer reliability (K=0.53).8 It showed that a learning curve was present and several recommendations were added in each classification stage to avoid controversy and confusion during usage.13 Therefore, using SMSS requires an organized teaching system with detailed descriptions, self-assessment examinations, viewing presentations and the newly added recommended modification guidelines to improve the reliability. Another scoring system "Distal Radius and Ulna (DRU)" score simplified the measurement to just using radial and ulnar epiphyses but results observed that DRU score had the least correlation of "Radio Ulna and Short bones" (RUS) growth centers with scoliosis behavior.9,10 In addition, the variable appearance of ulnar epiphysis were difficult to be seen clearly.

In this study, the investigators introduce Thumb Ossification Classification Index (TOCI). TOCI employed the measurements of epiphysis of distal phalange, proximal phalange, and adductor sesamoid, and results were analyzed together to form a composite stage (composite score) to predict maturity in patient at their peripubertal period. Ultimately the application of TOCI should not be limited to IS patients only. After the establishment of TOCI classification system, the staging system would be applied to radiographs from patients without spinal deformity or suffering from diseases not related to spine.