Intermediate-term annualized curve progression of adolescent idiopathic scoliosis curves measuring 40° or greater

STUDY DESIGN: Prospective cohort study. OBJECTIVES: The objective of this study was to examine intermediate-term progression for a large series of patients with adolescent idiopathic scoliosis (AIS) with curves 40 degrees or greater. BACKGROUND: Curve progression in AIS has been well documented for smaller curves in adolescence up to skeletal maturity; however, the data on curve progression past 40 degrees or into adulthood are limited. With many surgeons recommending surgical correction when patients reach this threshold, it is important to understand the radiographic progression of curves into adulthood. METHODS: A database of all patients seen by a single surgeon from 1984 through 2018 with AIS curves progressing to at least 40 degrees entered prospectively was utilized for this study. This included a total of 738 patients. Curve progression was analyzed overall and stratified by length of follow-up, curve location, and Risser stage at the time of presentation among other variables. Curve magnitude and Risser stage designations in this study were validated by performing a separate inter- and intrarater agreement study using four independent reviewers reading 50 patients' Cobb angle and Risser stage blinded in triplicate to examine the reliability of the study measurements. RESULTS: Annualized curve progression (ACP) averaged 6.3 +/- 10.4 degrees . ACP varied with length of follow-up: patients with up to 1 year of follow-up had an average ACP of 11.5 +/- 17.0 degrees , while those with 1-2 years had 8.2 +/- 8.8 degrees , and 2-5 years had 3.7 +/- 4.1 degrees , tapering off further from there. Risser stage 0 or 1 was associated with the highest ACP as compared to Risser stage 2-3 or 4-5. Intraclass correlation (ICC) values for Cobb angle measurement and Risser stage designations from four raters measuring 50 patients' measures, blinded and in triplicate, were all > 0.80, signifying a high degree of reliability within and between readers. CONCLUSIONS: Annualized curve progression for 40 degrees and greater curves was not linear over time; it was greatest immediately after a curve reaches 40 degrees and tapered off over the next decade. Immature Risser stage at presentation was strongly associated with increasing ACP at all time frames. LEVEL OF EVIDENCE: Prognostic Level I.