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Cobb Angle Progression in Adolescent Scoliosis Begins at the Intervertebral Disc

Study Design. Longitudinal radiographic study of patients with progressive idiopathic scoliosis. Objective. To determine the relative contributions of vertebral and disc wedging to the increase in Cobb angle during 3 phases of adolescent skeletal growth and maturation. Summary of Background Data. Both disc wedging and vertebral body wedging are found in progressive scoliosis, but their relative contribution to curve progression over time is unknown. Which occurs first is important for understanding how scoliosis progresses and for developing methods to halt progression. Previous studies have not properly identified maturity, and provide conflicting results. Methods. Eighteen girls were followed through their adolescent growth spurt with serial spine and hand skeletal age radiographs. Each Cobb angle was divided into disc wedge angles and vertebral wedge angles. The corresponding hand radiographs provided a measure of maturity level, the Digital Skeletal Age (DSA). The disc versus bone contributions to the Cobb angle were then compared during 3 growth phases: before the growth spurt, during the growth spurt and after the growth spurt. Significance of relative changes was assessed with the Wilcoxon 2-sided mean rank test. Results. Before the growth spurt, there was no difference in relative contributions of the disc and the bone (3° vs. 0°, P = 0.38) to curve progression. During the growth spurt, the mean disc component progressed significantly more than that of the vertebrae (15° vs. 0°, P = 0.0002). This reversed following the growth spurt with the vertebral component progressing more than the disc (10° vs. 0°, P = 0.01). Conclusion. Adolescent idiopathic scoliosis initially increases through disc wedging during the rapid growth spurt with progressive vertebral wedging occurring later.