Curve Progression in Adolescent Idiopathic Scoliosis with Cobb Angles Between 40 and 50 Degrees at the Late Stage of Skeletal Growth: A Minimum 5-Year Follow-Up Study

<b>Background/Objectives</b>: Surgical treatment is generally recommended for adolescent idiopathic scoliosis (AIS) when the Cobb angle exceeds 50 degrees even after skeletal maturity or 40 degrees with remaining growth potential. However, limited evidence exists regarding the natural history of curves between 40 and 50 degrees during the late stage of skeletal growth. This study aimed to evaluate the curve progression in AIS patients with a curve between 40 and 50 degrees at Risser stage IV or V. <b>Methods</b>: The inclusion criteria were as follows: (1) AIS patients at the late stage of skeletal growth (Risser IV or V) and a (2) curve between 40 and 50 degrees, with a minimum follow-up of 5 years. Sex, age, the magnitude of the curve, the location of the apex, Risser stage, height, and weight were measured at the baseline and the final follow-up. Curve progression was defined as an increase in the Cobb angle of ≥5 degrees. Patients were also categorized based on whether their final Cobb angle was <50 or ≥50 degrees to evaluate additional risk factors. <b>Results</b>: A total of 97 patients were included, with a mean follow-up of 97 months. Their mean age was 14.6 years at the baseline and 22.6 years at the final follow-up. The mean Cobb angle increased from 42.6 to 45.1 degrees, with a mean change of 2.7 degrees and an annual progression rate of 0.35 degrees. Curve progression was observed in 38 patients (39.2%), and 24 patients (24.7%) reached a final Cobb angle ≥ 50 degrees. Younger age (<i>p</i> = 0.004) and Risser stage IV (<i>p</i> = 0.014) were significantly associated with curve progression. In patients with a final Cobb angle ≥ 50 degrees, Risser stage IV (<i>p</i> = 0.050) and a larger baseline curve magnitude (<i>p</i> = 0.045) were also significant risk factors. <b>Conclusions</b>: In AIS patients at the late stage of skeletal growth, 39.2% experienced significant curve progression. A younger age and Risser stage IV were identified as risk factors for curve progression. A larger baseline curve magnitude and Risser stage IV were also associated with a final Cobb angle ≥ 50 degrees.