A retrospective controlled clinical study of Cobb angle distribution of the main thoracic curve in adolescent idiopathic scoliosis

To compare the characteristics of Cobb angle distribution of the main thoracic curve (MTC) in patients with Lenke 1 adolescent idiopathic scoliosis (AIS) and differences in fulcrum-bending flexibility, correction rate, and correction index between different segments. Included in this study were 40 consecutive patients with Lenke 1 AIS who received posterior correction and fusion with pedicle screws. Cobb angle based on the proximal (T5–T7 or T6–T8), apical (T7–T9 or T8–T10), and distal (T9–T11 or T10–T12) segments in the fulcrum-bending position was measured before and after surgery. The flexibility ([Cobb angle of each segment − residual Cobb angle on fulcrum bending]/Cobb angle of each segment × 100%), correction rate ([Cobb angle of each segment − postoperative residual Cobb angle]/Cobb angle of each segment × 100%]), and correction index (correction rate of each segment/preoperative flexibility of each segment) in different segments were calculated. Comparative analyses were conducted by variance analysis. The mean age before surgery, Cobb angle, Risser sign, and follow-up time were 14.15 ± 2.13 years, 51.17 ± 10.72°, 2.78 ± 1.73, and 43.75 ± 9.82 months, respectively. MTC Cobb angle of the proximal segments was similar to that of the distal ones (12.88 ± 4.81 vs 12.85 ± 5.00) versus 25.45 ± 5.90 in the middle segments (P < .001). The flexibility was higher in the distal segments than that in the proximal or apical segments (66.43 ± 0.22% vs 43.78 ± 0.20% or 32.55 ± 0.17%, P < .001). One week after surgery, the correction rate in these 3 segments was 69.55 ± 0.1%, 66.25 ± 0.17%, and 75.28 ± 0.16 (P = .067), and the correction index was 2.15 ± 1.78, 3.16 ± 3.60, and 1.53 ± 1.93 (P = .019); the correction rate during the 3-year follow-up period was 68.06 ± 0.19%, 69.98 ± 0.15%, and 73.29 ± 0.17 (P = .212); and the correction index was 2.12 ± 1.78, 3.20 ± 3.54, and 1.49 ± 1.93 (P = .012), respectively. The proximal, apical, and distal segments in Lenke 1 AIS accounted for about 25%, 50%, and 25% of MTC Cobb angle, respectively. The distal segments were found to be most flexible and the apical segments most rigid. The correction rate was similar between the proximal, apical, and distal segments, and the correction index in the apical segments was higher than that in the proximal and distal segments.