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Does Disc Wedging Contribute to the Flexibility of Thoracic Curve in Adolescent Idiopathic Scoliosis?

Introduction The appreciations of curve flexibility and segmental mobility in patients with adolescent idiopathic scoliosis (AIS) are important for determining structural curves, defining the safe range of correction, selecting the segment length for fusion, determining the most appropriate surgical procedure, and predicting the results of correction. However, the role of disc wedging in predicting spinal flexibility has not been well defined. The objective of this study was, therefore, to investigate the effect of disc wedging on curve flexibility and gain insight into segmental mobility by comparing disc wedging in upright and side bending radiographs in thoracic AIS. Patients and Methods This study included a total of 116 consecutive patients with AIS with main thoracic curve. According to thoracic curve flexibility index (TCFI), 56 patients with TCFI < 50% were assigned to group A (Stiff), and the other 60 patients with TCFI ≥ 50% were assigned to group B (Flexible). Age, curve magnitude, disc wedging, and disc wedging percentage were compared between the two groups. Segmental flexibility indices ([upright disc angle-bending disc angle]/[upright disc angle]) were computed and compared between periapical and remote levels. Correlations were estimated using Pearson correlation coefficients. After adjustment of covariates, logistic multivariate regression analysis of independent factors for TCFI was performed. Results Significant differences with respect to the age and curve magnitude were observed between the two groups (16.33 ± 3.08 vs. 13.50 ± 1.97, p < 0.01 and 56.06 ± 8.34 vs. 48.60 ± 7.87 degrees). The wedging angle and wedging percentage of discs in group B were significantly higher than those in group A (21.86 ± 6.18 vs. 17.52 ± 6.37 degrees, p < 0.01 and 43.55% ± 8.5 vs. 26.56% ± 9.1, p < 0.01). There was a significant correlation between disc wedging percentage and TCFI (r = 0.766, p = 0.001). Multivariate regression analysis showed that the disc wedging percentage was the single greatest predictor of TCFI. And the disc wedging showed symmetric periapical distribution with significant decrease (all p values < 0.01) for every cephalad (+) and caudad (−) level change. The apical level +1 and −1 wedged at 6.3 and 6.7 degrees (range, 3.5–10.8 degrees), respectively. We noted mean periapical flexibility indices of 41% (+1), 44% (−1), 53% (+2) and 97% (−2), which were significantly less (p < 0.01) than for the group of remote levels 90% (+3), 160% (−3), 116% (+4), and 208% (−4). Conclusion Disc wedging contributes to the spinal flexibility in thoracic AIS. The average flexibility indices of the periapical discs are significantly less than that of the remote discs. These may offer useful information in the evaluation of surgical options and preoperative planning.