Redefining Success and Characterizing Midterm Growth Modulation: Analysis of Radiographic Outcomes in 2 Years Following Vertebral Body Tethering.

Background: Vertebral body tethering (VBT) offers fusionless correction for idiopathic scoliosis, yet midterm growth modulation remains poorly characterized. This study established a clinically relevant success criterion and characterized growth modulation kinetics with associated predictors. Methods: This multicenter prospective study enrolled patients with idiopathic scoliosis with Sanders stage ≤7A, preoperative fulcrum-bending flexibility rate ≥50%, and ≥2-year follow-up after VBT. Parameters were collected before surgery and at 7 postoperative time points. Two-year success was defined as Cobb angle <35° (group 1) and correction rate (CR) ≥50% without overcorrection (Group 2). Group 3 included cases with an instrumented Cobb angle <35° but CR <50%. Curves were further classified as undercorrection (CR <50%), overcorrection (negative curve), and short-term clinical success (same as group 2) at 2 years postoperatively. Results: Thirty-eight patients (50 curves, mean age 11.9 ± 1.17 years; 89.5% female) were analyzed. Coronal CR was 63.2% at 24 months. T2-12 kyphosis (p = 0.637), thoracolumbar kyphosis (p = 0.104), lumbar lordosis (p = 0.869), and sacral slope (p = 0.091) remained unchanged. While pelvic obliquity improved at 12 months (2.1° vs. 1.6°, p = 0.001) and trended toward recurrence at 24 months (2.1° vs. 1.8°, p = 0.142), the difference was below the minimally clinically important difference. Apical (11.3° vs. 6.8°) and upper end vertebral (8.5° vs. 2.8°) rotation improved significantly, while lower end vertebral rotation remained unchanged. Although no significant differences were found between group 1 (n = 39) and group 2 (n = 25) in CR and postoperative revisions, among curves under group 1 but not group 2 (group 3, n = 14), a significantly higher revision rate was observed compared with group 2 (28.6% vs. 4.0%, p = 0.047). Eighty-four percent of successful operations under the new criterion remained successful at the 3.4-year follow-up. The change in correction (Δcorrection) diverged at 3 to 6 months postoperatively. Overcorrection was associated with greater flexibility, a younger Risser sign, lower body mass index (14.5 ± 0.86 kg/m Conclusion: VBT effectively corrects coronal and axial deformity. A short-term success criterion based on a Cobb angle CR ≥ 50% without overcorrection better reflects a favorable biological response than the <35° benchmark at 2 years. Skeletally immature, lean patients with high flexibility and continued correction beyond 6 months may signal overcorrection risk. Level of Evidence: Prognostic Level II. See Instructions for Authors for a complete description of levels of evidence.