Patient-specific biomechanical modeling of intraoperative scoliosis correction on the 2-year outcomes for thoracolumbar/lumbar vertebral body tethering.

Purpose: To biomechanically assess the influence of intraoperative correction and presenting Sanders maturity scores (SS) on growth modulation correction after 2 years in pediatric idiopathic scoliosis treated with Vertebral Body Tethering (VBT).

Methods: Lumbar VBT was simulated using patient-specific finite element models (FEMs) from 20 cases of pediatric idiopathic scoliosis (average thoracolumbar/lumbar Cobb 47°; min: 34°, max: 63°), calibrated for preoperative SS, weight, and spine flexibility. The validated FEM included lateral decubitus positioning and VBT instrumentation at the actual upper instrumented vertebra (UIV: T9-T12) and lower instrumented vertebra (LIV: L2-L4). Simulations tested three intraoperative nominal correction levels (35, 50, and 70%) across SS stages (3A, 3B, 4, 5), with immediate and 2-year postoperative corrections computed and analyzed.

Results: A 35% intraoperative correction resulted in an immediate post-operative Cobb angle of 37° (23°-54°) but led to under-correction, with a final deformity of 38° (22°-63°) at 2 years. Curve progression occurred in 40% of SS3A cases, particularly in heavier patients (54 kg vs. 38 kg, p < 0.05). A 50% intraoperative correction yielded an immediate post-operative Cobb angle of 27° (16°-40°), with significant improvement at 2 years only in SS3A (p < 0.05). Clinically successful growth modulation (>5° improvement) correlated with lower weight (40 ± 6 kg vs. 54 ± 6 kg, p < 0.05). A 70% intraoperative correction produced an immediate post-operative Cobb angle of 17° (11°-22°) and significant improvement across all SS levels (p < 0.05), with final 2-year angles of 1° (-27° to 10°) for SS3A, 10° (-5° to 10°) for SS3B, 12° (0°-18°) for SS4, and 13° (4°-19°) for SS5. Overcorrection occurred in SS3A (4 cases) and SS3B (1 case).

Conclusion: Successful outcomes at 2 years depend on the interaction of key factors, such as intraoperative correction, residual growth potential as defined by preoperative SS, patient weight, spinal flexibility, and mechanobiological growth modulation. The advanced and validated planning tool used for the simulations incorporates these elements, integrating both biomechanical and biological growth dynamics to support a more precise and personalized surgical approach.