Increased differential rod contouring reduces the effectiveness of surgical correction in patients with adolescent idiopathic scoliosis.

Purpose: To evaluate the impact of increasing the differential precontouring angle of concave and convex rods on three-dimensional correction during posterior spinal fusion in patients with adolescent idiopathic scoliosis (AIS) and determine the threshold beyond which additional contouring yields no further correctional benefit in a computational model.

Methods: Patient-specific computational biomechanical models were developed using radiographs from 10 thoracic Lenke 1 AIS patients. Posterior instrumentation included bilateral uniaxial pedicle screws from T4 to L1 and rods of various diameters and materials. First-order simulations of the primary correction maneuver using the rod translation technique were performed using MSC Adams. The effects of varying concave rod precontouring angles on the forces at play and resulting correction in the coronal, sagittal, and axial planes were analyzed.

Results: Coronal plane correction decreased progressively with increasing rod contouring angles (p < 0.05) due to posterior displacement of the rod relative to the spine, reducing coronally-directed torque at the curve apex. Aggressive rod contouring (> 55°) improved sagittal plane correction (thoracic kyphosis), with the benefit depending on presenting kyphosis (p < 0.05). Normo-kyphotic curves showed better outcomes with moderate rod contouring (< 55°), whereas hypo-kyphotic curves required more aggressive bending (55-85°) to restore kyphosis, despite reduced coronal correction. Transverse plane effects from these simulations appeared less influential (p > 0.05). Screw axial forces increased linearly with concave rod bending angle.

Conclusion: Increasing concave rod precontouring beyond 35° to 55° yields diminishing coronal plane correction, while offering selective benefit in restoring thoracic kyphosis in hypo-kyphotic AIS patients.

Level of evidence: Level III.