Biomechanical analysis of 3D correction and bone-screw forces as a function of rod insertion sequence and orientation relative to the sagittal plane in adolescent idiopathic scoliosis instrumentation

Abstract

Background: In adolescent idiopathic scoliosis instrumentation, rods are typically aligned with the sagittal plane during the initial translation maneuver. Surgeons often empirically orient the rod slightly opposite to the scoliotic curve, but the optimal orientation and insertion sequence, as well as their influence on 3D correction and forces, remain unclear. This study investigates the biomechanical influence of these rod parameters on scoliosis correction.

Methods: Patient-specific multi-body biomechanical models were developed for 30 adolescent idiopathic scoliosis patients (11 hypo-, 12 normo-, 7 hyper-kyphotic thoracic curves) to simulate posterior instrumentation with a primary segmental translation correction maneuver. Rod insertion was tested in two sequences: concave side first, followed by the convex side, and vice versa. The construct included pedicle screws and 5.5-mm Cobalt-Chromium rods contoured to 45° (concave side) and 15° (convex side). Four rod orientations relative to the sagittal plane (0°, 10°, 20°, 30°) were analyzed for 3D correction and bone-screw forces.

Findings: Main thoracic Cobb angle correction improved significantly with rod orientations ≥20° (p < 0.01). Apical vertebral rotation and thoracic kyphosis correction were unaffected by rod orientation (p > 0.05). Bone-screw forces increased slightly but not significantly (p > 0.05) with greater rod orientation. Convex-side-first rod insertion reduced bone-screw forces during initial insertion, achieving corrections similar to the concave-first sequence.

Interpretation: Pre-contoured rods oriented up to 30° opposite the scoliotic curve significantly improved coronal plane correction, while maintaining similar thoracic kyphosis and vertebral rotation, with non-significant increases in bone-screw forces. Convex-side-first insertion may reduce mechanical stress during surgery.

Keywords: 3D correction; Adolescent idiopathic scoliosis; Biomechanical modeling; Multibody modeling; Rod orientation; Spine instrumentation.