Biomechanical analysis of pedicle screw density and rod contouring in adolescent idiopathic scoliosis instrumentation.

Abstract

Pedicle screw density and rod contouring are crucial in the posterior spinal fusion (PSF) in adolescent idiopathic scoliosis (AIS) patients. This study evaluates the three-dimensional (3D) biomechanical interaction between rod contouring and the number/pattern of implants anchoring these rods to determine a range of advisable combinations that improve clinical outcomes. Patient-specific, deterministic computational models were developed based on ten Lenke 1 AIS surgical cases. First order assessment with various pedicle screw patterns and rod contouring angles was performed. Rod material properties consisted of cobalt chrome (CoCr) and titanium (Ti). The primary correction maneuvers were concave rod translation followed by convex rod translation. Four screw configurations were tested: a reference pattern with two screws per level, and three alternatives (alternate, convex alternate, and convex peri-apical dropout) averaging 1.11 to 1.56 screws per level. Across the four screw density patterns tested, no statistically significant differences were observed in all three anatomical planes (p > 0.05). Significant differences in thoracic kyphosis (TK) were observed with increasing rod angulations (p < 0.05). Coronal plane correction diminished with greater rod contouring (p < 0.05). Axial bone-screw forces were negatively influenced by screw density and varied significantly amongst rod contouring combinations (p < 0.05). Apical vertebral rotation (AVR) was not influenced by rod properties or screw density (p > 0.05). Screw density had the greatest impact on axial bone-screw forces. Regardless of the screw density pattern, more aggressive rod contouring angles reduced coronal plane correction but improved sagittal plane alignment.