Biomechanical Computational Study of Pedicle Screw Position and Density in Adolescent Idiopathic Scoliosis Instrumentation

Study Design: Computer simulation of adolescent idiopathic scoliosis instrumentation. Objective: To test the hypothesis that different screw densities would result in different apical vertebral rotation (AVR) corrections and bone-screw forces in adolescent idiopathic scoliosis instrumentation. Summary of Background Data: The "Minimize Implants Maximize Outcomes" Clinical Trial revealed that the use of more versus fewer screws resulted in similar coronal plane correction for Lenke 1A curves. However, the biomechanical impact of screw density on transverse plane correction is still unclear. Further investigation is needed to determine if and how transverse plane correction is correlated with screw density. Patients and Methods: We simulated apical vertebral derotation after segmental translation using patient-specific computer models of 30 patients from the "Minimize Implants Maximize Outcomes" Trial. For each case, 10 alternative screw patterns were tested with overall densities ranging between 1.2 and 2 screws per level fused, and local density at the 3 apical levels ranging between 0.7 and 2 (total: 600 simulations). Main thoracic (MT) Cobb angle, thoracic kyphosis (TK), AVR, and bone-screw forces were computed and compared. Results: The presenting MT (62 ± 11°; range: 45° to 86°), TK (27 ± 20°; -5° to 81°), and AVR (14±7°; -2° to 25°) were corrected through segmental translation to 22 ± 7° (10° to 41°), 26 ± 5° (18° to 45°), and 14 ± 7° (-4° to 26°). After apical vertebral derotation, they became 16 ± 8° (1° to 41°), 24 ± 4° (13° to 40°), and 4 ± 5° (-12° to 18°). There was no significant difference in MT among screw patterns; higher screw density had lower bone-screw forces (P < 0.05). The apical vertebral derotation maneuver reduced AVR by an average of 70%, positively correlated with apical screw density (r = 0.825, P < 0.05). There was no significant difference in TK. Conclusion: Screw density had no significant effect on 3-dimensional correction through the primary segmental translation maneuver. Transverse plane correction through subsequent apical vertebral derotation was positively correlated with screw density at the apical levels (r = 0.825, P < 0.05). Bone-screw forces were negatively correlated with overall screw density (P < 0.05).