A biomechanical study comparing combined S1AI and S3AI trajectories to other pelvic fixation techniques: A finite element analysis.

BACKGROUND

Pelvic fixation is fundamental to multi-level spinal fusion constructs. Established techniques include conventional iliac, S1-alar-iliac (S1AI), S2-alar-iliac (S2AI), and S3-alar-iliac (S3AI) fixation. Novel porous sacral-alar-iliac (SAI) screws with integrated tulips are increasingly used in long constructs requiring bilateral pelvic fixation. The optimal pelvic fixation strategy remains debated. Conventional techniques include iliac screws and sacral-alar-iliac (SAI) variants, while combined constructs such as S1AI + S3AI remain underexplored. This study presents a finite element analysis (FEA) comparing the biomechanical impact of various fixation methods, with emphasis on combined S1AI + S3AI fixation.

METHODS

Six L1-pelvis FEA models were created: one noninstrumented control (V1) and five instrumented variants-iliac screw (V2), S1AI (V3), S2AI (V4), 8.5 mm S1AI + S3AI (V5), and 10.5 mm S1AI + S3AI (V6). L5-S1 interbody fusion was modelled in all constructs. Loading conditions included flexion, axial-compression, and lateral bending at 100 N, 200 N, and 300 N. Von Mises Stress (VMS) and range of motion (ROM) were analyzed.

FINDINGS: All instrumented constructs reduced ROM versus control. V3 had the lowest ROM but showed construct imbalances internally, cranial to caudal. V6 offered the most uniform ROM reduction. V5 and V6 exhibited the lowest pelvic VMS; V2-V4 had higher stress than control. Instrumentation stress localized to L5-S1 in all variants, with V3 showing bilateral S1AI instrumentation stress. V5 and V6 demonstrated the most favorable stress profiles throughout.

INTERPRETATION: Combined S1AI + S3AI fixation outperformed other pelvic constructs biomechanically, with reduced stress and improved ROM distribution. These findings support its use in long spinal fusions to enhance construct durability and may help guide surgical decision-making. Additional, clinical studies are needed to validate these findings.