The Effect of Sacroiliac Fusion and Pelvic Fixation on Rod Strain in Thoracolumbar Fusion Constructs

Abstract

Study Design. In vitro biomechanical study. Objective. Investigate effects of sacroiliac joint (SIJ) fusion and iliac fixation on distal rod strain in thoracolumbar fusions. Summary of Background Data. Instrument failure is a multifactorial, challenging problem frequently encountered by spinal surgeons. Increased rod strain may lead to instrumentation failure and rod fracture. Methods. Seven fresh frozen human cadaveric specimens (T9–pelvis) used. Six operative constructs tested to investigate changes in rod strain at L5–S1 and S1–Ilium rods, posterior pedicle screws/rods from T10–S1 (PS), PS + bilateral iliac screw fixation, PS + unilateral iliac screw fixation (UIS), PS+UIS+3 unilateral SIJ screws, PS + 3 unilateral SIJ screws, and PS +6 bilateral SIJ screws. Uniaxial strain gauges were used to measure surface strain of rods during flexion-extension. Results. In flexion-extension, bilateral iliac screws added significant strain to L5–S1 compared with long fusion constructs ending at S1 (PS) (P < 0.05). Unilateral iliac fixation exhibited highest strain to L5–S1 ipsilateral rod, was significantly higher compared with bilateral iliac fixation and PS construct. Unilateral and bilateral SIJ fusion did not significantly change L5–S1 rod strain compared with PS. When measuring S1–Ilium rod strain, unilateral pelvic fixation had highest reported rod strain, approached significance compared with bilateral iliac screws (P = 0.054). Addition of contralateral SIJ fusion did not affect rod strain at S1–ilium on side with unilateral fixation. Conclusion. Results showed additional fixation below S1 to pelvis added significant rod strain. Unilateral pelvic screws had highest rod strain; SIJ fusion did not affect rod strain. Findings can help guide surgeons when associated risk of rod failure is a consideration. Level of Evidence: N/A