• Harry M Mushlin, Phelan Shea, Daina M Brooks, Gerald M Hayward, Bryan J Ferrick, Joshua Olexa, Brandon S Bucklen, and Charles Sansur.
• Department of Neurosurgery, University of Maryland School of Medicine, Baltimore, MD.
• Spine. 2021 Jul 15; 46 (14): E769-E775.

Study DesignIn vitro biomechanical study.ObjectiveInvestigate effects of sacroiliac joint (SIJ) fusion and iliac fixation on distal rod strain in thoracolumbar fusions.Summary Of Background DataInstrument failure is a multifactorial, challenging problem frequently encountered by spinal surgeons. Increased rod strain may lead to instrumentation failure and rod fracture.MethodsSeven 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.ResultsIn 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.ConclusionResults 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.Copyright © 2021 Wolters Kluwer Health, Inc. All rights reserved.

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