Supplemental Rods Are Needed To Maximally Reduce Rod Strain Across The Lumbosacral Junction With Tlif But Not Alif In Long Constructs



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Background Context: Rod fracture at the lumbosacral (LS) junction remains challenging in long segment fusions and likely stems from increased LS strain. Reduction of LS instrumentation strain may help reduce fracture rates. Purpose: The goal of this investigation was to assess the effect of supplemental posterior 4-rod (4R) construction on LS stability and rod strain compared with standard 2-rod (2R) construction in a long segment fusion model. Study Design/Setting: Cadaveric biomechanical study. Outcome Measures: Range of motion (ROM), rod strain, and sacral screw (SS) bending moments during flexion, extension, compression, lateral bending, and axial rotation. Methods: Standard nondestructive flexibility tests (7.5 Nm) were performed on 14 cadaveric specimens (L1-ilium) to assess ROM stability, rod strain, and SS bending moment of a supplemental 4R construction versus standard 2R construction. Specimens were equally divided into L5-S1 anterior lumbar interbody fusion (ALIF) or L5-S1 transforaminal lumbar interbody fusion (TLIF) groups. Three conditions were tested in each group: (1) no lumbar interbody fusion (No LIF)+2R, (2) ALIF or TLIF+2R, and (3) ALIF or TLIF+4R. Data were analyzed using repeated measures analysis of variance (ANOVA) or ANOVA. Results: No differences were observed between groups 1 and 2 for age, sex, bone mineral density, or baseline ROM (p>.09). Overall, TLIF+2R demonstrated greater ROM than ALIF+4R in extension (p=003), with greater rod strain in flexion, extension, and compression (p<.001), and greater SS in compression and AR (p<.04). Compared with TLIF+2R, TLIF+4R resulted in reduced rod strain in flexion, extension, compression, and LB (p<.04), as well as SS in AR (p<.001). The TLIF+4R yielded biomechanics comparable to ALIF+2R in ROM and rod strain but SS inflexion, extension, compression, and AR remained elevated (p<001). The ALIF+4R did not significantly improve ROM, rod strain, or SS (p>.11). Conclusions: The use of ALIF and adding accessory rods with TLIF significantly reduced LS rod strain in a long segment cadaveric model with iliac fixation. Clinical Significance: Reducing strain could decrease the risk of failure associated with long segment fixation.

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Spine Journal







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