Supplemental rods are needed to maximally reduce rod strain across the lumbosacral junction with TLIF but not ALIF in long constructs

Publication date: Available online 23 January 2019

Source: The Spine Journal

Author(s): Jakub Godzik, Randall J. Hlubek, Anna G.U.S. Newcomb, Jennifer N. Lehrman, Bernardo de Andrada Pereira, S. Harrison Farber, Lawrence G. Lenke, Brian P. Kelly, Jay D. Turner

Abstract

Background Context

Rod fracture at the lumbosacral (LS) junction remains challenging in long segment fusions and likely stems from increased lumbosacral 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 to standard 2-rod (2R) construction in a long segment fusion model.

Study Design/Setting

Cadaveric biomechanical study.

Outcome Measures

Range of motion, rod strain, and sacral screw 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 range of motion stability (ROM), rod strain, and sacral screw bending moment (SS) of a supplemental 4R construction versus standard 2R construction (Fig. 1). 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 ANOVA or ANOVA.

Results

No differences were observed between groups 1 and 2 for age, sex, bone mineral density, or baseline ROM (p>0.09). Overall, TLIF+2R demonstrated greater ROM than ALIF+4R in extension (p=0.03), with greater rod strain in flexion, extension, and compression (p<0.001), and greater SS in compression and AR (p<0.04). Compared to TLIF+2R, TLIF+4R resulted in reduced rod strain in flexion, extension, compression, and LB (p<0.04), as well as SS in AR (p<0.001). TLIF+4R yielded biomechanics comparable to ALIF+2R in ROM and RS, but SS inflexion, extension, compression, and AR remained elevated (p<0.01). ALIF+4R did not significantly improve ROM, rod strain, or SS (p>0.11).

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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