Iliac screws may not be necessary in long segment constructs with L5-S1 ALIF: Cadaveric study of stability and instrumentation strain

Publication date: Available online 9 November 2018

Source: The Spine Journal

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

ABSTRACT

Background Context

Lumbosacral pseudoarthrosis and instrumentation failure is common with long segment constructs. Optimizing lumbosacral construct biomechanics may help to reduce failure rates. The influence of iliac screws and interbody type on range of motion (ROM), rod strain (RS), sacral screw strain (SS) is not well-established.

Purpose

Investigate the effects of TLIF, ALIF, and iliac screws on long segment lumbosacral construct biomechanics.

Study Design

Biomechanical study

Patient Sample

Fourteen human cadaveric spine specimens

Outcome Measures

Lumbosacral ROM, RS, and SS

Methods

Specimens were potted at L1 and the ilium. Specimens were equally divided into either an L5-S1 ALIF or TLIF group and underwent testing in the following conditions: 1) intact 2) L2-S1 pedicle screw rod fixation(PSR-S) 3) L2-ilium (PSR-I) 4) PSR-S+ALIF (ALIF-S) or TLIF (TLIF-S) 5) PSR-I + ALIF (ALIF-I) or TLIF (TLIF-I). Pure moment bending (7.5Nm) in flexion, extension, lateral bending, axial rotation and compressive loads (400N) were applied and ROM, SS, and RS were measured. Comparisons were performed using a one-way ANOVA (p<0.05).

Results

ALIF-S and TLIF-S provided similar decreases in ROM as TLIF-I (p>0.05). Compared to PSR-S, PSR-I significantly decreased SS during bending in all directions (p<0.02) but increased RS in flexion and extension (p≤0.02). ALIF-S provided similar decreases in SS as TLIF-I in all directions (p>0.40) but had significantly less RS than TLIF-I in flexion, extension, compression (p<0.01). TLIF-S had more SS than TLIF-I in flexion, extension, axial rotation (p<0.02), while TLIF-S had less RS only in flexion (p=0.03). Compared to PSR-I, ALIF-I decreased the RS (p<0.02) but TLIF-I did not (p>0.67).

Conclusions

Iliac screws were protective of SS but increased RS at the lumbosacral junction. Constructs with ALIF and no iliac screws result in comparable SS as constructs with TLIF and iliac screws with significantly reduced RS. If iliac screws are utilized, ALIF but not TLIF reduces the iliac screw-induced RS.

Clinical Significance

There is a relatively high incidence of lumbosacral instrumentation failure in adult spinal deformity. Optimizing lumbosacral construct biomechanics may help to reduce failure rates. Iliac screws induce lumbosacral rod strain and may be responsible for instrumentation failure. Constructs with lumbosacral ALIF reduce iliac-screw induced rod strain and may obviate the need for fixation to the ilium.

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