Tailoring selection of transforaminal interbody spacers based on biomechanical characteristics and surgical goals: evaluation of an expandable spacer

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Transforaminal lumbar interbody fusion (TLIF) is commonly used for lumbar fusion, such as for foraminal decompression, stabilization, and improving segmental lordosis. Although many options exist, surgical success is contingent on matching design strengths with surgical goals. The goal in the present study was to investigate the effects of an expandable interbody spacer and 2 traditional static spacer designs in terms of stability, compressive stiffness, foraminal height, and segmental lordosis.


Standard nondestructive flexibility tests (7.5 N⋅m) were performed on 8 cadaveric lumbar specimens (L3–S1) to assess intervertebral stability of 3 types of TLIF spacers at L4–5 with bilateral posterior screw-rod (PSR) fixation. Stability was determined as range of motion (ROM) in flexion-extension (FE), lateral bending (LB), and axial rotation (AR). Compressive stiffness was determined with axial compressive loading (300 N). Foraminal height, disc height, and segmental lordosis were evaluated using radiographic analysis after controlled PSR compression (170 N). Four conditions were tested in random order: 1) intact, 2) expandable interbody cage with PSR fixation (EC+PSR), 3) static ovoid cage with PSR fixation (SOC+PSR), and 4) static rectangular cage with PSR fixation (SRC+PSR).


All constructs demonstrated greater stability than the intact condition (p < 0.001). No significant differences existed among constructs in ROM (FE, AR, and LB) or compressive stiffness (p ≥ 0.66). The EC+PSR demonstrated significantly greater foraminal height at L4–5 than SRC+PSR (21.1 ± 2.6 mm vs 18.6 ± 1.7 mm, p = 0.009). EC+PSR demonstrated higher anterior disc height than SOC+PSR (14.9 ± 1.9 mm vs 13.6 ± 2.2 mm, p = 0.04) and higher posterior disc height than the intact condition (9.4 ± 1.5 mm vs 7.1 ± 1.0 mm, p = 0.002), SOC+PSR (6.5 ± 1.8 mm, p < 0.001), and SRC+PSR (7.2 ± 1.2 mm, p < 0.001). There were no significant differences in segmental lordosis among SOC+PSR (10.1° ± 2.2°), EC+PSR (8.1° ± 0.5°), and SRC+PSR (11.1° ± 3.0°) (p ≥ 0.06).


An expandable interbody spacer provided stability, stiffness, and segmental lordosis comparable to those of traditional nonexpandable spacers of different shapes, with increased foraminal height and greater disc height. These results may help inform decisions about which interbody implants will best achieve surgical goals.

ABBREVIATIONS BMI = body mass index; DEXA = dual-energy x-ray absorptiometry; EC = expandable interbody cage; FE = flexion-extension; PSR = posterior screw-rod; ROM = range of motion; SOC = static ovoid cage; SRC = static rectangular cage; TLIF = transforaminal interbody fusion.

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Article Information

Correspondence Laura A. Snyder: c/o Neuroscience Publications, Barrow Neurological Institute, St. Joseph’s Hospital and Medical Center, Phoenix, AZ. neuropub@barrowneuro.org.

INCLUDE WHEN CITING Published online April 12, 2019; DOI: 10.3171/2019.1.SPINE181008.

Disclosures Dr. Snyder receives research support from Medtronic.

© AANS, except where prohibited by US copyright law.



  • View in gallery

    Photograph of the 3 interbody spacer types that were compared in this study: EC (A), SOC (B), and SRC (C).

  • View in gallery

    Photograph of the biomechanical flexibility testing setup. Anterior view of a potted cadaveric specimen in the test frame under load. Desired directional loads within anatomical planes were induced by reorientation of cranially attached cable and pulleys. Specimens were tested in flexion-extension, lateral bending, and axial rotation.

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    Comparison of foraminal height (mean ± SD) at L4–5 between the intact condition and each of the 3 interbody spacer types.

  • View in gallery

    Comparison of anterior and posterior disc height (mean ± SD) at L4–5 between the intact condition and each of the 3 interbody spacer types.

  • View in gallery

    Comparison of segmental lordosis (mean ± SD) across the L4–5 segment between the intact condition and each of the 3 lumbar interbody fusion (LIF) spacer types, both with and without PSR fixation. Note that there were no significant differences among conditions.




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