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Biomechanical Effect of Transforaminal Lumbar Interbody Fusion and Axial Interbody Threaded Rod on Range of Motion and S1 Screw Loading in a Destabilized L5–S1 Spondylolisthesis Model

Fleischer, Gary D. MD*; Hart, David MD; Ferrara, Lisa A. PhD; Freeman, Andrew L. MS§; Avidano, Eugene E. BS

doi: 10.1097/BRS.0000000000000077

Study Design. A cadaveric lumbosacral spondylolytic spondylolisthesis model was used to evaluate the biomechanical function of 2 different interbody spacers.

Objective. To analyze and compare the reduction in pedicle screw strain and spine range of motion (ROM) between transforaminal lumbar interbody fusion (TLIF) and an axial interbody threaded rod (AxialITR) in a destabilized L5–S1 spondylolisthesis model.

Summary of Background Data. Symptomatic spondylolytic spondylolisthesis is often treated with posterior instrumented fusion augmented by a variety of different interbody devices. Interbody spacers rely primarily on posterior instrumentation to stabilize the spine during fusion, but there may be advantages to the more rigid support offered by an anterior threaded rod.

Methods. Pure-moment flexibility testing was performed on L3–S1 cadaveric specimens in 4 conditions: (1) Intact, (2) L5–S1 pedicle screws (PS) + L5–S1 disc destabilization (DDS), (3) TLIF at L5–S1 + PS + DDS, and (4) AxialITR at L5–S1 + PS + DDS. Specimens were destabilized by performing a complete denucleation at L5–S1 and sectioning two-thirds of the annulus' width from anterior to posterior. The S1 PSs were instrumented with strain gauges to measure screw-bending moments and ROM was quantified with a noncontact camera system.

Results. S1 screw strains were highest with PS but were significantly reduced by 73% in flexion and 31% in extension with TLIF (P ≤ 0.004). AxialITR significantly reduced strain by 78% in flexion and 81% in extension (P ≤ 0.001). ROM was smallest with AxialITR in each test direction at 1.7 ± 1.8° in flexion-extension, 1.6 ± 0.9° in lateral bending and 1.3 ± 0.8° in torsion.

Conclusion. This study demonstrated that ROM and S1 screw-bending moments were reduced with the use of AxialITR and TLIF. Although the TLIF and AxialITR both reduced strains and motion, the AxialITR provided a significant reduction in extension strain when compared with TLIF.

Level of Evidence: N/A

Biomechanical testing was performed to compare transforaminal lumbar interbody fusion (TLIF) and an axial threaded rod (AxiaLITR) in a cadaveric spondylolytic spondylolisthesis model. S1 screw strains were similar between TLIF and AxiaLITR in flexion but significantly reduced in extension with AxiaLITR. The additional biomechanical stability of AxiaLITR in all loading modes may be beneficial.

*Southern New Hampshire Medical Center, Nashua, NH

Cleveland Clinic, Cleveland, OH

OrthoKinetic Technologies, Southport, NC

§Excelen Center for Bone and Joint Research, Minneapolis, MN; and

TranS1, Wilmington, NC.

Address correspondence and reprint requests to Gary D. Fleischer, MD, Spine Surgery, Southern New Hampshire Medical Center, Nashua, NH; E-mail:

Acknowledgment date: January 8, 2013. First revision date: June 21, 2013. Second revision date: September 17, 2013. Acceptance date: September 27, 2013.

The device(s)/drug(s) is/are FDA-approved or approved by corresponding national agency for this indication.

TranS1 funds and implant donations were received in support of this work.

Relevant financial activities outside the submitted work: board membership, consultancy, payment for lectures, payment for development of educational presentations, and provision of writing assistance, medicines, equipment or administrative support.

© 2014 by Lippincott Williams & Wilkins