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

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 fl exibility 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 L5S1 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 quantifi ed with a noncontact camera system. Results. S1 screw strains were highest with PS but were signifi cantly reduced by 73% in fl exion and 31% in extension with TLIF (P = 0.004). AxialITR signifi cantly reduced strain by 78% in fl exion and 81% in extension (P = 0.001). ROM was smallest with AxialITR in each test direction at 1.7 +/- 1.8 in fl exion-extension, 1.6 0.9 in lateral bending and 1.3 0.8 in torsion. Conclusion. This study demonstrated that ROM and S1 screwbending moments were reduced with the use of AxialITR and TLIF. Although the TLIF and AxialITR both reduced strains and motion, the AxialITR provided a signifi cant reduction in extension strain when compared with TLIF.