Biomechanical Evaluation of ALIF, TLIF, and LLIF fusion with Supplemental Pedicle Screws versus Interspinous Fixation

Introduction
Surgical techniques and constructs for lumbar fusion vary and are dependent on patient needs and surgeon preference. An interbody spacer with posterior pedicle screw and rod fixation, known as a 360-degree fusion, is standard. Common approaches for spacer placement within the disc space include anterior lumbar interbody fusion (ALIF), transforaminal lumbar interbody fusion (TLIF), and lateral lumbar interbody fusion (LLIF). Use of pedicle screw and rod fixation to augment these interbody cages can result in poor outcomes due to truncal musculature dissection, facet violation, neurologic injury, post-operative pain, infection, or other complications. The purpose of this study is to compare the effectiveness of the three interbody approaches (ALIF, TLIF, and LLIF) at reducing segmental motion when supplemented with pedicle screws and rods versus a minimally invasive alternative using interspinous process fixation. The purpose of this study was two-fold: Compare the relative stability provided by ALIF, TLIF, and LLIF when combined with posterior fixation and also compare the effectiveness of posterior interspinous process fixation to traditional bilateral pedicle screws and rods when used as an adjunct to interbody fusion.

Methods
We conducted an in vitro biomechanical study using eighteen (18) cadaveric lumbar spines (L1-S1) divided into three experimental groups: ALIF (age: 44.3±8.8 yo; 4M, 2F), TLIF (age: 44.3±5.5 yo; 4M, 2F), LLIF (age: 43.0±7.9 yo; 4M, 2F). The motion of each vertebral body was measured optoelectronically in each of the following protocol steps: 1) Intact L1-S1 spine; 2) L4-L5 interbody + bilateral pedicle screw and rod fixation; 3) L4-L5 interbody + interspinous fixation. These steps were repeated for each interbody test group (ALIF, TLIF, LLIF). Segmental RoM were measured under applied moment forces in flexion (8Nm), extension (6Nm), lateral bending (±6Nm), and axial rotation (±5Nm).

Results
There was no difference in flexion-extension or axial rotation RoM between supplemental pedicle screws and interspinous process fixation for any of the interbody cages (ALIF, LLIF, TLIF). Regardless of the fixation used, flexion-extension and axial RoM was at most 1.4±0.5 and 1.4±0.6 degrees respectively. In lateral bending, pedicle screws provided significantly better stabilization than interspinous fixation for all interbody types (p < 0.05). The effectiveness of supplemental interspinous fixation in lateral bending was significantly improved with ALIF as compared to TLIF (2.7±0.5 vs 4.1±0.7 degrees, p< 0.05). There was no difference in lateral bending RoM between ALIF and LLIF (3.5±1.7 degrees) with supplemental interspinous fixation (p>0.05).

Conclusions
Lumbar fusion using an interbody spacer and posterior supplemental fixation is very effective at reducing segmental RoM. Complications associated with pedicle screw and rod fixation are reduced with a minimally invasive percutaneous approach. However, the accuracy of screw placement is compromised with these techniques, leading to a higher incidence of other complications such as adjacent facet violation. An alternative posterior stabilizing method such as interspinous fixation may eliminate the complications associated with pedicle screws while providing sufficient immobilization to promote fusion.