The Dynamic Stability of Lumbosacral ALIF vs Bilateral TLIF in “Top-Heavy” Prone Lateral Constructs

Introduction
The prone transpsoas (PTP) approach to lateral lumbar interbody fusion (LLIF) has gained popularity as a technique to achieve desired radiographic and clinical outcomes with the benefits of familiar positioning, direct decompression, and operative efficiency. L5/S1 can be accessed in the supine or lateral position as an anterior lumbar interbody fusion (ALIF), whereas in PTP, only posterior approaches are achievable at L5/S1 without patient repositioning. Thus, transforaminal lumbar interbody fusion (TLIF) is commonly utilized. This difference in approach results in different fusion constructs (ALIF+LLIF vs TLIF+LLIF) and it is unclear how this affects the resulting biomechanics and radiographic parameters.

Aims/

Objectives:
To compare primary and dynamic biomechanics and radiographic parameters between multilevel LLIF constructs with L5/S1 ALIF and straight bilateral TLIF spacer configurations.

Methods
Twelve healthy fresh-frozen human cadaveric lumbar spine specimens (L2-S1) were tested. Specimens were randomly assigned to the ALIF or bilateral TLIF (BiTLIF) cohort and had similar intact range-of-motion (ROM), age, and DEXA scores. ALIF cohort was treated with static, integrated ALIF at L5/S1. BiTLIF cohort was treated with expandable bilateral straight TLIF at L5-S1. All specimens were treated with LLIFs at L3-5 and posterior fixation via pedicle screws and rods at L3-S1 (ALIF+2LLIF and BiTLIF+2LLIF). Specimens were then fatigued for 3000 cycles of motion in each plane using a servo-hydraulic device. ROM was measured for intact and treated specimens in flexion-extension (FE), lateral bending (LB), and axial rotation (AR) using a motion simulator (±7.5Nm), capture system, and analysis software. Radiographic parameters were evaluated with fluoroscopy. These included intervertebral angle (IVA), posterior disc height (PDH), and foraminal height (FH).

Results
Intact specimen L5/S1 ROM were similar between ALIF (FE: 9.72°, LB: 7.07°, AR: 4.32°) and BiTLIF cohorts (FE: 10.19°, LB: 7.14°, AR: 4.14°). Following L3-5 LLIF and L3-S1 posterior fixation, L5/S1 ROM was comparable between ALIF+2LLIF (FE: 3.51°, LB: 1.64°, AR: 1.43°) and BiTLIF+2LLIF (FE: 2.23°, LB: 0.93°, AR: 1.43°). Post-fatigue, both ALIF+2LLIF (FE: 4.81°, LB: 2.28°, AR: 2.43°) and BiTLIF+2LLIF (FE: 3.38°, LB: 1.72°, AR: 2.12°) saw increased L5/S1 ROM. There were no significant differences in lumbosacral stiffness between ALIF or BiTLIF cohorts (FE: p=0.66, LB: p=0.65, AR: p=0.76).
Compared to intact specimens, ALIF+2LLIF demonstrated correction in PDH (pre-fatigue: +0.96mm, post-fatigue: +0.44mm), FH (pre-fatigue: +0.29mm, post-fatigue: +0.16mm), and IVA (pre-fatigue: +4.07°, post-fatigue: +5.83°). Compared to intact specimens, BiTLIF+2LLIF demonstrated correction in PDH (pre-fatigue: +3.34mm, post-fatigue: +3.24mm), FH (pre-fatigue: +2.30mm, post-fatigue: +2.06mm), and IVA (pre-fatigue: +1.23°, post-fatigue: +1.22°).

Conclusions
Primary and dynamic lumbosacral stiffness was comparable between ALIF and bilateral TLIF configurations in “top-heavy” LLIF constructs. ALIF allowed for the greatest correction in intervertebral angle, but minimal changes in posterior and foraminal height, while the bilateral TLIF allowed for the greatest posterior and foraminal height correction, but less intervertebral angle correction. No significant differences were observed between ALIF or BiTLIF cohorts.