33 - Sequential ALL Release with Expandable Spacers for Lordosis Correction in Anterior-to-Psoas Lumbar Interbody Fusion: A Radiographic and Biomechanical Study

Introduction
Anterior column realignment (ACR) is an attractive, minimally invasive alternative to pedicle subtraction osteotomies (PSO) for correcting sagittal imbalance. ACR can be performed laterally or with anterior-to-psoas lumbar interbody fusion (ATP-LIF). Expandable spacers offer more controlled tensioning of the anterior longitudinal ligament (ALL) during release which could optimize sagittal correction and anterior column stability.

Aims/Objectives
To investigate biomechanical stability and sagittal correction of ATP-LIF with sequential ALL release.

Methods
Seven fresh-frozen human cadaveric lumbar spine specimens (L2-5) with similar age and DEXA scores were used. Specimens with previous spinal trauma, malignancy, deformity, or fractures were excluded. Range-of-motion (ROM) of intact specimens was measured before being treated with a single-level ATP-LIF at L3-4 via an expandable spacer, posterior fixation via pedicle screws and rods, and integrated fixation via lateral screws. ROM was measured for ATP-LIF constructs with intact ALL. Posterior fixation was removed and the width of the ALL was divided into three equal sections and sequentially resected one section at a time to achieve 1/3, 2/3, and complete ALL release. After each resection, spacers were expanded until the torque limit of the expansion driver was reached (3.0Nm). Posterior fixation was reapplied before testing. ROM was measured for intact and treated specimens in flexion-extension (FE), lateral bending (LB), and axial rotation (AR) using a custom-built motion simulator (±7.5Nm), capture system, and analysis software. Radiographic parameters were evaluated with fluoroscopy. These included anterior disc height (ADH), posterior disc height (PDH), foraminal height (FH), and intervertebral angle (IVA).

Results
Following ATP-LIF, constructs with intact ALL had significantly less ROM than intact in FE (17.8% intact, p=0.02), LB (13.8% intact, p=0.02), and AR (29.5% intact, p=0.01), and increased ADH (+2.64mm), PDH (+2.28mm), FH (+2.13mm), and IVA (+0.82°) compared to intact. LB and AR remained stable despite sequential ALL release. FE increased slightly following 2/3 ALL release (32.9% intact, p=0.09 vs intact), but remained stable with complete release.
Compared to intact ALL, expanded spacer height increased linearly with sequential ALL release (1/3 cut: +1.16mm, 2/3 cut: +2.16mm, 3/3 cut: +3.18mm). ADH also increased linearly with sequential ALL release (1/3 cut: +3.95mm, 2/3 cut: +5.92mm, 3/3 cut: +8.92mm). PDH also increased linearly with sequential ALL release (1/3 cut: +2.85mm, 2/3 cut: +3.24mm, 3/3 cut: +3.67mm). FH remained stable despite sequential ALL release (1/3 cut: +2.85mm, 2/3 cut: +3.08mm, 3/3 cut: +3.46mm), while IVA increased exponentially with sequential ALL release (1/3 cut: +2.26°, 2/3 cut: +4.98°, 3/3 cut: +8.78°).

Conclusions
Stiffness was comparable across all constructs despite sequential ALL release. There were no significant differences in stiffness between ATP-LIF constructs with different levels of ALL release. Complete ALL release allowed for the most correction in ADH, PDH, FH, and IVA. Spacer expansion allowed for controlled tensioning of the ALL in order to achieve partial and complete ligament release without removing or repositioning the spacer.