Biomechanical Analysis of Transforaminal Lumbar Interbody Fusion with Unilateral vs Bilateral Pedicle Screw Fixation in the Setting of Bilateral Spondylolysis: An In-vitro Human Cadaveric Model

Introduction: It is debated whether minimally invasive transforaminal lumbar interbody fusion (MIS TLIF) achieves better outcomes with unilateral (UPS) or bilateral (BPS) pedicle screws. While UPS are less invasive, it is questioned how effective they are for isthmic spondylolisthesis due to higher likelihood of cage migration, pain, and lower fusion rates (Ren et al., Eldin et al., Aoki et al.). This study compares spinal stability in cadaveric models with MIS TLIF using UPS versus BPS for pars defects.

Methods: Eleven lumbosacral spine specimens were tested on flexion-extension, lateral bending, and axial rotation. In preparation for biomechanical testing, the proximal lumbar vertebral bodies were secured in foundations using four-point compression screws. Five InfraRed Emitting Diodes (IRED) motion detection markers were then placed on the top/base containers and intervening vertebral bodies (L3, L4 and L5). The markers were equipped with three non-co-linear infrared emitting diodes designed for detection by an optoelectronic motion analysis system. Reconstructions at the operative L4-L5 level were performed and motions quantified for the following treatment conditions.

1. Intact pars interarticularis
2. Intact pars with UPS
3. Intact pars with BPS
4. Bilateral pars defects with UPS
5. Bilateral pars defects with BPS
6. Bilateral pars defects + TLIF cage + UPS
7. Bilateral pars defects + TLIF cage + BPS

Results: In the setting of BL pars defects, BPS fixation significantly reduced range of motion compared to UPS with a mean difference 3.48° for flexion-extension, 2.941° for lateral bending, and 2.475° for axial rotation. No significant difference was found between TLIF+UPS and TLIF+BPS for flexion-extension or lateral bending for BL pars defects. However, TLIF+BPS showed significantly greater stability at the operative level compared to TLIF+UPS, with a mean difference of 1.723°.

Conclusion : Our findings show that TLIF+BPS offers superior axial rotation stability compared to TLIF+UPS for pars/isthmic defects. Adding contralateral percutaneous pedicle screws during unilateral MIS TLIF with robotic or navigation technologies enhances stability while keeping the procedure minimally invasive.

Bilateral fixation may address issues like cage migration and lower fusion rates associated with unilateral MIS TLIF. Additional studies on migration, fusion rates, and patient outcomes are needed to confirm these benefits.