A computational comparison of lumbar interbody fusion utilizing different interbody cages with unilateral or bilateral fixation

Abstract

Purpose: Extreme lateral lumbar interbody fusion (XLIF) is a surgical approach to manage severe low back pain associated with lumbar degenerative diseases. In comparison with transforaminal interbody fusion (TLIF), XLIF reduced the range of motion thus achieving improved stability postoperatively. Observational studies also suggested that unilateral pedicle screw fixation (PSF) is similar to bilateral screw fixation in terms of fusion rate, but with significant reduction in operative time and blood loss. However, its fixation stability is unknown. The aim of study is to perform a computational simulation of the stability of LIF with unilateral or bilateral PSF. Materials and Methods: Computed tomography scans of a human L4 and L5 were acquired with an isotropic spatial resolution of 1 mm. Two interbody cages (22 mm and 16 mm) were developed to represent XLIF and TLIF, respectively. A cylinder (length, 45 mm; diameter, 5.5 mm) was built as an idealized shape of the pedicle screw (PS) implant and connected by a third cylinder (radius, 5.5 mm) representing the rod. Axial compression was performed across these models. Results: The maximum stress at rods with XLIF was reduced (23.92 MPa) compared with TLIF (17.49 MPa) with bilateral PSF. The maximum strain at the facet joint was reduced using XLIF (0.096) rather than TLIF (0.14). The maximum bone deformation was similar. Maximum rod stress and facet joint strain were comparable with bilateral and unilateral PSF on the left side but stability reduced on right side (no fixation with unilateral PSF) suggesting possible effects of facet tropism. Conclusion: This biomechanical simulation suggests that XLIF, with bilateral or unilateral PSF, provided improved stability over TLIF constructs. Bilateral and unilateral PSF can achieve similar postoperative stability, but it may be subjected to variable facet orientation. Unilateral PS with XLIF is less invasive and more economical with equal stability achieved. All while keeping the contralateral side available in case of future revision surgery.