Biomechanical comparison of unilateral and bilateral pedicle screws fixation for transforaminal lumbar interbody fusion after decompressive surgery--a finite element analysis

Abstract

Background: Little is known about the biomechanical effectiveness of transforaminal lumbar interbody fusion (TLIF) cages in different positioning and various posterior implants used after decompressive surgery. The use of the various implants will induce the kinematic and mechanical changes in range of motion (ROM) and stresses at the surgical and adjacent segments. Unilateral pedicle screw with or without supplementary facet screw fixation in the minimally invasive TLIF procedure has not been ascertained to provide adequate stability without the need to expose on the contralateral side. This study used finite element (FE) models to investigate biomechanical differences in ROM and stress on the neighboring structures after TLIF cages insertion in conjunction with posterior fixation.

Methods: A validated finite-element (FE) model of L1-S1 was established to implant three types of cages (TLIF with a single moon-shaped cage in the anterior or middle portion of vertebral bodies, and TLIF with a left diagonally placed ogival-shaped cage) from the left L4-5 level after unilateral decompressive surgery. Further, the effects of unilateral versus bilateral pedicle screw fixation (UPSF vs. BPSF) in each TLIF cage model was compared to analyze parameters, including stresses and ROM on the neighboring annulus, cage-vertebral interface and pedicle screws.

Results: All the TLIF cages positioned with BPSF showed similar ROM (<5%) at surgical and adjacent levels, except TLIF with an anterior cage in flexion (61% lower) and TLIF with a left diagonal cage in left lateral bending (33% lower) at surgical level. On the other hand, the TLIF cage models with left UPSF showed varying changes of ROM and annulus stress in extension, right lateral bending and right axial rotation at surgical level. In particular, the TLIF model with a diagonal cage, UPSF, and contralateral facet screw fixation stabilize segmental motion of the surgical level mostly in extension and contralaterally axial rotation. Prominent stress shielded to the contralateral annulus, cage-vertebral interface, and pedicle screw at surgical level. A supplementary facet screw fixation shared stresses around the neighboring tissues and revealed similar ROM and stress patterns to those models with BPSF.

Conclusions: TLIF surgery is not favored for asymmetrical positioning of a diagonal cage and UPSF used in contralateral axial rotation or lateral bending. Supplementation of a contralateral facet screw is recommended for the TLIF construct.

Figure 1

The finite-element model used in this study. (A) Front view. (B) Lateral view.

Figure 2

Finite element models show: Intact model of the L1-S1 segments; transforaminal lumbar interbody fusion (TLIF) cage implanted in models: (A) TLIFm (moon-shaped cage; middle); (B) TLIFa (moon-shaped cage; anterior); (C) TLIFo (left diagonal cage); and supplemented with (D) Bilateral pedicle screw fixation after minimal invasive TLIF surgery; or (E) Unilateral pedicle screw plus translaminar facet screw fixation contralaterally in TLIFo model (TLIFo + f).

Figure 3

Comparison of ROM calculated for the five levels of intact lumbar spine (INT) with previous studies: (A) loading of 3.75 N-m and 7.5 N-m pure moments in the INT models; (B) loading of 10 N-m moments with 150 N preload in the INT models.

Figure 4

The calculated ROM at surgical L4-5 level in the INT, TLIFm, TLIFa, TLIFo models supplemented with: (A) Bilateral pedicle screw fixation or (B) Unilateral pedicle screw fixation. Translaminar facet screw was used in the model of TLIFo plus unilateral pedicle screw fixation (TLIFo + f). (Fl = Flexion; Ex = Extension; LLB = Left lateral bending; RLB = Right lateral bending; LAR = Left axial rotation; RAR = Right axial rotation).

Figure 5

The maximum von Mises stress of annulus in all models (1 = with unilateral pedicle screws fixation; 2 = with bilateral pedicle screw fixation).

Figure 6

For the unilateral fixation, the stresses distribution in annulus at surgical level of TLIFo model under (A) right axial rotation (RAR) or (B) right lateral bending (RLB), and the cage-bone interface under (C) RAR or (D) RLB. The stresses distribution in annulus at surgical level of TLIFo model supplemented with a translaminar facet screw contralaterally under (E) RAR or (F) RLB, and the cage-bone interface under (G) RAR or (H) RLB.

Figure 7

The maximum von Mises stress of pedicle screws in all models. (1 = with unilateral pedicle screws fixation; 2 = with bilateral pedicle screw fixation) (FL = Flexion; EX = Extension; LLB = Left lateral bending; RLB = Right lateral bending; LAR = Left axial rotation; RAR = Right axial rotation).