Biomechanical comparison of anterior lumbar interbody fusion: stand-alone interbody cage versus interbody cage with pedicle screw fixation -- a finite element analysis

Abstract

Background: Anterior lumbar interbody fusion (ALIF) followed by pedicle screw fixation (PSF) is used to restore the height of the intervertebral disc and provide stability. Recently, stand-alone interbody cage with anterior fixation has been introduced, which eliminates the need for posterior surgery. We compared the biomechanics of the stand-alone interbody cage to that of the interbody cage with additional PSF in ALIF.

Methods: A three-dimensional, non-linear finite element model (FEM) of the L2-5 segment was modified to simulate ALIF in L3-4. The models were tested under the following conditions: (1) intact spine, (2) destabilized spine, (3) with the interbody cage alone (type 1), (4) with the stand-alone cage with anterior fixation (SynFix-LR®; type 2), and (5) with type 1 in addition to PSF (type 3). Range of motion (ROM) and the stiffness of the operated level, ROM of the adjacent segments, load sharing distribution, facet load, and vertebral body stress were quantified with external loading.

Results: The implanted models had decreased ROM and increased stiffness compared to those of the destabilized spine. The type 2 had differences in ROM limitation of 8%, 10%, 4%, and 6% in flexion, extension, axial rotation, and lateral bending, respectively, compared to those of type 3. Type 2 had decreased ROM of the upper and lower adjacent segments by 3-11% and 3-6%, respectively, compared to those of type 3. The greatest reduction in facet load at the operated level was observed in type 3 (71%), followed by type 2 (31%) and type 1 (23%). An increase in facet load at the adjacent level was highest in type 3, followed by type 2 and type 1. The distribution of load sharing in type 2 (anterior:posterior, 95:5) was similar to that of the intact spine (89:11), while type 3 migrated posterior (75:25) to the normal. Type 2 reduced about 15% of the stress on the lower vertebral endplate compared to that in type 1. The stress of type 2 increased two-fold compared to the stress of type 3, especially in extension.

Conclusions: The stand-alone interbody cage can provide sufficient stability, reduce stress in adjacent levels, and share the loading distribution in a manner similar to an intact spine.

Figures

Figure 1

Three-dimensional finite element model of (A) a normal spine model (L2–5), (B) a destabilized model, and (C) post-operated models: type 1 (SynCage-LR®), type 2 (SynFix-LR®), and type 3 (SynCage-LR® + pedicle screws).

Figure 2

Intact model validation based on cadaveric study: stiffness under axial compressive load (A), various loading modes (B), and intradiscal behavior under axial compressive load (C).

Figure 3

Moment-rotation curve in flexion/extension with 5 models (destabilized spine, intact spine, type 1, type 2, and type3) at an operated segment (L3–4) in 10 Nm flexion/extension superimposed a follower load of 400 N.

Figure 4

ROM (A) and stiffness (B) among various surgical models at the operated level under flexion, extension, lateral bending, and axial rotation.

Figure 5

Normalized intersegmental rotation angle of types 1, 2, and 3 in flexion (A), extension (B), bending (C), and axial rotation (D) using a hybrid multidirectional test [[11]].

Figure 6

Contour plots of peak von Mises stress (PVMS) of the lower vertebral body (L4) when tested with flexion load of 10 Nm (A) and extension load of 10 Nm (B) after applying a compressive follower load (400 N).