Posterior spinal instrumentation and decompression with or without cross-link?

Abstract

Background: Posterior lumbar instrumentation requires sufficient primary stiffness to ensure bony fusion and to avoid pseudarthrosis, screw loosening, or implant failure. To enhance primary construct stiffness, transverse cross-link (CL) connectors attached to the vertical rods can be used. Their effect on the stability of a spinal instrumentation with simultaneous decompression is yet not clear. This study aimed to evaluate the impact of CL augmentation on single-level lumbar instrumentation stiffness after gradual decompression procedures.

Methods: Seventeen vertebral segments (6 L1/2, 6 L3/4, 5 L5/S1) of 12 fresh-frozen human cadavers were instrumented with a transpedicular screw-rod construct following the traditional pedicle screw trajectory. Range of motion (ROM) of the segments was sequentially recorded before and after four procedures: (A) instrumented before decompression, (B) instrumented after unilateral laminotomy, (C) instrumented after midline bilateral laminotomy, and (D) instrumented after unilateral facetectomy (with transforaminal lumbar interbody fusion [TLIF]). Each test was performed with and without CL augmentation. The motion between the cranial and caudal vertebrae was evaluated in all six major loading directions: flexion/extension (FE), lateral bending (LB), lateral shear (LS), anterior shear (AS), axial rotation (AR), and axial compression/distraction (AC).

Results: ROM was significantly reduced with CL augmentation in AR by Δ0.03-0.18° (7-12%) with a significantly higher ROM reduction after more extensive decompression. Furthermore, slight reductions in FE and LB were observed; these reached statistical significance for FE after facetectomy and TLIF insertion only (Δ0.15; 3%). The instrumentation levels did not reveal any subgroup differences.

Conclusion: CL augmentation reduces AR-ROM by 7-12% in single-level instrumentation of the lumbar spine, with the effect increasing along with the extensiveness of the decompression technique. In light of the discrete absolute changes, CL augmentation may be warranted for highly unstable vertebral segments rather than for standard single-level posterior spinal fusion and decompression.

Keywords: Biomechanical; Cross-connector; Cross-link; Instrumentation; Segmental stability; Spine; lumbar.

Fig. 1

Biomechanical setup. A) Functional spine unit–Here, a sawbones model–In the vertical position for testing of axial rotation and axial compression. B) Spine segment installed in the horizontal position to measure flexion/extension and lateral shear. The caudal vertebra was attached to the semiconstrained test rig, which allowed for free translational movements in the horizontal plane. The cranial and caudal vertebral bodies were each installed with a marker, and additional markers were set on the cranial and caudal 3D-printed mounting clamps to control for excess movement between the vertebrae and the mounting system.

Fig. 2

Cross-link augmentation after varying degrees of surgical decompression. A = intact, B = unilateral laminotomy, C = midline decompression, D = unilateral facetectomy and insertion of a transforaminal interbody fusion cage. 1 = instrumentation without cross-link, 2 = instrumentation with cross-link.

Fig. 3

Absolute reduction in range of motion (ROM) after cross-link augmentation. Change is calculated as Δ_ROM = ROM_Crosslink − ROM_{Instrumentation}. The medians of the single differences are shown, along with the 25^th and 75^th percentiles. Bars below 0 indicate less motion with than without a cross-link. Asterisks (*) indicate statistical significance.