Effects of facetectomy and crosslink augmentation on motion segment flexibility in posterior lumbar interbody fusion.

STUDY DESIGN: Biomechanical assessment using calf lumbar motion segments.

OBJECTIVE: To determine whether facetectomy affects the primary stability of posterior lumbar interbody fusion.

SUMMARY OF BACKGROUND DATA: To improve visualization and access to the disc space, the facet joints frequently are removed. Previous biomechanical studies have indicated a fundamental role for the facet joints in maintaining spinal segment stability.

METHODS: Single motion segments from calf lumbar spines were tested for pure-moment flexibility in flexion-extension (FE), lateral bending (LB), and axial rotation (AR). After testing intact, an interbody cage and pedicle screw system were implanted. Next, a bilateral facetectomy was performed, and finally a crosslink was added. Flexibility testing was repeated at each stage of implantation. Data are reported for range of motion (ROM), neutral zone (NZ), and a new compliance parameter (COM), based on the slopes of the moment-angle curve in the neutral and elastic regions.

RESULTS: With posterior lumbar interbody fusion implantation, ROM in FE was reduced 82% +/- 4% (mean +/- standard deviation) and NZ 78% +/- 7% over intact (P < 0.015: Wilcoxon). Reduction in LB was slightly more, whereas reduction in AR was considerably less and did not achieve statistical significance for NZ. After facetectomy, ROM in FE increased 0.3 degrees (P < 0.05), on average, and NZ did not change. In LB neither changed significantly. In AR, ROM increased 0.6 degrees (P < 0.05), and NZ increased 0.2 degrees (P < 0.05). The addition of a crosslink changed ROM and NZ less than 0.1 degrees in FE and LB, whereas in AR it restored half of the stability lost due to facetectomy in ROM (P < 0.05), and had a similar trendwise effect on NZ. The new compliance measure, COM, was found to agree with the direction of change in ROM more consistently than did NZ.

CONCLUSION: Facetectomy causes a nominal increase in ROM and NZ in FE and LB, which are not affected by the addition of a crosslink. Although the effect of facetectomy is greater in AR-and crosslink has a measurable restoring effect-all differences are within a few tenths of a degree under this loading paradigm. Thus, the clinical utility of adding a crosslink may not be justified based on these small biomechanical changes. COM can serve as a complement to ROM and NZ, or even as a surrogate when its 2 components are reported together, as it shows strong agreement with ROM, effectively distinguishes between lax and elastic region behaviors, and provides a measure of flexibility independent of the load range.