Motion compensation associated with single-level cervical fusion: where does the lost motion go?

STUDY DESIGN Seven adult human cadaveric cervical spines (C2-T1) were biomechanically tested in a programmable testing device. OBJECTIVE Compare the effects of incremental single-level fusion at different levels of the cervical spine. SUMMARY OF BACKGROUND DATA Clinical studies have reported degenerative symptomatic disc disease at disc levels adjacent to fusion. No known study has attempted to delineate the effects of single-level fusion at different levels of the cervical spine. METHODS The spines were tested in flexion, extension, right and left lateral bending, and right and left axial rotation for 7 different conditions: harvested and 6 independent single-level fused conditions (i.e., C2-C3, C3-C4, C4-C5, C5-C6, C6-C7, and C7-T1). Segmental motion and global stiffness data were normalized to the harvested condition and compared using a 1-way analysis of variance followed by a SNK test (P < 0.01). RESULTS Motion compensation was distributed among the unfused segments with significant compensation at the segments adjacent to fusion. Significant increases occurred at the level above C3-C4 and C4-C5 fusions, and below for C5-C6 and C6-C7 fusions in both flexion and extension. CONCLUSIONS Increase motion compensation occurred at segments immediately adjacent to a single-level fusion. Significant differences occurred at the level above the fusion site for the C3-C4 and C4-C5 fusion in both flexion and extension. When the lower levels (C5-C6, C6-C7) were fused, a significant amount of increased motion was observed at the levels immediately above and below the fusion. However, greater compensation occurred at the inferior segments than the superior segments for the lower level fusions (C5-C6, C6-C7).