Effect of lower two-level anterior cervical fusion on the superior adjacent level

OBJECT: Symptomatic multisegment disease is most common at the C5-6 and C6-7 levels, and two-level anterior cervical discectomy and fusion (ACDF) is performed most often at these levels. Therefore, it may be clinically important to know whether a C5-7 fusion affects the superior C4-5 segment. A biomechanical study was carried out using cadaveric cervical spine specimens to determine the effect of lower two-level anterior cervical fusion on intradiscal pressure and segmental motion at the superior adjacent vertebral level. METHODS: Five cadaveric cervical spine specimens were used in this study. The specimens were stabilized at T-1 and loaded at C-3 to 15 degrees flexion, 10 degrees extension, and 10 degrees lateral bending before and after simulated two-level ACDF with plate placement at C5-7. Intradiscal pressure was recorded at the C4-5 level, and segmental motion was recorded from C-4 through C-7. Differences in mean intradiscal pressures were calculated and analyzed using a paired Student t-test. When the maximum calibrated intradiscal pressures were exceeded ("overshot") during measurements, data from the specimens involved were analyzed using the motion data with a Student t-test. Values for pressure and motion obtained before and after simulated ACDF were compared. RESULTS: During flexion, the mean intradiscal pressure changes (+/- standard deviations) in the pre- and post-ACDF measurements were 1275 (+/- 225) mm Hg and 2475 (+/- 75) mm Hg, respectively (p < 0.05). When the results of pre-ACDF testing were compared with post-ACDF results, no significant difference was found in the mean changes in the intradiscal pressure during extension and lateral bending. The maximum calibrated intradiscal pressures were exceeded during the post-ACDF testing in four specimens in extension, three in flexion, and two in lateral bending. Comparison of pre- and post-ACDF data for all five specimens revealed significant differences in motion and intradiscal pressure (p < 0.05) during flexion, significant differences in motion (p < 0.05) but not in intradiscal pressure during extension, and significant differences in intradiscal pressure changes (p < 0.05) but not in motion during lateral bending. CONCLUSIONS: Simulated C5-7 ACDF caused a significant increase in intradiscal pressure and segmental motion in the superior adjacent C4-5 level during physiological motion. The increased pressure and hypermobility might accelerate normal degenerative changes in the vertebral levels adjacent to the anterior cervical fusion.