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° flexion, 10° extension, and 10° 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.