Biomechanical study on the effect of cervical spine fusion on adjacent-level intradiscal pressure and segmental motion

Study Design: A biomechanical study was performed using cadveric cervical specimens. Objective: To determine the effect of cervical spine fusion adjacent level intradiscal pressure. Summary of Background Data: Clinical studies have reported that patients with spinal fusion are at greater risk of pathology and early disc degeneration at adjacent levels. It is hypothesized that eliminating motion at one level leads to hypermobility and increased forces at adjacent levels, thus increasing the rate of disc degeneration. Methods: Six cadaveric cervical spine specimens were tested. Specimens were stabilized at T1 and loaded at C3 to 20degrees of flexion and 15degrees of extension. Intradiscal pressures and segmental motion at C4-C5 and C6-C7 were recorded first on intact specimens, and then after anterior cervical plating at C5-C6. Changes in intradiscal pressure and segmentation motion were calculated and statistically analyzed using a paired Student t test. Results: Intradiscal pressures were significantly increased during flexion at both adjacent levels. The pressure increased by 73.2% at C4-C5 (P = 0.002), and by 45.3% at C6-C7 (P = 0.006). Intradiscal pressures increased at both adjacent levels during extension, but not significantly. During flexion, segmental ,motion increased at C4-C5. During extension, segmental motion increased at both adjacent levels, with greater increases at C6-C7. Conclusions: Clinical studies have reported increased rates of disc degeneration at levels adjacent to fusion. It is believed that eliminating motion through fusion shifts the load to the adjacent levels, causing earlier disc degeneration. This study has shown that significant increases in intradiscal pressure and segmental motion occur at levels adjacent to fusion during normal range of motion. These results may partially explain the mechanism of early disc degeneration at levels adjacent to cervical spine fusion.