The Effect of Single Level and Multilevel Cervical Fusion on Adjacent Segment Motion: A Biomechanical Analysis

Cervical arthrodesis is a common surgical technique utilized to treat many cervical spinal pathologies. The risk of re-operation due to symptomatic adjacent segment disease (ASD) remains steady, and the cause of ASD, whether rigid fixation or disease progression, remains unknown. The purpose of this study was to develop a cervical fusion model and analyze motion of the subaxial cervical spine adjacent segments following single level, two-level, and three-level spine fusion.

A in vitro cervical arthrodesis biomechanics model was developed utilizing nine fresh-frozen cervical spine specimens. Each spine underwent non-destructive testing in flexion, extension, axial rotation, and lateral bending using a six degree-of-freedom robotic manipulator. Following intact specimen testing, testing was repeated after C5-C6, C4-C6, C5-C7, C4-C7 fusions. Changes in range of motion among tests were calculated at each segment.

The cervical arthrodesis model showed reduced motion significantly at all fused levels. Following single level fusion at C5-C6, no statistically significant increases in adjacent segment motion occurred in any testing condition (p > 0.05). Fusion between C4-C6 resulted in increased motion during flexion at C2-C3 (p < 0.001). Two-level fusion between C5-C7 resulted in increased flexion at C2-C3 (p < 0.001) and C4-C5 (p < 0.05). Lastly, three-level fusion from C4-C7 produced significant changes in extension at C2-C3 (p < 0.01), C3-C4 (p < 0.01), and C4-C5 (p < 0.01), in flexion at C2-C3 (p < 0.01) and C4-C5 (p < 0.001), right axial rotation at C2-C3 (p < 0.001) and left axial rotation at C2-C3 (p = 0.001), C3-C4 (p < 0.02), and C4-C5 (p < 0.001).

We successfully developed a rigid cervical arthrodesis model and tested specimens in single, two-level, and three-level cervical fusion. No increases were found in adjacent segments following single level fusion and non-contiguous motion was increased following C4-6 fusion. Three-level fusion showed the greatest change in adjacent segment motion.