Study design: Biomechanics of normal vertebral segments adjacent to a degenerated segment in the cervical spine.
Objective: To test the hypothesis of higher motion changes in the segment immediately inferior to a degenerated segment.
Summary of background data: Past research has shown how disc degeneration (DD) affects adjacent segments; however, these studies are conducted only on the lumbar spine or the experimental protocols used are characterized by the presence of degeneration in adjacent segments. The question arises as to how much of the degenerative effect in a particular segment is internal to degeneration at that segment and how much is caused by degeneration at adjacent segments. It would be clinically relevant to analyze biomechanical changes in adjacent segments in the cervical spine by considering DD at only one segment, where adjacent segments remain normal.
Methods: A poroelastic, 3-dimensional finite element model of a normal C3-T1 segment was validated and then used for the degenerative study. Two additional C3-T1 models were developed with moderate and severe degenerative C5-C6 disc grades. Disc geometry and tissue material properties were modified to simulate C5-C6 DD. Intersegmental rotational motions (C4-C5, C5-C6, and C6-C7) for the 3 C3-T1 models were computed under moment loads.
Results: With progressive C5-C6 DD, motion decreased at that segment. At adjacent segments, higher motion changes were observed mainly in flexion/extension. Inferior C6-C7 motion changes were higher than superior C4-C5 motion changes. The inferior C6-C7 motion was affected even when C5-C6 DD was moderate, and it was further affected by severe C5-C6 DD. The superior C4-C7 motion, however, was mostly affected by severe C5-C6 DD.
Conclusion: The hypothesis of higher motion changes in the normal C6-C7 segment immediately inferior to a degenerated C5-C6 segment was found to be true. Future experiments on multisegmental cervical spines are recommended to verify the current data.