Is intervertebral disc degeneration related to segmental instability? An evaluation with two different grading systems based on clinical imaging.

Background Several in vitro studies investigated how degeneration affects spinal motion. However, no consensus has emerged from these studies. Purpose To investigate how degeneration grading systems influence the kinematic output of spinal specimens. Material and Methods Flexibility testing was performed with ten human T12-S1 specimens. Degeneration was graded using two different classifications, one based on X-ray and the other one on magnetic resonance imaging (MRI). Intersegmental rotation (expressed by range of motion [ROM] and neutral zone [NZ]) was determined in all principal motion directions. Further, shear translation was measured during flexion/extension motion. Results The X-ray grading system yielded systematically lesser degeneration. In flexion/extension, only small differences in ROM and NZ were found between moderately degenerated motion segments, with only NZ for the MRI grading reaching statistical significance. In axial rotation, a significant increase in NZ for moderately degenerated segments was found for both grading systems, whereas the difference in ROM was significant only for the MRI scheme. Generally, the relative increases were more pronounced for the MRI classification compared to the X-ray grading scheme. In lateral bending, only relatively small differences between the degeneration groups were found. When evaluating shear translations, a non-significant increase was found for moderately degenerated segments. Motion segment segments tended to regain stability as degeneration progressed without reaching the level of statistical significance. Conclusion We found a fair agreement between the grading schemes which, nonetheless, yielded similar degeneration-related effects on intersegmental kinematics. However, as the trends were more pronounced using the Pfirrmann classification, this grading scheme appears superior for degeneration assessment.