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The Basis of Mechanical Instability in Degenerative Disc Disease: A Cadaveric Study of Abnormal Motion Versus Load Distribution

Sengupta, Dilip K., MD, Dr. Med; Fan, Haibo, PhD

doi: 10.1097/BRS.0000000000000292
Biomechanics
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Study Design. A biomechanical study in cadaveric lumbar spine.

Objective. To establish the basis of mechanical stability in degenerative disc disease from the relationship between range of motion (ROM), neutral zone motion (NZ), intradiscal pressure profile, and instantaneous axis or rotation (IAR) in advancing grades of disc degeneration.

Summary of Background Data. The basis of mechanical instability in lumbar disc degeneration remains poorly understood. Controversy exists between abnormal motion and abnormal loading theories.

Methods. Thirty-nine lumbar motion segments were graded for staging of disc degeneration with magnetic resonance scan. These specimens were tested for ROM and NZ in a 6 df spine simulator, with 7.5 N·m unconstrained, cyclical loading. Continuous tracking of IAR was derived from ROM data. Intradiscal pressure profiles were determined using needle-mounted pressure transducer, drawn across the disc space under constant loading.

Results. The ROM showed insignificant change, but a trend of increase from grade I through III and a decrease with advanced degeneration. NZ increased significantly with advancing disc degeneration. Intradiscal pressure profile showed an even distribution of the load in normal discs but a depressurized nucleus and irregular spikes of excessive loading, with advancing degeneration. The IAR showed a smooth excursion in normal versus irregular jerky excursion in degenerated discs, without significant change in excursion. The center of rotation, derived from IAR, showed significantly increased vertical translation with advancing degeneration, indicating an abnormal quality of motion.

Conclusion. The study established a basis of mechanical instability in the lumbar spine with advancing disc degeneration as an abnormal quality of motion represented by variation in IAR and center of rotation, increased NZ motion without any increase in quantity of motion, and abnormal load distribution across the disc space with spikes of high load amidst depressurized nucleus. The study cannot identify clinical instability but finds an association between the abnormal motions and the abnormal load distribution in mechanical instability.

Level of Evidence: N/A

This biomechanical study on cadaveric lumbar spine analyzes range of motion, neutral zone motion, instantaneous axis or rotation, center of rotation, and intradiscal pressure profile to characterize advancing grades of disc degeneration identified by magnetic resonance image and establish a basis of mechanical instability in lumbar disc degeneration.

From the Department of Orthopedics, Dartmouth-Hitchcock Medical Center, Lebanon, NH.

Address correspondence and reprint requests to Dilip K. Sengupta, MD, Dr. Med, Department of Orthopedics, Dartmouth-Hitchcock Medical Center, One Medical Center Dr, Lebanon, NH 03756; E-mail: dksg@hotmail.com

Acknowledgment date: October 21, 2013. First revision date: December 29, 2013. Second revision date: February 5, 2014. Acceptance date: February 12, 2014.

The manuscript submitted does not contain information about medical device(s)/drug(s).

NASS Biomechanical Research Grant (2008) funds were received to support this work.

Relevant financial activities outside the submitted work: board membership, royalties, and stock/stock options.

© 2014 by Lippincott Williams & Wilkins