Study Design. Mechanical study on cadaver motion segments.
Objective. To determine whether high gradients of compressive stress within the intervertebral disc are associated with progressive disc degeneration.
Summary of Background Data. Mechanical loading can initiate disc degeneration but may be unimportant in disease progression, because degenerative changes cause the disc to be increasingly “stress-shielded” by the neural arch. However, the most typical feature of advanced disc degeneration (delamination and collapse of the annulus) may not depend on absolute values of compressive stress but on gradients of compressive stress that act to shear annulus lamellae.
Methods. A total of 191 motion segments (T7–T8 to L5–S1) were dissected from 42 cadavers aged 19 to 92 years. Each was subjected to approximately 1 kN compression, while intradiscal stresses were measured by pulling a pressure transducer along the disc's midsagittal diameter. “Stress gradients” in the annulus were quantified as the average rate of increase in compressive stress (MPa/mm) between the nucleus and the region of maximum stress in the anterior or posterior annulus. Measurements were repeated before and after creep loading and in simulated flexed and erect postures. Disc degeneration was assessed macroscopically on a scale of 1 to 4.
Results. As grade of disc degeneration increased from 2 to 4, nucleus pressure decreased by an average 68%, and maximum compressive stress in the annulus decreased by 48% to 64%, depending on location and posture. In contrast, stress gradients in the annulus increased by an average 75% in the anterior annulus (in flexed posture) and by 108% in the posterior annulus (in erect posture). Spearman rank correlation showed that these increases were statistically significant.
Conclusion. Despite stress-shielding by the neural arch, gradients of compressive stress increase with increasing grade of disc degeneration. Stress gradients act to shear adjacent lamellae and can explain progressive annulus delamination and collapse.
Level of Evidence: N/A
With increasing grade of disc degeneration, stress-shielding of the disc by the neural arch reduces nucleus pressure and peak compressive stresses on the annulus. Nevertheless, gradients of compressive stress in the annulus increase substantially. Stress gradients act to shear adjacent lamellae and can explain progressive annulus delamination and collapse.
*School of Science and Engineering, University of Nicosia, Cyprus
†University of Roehampton, London, United Kingdom; and
‡Centre for Comparative and Clinical Anatomy, University of Bristol, Bristol, United Kingdom.
Address correspondence and reprint requests to Michael A. Adams, PhD, Centre for Comparative and Clinical Anatomy, University of Bristol, Southwell St, Bristol BS2 8EJ, United Kingdom; E-mail: M.A.Adams@bris.ac.uk
Acknowledgment date: October 15, 2013. First revision date: December 2, 2013. Second revision date: February 26, 2014. Third revision date: April 14, 2014. Acceptance date: April 15, 2014.
The manuscript submitted does not contain information about medical device(s)/drug(s).
BackCare, Action Medical Research, and the Greek Institute of Scholarships funds were received to support this work.
Relevant financial activities outside the submitted work: grants/grants pending, payment for lectures, royalties, and travel/accommodations/meeting expenses.