In Vivo Remodeling of Intervertebral Discs in Response to Short- and Long-Term Dynamic Compression

Karin Wuertz,1,2 Karolyn Godburn,1 Jeffrey J. MacLean,1 Ana Barbir,1 Justin Stinnett Donnelly,1 Peter J. Roughley,3 Mauro Alini,4 James C. Iatridis1

1Spine Bioengineering Lab, School of Engineering, University of Vermont, 33 Colchester Avenue, 201 Perkins Hall, Burlington, Vermont 05405,

2Spine Research Unit, University Hospital Balgrist, University of Zurich, Zurich, Switzerland, 3Genetics Unit, Shriners Hospital for Children,

Montreal, Quebec, Canada, 4AO Research Institute, Davos Platz, Switzerland

Received 16 July 2008; accepted 27 January 2009

Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/jor.20867 

Summary: 

Those suffering from lower back pain are often diagnosed with intervertebral disc degeneration, a condition affected by many variables including genetics, mechanical loading, and nutrient availability.  Further analysis of disc degeneration reveals characteristic losses in disc height, nucleus volume, and distinction between the nucleus pulposus and annulus fibrosus.  Since active cell metabolism and matrix remodeling are likely indicators of healthy discs, biochemical levels of matrix enzymes and proteins – more specifically, changes in levels of glysosaminoglycans (GAG) and the ratio of collagen-1 and collagen-2 – can be measured to determine whether or not a disc is degenerative.  While loading type, magnitude, duration, and frequency of compression all influence metabolic activities of the intervertebral disc, this study focuses on the effects of short-term and long-term dynamic compression on intervertebral disc degeneration. 

Two sets of experiments were performed on rat tails, the first of which investigated the effects of applying prolonged daily exposure to dynamic compression for two weeks and was comprised of three groups: 1.5 hrs (dynamic compression applied for 1.5 hrs), 8 hrs (dynamic compression applied for 8 hrs), and sham (identical conditions except experienced no loading).  The second experiment investigated the effects of prolonged experimental duration (2 weeks vs. 8 weeks) on both the sham group and discs exposed to dynamic compression 8 hrs/day.  After each experiment, discs were analyzed for changes in gene expression, biomechanical composition, mechanical properties, intervertebral disc height, and morphology. 

Results of the first experiment illustrated that both groups exposed to either 1.5 hrs or 8 hrs of dynamic compression for two weeks exhibited an increase in anabolic gene expression (mainly collagen-2), which did not differ much between the two groups.  Therefore, it can be concluded that despite an increase in daily exposure time to dynamic compression, anabolic remodeling increases.  

The second experiment hinted at an ideal amount of dynamic compression, which if exceeded could surpass the anabolic remodeling stage and lead to signs of disc degeneration.  

Significant losses in biochemical activity and intervertebral disc height in the sham groups of both experiments suggest a correlation between disuse and degeneration.  Therefore, while exposure to dynamic compression for significant periods of time can lead to disc degeneration, immobilizing and thus preventing disc movement (the sham group) can also result in similar observations of degeneration.  These results signify the notion of a healthy amount of loading to provide optimal conditions for sustained integrity of intervertebral discs. 

Moderate levels of physical activity can potentially prevent or postpone disc degeneration as a healthy amount of compressive loading can promote matrix enzymatic and protein activity; however, since anabolic remodeling alone is not sufficient to restore disc height, dynamic compression would do little service to repairing already degenerative or damaged discs.  

Significance:

 Lower back pain, which can often be attributed to degenerative disc disease, plagues many people and is completely relevant to sustaining good spinal health.  Further research into the mechanics of intervertebral discs can provide valuable insight into the effects of varying durations and levels of physical activity on the spine.  Moreover, future studies may be able to shed light on new methods to repair and improve already damaged or degenerative discs.