Low back pain is one of the most prevalent health complaints in the US, with an estimated 70-85% of the population developing back pain at some point in their life, creating a significant financial burden. Although the causes of low back pain are poorly defined and indistinct, most often implicated as the origin of pain, is the intervertebral disc. The disc affords the spine its extensive multidirectional motion due to the complex interaction between two morphologically, biomechanically, and biochemically distinct tissues: the annulus fibrosus and the nucleus pulposus. With advancing age, injury, pathology or a combination of these, a degenerative cascade of biomechanical, biochemical, and nutritional alterations diminish the discs' ability to maintain its structure and function. Unfortunately, measurement of these properties in vivo is currently not a viable option due to the invasiveness of the procedures. Therefore, an indirect method is needed to evaluate the multifarious characteristics of a patient's disc health. Of critical interest is the relationship that functional spinal mechanics has with the morphologic, biochemical, and biomechanical properties of the intervertebral disc as they change with degeneration.
Eighteen osteoligamentous cadaveric lumbar spines that spanned the degenerative spectrum were utilized in a correlation study design to evaluate the relationships between each factors of disc health: imaging, biochemical content, biomechanical competency, and functional mechanics. Each specimen was first imaged using quantitative T2* MRI techniques, where the site-specific relaxation times and features of the Pfirrmann grading system, including signal intensity and distinction between the nucleus pulposus and surrounding annulus fibrosus, were measured. Then their functional spinal mechanics were evaluated and range of motion, neutral zone ratio, bending stiffness and helical axis patterns were computed. Local biochemical content and compressive biomechanical properties were subsequently analyzed. Each outcome measure was then assessed with respect to the others using correlation statistical methods in an effort to understand the multifactorial relationships surrounding disc degeneration.
The T2* relaxation times and newly defined variables, T2* Intensity Area and Transition Zone Slope, were significantly correlated to the standard Pfirrmann grading, showing the T2* MR imaging parameters are sensitive to the morphological changes associated with disc degeneration. Also, these features enable the quantitative grading of disc degeneration without subjectivity or bias but with clinically recognized features of distinction. Furthermore, T2* relaxation times were found to have a high sensitivity for detecting the proteoglycan content of the intervertebral disc, which may potentially have a profound impact on the early diagnosis of degeneration. The T2* relaxation times were also significantly correlated to the residual stress and excised strain of the disc. These multi-faceted changes that occur with degeneration impact the global mechanics of the spinal unit by increasing the neutral zone to range of motion ratio, or joint instability, and altering the bending stiffness and range of motion. Even stronger correlations were measured with alterations in the helical axis patterns of lateral bending. There was a marked increase of out-of-plane rotations and a larger migration of the instantaneous axis of rotation with worsening degeneration evaluated by MRI, local biochemistry, and local residual mechanics.
Quantitative T2* MRI has the sensitivity to predict the local biochemical and biomechanical properties of the intervertebral disc. Complementary to MRI analysis, the measurement of the pathway of motion throughout the degenerative progress, using the helical axis approach, can enhance the disc assessment. Altogether, these clinically viable methods may immediately improve the characterization of the intervertebral disc for enhanced treatment and care.
University of Minnesota Ph.D. dissertation. Major: Biomedical Engineering. May 2013. Advisor: David J. Nuckley. 1 computer file (PDF); xii, 149 pages, appendix p. 143-149.
Ellingson, Arin Michael.
Intervertebral disc degeneration, quantified by T2* MRI, biochemistry, and compressive mechanics, correlated to global functional mechanics of the lumbar spine.
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