The role of the nucleus pulposus in neutral zone human lumbar intervertebral disc mechanics

Department of Materials Science and Engineering, Drexel University, 3141 Chestnut Street, Philadelphia, PA 19104, USA.
Journal of Biomechanics (Impact Factor: 2.5). 02/2008; 41(10):2104-11. DOI: 10.1016/j.jbiomech.2008.04.037
Source: PubMed

ABSTRACT To study the effect of denucleation on the mechanical behavior of the human lumbar intervertebral disc through a 2mm incision, two groups of six human cadaver lumbar spinal units were tested in axial compression, axial rotation, lateral bending and flexion/extension after incremental steps of "partial" denucleation. Neutral zone, range of motion, stiffness, intradiscal pressure and energy dissipation were measured; the results showed that the contribution of the nucleus pulposus to the mechanical behavior of the intervertebral disc was more dominant through the neutral zone than at the farther limits of applied loads and moments.

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    ABSTRACT: There is currently no universal model or fitting method to characterize the viscoelastic behavior of the lumbar spine observed in displacement versus load hysteresis loops. In this study, proposed methods for fitting these loops, along with the metrics obtained, were thoroughly analyzed. A spline fitting technique was shown to provide a consistent approximation of spinal kinetic behavior that can be differentiated and integrated. Using this tool, previously established metrics were analyzed using data from two separate studies evaluating different motion preservation technologies. Many of the metrics, however, provided no significant differences beyond range of motion analysis. Particular attention was paid to how different definitions of the neutral zone capture the high-flexibility region often seen in lumbar hysteresis loops. As a result, the maximum slope was introduced and shown to be well-defined. This new parameter offers promise as a more descriptive measurement of spinal instability in vitro and may have future implications in clinical diagnosis and treatment of spinal instability. In particular, it could help in assigning treatments to specific stabilizing effects in the lumbar spine.
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