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: 2.72). 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: The first objective of this study was to determine the effects of physiological cyclic loading followed by unloaded recovery on the mechanical response of human intervertebral discs. The second objective was to examine how nucleotomy alters the disc's mechanical response to cyclic loading. To complete these objectives, 15 human L5-S1 discs were tested while intact and subsequent to nucleotomy. The testing consisted of 10,000 cycles of physiological compressive loads followed by unloaded hydrated recovery. Cyclic loading increased compression modulus (3%) and strain (33%), decreased neutral zone modulus (52%), and increased neutral zone strain (31%). Degeneration was not correlated with the effect of cyclic loading in intact discs, but was correlated with cyclic loading effects after nucleotomy, with more degenerate samples experiencing greater increases in both compressive and neutral zone strain following cyclic loading. Partial removal of the nucleus pulposus decreased the compression and neutral zone modulus while increasing strain. These changes correspond to hypermobility, which will alter overall spinal mechanics and may impact low back pain via altered motion throughout the spinal column. Nucleotomy also reduced the effects of cyclic loading on mechanical properties, likely due to altered fluid flow, which may impact cellular mechanotransduction and transport of disc nutrients and waste. Degeneration was not correlated with the acute changes of nucleotomy. Results of this study provide an ideal protocol and control data for evaluating the effectiveness of a mechanically-based disc degeneration treatment, such as a nucleus replacement.
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