Assessment of compressive modulus, hydraulic permeability and matrix content of trypsin-treated nucleus pulposus using quantitative MRI

A clinical strength MRI and intact bovine caudal intervertebral discs were used to test the hypotheses that (1) mechanical loading and trypsin treatment induce changes in NMR parameters, mechanical properties and biochemical contents; and (2) mechanical properties are quantitatively related to NMR parameters. MRI acquisitions, confined compression stress–relaxation experiments, and biochemical assays were applied to determine the NMR parameters (relaxation times T1 and T2, magnetization transfer ratio (MTR) and diffusion trace (TrD)), mechanical properties (compressive modulus HA0 and hydraulic permeability k0), and biochemical contents (H2O, proteoglycan and total collagen) of nucleus pulposus tissue from bovine caudal discs subjected to one of two injections and one of two mechanical loading conditions. Significant correlations were found between k0 and T1 (r=0.75,p=0.03), T2 (r=0.78, p=0.02), and TrD (r=0.85, p=0.007). A trend was found between HA0 and TrD (r=0.56, p=0.12). However, loading decreased these correlations (r 0.4, p 0.2). The significant effect of trypsin treatment on mechanical properties, but not on NMR parameters, may suggest that mechanical properties are more sensitive to the structural changes induced by trypsin treatment. The significant effect of loading on T1 and T2, but not on HA0 or k0, may suggest that NMR parameters are more sensitive to the changes in water content enhanced by loading. We conclude that MRI offers promise as a sensitive and non-invasive technique for describing alterations in material properties of intervertebral disc nucleus, and our results demonstrate that the hydraulic permeability correlated more strongly to the quantitative NMR parameters than did the compressive modulus; however, more studies are necessary to more precisely characterize these relationships.