Elastin content correlates with human disc degeneration in the anulus fibrosus and nucleus pulposus

Study design Quantitative study of elastin content in nondegenerate and degenerate human intervertebral discs. Objective To measure the site-specific changes in elastin content that accompany disc degeneration using a quantitative, dye-binding assay to assess elastin levels. Summary of background data Recently, an abundant and organized network of elastic fibers was observed in nondegenerated human disc using immunostaining histochemistry, suggesting a functional role for elastin. While degenerative changes in the disc extracellular matrix composition are well known, changes in elastin content that may accompany degeneration have not been reported. Methods Human discs were assigned a degenerative grade by 3 independent orthopedic surgeons based on gross morphology. Samples were taken from the outer anulus fibrosus (OAF), inner AF (IAF) and nucleus pulposus (NP). Elastin content was measured using a specific, dye-binding assay and normalized to dry weight and collagen content, which was measured via a hydroxyproline assay. Samples were divided into 2 groups: nondegenerate (Grades 1-2.5) and degenerate (Grades 2.6-4.0). A 2-way analysis of variance was used to test for statistical significance where the 2 factors were disc location and degeneration. Correlations of composition with degeneration and age were analyzed. Results In nondegenerate tissue, elastin by dry weight was on average 2.0% +/- 0.3%, and there were no differences in elastin content among the locations of OAF, IAF, or NP. With degeneration, there was a significant increase in total disc elastin per dry weight at each location. The degenerate IAF had the largest amount of elastin (9.3% +/- 2.3%), significantly greater than the NP and OAF. Elastin content correlated with degenerative grade and age at each site. Conclusion Based on the location-dependent degenerative changes, with highest increases in the IAF, elastin may function to restore lamellar structure under radial loads that potentially cause delamination. Future work will focus on distinguishing the changes in elastin orientation with degeneration and understanding the mechanical functional role of elastin in the disc.