Is the chemistry of collagen in intervertebral discs an expression of Wolff's Law? A study of the human lumbar spine.

The collagen content, proportion of Types I and II collagen, and the relative concentrations of the reducible crosslinks of human lumbar intervertebral discs have been found to vary with age and location and to be highly dependent on the topography of the tissue. From adolescence to mature adulthood, the most significant change is an increase in the content of Type I collagen at the expense of genetically distinct Type II collagen in the outer lamella of the posterior quadrant, while just the reverse is true of the anterior quadrant. These changes are accompanied by similar but smaller alterations in the total collagen content and in the crosslink hydroxylysinohydroxynorleucine . The same differences in the distribution of Types I and II collagens occur in the annuli on the concave and convex sides of scoliotic curves. Together, these data establish that active cellular activity and tissue remodelling occur in the annuli fibrosi and suggest that these specific changes are initiated in response to overall increases in compressive loading on the concave side and tensile loading on the convex side of the spine and the subsequent changes they induce in the magnitude and distribution of internal stresses within the annuli. In its most general formulation, the biological behavior of annuli fibrosi to mechanical forces appears to follow Wolff's Law.