Cross-link profile of bone collagen correlates with structural organization of trabeculae

Little is known regarding the mechanisms that govern the structural organization of cancellous bone. In this study, we compare the nature of the collagen in vertebral cancellous bone with the structural organization of its trabecular network. Cylindrical specimens of cancellous bone from vertebrae were obtained from nine autopsy subjects (ages 46-88). In each subject, eight pairs of corresponding samples were obtained from three levels in the spine and three areas within the vertebral body, leading to a total of 68 pairs of samples. The cylinders from one side were used for morphometry and the classical morphometrical parameters were obtained (BV/TV, bone volume fraction; Tb.Th, trabecular thickness; Tb.N, number; Tb.Sp, trabecular spacing) and strut analysis (TSL, total strut length; Nd, number of nodes; Fe, number of free-ends). The amount of osteoid bone was also quantified. The cylinders from the other side were powdered and used for collagen assessment, including the amount of collagen (% w/w), and its content in immature cross-links; such as hydroxylysinonorleucine (mol/mol of collagen) and dihydroxylysinornorleucine, as well as stable mature cross-links, such as hydroxylysylpyridinoline (HP), lysylpyridinoline (LP), and the pyrrole cross-links. A random regression model was used to explore the correlations. None of the biochemical parameters correlated with the BV/TV except the ratio between immature and mature cross-links (eta(2) = 0.34, p < 0.05). There was no relationship between the amount of osteoid bone and the cross-link profile. However, the concentration of pyrrole and HP cross-links in the bone samples correlated with the structural organization of its trabeculae, but in an opposite direction. Hence, the pyrrole/HP ratio was a good predictor of Tb.Th, Tb.N, Tb.Sp, and TSL (eta(2) > 0.65 and p < 0.01) as well as Fe and star marrow space (eta(2) > 0.45 and p < 0.05). The cylinders from subjects with high pyrrole or low HP in their bone collagen had a relatively thick and simple structure. Those with low pyrrole and high HP had relatively thin trabeculae that were more numerous and spread over a complex network. The relative concentrations of the pyrrole and pyridinoline cross-links appear to reflect the structural organization of the trabeculae.