Bone microarchitecture and mechanical resistance

Abstract

Although bone mass is the main determinant of bone mechanical resistance, it explains only 30 to 40% of the variability of this characteristic, indicating that other factors are involved. Among these factors is bone tissue quality, which depends on bone mineralization, bone turnover, and bone microarchitecture. Several parameters for characterizing bone microarchitecture have been developed over the last 15 years. The simplest (Parfitt’s parameters) are trabecular count, width, and separation. A binary image (two levels of gray) of bone tissue can be expanded and used to determine the trabecular bone pattern factor. This method tends to overestimate the number of convex surfaces, which are characteristic of trabecular network disruption. The binary image can be further simplified (skeletonized) and used to count the number of nodes (anastomoses between trabeculae) or free ends (segments disconnected from the network). The bone marrow star volume, the marrow interconnectivity index, and the Euler-Poincaré number are useful for characterizing the bone marrow. These parameters can be measured on bone specimens or on computed tomography (CT) or magnetic resonance imaging (MRI) scans, although in-plane resolution is far lower with scans than with specimens. Two-dimensional analysis is widely used, although three-dimensional studies are more satisfactory. Finally, fractal analysis is an original approach in which fractal dimension measurement, which is fairly simple, is used to determine the degree of network disruption. Ex vivo histomorphometric data suggest that microarchitecture-related factors may explain 10 to 30% of the variability in bone mechanical resistance beyond the proportion explained by bone mass. Similar results have been obtained in microimaging, CT, and MRI studies. Discrepancies across studies exist, however, in the strength of the relationship between bone mass and bone mechanical resistance; they are probably ascribable to differences in measurement sites and to errors in the measurement of variables characterizing bone mechanical resistance. The finite element method may be a means of sidestepping these problems. It can be used, in particular, to calculate Young’s modulus of elasticity from three-dimensional bone segment reconstructions. The results of the few studies of the finite element method are promising but require confirmation. Finally, a more clinical approach consists in comparing bone architecture in patients with osteoporotic fractures and in controls matched on bone mass. A few cross-sectional studies have used this approach. Bone architecture was evaluated using histomorphometry, CT, or MRI. The results indicate that trabecular network disruption is more severe in patients with than without fractures.