Micromechanics of lamellar bone

Bone metabolic diseases and the related fractures cause significant morbidity and mortality among the elderly population. They represent an important and growing burden for our western health care systems that needs to be faced. While the effect of compact and trabecular bone loss on whole bone strength is increasingly well understood, the consequences of the altered remodeling process and the use of drugs on the mechanical properties of the bone extracellular matrix remain obscure. In this context, the aim of this project is to model and quantify the relationship between the morphological and mechanical properties of the bone extracellular matrix at the lamellar level of organization. To achieve this goal, a dual modeling and experimental approach is proposed that applies the following techniques to single and multiple lamellae: homogenization theory to compute the effective anisotropic elastic properties, quantitative backscattered electron imaging (qBEI) to assess the mean degree of mineralization, quantitative polarized light microscopy (qPLM) to measure the mean orientation of the collagen fibers and indentation along multiple directions to derive the anisotropic indentation moduli of the considered representative volume element. Beyond the innovative aspect of the qPLM technique, the unprecedented combination of the modeling and experimental methods at the lamellar level and the gained basic knowledge on the mechanics of bone tissue, the project will provide the first tools to predict the variations in mechanical properties of the human bone extracellular matrix from measurable morphological data. These tools will, in turn, allow to evaluate the biomechanical consequences of metabolic bone diseases on the strength of whole bones and hopefully benefit future patients. The project will contribute to train two PhD students in engineering sciences, involve a national cooperation with the Ludwig Boltzmann Institute of Osteology as well as an international cooperation with the Max Planck Institute of colloids and interfaces and take full advantage of a recently inaugurated laboratory for nano- and micromechanics of biological and biomimetic materials at the Vienna University of Technology.

Bone metabolic diseases and the related fractures cause significant morbidity and mortality among the elderly population. They represent an important and growing burden for our western health care systems that needs to be faced. While the effect of compact and trabecular bone loss on whole bone strength is increasingly well understood, the consequences of the altered remodeling process and the use of drugs on the mechanical properties of the bone extracellular matrix remain obscure. In this context, the aim of this project was to model and quantify the relationship between the morphological and mechanical properties of the bone extracellular matrix at the lamellar level of organization. From the experimental point of view, a novel quantitative polarized light microscopy (qPLM) technique was developed to measure the mean collagen orientation in thin sections of mineralized tissues. This technique was combined with quantitative backscattered electron imaging (qBEI) and nanoindentation (NI) in various morphological planes to examine the respective role of fiber orientation and degree of mineralization in the elastic properties of such tissues. On the modeling side, a stepwise homogenization scheme was developed using mean field and unit cell methods to calculate the full stiffness tensor of human bone from the unidirectional sub-lamellae to different lamellar architecture reported in the literature such as the orthogonal and rotated plywood. Given the complexity of lamellar architecture in human bone, the experimental and computational modelling methodologies were first applied on mineralized turkey leg tendon (MTLT), which has the benefit of presenting a unidirectional collagen fibre orientation. An unexpected binary morphology with distinct extent of mineralization and collagen fibre density was discovered, characterized and modelled in MTLT. The largest extent of elastic anisotropy was measured and the results of the homogenization method could be validated with macroscopic tensile tests. In contrast with the classical separation into longitudinal, alternate and circumferential types, a relatively uniform lamellar anisotropy was observed in human femoral bone, suggesting an average helicoid pathway of the collagen fibres with an angle of approximately 10 with the bone axis. Given their small physiological variations, degree of mineralization and mean collagen orientation could not explain the substantial variability in elastic properties in the transversal plane of osteons, which is believed to be associated with fluctuating porosity at the nanometer level. In conclusion, new methodologies for the investigation of the micromechanics of lamellar bone were developed and successfully applied to both MTLT and human osteonal bone.