Mechanical characterization of bone from the tissue down to the lamellar level by means of nanoindentation

Osteoporosis has become an important health and economic problem of our aging western society. This metabolic illness leads to a net bone loss and to a deterioration of trabecular architecture. Motivated by the fact that osteoporosis may also result in a degradation of intrinsic tissue quality this thesis focuses on the micro- and nanomechanical properties of human, bovine and rat bone. For the first time, to the best of our knowledge, mechanical properties of single bone lamellae are tested under dry and physiological conditions. Stable thermal equilibrium conditions can be achieved for the latter that allow for tests at body temperature and under fully wet conditions. This study represents a first step towards extension of knowledge of the structure-function relationships down to the lamellar level. Adjacent thin and thick lamellae of the same bone structural unit (BSU) are significantly different in terms of hardness and indentation modulus. The two types of lamellae show a significantly different increase of mechanical properties when the water content is removed by drying. In this context morphological models are employed to discuss the mechanical properties (bone lamellation theory). The BSU is found to be the basic bone component with individual morphological and also mechanical properties. Significant differences are seen between BSU of osteonal, trabecular and interstitial microstructures dissected from the human femoral neck. Human interstitial and bovine plexiform bone do not show a significantly different indentation modulus. Depth-dependent indentation measurements, which are done for this purpose, further extend our current knowledge of the technique. Structure-function relationships are investigated on the BSU-level by applying two morphological and two mechanical techniques on identical BSUs of two donors. The dependence of the mechanical properties on the mineral content (as measured by microradiography) and the orientation of the collagen fibers (as measured by polarized light microscopy) are investigated. The reported correlations between macroscopic mechanical properties and these morphological parameters are not generally confirmed on the BSU-level by the nanoindentation data of this two-case study. The indentation modulus is validated by a comparison with a traction experiment of a bovine bone microspecimen. This experiment raises confidence in the absolute value of this elastic parameter. The influence of the material anisotropy on the measured indentation modulus is determined for bovine cortical bone. As a first step to apply the nanomechanical tool in the context of preclinical studies, a set of rat vertebrae were tested. Given the small number of specimens, this study does not show a general significance of low protein diet, ovariectomy and essential aminoacids on intrinsic tissue properties. This thesis proposes further nanoindentation studies on bone tissue for future work. It is expected that relationships between nanomechanical properties and the degree of damage accumulation of aging and/or osteoporotic tissue can be established. This can contribute to development of new strategies aiming at improving tissue quality.