ABSTRACT of the Doctoral Thesis
In-depth understanding of morphology and micromechanics of bone is important in prediction and treatment of bone metabolic diseases, like osteoporosis - a disease causing high morbidity and mortality among the elderly population. In this thesis, an extensive experimental and computational research program has been achieved in order to better understand the structure-function relationships of mineralized tissues.
Mineralized turkey leg tendon (MTLT) was used in the study, as a mechanical model of mineralized collagen fibers, also present in bone.
Quantitative backscattered electron (qBEI) and light microscopy images revealed two morphology zones in this tissue. The zones differed in mineralization, amount of organic phase, size of the mineralized fibers and microporosity, as well as stiffness. A mean field homogenization model applied in prediction of the indentation stiffness as compared to the measured one, helped in interpretation of some features of the two phases that were not accessible experimentally, like the distribution of mineral between the collagen fibrils and the extrafibrillar space.
Indentation modulus of MTLT was measured using different final indentation depths, contributing to the knowledge about the indentation size effects in mineralized tissues. The measurements were performed in dried and re-hydrated state, providing insights in the elastic anisotropy of unidirectionally oriented mineralized fibers in different hydration states. Additionally, a multiscale verification of stiffness in MTLT was performed. At the macroscale, stiffness obtained with uniaxial tension tests was compared to the one predicted with a micro finite element models. The same samples were tested at the microscale, where the experimentally measured indentation modulus was compared with the one predicted with the mean field model, based on morphological parameters measured in the sites of indentation.
As the arrangement of collagen fibers in bone is complex and not fully understood, a technique allowing its assessment was adapted in this thesis to provide quantitative information on the collagen out-of-plane angle in bone sections. The technique employing circularly polarized light was calibrated on the uniaxial MTLT samples in order to provide quantitative information normalized to sample thickness and wavelength of the probing light to enable a universally applicable assessment. This collagen arrangement assessment technique - the quantitative polarized light microscopy (qPLM) was applied to osteons of human mid-shaft femurs, along with a site-matched mineralization assessment (qBEI), nanoindentation and the mean field homogenization method of indentation modulus prediction. A weak correlation between the measured and predicted indentation moduli revealed that additional factors that contribute to the tissue stiffness, like nanoporosity, collagen cross-links or non-collagenous proteins, need to be taken into account.
Generally, in the course of this thesis new protocols and techniques helpful in assessment of the structure-function relationships were developed contributing to a better understanding of the elasticity of mineralized tissues.