Micromechanics of compact bone-osteons and cement lines

Abstract

Osteoporosis is one of the most common bone pathologies that affects especially the elderly and increase the risk of fracture leading to traumatic injuries, disability and in some cases to mortality. Currently osteoporosis is diagnosed according to bone mineral density (BMD i.e. quantity of bone) of the patient. In most cases, however, a patient is diagnosed as osteoporotic post-fracture and thus BMD is not an ideal biomarker of osteoporosis. Ostoporosis has also been related to deterioration of bone quality at the microand nanoscale. Bone has complex structure. The structure of bone can be distinguished in seven hierarchical levels from the whole bone to the nanostructure level of bone. At the nanoscale bone consist of collagen, calcium phosphate crystals, non-collagenous proteins and water. The organization, structural arrangement and interplay of these components to a large extend define the mechanical properties of bone at the different levels of bone structural hierarchy. Osteons are hollow-cylindrical-like structures that are found at the microscale with distinct boundaries. These boundaries are also known as cement lines. The cement line is a layer with 1 μm to 5 μm thickness characterized by less mineral content. It has been shown that cement lines act as crack barriers preventing crack from crossing through an osteon, which increases the toughness of bone. The goal of this master thesis was to perform tensile experiments for obtaining the mechanical properties of cement lines. Cement lines and osteons are microscale features (mean osteon diameter approximately 200 μm). Because of this, it is important to acquire a specific image-guided technique to assess their mechanical properties. Moreover, in situ microscopy methods such as atomic force and light microscopy can be used to further complement the study of crack propagation on cement lines. To achieve in situ AFM and light microscopy experiment, a microtensile testing insturment was designed and constructed. This device allows simultaneous AFM experiments and microtensile testing of miniature-carved bone samples submerged in aqueous solution. In this master thesis, a protocol was developed to manufacture miniature bone samples. These samples were used to test the applicability of the designed instrument and gain knowledge about the mechanical properties of cement lines of samples with different structural features. Results show that while the number of cement lines that deflect the crack increases, the stiffness, toughness and levels of ultimate stress also increase. Moreover, the mechanical properties (stiffness, toughness and ultimate stress) of bone where crack has penetrated the osteons and interstitial areas of their gauge region are lower. In this thesis a functional microtenstile device was constructed, with which tensile tests on miniature samples were conducted with results that complement finding from literature.