Mechanical testing of individual Osteons of cortical bone

Abstract

An increase in incidence of bone fractures, related to ageing or bone disease, is a signicant socio-economic burden. The reasons for the impaired mechanical function of cortical bone are dicult to identify, due to the limited understanding of mechanical properties of the basic functional unit of cortical bone, the osteon. Secondary osteons are formed as a result of bone remodelling and adaptation processes. Ageing and bone disease debilitate bone remodelling function leading to bone loss and increased bone fragility. The technique that is currently used to diagnose bone diseases and predict pathological fractures is based on the bone mineral density measure. As the diagnosis is often made only after the occurrence of the pathological fracture, the need in development of new techniques to assess the "quality" of bone is increasing. Understanding the mechanical function of the osteon, as a product of the bone remodelling process, can provide vital insights into mechanisms of bone change with ageing and disease. The compressive mechanical properties of single osteons were studied by Ascenzi et al. in 1968 [Ascenzi et. al 1968] and the results reported by the research group were not validated so far. In this master thesis, a methodology was developed to extract individual osteons from the bulk material of bone and test them in uni-axial compression. Bovine bone was used to calibrate and optimize the developed procedure. Furthermore, this thesis provides insights on how ageing can possibly affect the mechanical properties of human osteons. Bone tissue from two male donors, ages 64 and 93, was used to harvest two sets of osteons. The results of continuous and step-wise uni-axial compression testing suggest that there is a difference in mechanical properties between the two age groups of osteons. The younger osteon group consistently showed a higher variation in the measured parameters compared to the older group. Additionally, the younger donor had more osteons with lower stresses and higher strains and, therefore, the younger osteon group was more ductile. Elastic modulus and bone mineral density of the older osteon group (E = 6.6 [1.5] GPa, BMD = 1214 [32] mg HA/ccm) were found to be higher (ANOVA, p < 0:05) than those of the younger osteon group (E = 4.7 [1.1] GPa, BMD = 1186 [31] mg HA/ccm). However, recent research suggest that damage may accumulate already within an apparent elastic region leading to possibly underestimated values of elastic modulus [Luczynski et. al 2015]. In this thesis, the results of the step-wise compression testing suggested that the values of elastic modulus determined by the uniaxial compression testing are underestimated up to 1.7 times and that micro-damage accumulates within the apparent elastic region. Furthermore, BV/TV and SHG intensity were found to be the only significant predictors of the osteonal mechanical properties (regression analysis, p < 0.05). Investigating the differences in osteonal structural and mechanical properties may be the key to a better understanding of bone ageing and pathological processes.