Biomechanical assessment of cortical bone under dynamic loading conditions

Abstract

The human skeleton experiences a wide range of strain rates during various activities and events. The mechanical response of bone considerably depends on the rate of applied loading. Though various studies are available on the rate-dependent mechanical behaviour of bone, the knowledge on factors influencing this behaviour remains limited. Therefore, this thesis aimed to investigate the factors influencing rate dependent behaviour of cortical bone. A series of studies were conducted that utilised ex-vivo mechanical tests and various characterisation techniques to understand the deformation and damage behaviour of bone over a wide range of strain rates. First, using the micro-CT technique, the relationship between cortical bone microarchitecture and its damage behaviour was investigated. For this, mid-diaphysis of rat tibia bones was scanned using high resolution micro-CT and intracortical porosity network was analysed. Further, a linear micro-FE model was developed to understand the damage behaviour of bone. The results show that the damage behaviour of bone strongly depends on volume fraction and orientation of porosity. This study highlights the role of intracortical porosity on damage behaviour of cortical bone. In the next study, effect of cyclic loading strain rate on deformation and damage behaviour of cortical bone was investigated. Cortical bone was fatigued at physiologically relevant frequencies and it was observed that bone accumulates higher damage at low frequency as compared to high frequency. The atomic force microscopy (AFM) results revealed that collagen fibril d-spacing increases on the tensile region but not on the compressive region. Histological assessment of microdamage revealed significant higher crack density and crack length in the compressive region of the low-frequency group. Also, the indentation devices were found sensitive to detect fatigue loading-induced damage only on the compressive regions. The findings of this study provide insight into the frequency-dependent deformation and damage behaviour of cortical bone. Since the organic phase of bone contributes significantly to its rate-dependent mechanical behaviour, therefore in the next work, the role of organic matrix integrity on rate-dependent behaviour was investigated. The organic matrix phase of bone was denatured through thermal treatment and the changes that occurred in the constituents were quantified using FTIR and XRD techniques. FTIR results show reduction in the helical structure of collagen molecules with denaturation whereas XRD data revealed no alteration in the mineral phase. Compression test results show that failure strength and strain rate sensitivity of bone decreases significantly with increase in denaturation of organic matrix. This study provides insights into the effect of organic matrix integrity on the rate-dependent mechanical behaviour of bone. Further, the effect of inelastic deformation of bone on its rate-dependent behaviour was investigated. Human cortical bone samples were fatigued to accumulate inelastic strains and subsequently, compression tests were performed at low and high strain rates. Results show that the strength of bone reduced at low strain rates but not at high strain rates. These observations indicate that inelastic deformation mechanisms are more prominent at low strain rates as compared to high strain rates. Overall, this thesis explored the factors influencing the loading rate-dependent behaviour of cortical bone.