Human bone is a complex biological material with up to seven levels of hierarchical structure. Due to this complexity, it is still not fully understood how the various structures contribute to the macroscopic mechanical response. Such understanding is important to assess the mechanical contributions of the bone material to whole bone fractures.

It is well known that microcracking is associated with bone’s inelastic deformation and contributes to its resistance to fracture. Multiple microcracks suggest control over their development. Yet, the structure – microcracking interactions in cortical bone, particularly at the lamellar and Haversian systems levels, are still unclear. Following a qualitative, structure – mechanical function relations approach, the present dissertation provides further insight into how bone resists fracture by distributed microcracking.

This was achieved through a detailed study of bone’s deformation and fracture processes using mechanical testing on human cadaver bones and a combination of microscopy techniques, including laser scanning confocal microscopy, to characterize the structure – microcracking relations. Particular interest was given to compression and bending, two loading modes involved in falls resulting in hip fractures.

Haversian bone derived part of its fracture resistance through microcracking largely controlled by the concentric lamellae and underlying fibrillar organisation surrounding each Haversian canal. Multiple microcracks developed stably within the osteonal wall due to different fibrillar orientation in each lamella. Such process happened to most osteons resulting in well-distributed damage, hence providing inelastic deformation to the tissue. Haversian bone’s resistance to fracture would thus depend on its intact lamellar structure. Changes in number and organisation of the lamellae would likely alter bone’s ability to control microcracks and may lead to bone fragility.

Based on a tibia study, long bones’ fracture resistance in bending was found to be linked to Haversian bone’s behavior. As a result of post-yield strain redistribution associated with tensile and compressive microcracking, bone’s compressive behavior was also found to play an important role in the bending response. Directly applying fundamental research to the clinical field, a preliminary analysis of the superior cortex of fractured femoral necks retrieved from patients revealed compressive microcracking. Such evidence emphasizes the importance of bone’s hierarchical structure in hip fracture.