The fracture mechanics of human bone: influence of disease and treatment

Abstract

Aging and bone diseases are associated with increased fracture risk. It is therefore pertinent to seek an understanding of the origins of such disease-related deterioration in bone's mechanical properties. The mechanical integrity of bone derives from its hierarchical structure, which in healthy tissue is able to resist complex physiological loading patterns and tolerate damage. Indeed, the mechanisms through which bone derives its mechanical properties make fracture mechanics an ideal framework to study bone's mechanical resistance, where crack-growth resistance curves give a measure of the intrinsic resistance to the initiation of cracks and the extrinsic resistance to the growth of cracks. Recent research on healthy cortical bone has demonstrated how this hierarchical structure can develop intrinsic toughness at the collagen fibril scale mainly through sliding and sacrificial bonding mechanisms that promote plasticity. Furthermore, the bone-matrix structure develops extrinsic toughness at much larger micrometer length-scales, where the structural features are large enough to resist crack growth through crack-tip shielding mechanisms. Although healthy bone tissue can generally resist physiological loading environments, certain conditions such as aging and disease can significantly increase fracture risk. In simple terms, the reduced mechanical integrity originates from alterations to the hierarchical structure. Here, we review how human cortical bone resists fracture in healthy bone and how changes to the bone structure due to aging, osteoporosis, vitamin D deficiency and Paget's disease can affect the mechanical integrity of bone tissue.

Bone Material Properties in Osteogenesis Imperfecta

“…(16) Individual fibrils are 80 to 100 nm in diameter; fibril length is indeterminate. (17) Molecular changes in osteogenesis imperfecta…”

“…Fracture resistance extends beyond the intrinsic material properties of bone to encompass all levels of scale up to the whole bone. (17) Although stiffness can be considered as an averaged property across a material, strength and toughness cannot. Strength is affected by the "weakest link" problem.…”

Role of cortical bone in hip fracture

“…The reasons normal bone tissue is resistant to crack growth at the more microscopic levels have been recently reviewed by Ritchie and colleagues in this journal 14 and elsewhere. 15 As a consequence of these mechanisms, human cortical bone accumulates microcracks, each of which is thought to record a damaging loading event that was aborted.…”

“…124 At the nanoscale level, collagen molecules undergo increased glycation with ageing, which is likely to make bone harder and reduce toughness. 14,125 Femoral cortical bone tissue turnover has only rarely been studied, Wand et al 79 estimating its half life as approaching two decades, which gives plenty of time for exposure to circulating glycating sugars to increase collagen cross links. 126 …”

“…14,15,111,112 The prevalence of mechanical stressconcentrators such as notches and flaws must inevitably be increased in the cortex by increased remodelling, typically seen after menopause. Normal mature human femoral bone subjected to cyclic loading develops microcracks that grow then rapidly decelerate, 113 reflecting multiple toughening mechanisms absorbing the energy associated with cracking without excessive sacrifice of strength.…”