Clinical Importance of Bone Matrix Damage Mechanisms for Fracture Prevention

A Cross-Sectional Study of Bone Nanomechanics in Hip Fracture and Aging

Bone mechanics is well understood at every length scale except the nano-level. We aimed to investigate the relationship between bone nanoscale and tissue-level mechanics experimentally. We tested two hypotheses: (1) nanoscale strains were lower in hip fracture patients versus controls, and (2) nanoscale mineral and fibril strains were inversely correlated with aging and fracture. A cross-sectional sample of trabecular bone sections was prepared from the proximal femora of two human donor groups (aged 44-94 years): an aging non-fracture control group (n = 17) and a hip-fracture group (n = 20). Tissue, fibril, and mineral strain were measured simultaneously using synchrotron X-ray diffraction during tensile load to failure, then compared between groups using unpaired t-tests and correlated with age using Pearson's correlation. Controls exhibited significantly greater peak tissue, mineral, and fibril strains than the hip fracture (all p < 0.05). Age was associated with a decrease in peak tissue (p = 0.099) and mineral (p = 0.004) strain, but not fibril strain (p = 0.260). Overall, hip fracture and aging were associated with changes in the nanoscale strain that are reflected at the tissue level. Data must be interpreted within the limitations of the observational cross-sectional study design, so we propose two new hypotheses on the importance of nanomechanics. (1) Hip fracture risk is increased by low tissue strain, which can be caused by low collagen or mineral strain. (2) Age-related loss of tissue strain is dependent on the loss of mineral but not fibril strain. Novel insights into bone nano- and tissue-level mechanics could provide a platform for the development of bone health diagnostics and interventions based on failure mechanisms from the nanoscale up.

Association between nanoscale strains and tissue level nanoindentation properties in age-related hip-fractures

Measurement of the properties of bone as a material can happen in various length scales in its hierarchical and composite structure. The aim of this study was to test the tissue level properties of clinically-relevant human bone samples which were collected from donors belonging to three groups: ageing donors who suffered no fractures (Control); untreated fracture patients (Fx-Untreated) and patient who experienced hip fracture despite being treated with bisphosphonates (Fx-BisTreated). Tissue level properties were assessed by (a) nanoindentation and (b) synchrotron tensile tests (STT) where strains were measured at the 'tissue', 'fibril' and 'mineral' levels by using simultaneous Wide-angle - (WAXD) and Small angle- X-ray diffraction (SAXD). The composition was analysed by thermogravimetric analysis and material level endo- and exo-thermic reactions by differential scanning calorimetry (TGA/DSC3+). Irrespective of treatment fracture donors exhibited significantly lower tissue, fibril and mineral strain at the micro and nanoscale respectively and had a higher mineral content than controls. In nanoindentation only nanohardness was significantly greater for Controls and Fx-BisTreated versus Fx-Untreated. The other nanoindentation parameters did not vary significantly across the three groups. There was a highly significant positive correlation (p < 0.001) between organic content and tissue level strain behaviour. Overall hip-fractures were associated with lower STT nanostrains and it was behaviour measured by STT which proved to be a more effective approach for predicting fracture risk because evidently it was able to demonstrate the mechanical deficit for the bone tissue of the donors who had experienced fractures.