Partial removal of pore and loosely bound water by low-energy drying decreases cortical bone toughness in young and old donors

Abstract

With an ability to quantify matrix-bound and pore water in bone, 1H nuclear magnetic resonance (NMR) relaxometry can potentially be implemented in clinical imaging to assess the fracture resistance of bone in a way that is independent of current X-ray techniques, which assess bone mineral density as a correlate of bone strength. Working towards that goal, we quantified the effect of partial dehydration in air on the mechanical and NMR properties of human cortical bone in order to understand whether NMR is sensitive to water–bone interactions at low energy and whether such interactions contribute to the age-related difference in the toughness of bone. Cadaveric femurs were collected from male and female donors falling into two age groups: 21–60 years of age (young) and 74–99 years of age (old). After extracting two samples from the medial cortex of the mid-shaft, tensile tests were conducted on Wet specimens and paired, Partially Dry (PtlD) specimens (prepared by low-energy drying in air to remove ∼3% of original mass before testing). Prior analysis by micro-computed tomography found that there were no differences in intra-cortical porosity between the Wet and PtlD specimens nor did an age-related difference in porosity exist. PtlD specimens from young and old donors had significantly less toughness than Wet specimens, primarily due to a dehydration-related decrease in post-yield strain. The low-energy drying protocol did not affect the modulus and yield strength of bone. Subsequent dehydration of the PtlD specimens in a vacuum oven at 62°C and then 103°C, with quantification of water loss at each temperature, revealed an age-related shift from more loosely bound water to more tightly bound water. NMR detected a change in both bound and pore water pools with low-energy air-drying, and both pools were effectively removed when bone was oven-dried at 62°C, irrespective of donor age. Although not strictly significant due to variability in the drying and testing conditions, the absolute difference in toughness between Wet and PtlD tended to be greater for the younger donors that had higher bone toughness and more bound water for the wet condition than did the older donors. With sensitivity to low-energy bone–water interactions, NMR, which underpins magnetic resonance imaging, has potential to assess fracture resistance of bone as it relates to bone toughness.