Age-related bone fragility is associated with increased cortical porosity, which varies regionally across the mid-femoral cortex. However, biomechanical studies have shown that bone fragility is not fully explained by this bone loss, and altered material composition is thought to play a role. Whether compositional defects in aged bone reflect a degradation in the quality of existing bone tissue, or an inherent difference in the bone deposited is unknown. We aimed to determine whether newly deposited osteonal bone differs in composition between older and younger women, and whether this difference is exaggerated in regions of bone loss.
Cadaveric femoral midshaft samples from 5 healthy young women (20-40 years old) and 5 healthy elderly women (77-95 years old) were obtained from the Melbourne Femur Research Collection. Cortical bone wedges from the posterior octant (which exhibits the greatest age-related increase in porosity) and lateral octant (which exhibits the least) were assessed by micro-computed tomography. Synchrotron-based Fourier-transform infrared (FTIR) microspectroscopy was then used to measure mineral:matrix and carbonate:phosphate ratios within newly deposited osteons, identified by histology and low resolution scanning.
In elderly women, mean osteonal wall thickness was 37.5% lower than young women in both regions measured, even though porosity was only significantly increased in the posterior region. Within this region, new bone deposited in older women had a significantly (4.5%) lower mineral:matrix ratio and 10% higher carbonate:phosphate ratio than bone built in the same region by young women. Contrastingly, the lateral region did not exhibit any differences in composition between the two age groups.
In summary, osteonal bone built by older women in a region of bone loss is less mineralised and has a higher carbonate content than that built by younger women. This region-specific change in bone material quality in older women may contribute to their higher susceptibility to fragility fractures.