Differences in the degree of bone tissue mineralization account for little of the differences in tissue elastic properties

Abstract

Study of postmortem samples of cortical bone from the trochanters of 12 Caucasian females revealed that tissue mineral density (TMD) and tissue elastic modulus correlate weakly within and between individuals. Other material properties need to be taken into account to more fully predict variation in tissue elastic modulus. Bone is a composite material that varies in its material composition and structural organization at the macro-, micro-, and nano-scales. This hierarchical organization is essential for bone's resistance to crack initiation and propagation. We quantified the relationship between regional heterogeneity in TMD and tissue elastic modulus in cortical bone of the trochanter to determine whether TMD can be used as a predictor of tissue elastic modulus. Measurements of tissue elastic modulus and hardness were made using nanoindentation at 5 × 20 indent points spaced 100 μm apart. TMD at the same location was computed from quantitative backscattered scanning electron microscopy imaging of cortical samples from trochanters obtained at postmortem from 12 Caucasian females (mean age: 69 years; range: 29 to 85 years). Within an individual, the variance in tissue elastic modulus (CV = 18.7%; range: 9 to 41.5%) was five times greater than the variance in TMD (3.6%, range: 1.8 to 5.7%). On average, only 45% of the variance in tissue elastic modulus was explained by TMD. From individual to individual, the proportion of the variance in tissue elastic modulus explained by TMD ranged from 0 to 64%. In 6 of 12 samples, TMD explained less than 30% of the variance in tissue elastic modulus. Results were similar for tissue hardness. Tissue mineral density is an incomplete surrogate for tissue elastic modulus. Other material properties need to be accounted for to more fully predict regional variation in tissue elastic modulus.