Anatomic variation in the elastic inhomogeneity and anisotropy of human femoral cortical bone tissue is consistent across multiple donors

Numerical models commonly account for elastic inhomogeneity in cortical bone using power-law scaling relationships with various measures of tissue density, but limited experimental data exists for anatomic variation in elastic anisotropy. A recent study revealed anatomic variation in the magnitude and anisotropy of elastic constants along the entire femoral diaphysis of a single human femur (Espinoza Orías et al., 2009). The objective of this study was to confirm these trends across multiple donors while also considering possible confounding effects of the anatomic quadrant, apparent tissue density, donor age, and gender. Cortical bone specimens were sampled from the whole femora of 9 human donors at 20%, 50%, and 80% of the total femur length. Elastic constants from the main diagonal of the reduced fourth-order tensor were measured on hydrated specimens using ultrasonic wave propagation. The tissue exhibited orthotropy overall and at each location along the length of the diaphysis (p<0.0001). Elastic anisotropy increased from the mid-diaphysis toward the epiphyses (p<0.05). The increased elastic anisotropy was primarily caused by a decreased radial elastic constant (C11) from the mid-diaphysis toward the epiphyses (p<0.05), since differences in the circumferential (C22) and longitudinal (C33) elastic constants were not statistically significant (p>0.29). Anatomic variation in intracortical porosity may account for these trends, but requires further investigation. The apparent tissue density was positively correlated with the magnitude of each elastic constant (p<0.0001, R2>0.46), as expected, but was only weakly correlated with C33/C11 (p<0.05, R2=0.04) and not significantly correlated with C33/C22 and C11/C22.