Effects of sex, age, and two loading rates on the tensile material properties of human rib cortical bone

The objective of this study was to evaluate the effects of sex, loading rate, and age on the tensile material properties of human rib cortical bone over a wide range of subject demographics. Sixty-one (n = 61) subjects (M = 32, F = 29) ranging in age from 17 to 99 years of age (Avg. = 56.4 ± 26.2 yrs) were used in this study. Two rectangular coupons of cutaneous rib cortical bone were obtained from each subject and milled into dog-bone coupons for testing. For each subject, one coupon was tested to failure in tension on a material testing system at a targeted strain rate of 0.005 strain/s, while the other coupon was tested at 0.5 strain/s. A reaction load cell was used to measure axial load, and an extensometer was used to measure displacement within the gage length of the coupon. Data were obtained from fifty-eight (n = 58) subjects at 0.005 strain/s and fifty-eight (n = 58) subjects at 0.5 strain/s, with fifty-five (n = 55) matched pairs. The elastic modulus, yield stress, yield strain, failure stress, failure strain, ultimate stress, elastic strain energy density (SED), plastic SED, and total SED were then calculated for each test. There were no significant differences in material properties between sexes and no significant interactions between age and sex. In regard to the differences in material properties with respect to loading rate, yield stress, yield strain, failure stress, ultimate stress, elastic SED, plastic SED, and total SED were significantly lower at 0.005 strain/s compared to 0.5 strain/s. Spearman correlation analyses showed that all material properties had significant negative correlations with age at 0.005 strain/s except modulus. At 0.5 strain/s, all material properties except yield strain had significant negative correlations with age. Although the results revealed that the material properties of human rib cortical bone varied significantly with respect to chronological age, the R2 values only ranged from 0.15 to 0.62, indicating that there may be other underlying variables that better account for the variance within a given population. This is the first study to analyze the effects of sex, loading rate, and age on tensile material properties of human rib cortical bone using a reasonably large sample size. Overall, the results of this study provide data that will allow FEMs to better model and assess differences in the material response of the rib cage for nearly all vehicle occupants of driving age.