Effects of Sex, Strain Rate, and Age on the Tensile and Compressive 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 and compressive material properties of human rib cortical bone over a wide range of subject demographics. Tension coupons were tested from 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.). Compression samples were tested from thirty (n = 30) subjects (M = 19, F = 11) ranging in age from 18 to 95 years of age (Avg. = 49.0 +/- 23.9 yrs.). For each subject, one coupon/sample was tested to failure on a material testing system at a targeted strain rate of 0.005 strain/s, while a second coupon/sample was tested at 0.5 strain/s. A load cell was used to measure axial load for both the tension coupons and compression samples. An extensometer was used to measure displacement within the gage length of the tension coupons and a deflectometer was used to measure displacement of the compression samples. Tension data were obtained from fifty-eight (n = 58) coupons at 0.005 strain/s and fifty-eight (n = 58) coupons at 0.5 strain/s, with fifty-five (n = 55) matched pairs. Compression data were obtained from thirty (n = 30) compression samples at 0.005 strain/s and thirty (n = 30) samples at 0.5 strain/s. The elastic modulus, yield stress, yield strain, ultimate stress, elastic strain energy density (SED), plastic SED, and total SED were then calculated for each tensile and compression test. In addition, failure stress and failure strain were calculated for each tension test. There were no significant differences in the tensile material properties between sexes and no significant interactions between age and sex for either method of loading. In regard to the differences in tensile 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. All material properties were significantly lower at 0.005 strain/s compared to 0.5 strain/s in compression. Spearman correlation analyses showed that all tensile material properties had significant negative correlations with age at 0.005 strain/s except modulus. At 0.5 strain/s, all tensile material properties except yield strain had significant negative correlations with age. No significant correlations were observed in material properties with respect to advanced age in compression at either loading rate. Although the results revealed that the tensile material properties of human rib cortical bone varied significantly with respect to chronological age, the R2 values only ranged from 0.15 - 0.62, indicating that there may be other underlying variables that better account for the variance within a given population. Overall, 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 and the first study to test the compressive material properties of human rib cortical bone. The results of this study provide data that allows FEMs to better assess thoracic injury risk for all vehicle occupants. Additionally, this study provides the necessary data to more accurately model and assess differences in the material response of the rib cage for nearly all vehicle occupants of driving age.