Motor vehicle crashes (MVCs) are a leading cause of death in the United States, especially for children, young adults, and the elderly. Despite this, very few studies have focused on defining biomechanical properties at the extremes of the age spectrum. The thorax, specifically, is the most common fatal injury region for older drivers and the second most common for children. Rib fractures are frequent thoracic injuries and have been linked to high mortality rates, especially for the elderly.

Anthropomorphic tests devices (ATDs) and computational models are currently used to develop safer cars. These models, however, have been primarily scaled down from middle-aged adult data, since pediatric and very elderly biomechanical properties have not been extensively studied. To address this, structural properties of human ribs were examined across the entire age spectrum using the largest sample of ribs to date.

One-hundred eighty-four ribs from 93 post mortem human surrogates (PMHS) (70 male, 23 female;​ ages 4 to 99) were loaded in a fixture simulating a dynamic frontal impact to the thorax, and structural properties were calculated. A multi-level statistical model was used to determine differences with age and sex of subjects. Percent displacement in the loading direction (δ_X), energy absorption to fracture (U_tot), and plastic properties including post-yield energy absorption (U_Pl), plastic displacement (δ_Pl), and the ratio of elastic stiffness to secant stiffness (K-ratio) had a significant relationship only with age (p<0.01 for all) and were all found to be higher for younger individuals. Peak force (F_peak) had a significant relationship with both age and sex, however stiffness (K) had a significant relationship only with sex.

Rib curvature was calculated using two different methods and compared to structural properties. The first method, a simple ratio of the height of the rib to the span (h/s) had a linear relationship with percent displacement in the Y-direction (δ_Y) and the ratio of Y-displacement to X-displacement (dy/dx), but no other structural properties such as F_peak or K. The second method measured curvature for the anterior, lateral, and posterior sections of the rib throughout the test by tracking points marked on the rib’s surface. Non-linear relationships between structural properties and initial curvature are possible, but must be statistically tested using the multi-level model to confirm. The section with the greatest total change in curvature (ΔC) corresponded to the location of fracture for a given rib. Eight ribs that did not break during the first impact were impacted a second time, and structural properties were compared between first and second impacts. δ_Y, yield force (F_yield), U_tot, and elastic energy (U_e) all significantly decreased during the second impact, while δ_X and lateral curvature increased. This result indicates that damage may be accrued during the first impact, changing the structural properties.

Results of this study are applicable to ATDs, computational modeling, PMHS testing, and bone mechanics and will lead to improved tools to increase car safety through age- and sex-specific injury criteria and modeling.