Abstract
To evaluate vehicle occupant injury risk, finite element human body models (HBMs) can be used in vehicle crash simulations. HBMs can predict tissue loading levels, and the risk for fracture can be estimated based on a tissue-based risk curve. A probabilistic framework utilizing an age-adjusted rib strain-based risk function was proposed in 2012. However, the risk function was based on tests from only twelve human subjects. Further, the age adjustment was based on previous literature postulating a 5.1% decrease in failure strain for femur bone material per decade of aging. The primary aim of this study was to develop a new strain-based rib fracture risk function using material test data spanning a wide range of ages. A second aim was to update the probabilistic framework with the new risk function and compare the probabilistic risk predictions from HBM simulations to both previous HBM probabilistic risk predictions and to approximate real-world rib fracture outcomes. Tensile test data of human rib cortical bone from 58 individuals spanning 17–99 years of ages was used. Survival analysis with accelerated failure time was used to model the failure strain and age-dependent decrease for the tissue-based risk function. Stochastic HBM simulations with varied impact conditions and restraint system settings were performed and probabilistic rib fracture risks were calculated. In the resulting fracture risk function, sex was not a significant covariate—but a stronger age-dependent decrease than previously assumed for human rib cortical bone was evident, corresponding to a 12% decrease in failure strain per decade of aging. The main effect of this difference is a lowered risk prediction for younger individuals than that predicted in previous risk functions. For the stochastic analysis, the previous risk curve overestimated the approximate real-world rib fracture risk for 30-year-old occupants; the new risk function reduces the overestimation. Moreover, the new function can be used as a direct replacement of the previous one within the 2012 probabilistic framework.
Highlights
Despite improvements in vehicle occupant safety (Kullgren et al, 2019), rib fractures remain a prevalent outcome in motor vehicle collisions (MVCs) (Forman et al, 2019; Pipkorn et al, 2020)
Epidemiological studies reveal that risk of thoracic injury, including rib fractures, in MVCs increases with impact speed and age—and is greater for females than for males (Bose et al, 2011; Carter et al, 2014; Weaver et al, 2015; Brumbelow, 2019; Forman et al, 2019)
The increased rib fracture risk with age can be partly explained by findings from studies of human bone’s mechanical properties, which show that tolerance to mechanical load until fracture decreases with age (Lindahl and Lindgren, 1967; Burstein et al, 1976; Carter and Spengler, 1978; McCalden et al, 1993; Kemper et al, 2005)
Summary
Despite improvements in vehicle occupant safety (Kullgren et al, 2019), rib fractures remain a prevalent outcome in motor vehicle collisions (MVCs) (Forman et al, 2019; Pipkorn et al, 2020). The increased rib fracture risk with age can be partly explained by findings from studies of human bone’s mechanical properties, which show that tolerance to mechanical load until fracture decreases with age (Lindahl and Lindgren, 1967; Burstein et al, 1976; Carter and Spengler, 1978; McCalden et al, 1993; Kemper et al, 2005) Among these studies, Kemper et al (2005) reported a difference in bone’s ultimate strain due to sex, with the females showing reduced deformation before failure, but here the three female bone material donors were on average older than the three male donors, suggesting that the noted reduction may have been an effect of age rather than sex (Kemper et al, 2005). Among these studies, Kemper et al (2005) reported a difference in bone’s ultimate strain due to sex, with the females showing reduced deformation before failure, but here the three female bone material donors were on average older than the three male donors, suggesting that the noted reduction may have been an effect of age rather than sex (Kemper et al, 2005). McCalden et al (1993) reported a small increase in ultimate stress for female femoral specimens, while Lindahl and Lindgren (1967) and Burstein et al (1976) did not find any significant differences in femoral cortical bone ultimate stress between the sexes
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