Rib Cortical Bone Fracture Risk as a Function of Age and Rib Strain: Updated Injury Prediction Using Finite Element Human Body Models

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.

Protection challenges in seat positions with large rearward adjustment in frontal collisions: An approach using stochastic human body model simulations

“…The model was originally derived from laser scans of 14 cars and validated with their respective crash test data to analyze driver rib fractures in nearside oblique frontal accidents (Iraeus, 2015;Iraeus and Lindquist, 2016). Variants of the model have since been used to analyze the influence of crash pulse parameters on rib fractures (Iraeus and Lindquist, 2020) and study rib fracture risk as a function of age and rib strain (Larsson et al, 2021). For use in the Therefore, the open section on the inboard side of the footwell, which could accommodate for the deformations of the footwell material, was closed with elements of the same material as the footwell part.…”

“…The strains used in this evaluation were the maximum tensile principal strains based on the maximum integration point per rib. These were subsequeently converted into an injury risk per rib using an age dependent log-normal distribution (Larsson et al, 2021) and combined to an overall risk of 2 + rib fractures using the binomial function from Forman et al (2012).…”

“…One important aspect in this could be the expected under-reporting of mTBI in accident databases (Wu et al, 2022), resulting in lower MAIS2+ head injury rates in the IGLAD sample. On the other hand, while the GVI was validated with crash test data from 14 vehicles using crash test dummies, the model was subsequently utilized primarily for strain-based rib fracture prediction (Iraeus, 2015;Iraeus and Lindquist, 2016;Iraeus and Lindquist, 2020;Larsson et al, 2021). Additional GVI model validation focusing on the head-airbaginteraction could potentially improve the results.…”

“…The two distributions are based on two age groups (mean of 29, and mean of 77 years). Additionally, the risk for NFR1+ and NFR2+ for a 77-year-old occupant, evaluated for each parameter combination in the stochastic simulation study, and a NFR2+ risk curve based on data from the United States' National Automotive Sampling System Crashworthiness Data System (NASS-CDS) (Larsson et al, 2021) are shown.…”

“…This might be caused by the fact that MAIS2+ injury risk in IGLAD does not originate from rib fractures alone. Using data from NASS-CDS, rib fracture specific risk curves for delta-v have been published (Larsson et al, 2021). The NASS-CDS-based risk curve for a 70-year-old occupant for NFR2+ is also shown in Figure 6.…”