Thorax Response in Young and Aged Small Females Under Side Impact, Isolated Ribs and Full-Body Computational Studies

Abstract

Small-stature females and aged individuals face a disproportionate risk of thoracic injury in automotive crashes. Epidemiological studies indicate the heightened susceptibility observed in the aged small statured individuals compared to average statured males through a significantly increased number of rib fractures (NRF), especially in side impacts. The present thesis investigates the biomechanical factors contributing to the increased NRF observed in the aged small statured female individuals, focusing on the age-related changes in thoracic geometry and material properties. Advanced finite element human body models (HBMs) were employed to quantify these effects under side-impact loading conditions, with a specific emphasis on young and aged small female modelling. First, age and sex-specific isolated rib models were developed including a 26-year-old mid-stature male (M5026IR), 26-year-old small female (F0526IR) and a 75-year-old small female (F0575IR) 6th rib. Using morphing techniques, with targets based on regression models, the rib gross geometry, cortical bone thickness, and rib cross-sectional area were enhanced to reflect population-specific characteristics. The Cortical Bone Fracture and Continuum Damage Mechanics Constitutive Model (CFraC) was implemented to enhance the representation of rib cortical bone material behaviour, integrating age-specific mechanical properties. The isolated rib models were loaded in anterior-posterior compression and compared to available population-specific experimental data. The isolated rib models served as a stepping stone for full body model development and as foundational model validation exercise.

Subsequently, three novel HBMs were developed by integrating the isolated rib developments and performing full-body morphing: a young adult small female (F0526), an aged adult small female based on statistically derived population data (F0575), and an aged adult small female based on subject-specific imaging data (F0584). These three models, alongside the baseline small female GHBMC model (F05B), were assessed in two environments: a simplified side impact sled using rigid plates, and an Advanced Side Impact System (ASIS) representing a realistic vehicle environment with a deformable seat, seatbelt with pretensioner, thorax airbag, and intruding door. Isolated rib results demonstrated good correlation to the age-, sex- and size-specific experimental data (i.e. young mid-size male, young small female and aged female) predicting the overall rib stiffness, force-displacement to fracture and fracture location. The rib models predicted the expected age and size/sex differences, that is, reduced stiffness and lower force and displacement to fracture with increasing age and decreasing size. Transition zones from thick to thin cortical bone led to stress concentrators that dictated the fracture location in the isolated rib models. The CFraC material model was shown to be a foundational improvement allowing for the prediction of cortical bone fracture. At the full body level, the baseline F05B model demonstrated limited sensitivity to impact severity, underestimating injury outcomes attributed to its simplified cortical bone model. The F0526 demonstrated sensitivity to impact severity. The F0575 model predicted NRF, chest compression maximum magnitude and timing, and AIS injury scores higher than the F0526 with values aligned closely to experimental Post-Mortem Human Subject (PMHS) data for aged small females. The subject-specific model (F0584), experienced higher rib torque owing to increased rib angles associated with this subject specific model, relative to the population average F0575 model. Increased rib torque, led to higher shear stresses in the cortical bone, resulting in the initiation of rib fracture earlier in the impact an increased NRF. These findings quantified and suggested that age-related reductions in cortical bone strain to failure, rib cortical bone thickness, and inter-subject variability can increase NRF. The present thesis demonstrates, for the first time, a methodology to develop predictive thorax models capable of capturing age-, sex-, and size-related differences considering rib fracture and thorax response. The presented results suggest that the development of population-specific tissue-level-predictive HBMs require a combination of appropriate material models coupled with population-specific material properties and geometry. Importantly, the HBMs developed in this work can be used to evaluate existing safety systems and aid in the development of future inclusive safety systems