Investigation of Potential Predictors of Iliac Wing Fracture Tolerance in Frontal Lap Belt Loading

Introduction: Occupant environments in future automobiles are likely to create a situation where the seatbelt is taking more of the load in restraining the occupant and previous studies have suggested that the lap belt could cause fracture to the pelvis. Lap belt loading to the pelvis has been studied previously in male subjects, where a large variation of fracture tolerance was identified (Moreau et al. 2023). A general bone density calculation was the only parameter previously investigated that displayed a relationship to iliac wing fracture tolerance. Additionally, sex-based differences in iliac wing fracture tolerance were not assessed in the previous study; however, females have been shown to have differences in pelvis shape and a greater risk of injury in automotive crashes compared to males.
Goals: As a result, this thesis aims to identify the fracture tolerance of female iliac wings in a frontal lap belt loading scenario and compare the results to that of the males. In addition, this thesis aims to evaluate the effects of numerous parameters on iliac wing fracture tolerance and investigate how bone microstructure in the area of fracture relates to both sex and iliac wing fracture tolerance.
Methods: Component-level female iliac wing fracture experiments were conducted in agreement with Moreau et al. (2023), so that male and female fracture data could be combined to evaluate the influence of sex. Bone microstructure at the ASIS and AIIS was evaluated using micro-computed tomography and statistical analyses were performed to determine the highest sources of variance in the microstructure. Weibull survival models were used to model force at fracture, evaluate the effects of different parameters on fracture tolerance, and create injury risk functions (IRFs) that predict the probability of iliac wing injury due to lap belt loading.
Results: The female iliac wing specimens displayed a wide range in fracture tolerance (1135 N to 8759 N) that was similar to that of the male dataset. A 50% probability of female iliac wing injury in the IRF corresponded to a force of 5185 N. This research did not identify a sex-effect in the fracture tolerance of the iliac wing under frontal lap belt loading, but it did identify an age-effect in this loading condition. The feature that accounted for the largest variation in anterior iliac spine bone microstructure across the sample population was a measure of bone quantity, which also displayed a significant relationship to fracture tolerance. The bone quantity feature did not display relationships to sex or age.
Conclusions: IRFs were created from the female and combined male-female iliac wing fracture tolerance data. In general, male and female pelvises have similar likelihood of fracture in frontal lap belt loading, and older occupants have a lower iliac wing fracture tolerance in this loading condition. Including bone quantity in a risk function that utilized loading force to predict injury significantly improved the prediction models, which suggests the variation in fracture tolerance was largely explained by the variation in bone quantity. There was no sex-effect in the bone quantity feature, which suggests males and females have similar bone quantities in the anterior iliac spine region of the pelvis. An IRF that incorporated both bone quantity and age produced the best predictive performance among all combined male-female models. No significant correlation was found between the bone quantity feature and age, which suggests there was an effect of bone quantity on fracture tolerance that was not accounted for by age.