Pedestrian Lower Extremity Fracture Prediction: Development and validation of strain based Injury Risk Functions for Femur and Tibia

Abstract

Road traffic accidents remain a leading cause of death and serious injury globally, with vulnerable road users including pedestrians accounting for a disproportionate share of the fatalities. In pedestrian-vehicle collisions, the lower extremities are among the most frequently injured body regions, with fractures of the femoral and tibial shafts representing major injury outcomes. While finite element based human body models are widely used in automotive safety research, their pedestrian injury assessment capabilities are limited by the absence of validated fracture risk functions for the lower extremity long bones. This thesis addresses this gap by developing and validating age dependent, strain based injury functions for femoral and tibial shafts for the SAFER HBM. Probabilistic Weibull survival models were fitted to cortical bone coupon test data using a Monte Carlo reconstruction framework that accounts for the uncertainty associated with the use of aggregated experimental data. The resulting injury risk functions express fracture probability as a function of maximum principal strain and age. Component level finite element simulations of isolated femur three point bending and tibia four point bending experiments were conducted to validate the developed framework. The predicted force from the femur model was in agreement with the test results. The femur injury risk functions predicted fracture probability above 50% for four of the eight fracturing specimen, providing validation of the framework. The tibia simulations showed variable force time agreement, and the extracted cortical strain at the time of experimental peak force were clustered in the range of 1% to 1.7% across all tibia specimen which was below the fracture threshold. This outcome is however attributed to the highly dynamic nature of the experimental setup, where direct impactor contact without a foam padding introduced loading conditions that the current modeling framework could not fully replicate. The developed injury risk functions represent a first step toward biofidelic pedestrian fracture assessment with SAFER HBM, with the femur IRF considered suitable for use and tibia IRF providing reasonable first estimate pending improved experimental validation.