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

Sammanfattning

Road traffic accidents remain a leading cause of death and serious injury globally,with vulnerable road users including pedestrians accounting for a disproportionateshare of the fatalities. In pedestrian-vehicle collisions, the lower extremities areamong the most frequently injured body regions, with fractures of the femoral andtibial shafts representing major injury outcomes. While finite element based humanbody models are widely used in automotive safety research, their pedestrian injuryassessment capabilities are limited by the absence of validated fracture risk functionsfor the lower extremity long bones.This thesis addresses this gap by developing and validating age dependent, strainbased injury functions for femoral and tibial shafts for the SAFER HBM. ProbabilisticWeibull survival models were fitted to cortical bone coupon test data using aMonte Carlo reconstruction framework that accounts for the uncertainty associatedwith the use of aggregated experimental data. The resulting injury risk functionsexpress fracture probability as a function of maximum principal strain and age.Component level finite element simulations of isolated femur three point bendingand tibia four point bending experiments were conducted to validate the developedframework. The predicted force from the femur model was in agreement with the testresults. The femur injury risk functions predicted fracture probability above 50% forfour of the eight fracturing specimen, providing validation of the framework. Thetibia simulations showed variable force time agreement, and the extracted corticalstrain at the time of experimental peak force were clustered in the range of 1% to1.7% across all tibia specimen which was below the fracture threshold. This outcomeis however attributed to the highly dynamic nature of the experimental setup, wheredirect impactor contact without a foam padding introduced loading conditions thatthe current modeling framework could not fully replicate.The developed injury risk functions represent a first step toward biofidelic pedestrianfracture assessment with SAFER HBM, with the femur IRF considered suitable foruse and tibia IRF providing reasonable first estimate pending improved experimentalvalidation.