Human Foot-Ankle Injuries and Associated Risk Curves from Under Body Blast Loading Conditions

Under body blast (UBB) loading to military transport vehicles is known to cause foot-ankle fractures to occupants due to energy transfer from the vehicle floor to the feet of the soldier. The soldier posture, the proximity of the event with respect to the soldier, the personal protective equipment (PPE) and age/sex of the soldier are some variables that can influence injury severity and injury patterns. Recently conducted experiments to simulate the loading environment to the human foot/ankle in UBB events (~5ms rise time) with variables such as posture, age and PPE were used for the current study. The objective of this study was to determine statistically if these variables affected the primary injury predictors, and develop injury risk curves. Fifty below-knee post mortem human surrogate (PMHS) legs were used for statistical analysis. Injuries to specimens involved isolated and multiple fractures of varying severity. The Sanders classification was used to grade calcaneus severity and the AO/OTA classification for distal tibia fracture. Injury risk curves were developed using survival regression analysis and covariates were included whenever statistically significant (p<0.05). With peak force as the injury predictor and age and boot as covariates, the model was statistically significant. However, boot use changed the pattern of injury from predominately calcaneus to predominantly tibia. Also, a severity based risk curve showed tolerance differences between calcaneus (minor/major) and tibia (severity-I/ severity-II) injuries. The tibia demonstrated higher tolerance as compared to either minor or major calcaneus injury. These findings can play a vital role in development of safety systems to mitigate injuries to the occupant.