Anti-vehicular (AV) landmines and improvised explosive devices (IED) have accounted for more than half of the United States military hostile casualties and wounded in Operation Iraqi Freedom (OIF). The lower extremity is the predominantly injured body region following an AV mine or IED blast accounting for 26 percent of all combat injuries in OIF (Owens et al., 2007). Detonations occurring under the vehicle transmit high amplitude and short duration axial loads onto the foot-ankle-tibia region of the occupant causing injuries to the lower leg. The current effort was initiated to develop lower extremity injury criteria and biofidelic biomechanical surrogate to evaluate military occupant injury during an AV (axial) blast event.

Eighteen lower extremity post mortem human specimens (PMHS) were instrumented with an implantable load cell and strain gages and impacted at one of three incrementally severe AV axial impact conditions. Twelve of the 18 PMHS specimens sustained fractures of the calcaneus, talus, fibula and/or tibia. A tibia axial force of 2,650 N and impactor velocity of 8.2 m/s corresponds with a ten percent risk of an incapacitating injury.

Currently available lower extremity biomechanical surrogates were shown to lack biofidelity when impacted at simulated AV blast severities. A THOR-Lx underwent a series of modifications intended to reduce the overall stiffness of the surrogate. Its tibia compliant element was doubled in length to enable additional clearance for compression. The modified surrogate, MiL-Lx (military lower extremity), was loaded axially at three simulated AV axial loading rates using a piston driven linear impactor. The diameter and compressive modulus of the tibia compliant element was varied until the axial force measured by the surrogate was equivalent to the PMHS non-injury response in magnitude and duration. The MiL-Lx surrogate was capable of distinguishing between incrementally severe loading rates using tibia axial force. The MiL-Lx improves the accuracy and sensitivity needed to evaluate blast mitigation technologies designed to reduce injury to occupants of vehicles encountering AV landmines. The use of the MiL-Lx shall result in the development of new standards for the testing of blast mitigation technologies including underbelly protection, floor board materials, and vehicle structure.