Pedestrian Injury Mechanisms & Criteria a Coupled Experimental and Finite Element Approach

In pedestrian injury biomechanics, knees and lower legs are highly recruited, leading to joint damage and bones failures. Safety improvements should mainly focus on knee ligaments injury minimization. To investigate the corresponding injury mechanisms and postulate on injury criteria risk, both experimental and finite element simulation approaches were performed. The lower limb behavior was first studied in lateral bending and then in lateral shearing impact tests in order to isolate injury mechanisms effects. The tests consisted in evaluating lower limb forces and kinematic through a 37kg guided impact with velocities ranged between 15 & 20kph. 35 tests were performed on isolated PMHS lower limbs. Response corridors for the time history about the mean response curve ± one standard deviation with the Maltese procedure were established. The observed damages were contact injuries (head of fibula and lateral tibial condyle fractures), ligament injuries (cruciates and collaterals ligaments according to the tests) and bone fractures (extra and diaphysis). These experimental tests were simulated using a finite element model of the lower limb (with extended impact velocities). The model response analysis (bone Von Mises stress levels, Ligaments global and local strain levels, knee rotation and shearing measurements) was performed during each step of the impact chronology. It leads to postulate on injury criteria for knee soft tissues based on the knee ultimate lateral bending (~16°) and shearing levels (~15mm). These approaches by coupling PMHS experimentation and numerical simulation ensure an accurate description of pedestrian lower limb trauma in terms of injury chronology and threshold. These results were also relevant with accidentology and clinical knowledge, especially with the evaluated potential injuries. INTRODUCTION Although the number of pedestrian suffering severe or fatal injuries has decreased since 1991 in EU (Kallina, 2002), pedestrian safety has become a major issue of society and is one of the most discussed topics in vehicle safety. If pedestrian sustain multisystem injuries, head and lower extremities injuries are the most frequently injured body regions. Particularly, lower limbs are highly loaded during crash situations (AIS from 2 to 6) with joints damages and bones failures (IHRA 2001, Stutts 1999). To improve understanding of the mechanisms and establishing tolerance criteria for this specific body part, coupled experimental and numerical studies were conducted. Experimental studies were performed to represent condition of pedestrian accident focusing on the lower limb. The tests should have reflected the nature and the severity of the injuries sustained from this kind of impact. There is little data available from experimental studies measuring the response of the human knee joint in the pedestrian environment. Aekbote et al (Aekbote, 2003) reviewed the biomechanical studies conducted over the last three decades. He noted that mainly the injury mechanisms of the lower extremity were investigated. Tests were conducted in pure lateral shear loads, in pure bending moments or a combination of lateral shearing and bending of the knee (Kajzer 1990, Kajzer 1993, Grzegorz 2001). On cadavers full leg experiments, Kajzer (1990, 1993) focused on lower limb response under shearing and bending solicitation. He showed that pure shearing induces collateral tibial and anterior cruciate ligaments failure while a primarily bending mainly induces medial collateral ligament failure. More generally, three primary injury mechanisms were underlined: acceleration of the leg induced contact injuries as fracture of the femur and/or tibial shaft, shear force through the knee joint led to ACL rupture/avulsion, tibial intercondylar eminence fracture and femoral cartilage injury, and injuries due to bending moment at the knee joint are compression fracture of lateral femur condyle, tibial plateau fracture and MCL rupture.