Vehicle floor intrusion is a common cause of injury to the lower leg in automotive collisions. For the most part, experimental investigations into the fracture tolerance of the leg in these collisions have assumed that a vehicle occupant is seated in an idealized neutral posture, with force directed along the long axis of the leg (producing pure compression). This neglects the non-standard postures an occupant may actually assume during a crash, which result in bending in addition to compression. A new injury criterion that accounts for these non-standard postures and the associated combined loading is needed in order to better assess risk and evaluate protective measures in collision scenarios.
A custom-built apparatus was used to deliver impulses representative of automotive collisions to eight isolated human tibiae (four pairs, female, aged 48-73). Impacts were delivered at an average velocity of 6 m/s and over an average duration of 23 ms, in order to represent realistic conditions of a frontal collision. The mass of the impacting projectile (and correspondingly impact energy) was increased until fracture was achieved. One specimen from each pair was held at a posture of 15° from pure axial loading and the contralateral at 30°. Forces, moments, and impulse were collected from the tests and analyzed after fracture to determine the effect of posture on injury risk. It was found that specimens held at the smaller angle tended to withstand higher resultant forces applied to the bone before fracture, and that the Revised Tibia Index (RTI) in its current formulation was not reliably able to predict fracture.
This work to quantify the effects of leg posture on injury risk will lead to the development of more comprehensive design criteria for vehicle occupant protection measures.