The effect of impact duration on the axial fracture tolerance of the isolated tibia during automotive and military impacts.

Axial impacts to the lower leg during debilitating events such as frontal automotive collisions and military underbody blasts can cause significant injuries to the tibia. Several studies have conducted axial impact tests to determine the injury limits of the lower leg, mostly focused on automotive intrusions, resulting in an established force criterion for injury assessments. Due to the viscoelastic properties of bone, it remains unclear whether results from automotive experiments can be applied to higher-rate military blasts. Twelve male isolated cadaveric tibias (from six pairs, mean age: 62 ± 8 years) were subjected to axial impact loads using a custom-built pneumatic impactor, with one specimen from each pair tested at velocity and impact durations representative of a military blast condition, and the contralateral under conditions representing an automotive collision. Impacts were applied in increasing levels of intensity (defined using energy levels) until fracture occurred. Fracture risk was influenced by projectile velocity, kinetic energy, impulse, and load rate, and there was a significant difference in peak force (p = 0.023), impulse (p = 0.09), and load rate (p = 0.025) between the automotive and military test conditions causing fracture. A 10% risk of fracture corresponded to an impact force of 9.0kN for the automotive condition and 12.2kN for the military condition. These results suggest that fracture tolerances developed in studies that simulate automotive impacts cannot be directly applied to military impacts of shorter duration. The number of factors identified to predict injury also suggests that fracture is not controlled by a single variable.

The Injury Tolerance of the Tibia Under Off-Axis Impact Loading.

During a frontal collision, there are a range of lower extremity postures that the vehicle's occupant may assume, potentially changing the way load is transmitted to this region of the body. While most experimental studies on the tibia's injury tolerance assume that load is directed along the leg's long axis, the effects of off-axis loading due to non-standard postures have not been well quantified, and commonly-cited injury criteria such as the Tibia Index do not directly account for posture. Therefore, twelve cadaveric tibias (paired from six donors) were subjected to off-axis impact loading in a custom-built test apparatus. One specimen from each pair was held at an angle of 15° relative to the direction of loading, while the contralateral was held at an angle of 30°, with these angles representing ankle plantarflexion and corresponding knee extension in a vehicle occupant. Specimens held at 30° fractured at lower forces than the specimens held at 15° (mean force = 5.8 vs. 7.5 kN). This indicates that posture should be incorporated into injury criteria for the tibia in future safety assessments instead of using a single force value based on axial impacts.