Human Ankle Impact Response in Dorsiflexion

Although various automobile accident surveys showed between 20 to 30% of lower extremity injuries involved the foot or ankle, there is little information in the existing literature on the the injury mechanisms of ankle injuries for automobile occupants involved in frontal impacts. This study addresses the injury to ankles involving dorsiflexion caused by impact loading to the bottom of the foot. Types of injuries include malleolus fractures and ligament avulsions and ruptures.

Nine pair of cadaver and two Hybrid 3 lower limbs were impacted on the bottom of the foot with a 16 kg pneumatically propelled linear impactor. A horizontally oriented bar struck the foot 62 mm distally of the ankle joint with velocities between 3 and 8 m/s. The proximal end of the tibia/fibula was fixed to a rigid support through a triaxial load cell. Load cells on the foot and impactor along with high-speed photography provided the response data of the foot and ankle. Range of motion tests were done before and after testing.

Peak axial loads ranged between 1.1 and 5.0 kN and moments at the ankle between 52 and 300 N-m. Maximum dorsiflexion ranged between 25 and 74 degrees during impact. Inversion or eversion motions were not noticed in these tests where the foot was free to dorsiflect. There was no correlation between pre-impact range of motion values and injuries or peak forces. The best correlation for an injury pre-diction in these tests was the maximum angle of dorsiflexion; those with less than 45 deg of dorsiflexion did not sustain any injuries. Hybrid 3 responses were not comparable to cadaver responses.