Ankle Skeletal Injury Predictions Using Anisotropic Inelastic Constitutive Model of Cortical Bone Taking into Account Damage Evolution

The most severe ankle skeletal injury called pilon fractures can cause long term disability and impairment. Based on previous experimental studies, the pilon fractures are regarded as caused by a high-energy compressive force in the ankle joint and affected by a muscular tension force generated by emergency braking. However, quantitative injury criteria for the pilon fractures are still unknown. More accurate prediction of bone fractures in the distal tibia using a FE model of human lower leg can help us know the quantitative injury criteria. Therefore we newly proposed an anisotropic inelastic constitutive model of cortical bone including damage evolution and then implemented it to a FE code, LS-DYNA. The proposed model successfully reproduced most of anisotropy, strain rate dependency, and asymmetry of tension and compression on material and failure properties of human femoral cortical bone. However, the simplified model using an isotropic elasto-viscoplastic material, which has been used in previous studies, did not reproduce the characteristic features of the cortical bone. Two series of validation on axial impact cadaver tests for the foot and ankle indicate that the proposed model predicts the pilon fractures more accurately than the simplified model. Parametric studies on footwell impacts and pedal impacts for the foot using the proposed model show that the severity of the pilon fractures increases when the foot sustains normal and heel impacts with the impact velocity of 5 m/s and the pedal hits the forefoot with the impact velocity of 3 m/s regardless of the muscular tension force.