Deformation analysis of an ankle foot orthosis during stance phase by the dynamic finite element method using a human lower leg model

The purpose ofthis study is to simulate the deformation of a plastic shoehorn-type ankle-foot orthosis (AFO), because the contact of the AFO with the lower leg and deformation of the AFO affect gait. In this study, bone, muscle, and fat models ofa virtual human lower leg consisting of 6,650 elements were constructed. Further, a bone model composed ofthe tibia, fibula, tarsals, metatarsals, and phalanges was constructed, with the ankle joint and 5 metacarpophalangeal (MP) joints as rotatable joints. An AFO composed of1, 536 elements including belts was also developed. Using the dynamic finite element analysis program LS-DYNA (LSTC Corp.), we simulated the motion and deformation from the moment of heel contact to that of the heel-off position, where the lower leg, AFO, and its belt were in contact with each other. By loading the same weight as a subject onto the model at the center ofgravity ofthe human body and by defining the moment ofthe ankle joint resistance, we were able to simulate similar time variations in the ankle joint angle as observed in the subject's gait. In addition, by mounting a shortened belt after applying an inward-directed enforced displacement at the belt-mounting points ofthe orthosis, close contact was maintained between the lower leg and the AFO during movement. The results were compared with the measured data obtained from gait experiments performed on the normal subject wearing the AFO. The trend ofstrain distribution around the ankle portion ofthe AFO as determined by the simulation method coincided with the measured data, except in the outside region ofthe ankle. The results indicate the validity and effectiveness of the dynamic finite element method for analyzing an AFO by using a human lower leg model.