Structural stability prediction for thermoplastic ankle-foot orthoses

A thermoplastic ankle-foot orthosis (AFO) is an orthotic appliance which extends from the calf to the foot. The solid ankle AFO (SAAFO) is used to provide forces and moments to correctly position the lower leg and foot so that more normal stance is achieved. The SAAFO is a thin shell structure, usually 3/32 to 3/16 in. thick, thermally vacuum formed to a cast representing the patient´s lower leg, ankle and foot geometry. The SAAFO, then, is a thin shell structure of complicated geometry. The authors present the outcome of a comparison of instability loads obtained from testing and finite element analysis predicted buckling loads. The comparison indicates that while proper trends were predicted, the accurate prediction of buckling loads will require more sophisticated finite element models