The orthopedic prosthesis, known as orthoses, are an external aid used for the correction of diseases which are the cause of a motor malfunction. Nowadays, the classification of different orthoses is performed by grouping them by type of apparatus that is subject to correction and on the basis of the length of the orthosis. In this study we analyze a specific orthosis of composite material, belonging to the AFO (Ankle-Foot-Orthosis) family. Passing through a process of “reverse engineering”, we define a non-linear computational model of the orthosis that describes the large displacement, the composite material, and the contact with the ground. The validation of the model against experimental tests, allows to use it to correlate the stiffness of the orthosis to its geometry, thus providing a useful tool to guide the structural improvements needed for adaptation to the patient.