Permanent deformation of posterior leaf-spring ankle-foot orthoses: a comparison of different materials

Permanent deformation of posterior leaf-spring ankle-foot orthoses after prolonged use among foot-drop patients has been observed in Dundee, Scotland. A lack of evidence exists to suggest which material used to manufacture these orthoses best avoids this problem. This study aims to determine whether carbon fiber, polypropylene, or polyethylene has the best deformation resistance. Customized posterior leaf-spring ankle-foot orthoses of each material were tested on a universal testing machine to quantify material and structural properties, and subsequently attached to a custom-built cyclical loading machine and cyclically plantarflexed from 0° to 10°. After varying numbers of cycles, the orthosis was returned to the universal testing machine and its structural and material properties quantified again. Results indicate that after 30,000 cycles, no material properties had been affected. However, by 90,000 cycles and thereafter, both thermoplastics began to lose stiffness and had permanently deformed to a plantarflexed position. Carbon fiber remained relatively unaffected throughout, showing superior resistance. We recommend carbon fiber being used for more active foot-drop patients and thermoplastics for less active patients and growing children. It is anticipated this will ultimately result in a better use of resources, increased efficiency in orthosis prescription for foot-drop management, and most importantly, improved patient care.