Quantifying the effect of material stiffness and wall thickness on the mechanical properties of ankle–foot orthoses manufactured by material extrusion

Additive manufacturing (AM) is increasingly employed in orthopedics to manufacture patient-specific prostheses and orthoses while reducing time and production costs compared to traditional thermoforming. Nowadays, an ankle–foot orthosis (AFO) can be manufactured by material extrusion with various polymers and designs. The objective of this work is to understand the effect of the AFO’s material composition and wall thickness on its overall mechanical properties, such as stiffness and strength. A complete study is conducted through mechanical characterization of the polymers and orthoses, microstructure observations with X-ray tomography and numerical simulations. Particular attention is being paid to the use of recycled materials to reduce environmental impact while maintaining sufficient resistance. On the one hand, it is shown that the stiffness of the AFO varies linearly with that of the polymer and the wall thickness of the orthosis, resulting in an easy-to-use master curve, which could be used by orthotists. On the other hand, material, design and quality of printing appear to be crucial factors in the overall strength of the AFO and thus patient safety. Finally, PP and TPU are promising for the support and flexibility they can provide, while a sufficient thickness must be chosen for PET to avoid brittle fracture.