# Strength Evaluation and Modification of a 3D Printed Anterior Ankle Foot Orthoses
## 4 Discussion  
AFOs are widely used for patients with the stroke, cerebral palsy, and neuromuscular disorders. Previous study has mostly focused on tests of PAFOs [15; 16; 17], rather than AAFOs. Compared to PAFOs, the AAFO has a lesser volume and is more suitable for the development of 3D printing. However, the AAFOs tended to fracture at the neck during walking within two months. If a 3 DP-AAFO is expected to be developed for a patient, then the mechanical strength is important to realize. To figure out the mechanical properties of the 3 DP-AAFO, the mechanical test and FE analysis were conducted in the study. In the mechanical test, the 3 DP-AAFOs were stiffer than the TAAFOs due to the discrepancy of material properties. [PERSON] et al., in 2009 [21], reported that the stiffness of AFO should range from 0.20 to 1.56 Nm/degree to provide ankle stability and prevent drop foot. In this study, the stiffness of 3 DP-AAFO (1.09 Nm/degree) was within the range, but the stiffness of TAAFO was slightly smaller than the recommended range. In the ultimate load test, TAAFOs only exhibited remarkable deformation without any fracture because of the behavior of the thermoplastic material. However, 3 DP-AAFOs were fractured at the bilateral neck in ankle plantarflexion (28.65 \(\pm\) 6.2\({}^{\circ}\)). During most activities of daily living, normal people just walk in 20\({}^{\circ}\) of plantarflexion. Furthermore, the individuals with hemiplegia move in 10\({}^{\circ}\) of plantarflexion, which indicates that it is safe to use 3 DP-AAFOs in daily life without fracture. Overall, 3 DP-AAFOs offer more ankle stability than TAAFOs. Additionally, in the ultimate load test, 85% of 3 DP-AAFOs fractured at the bilateral neck, which was the same as the clinical observation. Only 15% of all specimens fractured at the base and shank of the 3 DP-AAFO. This means that an unexpected fracture is possible in the base of the 3 DP-AAFO. The reason for early fracture can be attributed to the process of additive manufacturing. The strength of the 3 DP-AAFO was influenced by the printing orientation of the 3 DP-AAFO. The fused line between the different layers was weak, and easily torn from the tensile force. As a result, unsmooth surfaces or some gaps appearing on the neck or base in the 3 DP-AAFO should be carefully checked. According to the FE results, the thicker the neck of the 3 DP-AAFO, the stiffer the 3 DP-AAFO was. This study found that the 4.7 mm-thickness of the 3 DP-AAFO would obtain a better stiffness. However, in dorsiflexion, the 3 DP-AAFO with a 5.2 mm-thickness demonstrated a decrease in stiffness and increase in stress. The reason could be attributed to locally thicken the neck and result in stress concentration due to a discrepancy in thickness between the neck and base. Therefore, the sudden change between small and larger areas should be avoided. If this is not avoidable, the remarkable area change around the neck should be smoothed to moderate stress. Additionally, if the neck of the 3 DP-AAFO is too thick, it is inconvenient to wear the 3 DP-AAFO and the patient will choose shoes. As a result, this study does not recommend the application of a 3 DP-AAFO with a 5.2 mm-thickness. Regarding the limitation of the study, the FE analysis only considered the model in an isotropic material, but the real 3 DP-AAFO is anisotropic and in-homogenous. This study underwent a static test rather than a dynamic test, so a fatigue or impact test is needed to figure out its dynamic response. Moreover, additive manufacturing is different from the machining manufacture because it existed in pore distributions in the process of 3D printing. Some nondestructive tests or nanoindentation analyses [22] can be considered to examine the hardness or effective modulus of 3 DP-AAFO in future study. Before using the 3 DP-AAFO, an evaluation of a patient's spasticity is important. In the clinic, the physical therapist should consider the patient's spasticity to fabricate the 3 DP-AAFO.