Design and Fabrication of a Cost-effective Ankle Foot Orthosis- Full

Cerebral Palsy (CP) is a neurodevelopmental disorder with a wide spectrum of severity. [1] Affected individuals experience irregular motor function, often to the detriment of proper gait. [1] Drop foot is a condition associated with CP in which the dorsiflexion in the leg falters, and the forefront of the foot drags during walking. [2] A common solution for this condition is the use of Ankle Foot Orthoses (AFOs)external devices that improve gait. [3] Although these orthoses have been improved over time to address a user's physical needs, material and financial restrictions are still an obstacle. This study sought to create an orthosis that achieved these improvements by comparing different designs, materials, and configurations for their stiffness and ability to function, given the loads predicted for their use. This study required the design of an AFO, sensitivity comparisons of material types using Finite Element Analysis (FEA) simulations, and the creation of a physical model by laser cutting thermoplastic resin in a 2D shape followed by hot drawing. The method used was ideal because it lowered manufacturing difficulty, time, and cost. Iterations of the AFO can be created easily based on stiffness tests and FEA, focusing on functional design improvements and fabrication cost reductions. INTRODUCTION Cerebral Palsy (CP) is a non-progressive disorder that affects cognitive and functional abilities. [3] It often leads to diminished gait performance, which, in its most extreme case, can completely impair one's ability to walk. One common observation in CP patients is drop foota condition in which the foot does not properly rotate during the swing phase of walking. A person with drop foot cannot lift their entire foot when walking, resulting in little to no clearance between the toes and the ground, causing the toes to drag and inhibiting any regular motion. One solution is providing proper foot support in the form of an Ankle Foot Orthoses (AFO). An AFO is used to restore walking ability by either keeping the peripheral joints in a range of motion that will reduce pathological reflex patterns or restricting motion the user would naturally perform incorrectly. [4] For example, the talus of the ankle is held to eliminate downward rotation caused by drop foot. [5] In terms of general walking improvement, an AFO specifically helps to improve gait kinematics, decrease user energy, [6] and facilitate the development of motor skills. Gait parameters improved are the range of motion of both the ankle and knee joints, as well as increased walking ease, speed, and stride length. [5] There are a variety of AFO designs on the market including the solid AFO (SAFO), dynamic AFO (DAFO), floor reaction orthosis (FRO), posterior leaf spring (PLS) and the hinged AFO (HAFO). [4] AFO's also have varying mechanical complexity ranging between passive, semi-active, and active devices. [2] A passive AFO can store and release energy elastically and functions by physically holding the ankle complex around the correct position. Semi-active AFOs have some sensor function, and active devices incorporate active movement of the joint (i.e. exoskeleton). AFO design optimization seeks maximum comfort, high resistance to mechanical failure, and sufficient support. One of the best ways to analyze the mechanical properties of a complex flexible mechanism is the process of finite element analysis (FEA). In this study, an educationl version of the software Fusion 360 (version 2.0.6670), was utilized to conduct an effective analysis used for design optimization. There are several materials commonly used for AFO design. Some are sturdy and provide ankle-joint stability, but split or crack along pressure points with repeated use. Others are flexible and provide comfort while sacrificing ankle-joint stability. This study compared several thermoplastic materials for stress tolerance and deformation resistance. One of the objectives of this study is to create an AFO with complex geometry by cutting and bending a plate of thermoplastic material. After the proper material and thickness of the plate were identified via the sensitivity FEA we designed a 2-dimensional AFO "unfolded" model in Fusion 360. The model was then converted via virtual folding into a 3-dimensional model that was used for a final FEA. In order to create the physical model, a lasercutter was used to cut a thermoplastic resin sheet into the desired 2-dimensional shape. Afterward, the final 3dimensional product was produced by bending along proper creasing lines. The paper is organized as such: the Methods section describes how the AFO was designed, how the 3dimensional model was created, how the FEA was performed, and the cost-effective process for making the physical AFO. The Results section presents the results of the simulations and the Discussion section explains their implications. METHODS Overall, this study was comprised of four parts. First, a sensitivity analysis was performed to identify the best combination of material and plate thickness to fabricate an AFO. Second, an anterior support AFO virtual model was created as a planar cutout as shown in Figure 1. Third, using the material and thickness pairs selected in point one the planar model was extruded at the proper thickness, virtually folded, and underwent an FEA simulation. Finally, a physical model was created with a cost-effective manufacturing technique to demonstrate the feasibility of the manufacturing technique. Description of the initial sensitivity analysis A sensitivity analysis via FEA was performed to find the optimal material based on performance and cost. The simulation considered 11 thermoplastic polymeric materials and was run on a vertical cantilever beam (300mm x 50mm x T), with three different thicknesses “T” (i.e. 3mm, 6mm, and 9mm). The cantilever was fixed on one side, and a 100N force was applied to it, orthogonally to the length and width. The force was applied at the end of the cantilever, 300mm from the constraint. We consider this structural arrangement and loading scheme as a first approximation of the behavior of a user applying a load on an AFO during walking [7], without the necessity to deal with more complex geometry. This information and respective results can be viewed in Table 1. We performed this sensitivity analysis to find an optimal combination of material and plate thickness that would guarantee linear, yet large deformations. A combination of material and plate thickness with maximal deformation on the simplified model between 100 and 250mm indicate a secant angle between the constraint end of the cantilever and its free tip between 18 and 40 degrees, which is compatible with unimpaired dorsiflexion range in walking [8]. Many thermoplastic polymers can deform within such boundries without plasticising. Combination of materials and thickness passing this initial screaning were considered suitable to be tested in an FEA analysis with more complex geometry. Constructing the 2-dimensional model of the AFO To create the basis of the 2-dimensional design, there was a reference to TOEOFF® (Allard USA)an orthosis similar to the desired design. To construct the footplate, geometrical reference points were placed on a sketching plane with appropriate dimensions based on the reference. Then, a spline curve tool was used to achieve the edges of the footplate (Figure 1). The upper structure of the AFO, composed of the ankle and shank support utilized the line, fillet, and dimension tools in Fusion 360. The final 2dimensional drawing is displayed in Figure 1. Constructing the 3-dimensional model of the AFO After the planar cutout was complete, it was transformed into a 3dimensional model. A 3-dimensional model was made using a sheet metal bending toolbox in Fusion 360’s. The 2-dimensional drawing was thickened using Fusion 360’s flange tool, as opposed to the more commonly used extrusion tool, and then bent into the shape of the AFO shown in Figure 2. It was bent at 90 degrees at the foot plate and then bent as needed to create a curvature in the shank and ankle support. This process was chosen because it allows to simulate the plastic deformation of the polymeric plate so to simulate the real fabrication process. Figure 1: Drawing of orthosis planar cutout Figure 2: Virtual model Sank suport Akle suport Foot plate