Development of modular wrist, hand and finger orthesis by additive manufacturing

Additive Manufacturing (AM) has been considered an innovative technology for the development of orthoses. Even so, the use of AM, utilizing low cost rigid and flexible material which can be used in different ways by the same user, to produce a modular orthosis has yet to be explored. Purpose: Develop a modular wrist, hand and finger orthosis that can be utilized as a functional or static orthosis, depending on the therapeutic objective. This being produced by low cost Additive Manufacturing, through a single anatomy acquisition process. Approach: Firstly, requirements for modularization and development were defined in a team with occupational therapists and mechanical engineers, After indirect anatomy acquisition of a volunteer, without disabilities, two parts of the same orthosis were modeled, one flexible (functional) and the other rigid (static). These were printed on PLA (rigid part) and flexible TPU (functional part) with an Open Source printer. In addition, fastening strips were also made in flexible TPU. Findings: Three parts of which make up the modular orthosis were produced. This can be used in two different ways; one being to maintain the static posture of the wrist, hand and fingers and the other to provide functionality of the hands, but with the correct positioning of the wrist and thumb. Originality: Even with low-cost material and an open source machine, it was possible to generate an innovative proposal with the use of AM as the orthosis manufacturing process.

In general, the upper limb orthoses made by AM, as reported in the literature, are of the static type, for limb immobilization, constructed with rigid materials such as ABS (Palousek et al., 2014;Kim and Jeong, 2015) or PLA (Rosenmann et al., 2018;Blaya et al., 2018). However, in some cases, it is desirable that the orthoses allow for a certain degree of movement, guaranteeing the functionality of the upper limb (Trujillo & Amini, 2013).
In clinical practice rehabilitation services, it is common to prescribe two types of orthoses for the same individual: rigid and flexible. The devices are used alternately, according to a rehabilitation protocol (Arakaki, Cardoso, Thinen, Imamura, & Battistella, 2012). In individuals with Cerebral Palsy (CP), for example, rigid orthoses are used to prevent the advance of deformities (Morris, Bowers, Ross, Stevens, & Phillips, 2011), however, concomitantly, flexible orthoses made with fabric (neoprene) can also be used to maintain joint stability while allowing the movement and functionality of the member (Schwartz, 2020). Each orthosis is made separately, with different processes, at different times.
In this context, the purpose of this study was to develop, with the aid of additive manufacturing, a modular orthosis model that could be used as a static and functional orthosis. Secondary objectives for the development of this new orthosis were the use of low-cost materials and methods and the elimination of materials traditionally used for finishing and fastening.

Methodology
This article is presented as an exploratory research conducted through a case study (Santos, 2018). We conduct the development of a modular orthosis, in order to structure a development process and validate ow-cost additive manufacturing technologies in the manufacture of this product. For the development of the proposal, a volunteer with no disability was recruited. This research was approved by the Human Research Ethics Committee (1.859.901). Using the analysis of static wrist, hand and finger orthoses and the functional orthoses used by people with upper limb deformities as a starting point, possibilities of modularization and modification through AM were explored. The study was then divided into five stages that consisted of defining requirements for orthoses, anatomy acquisition, 3D scanning, 3D CAD modeling, and 3D printing. Each phase will be described below.
Definition of requirements: From the analysis of the literature and clinical experience of two occupational therapists, the requirements were defined. The orthosis was designed to be composed of a rigid part, another flexible part and fastening fasteners. The requirements of the different parts can be seen in Table 1. In addition to these requirements for each part of the orthosis, it was also defined to use only elements produced by AM and that the use of velcro, elastics or additional comfort materials such as foams were not necessary.
Anatomy acquisition: In this stage, a static orthosis for positioning the wrist, hand and fingers was made by an occupational therapist (OT) with a plaster cast ( Figure 1). This technique allows for the correct positioning of the individual's wrist, hands and fingers, as the OT manages to avoid undesirable movements, and thus reduces errors that may occur due to the difficulty of scanning, directly, the upper limb in the correct position.   Figure 1 shows the plaster orthosis model made with the hand posture determined by the Occupational Therapist. This was submitted to 3D scanning, whose 3D mesh can be seen in Figure 2.    Source: Authors. Figure 5a shows the printing process of the strips for fixing the modular orthosis. In Figure 5b the flexible orthosis is printed. After printing, the orthosis was analyzed in relation to the defined requirements. The time to produce each part of the orthosis, the mass (Kg) and the cost (dollar) in material used for making the orthosis were also recorded.   Source: Authors. Figure 7 shows the two ways of using the developed orthosis on the volunteer without disabilities. In figure 7a, the user uses the flexible part of the modular orthosis, which allows the movement of the fingers and thumb. Figure 7b shows the use of the set with the inclusion of the rigid part, providing the static positioning of the wrist, hand and fingers. Table 2 shows the time required to perform each of the steps previously described for each component of the modular orthosis (fasteners, rigid and flexible orthosis). The times related to the anatomy acquisition and scanning steps are the same, as they were performed only once. 3D printing and post-processing 1h 30min 1h 30min 1h 30min

Results
Source: Authors. Table 3 shows the mass of each of the three modular orthosis components, distinguishing when they are with and without support material. The total mass of the assembled set is also shown. Lastly, the costs inherent in making the modular orthosis are shown in Table 4. Also shown is the cost per machine hour for each component.

Discussion
The orthosis proposal, developed for a volunteer without disabilities, fulfilled the objectives of modularization and the proposed requirements guiding the research. It was possible to make two orthosis models in the same production process. In In relation to the flow for the development and manufacture of orthoses with low-cost materials and machines, these were effective. The process used for anatomy acquisition with the plaster cast has disadvantages due to the mess generated by the plaster. However, this proved to be a low cost and low complexity option. Therapists, in general, are familiar with handling this material. One possibility is that therapists utilize the plastered cast in order to materialize the orthosis models, in accordance to the specifications of the clients, and then send them to reference centers so that they can scan, model and print the orthoses using additive manufacturing. Paterson et al. (2010) evaluated, through literature review, different scanning methods, these being computed tomography (CT), magnetic resonance imaging (MRI), 3D laser scanning and anthropometry.
In this regard, one approach considered successful was the use of molds and plaster to achieve the anatomy acquisition of the individuals. This technique reduces inaccuracies and noise caused by movements during acquisition (Paterson et al., 2010). Baronio et al., (2017) proposed the use of a support to enable direct scanning of the hand and wrist of people with spasticity, this being a future possibility for the making of this model of orthosis.
Regarding 3D CAD modeling, the use of different programs shows the complexity involved in correction and preparation for printing. The scanning of the plaster cast model facilitated modeling, however the mesh from the scanning process showed imperfections that were corrected, but required processing time with the risk of altering the original geometry (Palousek et al., 2014). Paterson et al. (2014) proposed the development of an exclusive CAD program to provide therapists with the possibility of making modifications to an orthosis model for 3D printing. This would be an interesting option, since traditional CAD programs are difficult to operate and depend on the operator's experience. In the present study, the fact that the team is transdisciplinary made it possible to exchange information at all stages, especially in CAD modeling so that clinical objectives were respected in the final product.
Regarding costs, it is believed that this can be considered low cost. And, when considering that the total value refers to two orthosis models, this can be considered even more advantageous.
The utilization of open source printers proved to be a viable option for making orthoses with flexible and rigid components. The use of TPU for orthosis fastening fasteners can contribute to the replacement of velcro, which presents negative aspects (Kelly et al., 2015), such as being noisy, allowing for the concentration of dirt, harming of the skin and damage to clothes. In the case of the fastening fasteners developed in this project, in spite of being made with flexible material, does not have great elasticity, and are presumed to be sufficient in order to maintain the desired posture of the wrist, hand and fingers.
The next phase of the study will be the evaluation of comfort, intuitive use, ease of cleaning, presence of pressure points and resistance in the context of real use in a longitudinal study. This study focused only on the development of the proposal, emphasizing the modularization of the orthosis.

Conclusion
The modular orthosis developed presents innovative elements regarding the different options of use and the application of low cost flexible 3D printing. The results obtained with the use of the flexible material showed that the proposal, being used as a comfort material for the rigid static orthosis, is possible, and now depends on the realization of evaluations with real users. It is concluded that, even with low cost processes, AM favors the development of orthoses with the necessary qualities and requirements, and with the consideration of clinical aspects.
Evaluating the functionality of use of the flexible part for people with indication for this model of orthosis has also been suggested as future work. It is also suggested to evaluate the use of modular orthosis to measure the advantages of modularity compared to traditional models.