## 4 Discussion
Due to that, we cannot compare our power-arms with commonly manufactured ones, as commonly produced power-arms are made of metal alloys and have completely different material properties. After gaining knowledge about the properties of the material, we redesigned the shape of the prototype power-arms. The strength of each prototype had to be calculated by computer. After complete tuning, the spatial shape was finally modeled with a cavity inside. We managed to model power-arms using 3D printing without the prototype cracking. We managed to achieve our goal, which is confirmed by our calculations and clinically applied arms. The disinfection of the 3D printed part is an important aspect to be discussed. As described in the Materials and Methods section, our procedures of power-arm disinfection were consisting of UV post-curing and IPA. The 3D printed power-arms are intended to be fixed in the oral cavity, which is not a sterile place. They are not considered to be implants which are required to follow strict rules, for example, prosthesis for hips or knees, dental implants, or spine screws [36]. Results of the analysis and design of increasing the tensile arm strength showed that deformations of the originally analyzed arm pointed to the strength deficiencies of the original model. The disadvantages were a weak layer of material in the notch area for the tension rubber and an insufficient amount of material for the vertical part of the arm under the application of lateral forces. This was followed by adjusting the shape of the base from square to circular to eliminate tension nodes. The vertical part was adjusted from a rectangular shape to a \"lemon\" shape. Thanks to this modification, we have achieved the largest increase in strength in our favor. The rounded edges and three-dimensional shape increased the tensile strength, especially in the lateral action of forces. We performed all strength calculations on FEM virtual calculation programs [37]. The finite element analysis was already used in orthodontic research; however, it was not applied to power-arm 3D design, it had merely been used to research an ideal power-arm location in the oral cavity [38]. We are not aware of any similar research in the world that has been published in regards to the 3D printing of customized orthodontic power-arms, that utilizes finite element modelling to upgrade an existing design of the individualized 3D printed power-arm. As a result of the technical part of this research, we have managed to increase the strength by 7% and reduce the stress by up to 82%. The modifications we make increase the service life of the given component several times, which in practice means a lesser load for the attending physician, due to the need to glue a new arm, but also a lesser load for the patient. The stress at the interface between tooth surface and the power-arm is important. One of the limitations of this study is the lack of analysis of this connection and adhesives [39]. Other limitations of this study are those characteristics of design and methodology that influenced the interpretation of the findings. For example, the individuality of behavior and eating habits of the patients has not been researched. These might have influence on result interpretations. Considering other pros and cons of this new approach, another limitation is the technical and computer skills of doctors. Capability to perform technical analysis of the material requires interdisciplinary cooperation, however, the new trend is the online availability of all certified material properties regarding biocompatible resins for 3D printing. So, the advantage of this situation is that only FEM analysis for redesigning current individualized intraoral appliances will be necessary. The novelty of the proposed approach rests not only in the biocompatible 3D printing of an intraoral appliance never 3D printed before, but also in the way of its individualization based on digital intraoral scans, and its redesign based on FEM analysis.