# 3D Printed Orthodontic Aligners--A Scoping Review
## 1 Introduction  
As the number of adults undergoing orthodontic treatment is steadily increasing, there is a high demand for less visible treatment options [1; 2]. Aligners are transparent, less-visible, removable orthodontic appliances that can be used for the treatment of mal-occlusions by splitting the movement into various steps. The principle is based on the idea of [PERSON] in 1945 of moving teeth with removable thermoplastic positioners [3]. Although their clinical efficiency remains limited compared to fixed orthodontic appliances, the introduction of auxiliaries such as attachments and the development of specific treatment protocols for different malocclusions have significantly improved clinical outcomes [4; 5; 6]. One considerable option is to outsource the set-up (treatment planning) and aligner production to a company [7]. The fast and continuous development of digital technologies in terms of the wide availability of intraoral scanners and the development of appropriate software and 3D printing technologies offer new opportunities to realize the whole workflow in-house [8]. As the state of the art, aligners are manufactured by vacuum-forming thermoplastic sheets over a series of printed models, each representing incremental tooth movements towards the planned outcome [9]. These sheets can bemade of polyethylene terephthalate glycol (PETG), polyurethane (PU), or polyethylene (PE) and can be single, double, or triple-layer sheets [10; 11]. In recent years, significant advancements have been achieved in the field of additive manufacturing (3D printing), enabling the precise manufacturing of a wide range of materials. In contrast to traditional methods, direct 3D printing of orthodontic aligners represents a new approach in aligner manufacturing that does not require models or a thermoforming process [12]. Initial reports explored the possibility of digitally designing aligners and directly printing them using splint materials [13]. However, the materials were not approved or authorized to perform orthodontic tooth movements. The first scientific studies investigating an approved material for orthodontic aligner treatments, Tera Harz TC-85 (Graphy, Seoul, Republic of Korea), were published in 2022 [14; 15]. Many potential advantages of direct 3D printing of orthodontic aligners compared to conventional thermoforming techniques have been suggested [12]. In contrast to thermoforming techniques, it might be possible to control and adjust the aligner thickness individually regarding biomechanical considerations as with different thicknesses the exerted force can be influenced [16]. For direct aligner printing, new materials with shape memory behaviors, so-called 4D printing, have been introduced, which could enable higher thresholds for tooth movement per aligner [17; 18; 19]. Shape memory polymers (SMPs) are able to alter their macroscopic shape upon a stimulus such as a temperature change [20]. The model-free manufacturing process is potentially more cost-effective and ecological [12; 20]. However, there are concerns, particularly regarding biocompatibility, as unprocessed resins are highly toxic, and adverse effects such as possible allergic reactions have been reported [20; 21]. While scientific evidence on 3D-printed aligners remains limited, especially regarding clinical trials, the literature on this topic is quickly growing, with new insights from both clinical and experimental studies. This scoping review offers a comprehensive summary of the existing literature on 3D-printed aligners, aiming to enhance understanding of the present state of research by identifying gaps and highlighting potential areas for future investigation.