# 3D Printed Orthodontic Aligners--A Scoping Review
## 4 Discussion  
This scoping review provides an updated summary of the current literature on in vitro and clinical studies, as well as existing review articles on 3D-printed aligners. Recent research has expanded significantly, covering various aspects of this novel approach for aligner manufacturing. In comparison to previous reviews, which provided narrative overviews of selected studies, this scoping review is the first to use a structured search strategy to systematically identify the available evidence regarding 3D-printed aligners and includes studies up to September 2024. To date, scientific data are only available for one material that has been approved to perform orthodontic tooth movements or aligner treatments, Tera Harz TC (Graphy, Seoul, Republic of Korea). Although other approved materials indicated for aligner manufacturing, such as Dental Clear Aligner (DCA, LuxCreo Inc., Chicago, IL, USA), were identified while conducting this review, no scientific studies were available. Other frequently studied materials, such as Dental LT clear (Formlabs, Somerville, MA, USA), are approved for intraoral use but are not intended to perform tooth movements. As more materials are expected to be approved or authorized for this indication, investigations of non-approved materials regarding aligner therapy do not appear to be justified, except for regulatory studies. The most prevalently studied aspect of 3D-printed aligners is the evaluation of their material properties. Numerous studies have investigated the mechanical characteristics of 3D-printed aligner materials compared to thermoformed ones, such as shape recovery, hardness, and thickness. These properties may directly affect the performance of aligners in clinical settings. In terms of dimensional accuracy, the current evidence suggests that 3D-printed materials seem to be superior to thermoformed appliances [13; 15]. However, there was an overall increase in the dimensions of the 3D-printed aligners compared to the virtually planned dimensions [13; 15]. Relaxation was found to be higher for 3D-printed Tera Harz TC than in the thermoformed group [17]. Relaxation is a commonly investigated parameter to evaluate the material's ability to maintain the force level and should therefore be low. Shape recovery seems to be superior in aligners printed using Tera Harz TC compared to thermoformed ones [17]. Given the heterogeneity of available studies, the results are not comparable given the various investigated parameters using different methodologies. The idea to vary aligner thickness depending on whether movement or anchorage is needed [55] is interesting. But, it needs to be borne in mind that integrating this aspect is very complex, and it becomes even more complex with increasing numbers of teeth that need to be moved. Furthermore, there is agreement in the available literature that thickness increases during the printing process [17; 36; 40]. In general, manufacturing techniques, including the 3D printing process, the printer itself, and post-processing procedures, havebeen identified as critical factors not only for ensuring the material properties [14; 46] but also for achieving high monomer conversion. It should be noted that the manufacturer's recommendation for post-processing of Tera Harz TC-85 has changed since its introduction to the market. Post-processing plays a crucial role in reducing the risk of adverse effects associated with residual monomers, particularly UDMA, as these substances may pose biocompatibility concerns [21]. Interestingly, extended post-curing time was shown to positively correlate with cytotoxicity [49]. Validated protocols and the manufacturers' recommendations should be strictly followed during production of 3D printed aligners. Biocompatibility is a central consideration when evaluating materials for intraoral use, especially for 3D-printed aligners, since 3D-printable resins are highly toxic before printing and post-processing [56].