## 3D-Printed Materials Dentistry  
**[PERSON]**  
Department of Orthodontics, University Hospital Dusseldorf, 40225 Dusseldorf, Germany;  
[EMAIL_ADDRESS]; Tel. : +49-211-8118160  
This editorial focuses on the Special Issue on 3D-printed materials in dentistry. The articles of this Special Issue cover a wide range of applications in the dental field. They range from applications in the clinical workplace to in vitro investigations on the impact of postcuring and storage time on accuracy and precision measurements to the indirect transfer of orthodontic brackets, as well as the application of 3D printing in dental education. Five out of eleven research papers deal with applications of 3D printing in orthodontics, one study presents a 3D-printed fitting system for FFP2 masks which were applied during the COVID-19 pandemic, while the remaining one addresses applications in prosthodontics and restorative dentistry. In orthodontics, 3D printing is becoming increasingly popular. It allows to create individualized appliances, to perform different treatment tasks at the same time, and much more [1]. The article by [PERSON] et al. [2] demonstrates a digital workflow for producing highly individualized, skeletally borne 3D-designed appliances for the upper and the lower jaw. As orthodontic mini-implants providing skeletal anchorage are often inserted using templates, the article also presents typical designs for insertion guides. Additionally, it highlights potential sources of errors within the digital workflow that are especially relevant to clinicians in their daily practice. The article by [PERSON] et al. [3] addresses the question of whether steam autoclaving impacts on the biomechanical properties of 3D-printed insertion guides. Autoclaving is necessary as the guides may be in contact with blood during orthodontic implant placements. The study compared two autoclaving cycles and different resin/printer combinations. For the protocol using a lower temperature and a longer autoclaving time, no biomechanical alterations could be observed, whereas in some groups, significant difference were noted for the faster autoclaving protocol at an increased temperature. The article by [PERSON] et al. [4] investigated whether the socket height of 3D-printed casts used to thermoform aligners has an impact on the aligner thickness and homogeneity. Thicker aligners would apply higher forces to the teeth, and varying thickness values might lead to inhomogeneous force applications across the aligners. Indeed, the study demonstrated that increased socket height was associated with thinner and more homogeneous aligners. Additionally, in all groups, the thickness values were highest at the incisal surfaces, and they were the lowest at the facial aspects, especially at the cervical margins. Future clinical studies are needed to assess the impact of local variation in the aligner thickness on the local force application and thus on treatment's success. The article by [PERSON] et al. [5] compared two 3D-printed bracket transfer trays with different shore hardnesses and assessed whether the crowding of the incisors impacts on the accuracy of indirect bracket transfer. Additionally, the workflow was tested using two different methods, i.e., intraoral scanning and Micro-CT, whereby intraoral scanning was inferior as the shape of the brackets was very different from that of the reference. The study found minor linear deviations for the indirect bracket transfer, whereas angular deviations and deviations in the torque reached values that were above the limits specified by the American Board of Orthodontists. Finally, the deviations were more pronounced for crowded teeth. [PERSON] et al. [6] proposed the fabrication of Polymer Polyether Ether Ketone (PEEK) transpalatal arches designed in a CAD process. The forces and moments of two transpalatal arch designs were within a range that appears to be clinically useful for expansion or for anchorage purposes.