## 4 Discussion  
Digital workflows and computer-aided design (CAD) are transforming the daily work in all dental disciplines. In orthodontics, they enable the provision of individualized appliances designed specifically to the patient's needs. Owing to the advancement of computer-aided manufacturing (CAM), new materials for 3D printing or 5-axis milling are becoming available [32]. Most of the printable polymers currently available on the dental market are limited to temporary use in the mouth. In orthodontics, they are utilized for insertion guides of orthodontic mini-implants [33; 34] or surgical splints for orthognathic surgery [35]. Fully digitally planned and CAM-fabricated appliances made of printed or milled materials have been described in the literature. They are utilized for active treatment and retention purposes [11; 12; 13; 36; 37; 38]. The biocompatible high-performance polymer polyether ether ketone (PEEK) that was analyzed in the present study has been used in dentistry for dental implants [18; 19; 20] as well as for permanent and removable dentures [14; 15]. In the orthodontic field, PEEK has been tested for the manufacturing of removable and fixed space maintainers. Owing to the low modulus of elasticity, it has also been tested for orthodontic wires made of or covered by PEEK [23; 24; 25]. Therefore, the present study investigated whether PEEK could also be suitable for the CAM fabrication of transpalatal arches. In contrast to the traditional transpalatal arch made of stainless steel or TMA wire, the use of digital design programs enables the user to create customized and precise treatment appliances for specific needs. The TPA geometries presented in this article reflect the wide range of design possibilities. PEEK, in contrast to stainless steel (160 GPa) or TMA (80 GPa), has a significantly lower Young's modulus of only 3 to 5 GPa. When employed as a material for orthodontic purposes, forces and moments of much smaller magnitudes can be expected. Accordingly, the activation range of the TPAs made out of PEEK that were measured in the present study was considerably higher compared to values previously reported for TPAs made of stainless steel (maximum activation \(<\) 3 mm) or TMA (maximum activation \(<\) 5-6 mm). Up to an activation of 6 mm, none of the TPA geometries presented in this study exceeded the recommended force limit of 2.5 N [5]. The _S-bow TPA_ and the _Omega-loop TPA_ generated expansion forces below 1.5 N up to an activation of 8 mm, implicating that a further activation would be conceivable for the respective geometries. Out of the TPAs tested in this study, the _Double-arch TPA_ and the _Triple-loop-arch TPA_ generated the highest forces, which were in the range of 2.5 to 3.0 N and appeared appropriate for orthodontic purposes. Among the two, the _Double-arch TPA_ showed rather low moments and could thus be well suited for expansion or--possibly with an enlarged or modified cross section--also as an anchorage unit. The _Triple-loop-arch TPA_ generated similar forces but higher moments (rotation \(-6.6\) Nmm) and may therefore also be suited for molar derotation. The _S-bow TPA_ generated lower forces and higher moments (rotation 3 to 6 Nmm), which may in principle be eligible for orthodontic treatment. However, the FEA revealed that the two \"S\" arches might end up in direct contact upon higher activation, which has to be considered in the case of clinical application. The _Omega-loop-arch TPA_, with its large spring, generated very low forces (about 1.0 N) and may therefore not be useful for expansion or for anchorage. The traditional TPA made of stainless steel or TMA is usually connected to the molars via palatal attachments. This plug-in connection has the advantage that the palatal arch can be easily removed and re-inserted for activation.