# Transfer Accuracy of Two 3D Printed Trays for Indirect Bracket Bonding--An In Vitro Pilot Study
## 4 Discussion
Deviations in torque were greater than in angulation. Especially when brackets were bonded with hard trays, values were frequently greater than \(2^{\circ}\) and also clearly higher as reported previously [20, 21, 29]. Besides the digitization method and measurement workflow, reasons for the greater angular deviations may also owe to the design of the trays, handling, crowding, and material properties, as discussed below. Few studies investigated the transfer accuracy of 3D printed transfer trays. [PERSON] et al. compared different variants (3D printed trays for each single tooth versus 3D printed trays for the whole dental arch) with two types of double-layer vacuum formed trays and found no significant differences [22]). However, this study performed caliper measurements in a single direction only. [PERSON] et al. found that 3D printed trays provided better transfer accuracy compared to vacuum-formed ones, and in line with the present investigation, linear control was superior to angular control, whereas angular values also exceeded the ABO-ranges [20]. In contrast, satisfactory angular control was reported for an L-type design of 3D printed trays that was employed in an in vivo study by [PERSON] et al. [29]. This approach is of particular interest, as it provided a positioning template. This allowed a combination of direct bonding and a-priori planning of bracket positions. When comparing 3D printed with conventional silicone trays, [PERSON] et al. reported that conventional silicone enabled a more accurate bracket transfer [24], and accurate bracket transfer using silicone trays was also reported by other authors [18, 38]. The high precision of this conventional laboratory workflow may owe to the material properties of silicone. Additionally, brackets are fully covered by the tray which may prevent bracket movement during transfer. In contrast, due to material properties of the 3D printed transfer trays, no full coverage of brackets was possible, and the trays overlapped the slots by 0.04 mm only. This design resulted in a difficult handling specifically at front teeth and canines in the presence of crowding because brackets were located remarkably close to each other. Additionally, placement in hard trays was particularly challenging as brackets had to be pressed with the fingertips into the tight molds. Whereas bracket placement was easier in the soft tray group, brackets did not remain stable in their position when minor deformations of the soft tray occurred. This might explain the more gingival placement of molar tubes in the presence of severe crowding. To the best knowledge of the authors, no previous study investigated the impact of crowding and hardness of the bonding tray on bracket transfer accuracy. Against our a-priori assumptions, linear bracket transfer accuracy was slightly but significantly higher in the crowding group. Interestingly, crowded front teeth seemed to improve the vertical fit of the trays, thus prohibiting a gingival bracket displacement. However, when hard transfers were utilized the tight fit of the transfer tray did not prevent deviations in torque, which were most pronounced at front teeth and canines in the present of crowding. The authors suspect that this might be explained by increased tension due to severely crowded incisors resulting in deformation of the lower part of the hard tray. In the presence of crowding bonding with the hard tray further resulted in increased angular deviations at front teeth, canines and premolars. One of limitations of the present study is that cases with mild crowding (LII 3-7) were not included since the present study aimed at assessing whether crowding has an overall impact. The fact that the present study reports in vitro findings, and that the micro-CT workflow cannot be established for clinical trials, needs to be considered as another limitation. Additionally, the present study did not evaluate different designs in terms of thickness, extension and bracket covering, and further materials.