# Transfer Accuracy of Two 3D Printed Trays for Indirect Bracket Bonding--An In Vitro Pilot Study
## 4 Discussion  
The aim of the present in vitro study was to assess whether the hardness (shore A 85 vs. shore D83 -86) of 3D printed transfer trays and severe crowding impact on bracket bonding accuracy. Additionally, it was aimed to validate the workflow. In the literature, different procedures for digitizing study models, e.g., intraoral scanners, 3D-model scanners or cone beam computed tomography (CBCT), were described for assessing the accuracy of transfer trays in digital workflows [29; 30; 31; 32]. Matching intraoral scans (IOS) with the virtually planned reference models seems to be used most frequently and has the advantage that this workflow can be easily applied to clinical as well as in vitro settings, whereas microcomputed tomography (micro-CT) is limited to in vitro studies only. Nevertheless, full arch scanning in the presence of brackets might have an impact on scanning accuracy [33; 34] and could therefore affect outcomes achieved with this technology. Previous studies did not evaluate the accuracy of workflows for assessing the accuracy of transfer trays or did not indicate whether the whole process including image registration was repeated to assess reliability. In our study, ICC values for intraorally  
Figure 8: Boxplots detailing the accuracy of bracket transfer for angular measurements. Results are summarized for the canines, front teeth, molars, and premolars and split up into hard/soft trays and no crowding/crowding. scanned models indicated a very low reliability likely owing to significant artifacts at the scanned metal brackets and tubes. To some extent scanning accuracy may be impeded by the thin layer of scanning powder applied to the brackets, as scanning powder is suspected to increase scanning errors [35]. Nonetheless, distortion of the bracket area in intraoral scans was reported recently [36]. In the present analysis, the references and scanned surfaces showed a visible incongruence. Although only the apparently matching regions from the brackets were utilized for comparison in GOM software as described in [29], the intra- and interrater reliability ranged from excellent to poor in our experiment. Therefore, and despite having been used in the majority of previous experiments, the IOS approach was not found eligible to assess the accuracy of transfer trays in the present investigation. Instead, micro-CT scanning proved to be a highly reliable approach. Therefore, only the deviation measurements performed with this approach were reported. The micro-CT workflow revealed minor linear discrepancies between planned and achieved bracket positions ranging from \(-0.06\) to \(0.28\) mm (1 st-3 rd quartile). Interestingly, linear deviations were slightly but significantly lower in severe crowding situations, whereas no overall impact of crowding was identified. In the subgroup analysis, significant differences were most frequently observed at front teeth, where brackets were bonded more accurately in occlusal/gingival direction when severe crowding was present. In contrast, in crowding groups, tubes were located significantly more gingival when soft trays were utilized. Angular deviations were by trend higher when crowding was present, and torque discrepancies reached significance at front teeth. Bracket transfer with hard trays resulted in greater angular deviations compared to soft trays. The greatest impact of bracket hardness was noted on canines, where soft trays improved median angular discrepancies by \(2.16^{\circ}\) and \(3.03^{\circ}\) in no crowding and crowding situations, respectively. Regarding the amount of deviations, linear discrepancies \(\leq\) 0.5 mm and angular deviations of \(\leq\)\(2^{\circ}\) were considered to be acceptable as suggested by The American Board of Orthodontists (ABO) [37] and further studies [21, 32], despite some authors recommending more strict ranges [30, 38]. In the present analysis, linear discrepancies were within the ABO range of 0.5 mm, which is also in line with previous findings [20, 21, 29].