## 4 Discussion
They also noted that some samples from the same manufacturers had convergent and others divergent walls, resulting in a variation in height between the slot entrance and exit. Similarly, [PERSON] et al. compared 100 different brackets from 10 different manufacturers, concluding that manufacturing anomalies can affect both individual brackets and entire series of brackets [28]. This disparity was reflected in Group 1 (conventional metal brackets) of this study, in which a direct comparison between the conventional metal brackets Primo and Legend Mini revealed different degrees of inaccuracy, whereas Group 2 brackets (self-ligating metal) were more homogeneous. While the direct comparison of metal and ceramic brackets (Groups 1, 2 and 3) revealed similar levels of inaccuracy, with no statistically significant differences, a comparison between groups revealed that in-house 3D-printed resin brackets (Group 4) presented significantly greater slot height accuracy as compared to conventional brackets. A curious aspect of the study presented here is that the IH3 DB slots were, in some cases, slightly undersized, with negative percentage differences from nominal values (\(-0.36\%\)). This aspect, never yet reported in the literature, seems to depend on contraction during the post-curing phase of the 3D printing process [29]. In fact, contraction rates of 6-10% have been reported by [PERSON] et al. [29], likely arising from thermal expansion and contraction phenomena during bracket production [30; 31; 32; 33]. Several other factors must be taken into account in order to limit this phenomenon, including exposure time, wavelength and power supply, as well as differences in filler particles and polymerisation methods [34; 35; 36]. That being said, [PERSON] et al. have shown that DLP technology printers are associated with fewer contraction phenomena than the others [28], despite a degree of contraction on both the X and Y axes, as compared to the Z axis [7], making the position of the object to be printed on the print plate fundamental. Despite the inherent limitations in the 3D-printing process, the findings from this study are encouraging. In fact, the analysis performed revealed that although there was a significant difference between the nominal and measured slot heights of the five types of commercially available brackets (conventional and self-ligating metal and ceramic) tested (\(p<0.05\)), the same could not be said for the 3D-printed brackets. Indeed, the slot heights measured for both IH3 DB1 and IH3 DB2 brackets were very similar to those included in the CAD design. Moreover, this reproduction accuracy was reflected across the sample of IH3D brackets, as evidenced by the very low standard deviation. This compares favourably with the commercially available brackets, which all present oversized slots and inconsistent measurements within the same sample, and indicates that, with current technologies, 3D printing is a precise and reproducible means of manufacturing custom brackets. As the 3D printing of resin brackets is of very recent application, the literature on the subject is still very scarce. However, the results obtained appear to be very encouraging, as no obvious differences were found when using a printer at 50 \(\upmu\)m resolution or 100 \(\upmu\)m on the Z axis with the models reported herein. Further studies should aim to investigate the printing accuracy of other professional printers used in dentistry. Other aspects of IH3D brackets should also be investigated, including wear resistance, the quantification of frictional forces generated with the archive, the in vivo and in vitro testing of the colour stability, and any deformation following the application of torque bends.