## 4 Discussion
For horizontal positioning, trueness RMSE values of 71.30 um [SD \(\pm\) 2.11 um] (OrthoClear, NextDent, Soesterberg, The Netherlands), 101.99 um [SD \(\pm\) 4.79 um] (Freeprint Split, Detax, Ettlingen, Germany) and 77.17 um [SD \(\pm\) 3.22 um] (V-Splint, VOCO, Cuxhaven, Germany) were achieved using different DLP printers [17]. SLA resulted in RMSE values of 76.61 um [SD \(\pm\) 4.46 um] (Dental LT, Formlabs, MA, USA) [17]. The DLP printer investigated in the present study revealed better or similar results and was competitive to the SLA printer. [PERSON] et al. also presented precision RMSE values of 31.12 um [SD \(\pm\) 3.25 um] (OrthoClear, D20 II), 41.22 um [SD \(\pm\) 8.27 um] (Freeprint Split, D20 II), and 58.22 um [SD \(\pm\) 3.53 um] (V-Splint, SolFlex 350) for horizontal positioning. The SLA printer revealed precision RMSE values of 51.80 um [SD \(\pm\) 8.01 um] (Dental LT, Form 2) [17]. The vertical orientation presented RMSE values of 19.42 um [SD \(\pm\) 2.62 um] (OrthoClear, D20 II), 19.47 um [SD \(\pm\) 2.82 um] (Freeprint Split, D20 II) and 24.40 um [SD \(\pm\) 2.48 um] (V-Splint, SolFlex 350), and 109.20 um [SD \(\pm\) 13.50 um] (Dental LT, Form 2) were described [17]. The study of [PERSON] et al. revealed that a different orientation of the printed object may lead to different trueness and precision values [17]. In our study, precision RMSE values were 19 um [SD \(\pm\) 10 um] for [2.5_INTERNAL] and 31 um [SD \(\pm\) 13 um] for [4.5_INTERNAL], so that the precision performed was comparable or better. Studies evaluating the accuracy of CAD/CAM occlusal devices are limited. However, there are remotely comparable investigations focusing on the accuracy of denture bases [42]. [PERSON] et al. assessed the trueness of the intaglio surfaces of milled (MIL) and printed (DLP) mandibular complete denture bases [43]. They revealed negative and positive mean trueness values of 93 um [SD \(\pm\) 10 um] and \(-\)68 [SD \(\pm\) 10 um] for DLP and 21 [SD \(\pm\) 4 um] and \(-\)43 um [SD \(\pm\) 4 um] for MIL. In the present study, the intaglio surfaces showed an absolute mean trueness of 59 um [SD \(\pm\) 5 um] for [2.5_INTERNAL] and of 80 um [SD \(\pm\) 9 um] for [4.5_INTERNAL]. The comparison of the values of the present study with those of [PERSON] et al. have to be considered very carefully. Although mandibular denture bases have a similar extension to occlusal devices, their intaglio surface is less structured. Different test methods could have been applied in detail. When comparing the results for printed denture bases and printed occlusal devices, the transparency of the respective resin must be addressed [42]. Transparent resins can affect printing resolution due to the absence of absorbing pigments [42]. The depth of cure, determining the z-resolution, is influenced by the photointiator, pigments, dyes, or added UV absorbers, as well as exposure conditions such as wavelength, power, and exposure time/velocity [31]. UV absorbers are required when printing transparent materials [31]. Next to material qualities, there are other parameters which might compromise printing results. The orientation on the printer platform, layer thickness, the post-processing method, exposure times, x, y resolution, z resolution, detaching forces and printing technology must be acknowledged [17, 45, 46]. Thermal expansion or contraction during polymerization can cause dimensional errors, too, since these factors define the degree of polymerization shrinkage [44]. During post-processing additional polymerization using UV light and heat can result in further shrinkage with the occurrence of warping [45, 46]. [PERSON] et al. analyzed the three-dimensional accuracy within the \(x,y,\) and \(z\)-axis [45]. Based on their findings, deviations of the \(z\)-axis were generally smaller than those in the \(x\)- and \(y\)-axis [45]. They stated that the \(z\)-axis is stationary during material application and movesto the next layer when printing pauses which they considered as a possible explanation [45].