Effect of printing layer thickness on the trueness of 3-unit interim fixed partial dentures

Statement of problem. Three-dimensional printing has facilitated the fabrication processes in dentistry. However, knowledge on the effect of layer thickness on the trueness of 3D printed fixed partial dentures (FPDs) is lacking. Purpose. The purpose of this in vitro study was to compare the effect of printing layer thickness on the trueness of 3-unit interim FPDs fabricated by using additive manufacturing with that of those fabricated by subtractive manufacturing. Material and methods. The right first premolar and first molar teeth of a dentate mandibular model were prepared for a 3-unit restoration and then digitized by using an intraoral scanner. A 3-unit interim FPD was designed to fabricate 40 restorations by using either the additive (NextDent C&B MFH) with layer thicknesses of 20 mu m (n=10), 50 mu m (n=10), and 100 mu m (n=10) or subtractive manufacturing technique (Upcera) (milled, n=10). After fabrication, the interim FPDs were digitized by using the same intraoral scanner and were superimposed over the design data by using a 3D analysis software program. Root mean square (RMS) was used to analyze the trueness of the restorations at 4 different surfaces (external, intaglio, marginal area, and intaglio occlusal) and as a complete unit (overall). Data were analyzed with the Kruskal-Wallis and Wilcoxon tests with Bonferroni correction (a=.05). Results. The 100-mu m-layer thickness interim FPDs showed the greatest overall (P<.015), external (P<.021), and intaglio occlusal (P<.021) deviations, whereas the milled interim FPDs showed the lowest (P=.001). No significant differences were found among the test groups for marginal RMS (P >=.108). The differences between the 50-<mu>m-layer thickness and 100-mu m-layer thickness interim FPDs for the intaglio surface deviations (P=.064) and between the 20-mu m-layer thickness and 50 mu m-layer thickness interim FPDs for each surface tested were not statistically significant (P>.108). Conclusions. The printing layer thickness had a significant effect on the trueness of the additively trueness than those additively manufactured, regardless of the printing layer thickness.