Effect of print layer thickness and finishing and polishing protocols on the flexural strength, cell adhesion, and surface properties of a 3D printed resin

Statement of problem: Evidence regarding standardization of finishing and polishing protocols for 3-dimensionally (3D) printed interim resins is sparse. Purpose: The purpose of this in vitro study was to evaluate the influence of print layer thickness and aging on the flexural strength, cell adhesion, and surface roughness of a 3D printed resin for interim crowns submitted to different finishing and polishing protocols. Material and methods: A total of 150 bars were printed (Flash Forge Foto 6.0), (25×2×2 mm) following the International Organization for Standardization (ISO) 4049 standard and divided into 10 groups (n=15) based on the print layer thickness (25 µm or 50 µm) and finishing and polishing protocols (FP) (C: control - no treatment; T: tungsten carbide bur; TG: T + glaze; TP: T + rubber polisher; and TPG: T + P + glaze). All specimens were then stored in distilled water at 37 °C for 180 days. The specimens were then subjected to a flexural strength test (980.6 N; 1 mm/minute). Weibull analysis, surface scanning electron microscopy (SEM), optical profilometry, and microbial adhesion analysis using Candida albicans were also performed. Data for σ (MPa) and roughness (µm) were analyzed using 2-way ANOVA followed by the Tukey post hoc test (α=.05). Data for microbial adhesion analysis were evaluated using the Kruskal-Wallis test. Results: For σ, the ANOVA revealed that the interaction between the factors was not significant (P=.088), however, factors the FP and print layer thickness were significant (P<.001). The groups 25 _C (99.58 MPa) and 50 _C (97.52 MPa) presented the highest σ, which were higher (P<.05) than T_25 (70.68 MPa) and T_50 (67.09 MPa), that showed the lowest strength among the experimental groups. For roughness, T_50 (0.068 ±0.030 µm) and T_25 (0.076 ±0.073 µm) groups obtained the highest average roughness, that were different between them (P<.05). Moreover, microbial adhesion analysis found that surface polishing procedures did not significantly affect microbial adhesion (P>.05). Conclusions: The print layer thickness of 25 µm and the glaze application after finishing are recommended to increase the material's strength and reduce roughness surface. In addition, after finishing with tungsten carbide drills, a glaze layer is indicated. Microbial adhesion was not influenced by the factors studied.