The effect of coffee thermal cycling on the microhardness, surface roughness, surface properties, and color stability of permanent crown materials manufactured via additive and subtractive techniques

The aim of this study is to comparatively evaluate the color stability, surface properties, surface roughness, and microhardness values of permanent restorations fabricated using CAD/CAM hybrid ceramic blocks and three-dimensional (3D) printer resins, whose usage has increased in recent years, after the application of coffee thermal cycling. In this study, four different crown resins used in additive manufacturing methods (VarseoSmile TriniQ, C&B MFH, CROWNTEC, Formlabs Permanent resin) and one CAD/CAM hybrid ceramic block, CeraSmart 270 (GC Corp., Tokyo, Japan), were employed. For the subtractively manufactured specimens, 10 mm diameter cylinders were digitally designed using Blender 4.0 software and exported in STL (Standard Triangle Language) format. The designs were milled using the inLab MC X5 milling unit and sectioned using a precision cutting device. For the additively manufactured specimens, 10 x 2 mm disks were designed in Blender 4.0 and exported in STL format. The NextDent 5100 printer, equipped with DLP technology, was used for fabricating the Nextdent C&B and Saremco Crowntec resins. The Bego VarseoSmile TriniQ resin was fabricated using the Asiga Ultra (50) 3D printer (DLP technology), and the Formlabs Permanent Crown resin was printed using the SLA-based Form 3B printer. Washing and post-curing procedures were carried out in accordance with the manufacturers' instructions. Color measurements of the prepared specimens were performed using a spectrophotometer. Surface roughness was analyzed using a profilometer and atomic force microscope (AFM), while surface characteristics were evaluated with a scanning electron microscope (SEM). Microhardness was measured using a Vickers microhardness tester. After the initial measurements, all specimens underwent thermal aging. The samples were placed in a thermal cycling machine and exposed to a coffee solution (prepared by adding one tablespoon of instant coffee to 177 mL distilled water in each tank) at temperatures alternating between 5°C and 55°C for 10,000 cycles. Post-aging measurements were conducted using the same instruments and parameters, and the pre- and post-aging values were compared statistically. The IBM SPSS Statistics 22 software was used for statistical analyses. The normality of the data distribution was evaluated using Kolmogorov-Smirnov and Shapiro-Wilk tests. One-way ANOVA was used for intergroup comparisons, with the Tukey HSD test employed for post hoc analysis. Paired samples t-tests were conducted for intragroup comparisons. The level of significance was set at p<0.05. There was a statistically significant difference in post-aging surface roughness among the material groups (p=0.001; p<0.05). The Formlabs group showed significantly higher roughness values compared to other groups (p<0.05). For all groups, the increase in roughness values after thermal aging compared to the pre-aging values was statistically significant (p=0.001; p<0.05). There was a statistically significant difference in post-aging microhardness among the material groups (p=0.001; p<0.05). The CeraSmart group exhibited significantly higher microhardness values compared to the other groups (p<0.05). In both the Formlabs and NextDent groups, the increase in microhardness values after thermal aging compared to the initial values was statistically significant (p=0.001; p<0.05). There was a statistically significant difference in mean ∆E values (color changes) among the material groups (p=0.001; p<0.05). The Formlabs group had significantly higher ∆E values than the other groups (p<0.05). Keywords: additive manufacturing, coffee thermal cycling, surface roughness, color stability, microhardness