Effect of acidic solutions on surface microhardness of CAD/CAM ceramic materials with different finishing protocols

This in vitro study intended to evaluate the effects of two distinct finishing protocol, mechanical polishing and glazing, on the microhardness changes of various dental ceramics with different compositions following exposure to an acidic pH. A lithium disilicate glass-ceramic (IPS e.max CAD, IPS), a polymer-infiltrated ceramic network (Vita Enamic, VE) and two resin-matrix nanohybrid materials (GC Cerasmart, GC; Lava Ultimate, LU) were tested (168 specimens; n = 7 per group). The specimens were separated into two subgroups according to the type of finishing protocol used: glazing and mechanical polishing. Subsequently, the specimens were immersed in artificial saliva (control), citric acid, or Cola for 168 h at 37 °C. Surface microhardness was measured using a Vickers hardness tester (1000 g load, 15 s dwell time) both before and after immersion. The within-specimen change (log of the post/pre ratio) was analyzed with a repeated-measures model (material × solution × finishing protocol) with heteroscedasticity-robust tests; pairwise contrasts used Welch tests with Holm adjustment. Baseline-adjusted ANCOVA and an analysis without outlying specimens were sensitivity analyses. Normality was assessed with the Shapiro-Wilk test and homogeneity of variance with the Brown-Forsythe test (α = 0.05). Results. IPS was harder (595.6–672.3 HV after immersion) than the resin-matrix materials (134.4–382.1 HV). Hardness decreased overall by 10.7% (95% CI 8.3–13.2%); group-level changes ranged from − 32.2% to + 2.1%, and the adjusted mean change was smallest for IPS (− 4.3%) and largest for LU (− 15.9%). The change depended on material ( p = 0.013) and solution ( p = 0.018; saliva − 6.6%, citric acid − 10.3%, Cola − 15.2%) but not on the finishing protocol ( p = 0.31). Hardness also decreased in artificial saliva. After Holm adjustment no acid-versus-saliva contrast was significant (smallest adjusted p = 0.10, VE glaze-based protocol in Cola). Glaze-based finishing resulted in lower baseline hardness than polishing ( p < 0.001), and the only significant protocol contrast in the change was in VE exposed to Cola (− 25.6%, adjusted p = 0.035). Three outlying specimens influenced some results. Within the limitations of this in vitro study, lithium disilicate showed higher hardness and the smallest relative change after 168 h of immersion. Hardness decreased also in the control, solution effects were small relative to variability and mostly not significant after adjustment for multiplicity, and the finishing protocol did not generally alter the change in hardness. These findings are exploratory and limited to the materials, protocols and conditions investigated.