Previous studies have shown that conventional hardness measurements are load-dependent. Hardness values obtained at low loads are usually very high, while at higher loads the hardness becomes constant. Objective: To determine the effect of indentation load on the Vickers hardness of three types of ceramic materials: a lithium disilicate-based ceramic (OPC 3G), a mica-based glass-ceramic (Dicor MGC) and a zirconia-based ceramic (Cercon zirconia). Soda lime glass was used as control. Method: Specimens of each material (n=5) were embedded in epoxy resin; polished; then indented at loads ranging between 1.96 and 98.1 N with a Vickers hardness tester (n=10 per load). For better accuracy indentation diagonals were measured directly from digital optical micrographs that were taken immediately after indentation. Hardness values were then calculated for each material. Results: ANOVA and Tukey's test showed that above a certain load, the hardness values were not significantly different for each material. The load corresponding to the transition point in the hardness vs. load curve was 4.9 N for both soda lime glass and the mica glass-ceramic, 19.6N for the lithium disilicate ceramic and 29.4N for the zirconia-based ceramic. Hardness values corresponding to the transition point were then compared between materials. The mean hardness of the zirconia-based ceramic (13.6 GPa at 29.4N) was significantly higher than that of the lithium disilicate-based ceramic (4.90 GPa at 19.6N), which was greater than that of the mica-based glass-ceramic (2.98 GPa at 4.9N) (p<0.05). The mean hardness obtained for soda lime glass compared favorably with previously published data (5.02 GPa at 4.9N). Conclusions: Statistically significant differences were observed for each material between the mean Vickers hardness at low loads and the mean Vickers hardness at high loads. The load needed to reach a constant hardness value varied with the material tested. Supported by NIH-NIDCR Grant No. DE 13570

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