Knoop Hardness of Different CAD/CAM Crown Materials

Objectives: This study determined the surface hardness of a group of CAD/CAM crown materials representing different categories of blocks: glass-ceramic, porcelain, zirconia and resin composite.
Methods: Rectangular specimens 2 mm thick were cut from CAD/CAM blocks: Lava Plus High Translucency Zirconia (3M/ESPE)(LPZ), IPS e.max-CAD (Ivoclar-Vivadent) tested before(E-max-U) and after (E-max) crystallization, Celtra-Duo (Dentsply) unfired(CD-U), Vita Mark-II (Vident) (VMII), Lava-Ultimate(3M/ESPE) (LU) and Vita-Enamic (Vident) (VE). Specimens were highly polished using a special diamond-particle loaded polishing paste. Specimens were then subjected to surface microhardness testing in a microhardness tester (Tukon, Page-Wilson Corp.) with a Knoop indenter. Load was selected according to the material being tested and ranged from 100-800g. Five indentations were made on each specimen and 2 specimens were tested per material (n=10). Load was maintained for 15s and indentation length was measured under microscope. Knoop Hardness Number (KHN), means and SDs were determined. Data were statistically-analyzed using ANOVA and Games-Howell multiple comparison tests (p = 0.05).
Results:
Means and SDs of KHN for LPZ, E-max, E-max-U, LU, VMII, CD-U and VE were: 3,269(427), 1,331.7(49), 795(62.2), 119.5(9.24), 1,301(47.9), 499.5(86.8) and 232.2(14.5), respectively. ANOVA revealed significant difference among means (p<.0001). LPZ had significantly highest mean KHN(3,269) while LU had the lowest mean KHN(119.5).
Conclusions: Mean KHN values of LPZ, E-Max, E-Max-U, VMII and CD-U were higher than mean KHN value reported for enamel(343), while mean KHN values for VE and LU were lower than mean KHN value reported for enamel but greater than mean value reported for dentin(68). Significance of extremely high mean KHN for some CAD/CAM blocks in terms of potential for excessive abrasion of opposing dentition warrants further investigation.