Effect of Surface Treatment on Load to Failure of Lithium-disilicate

Objectives: To examine the effect of occlusal grinding and glazing on the fracture load of CAD/CAM lithium-disilicate glass-ceramic crowns. Methods: A standard maxillary 1st molar crown preparation was scanned (inEos, Sirona) and the crown designed (inLab3D, Sirona). Forty-five IPS e.max CAD (Ivoclar Vivadent) crowns were milled (inLab MC XL, Sirona) and arbitrarily assigned to one of the following three groups (n=15): (1) Group 1: no occlusal adjustment; crystallization/glaze firing (control); (2) Group 2: occlusal adjustment and polishing; crystallization/glaze firing; and (3) Group 3: occlusal adjustment and polishing; crystallization firing without glaze. Occlusal grinding was performed by hand on Groups 2 and 3 in the pre-crystallized state using a fine diamond bur (60µm grit) at 15,000 rpm and a series of ceramic polishers at 12,000 rpm (coarse, medium, and fine) (CeramiPro Dialite, Brasseler USA) and a silicon carbide brush with polishing paste (Diashine, VH Technologies). Glaze paste (IPS e.max CAD Crystall./Glaze, Ivoclar Vivadent) was applied to Groups 1 and 2, and all groups underwent crystallization firing in a porcelain furnace (Programat P500, Ivoclar Vivadent). Crowns were backfilled with composite resin (Tetric EvoCeram, Ivoclar Vivadent), and cured. Crowns were etched with hydrofluoric acid (4.9%, 20s), silinated 60s (Monobond Plus, Ivoclar Vivadent), cemented with a self-adhesive resin cement (SpeedCEM, Ivoclar Vivadent), stored in water (37°C, 1 week), and loaded in an Instron Universal Testing machine using a 15mm diameter steel ball at a crosshead speed of 0.5mm/min until fracture occurred. Results: The mean failure loads (N) were as follows: Group 1: 2,309±316; Group 2: 2,350±480; and Group 3: 2,309±322. No significant differences were found among groups (ANOVA, p<0.05). Conclusions: Occlusal adjustment of lithium-disilicate crowns in the pre-crystallized state followed either by (1) polishing and combination crystallization/glaze firing or (2) polishing and crystallization firing without glaze has no significant effect on fracture load.