Effect of Heat Treatment on Strength of Dental Glass-Ceramic Discs

Objective: To examine the effect on biaxial flexure strength by altering the heat treatment of the e.maxCAD by changing the closing time, heating rate and long term cooling.

Methods: Four groups were used for this study: (1) Control – manufacturer's instructions (2) Rapid Closing - manufacturer's instructions but the closing time is reduced from 6 min to 18 s (3) Increased Heating Rate - manufacturer's instructions except the initial heating rate is changed from 30ºC/min to 90ºC/min and (4) No long term cooling - manufacturer's instructions except no long term cooling is used. An ISO 6782 sized disc (1.2 mm in thickness and 15.5 mm in diameter) was scanned using the Cerec InLab (Sirona). Twenty specimens were milled for each group in four shades (five in each of A3, A3.5, B1 and BL2) using standard milling. The specimens were fired and glazed in one firing according to the firing parameters of each group in a P500 Programat oven (Ivoclar Vivadent). Specimens were tested using a universal testing machine (Instron Corporation) at a crosshead speed of 0.5 mm/min using a pin on three ball configuration. Mean biaxial flexure strengths were calculated and subjected to statistical analysis.

Results: The mean biaxial flexure strength values (MPa) of the groups were Group 1: 376.0.3±77.8, Group 2: 312.0±78.3, Group 3: 302.9±44.5 and Group 4: 331.4±71.1. No statistical difference was seen between the study groups using ANOVA (p≤0.05) due to the large standard deviation from no surface finishing after glazing, though a reduction in strength of 10 to 20% was measured by altering from the manufacturer's. instructions

Conclusion: Though no statistical difference between the heat treatment groups due to a large standard deviation. It is interesting to note a strength reduction of 10 to 20% was observed when deviating from the manufacturer's instructions for firing.