Comparison of Mechanical and Color Properties of Lithium Disilicate (IPS e.max ® CAD) Obtained by Microwave and Conventional Oven Crystallization
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The CAD/CAM restorative technique has become increasingly popular in the dental profession. Lithium disilicate (emax CAD) is frequently employed as a material of choice due to excellent material and esthetic properties. One of the final steps in the fabrication process of this restoration is the crystallization process which requires a heating phase to convert partially crystallized lithium metasilicate to lithium disilicate crystals (LS 2 ). Conventional ceramic ovens transfer heat to the ceramic by conduction heating from the outside-in. Microwave processing ovens represent a system that combines radiating conduction heating elements with microwave heating, resulting in volumetric heating. The advantages of heating ceramics with a microwave oven include greater energy efficiency, faster sample heating, more uniform heating, and improved material mechanical and optical properties. Purpose: This study compares mechanical and color properties of lithium disilicate crystallized in a microwave furnace with that fired in a conventional furnace. Material and Method: Pre-crystallized lithium disilicate blocks were milled into 12 mm diameter by 1.4 mm thick discs (ISO:6872:1995(E)). Specimens (N=10 discs) were crystallized in a conventional furnace for 7 minutes (manufacturer's recommendation) and a microwave furnace for three different test periods of time 2 minutes, 3.5 minutes and 5 minutes. Samples were polished then etched with 0.5% hydrofluoric acid for 1 minute and observed under scanning electron microscope. A spectrophotometer was used to evaluate the color properties. Biaxial strength tests and Vicker's hardness test were employed to evaluate the mechanical properties. One-Way ANOVA and Tukey HSD tests were used for statistical analyses. Result: Scanning electron microscope observation of samples crystallized in a microwave oven for 3.5 and 5 minutes showed similar crystal size and shape compared with conventionally crystallized samples. CIELAB values were calculated from spectrophotometer observations. ΔE values determined between conventional oven crystallization samples and samples crystalized for 2 minutes, 3.5 minutes and 5 minutes in a microwave oven were 12.70, 1.19 and 1.33 respectively. There was no statistically significant difference in biaxial strength among lithium disilicate samples (F 3,36 =.535, P=.661) for the microwave or conventional crystallization. A significant difference in surface hardness (Vicker's hardness) was found (F 3,36 = 3.917, P=.016). Follow up tests showed a significant difference in surface hardness between 2 minutes crystallization in microwave oven samples (5719 ± 147 Mpa) and conventional oven crystallization samples (5882 ± 144 Mpa) (p=.38) as well as with 3.5 minutes (5887 ± 95 MPa). (p= .31) and 5 minutes (5873 ± 119 MPa) ( p=.054) crystallization in microwave samples. Conclusion: Lithium disilicate specimens crystallized in a microwave oven for 3.5 minutes and 5 minutes have similar micro-crystal morphology compared with conventionally crystallized samples under scanning electron microscope observation. Lithium disilicate specimens, crystalized for 3.5 and 5 minutes in a microwave oven, produced a minimal ΔE value when compared to lithium disilicate specimens that were crystalized in a conventional furnace. The ΔE values obtained are in clinically acceptable range. Lithium Disilicate specimens crystallized in a microwave furnace for 3.5 minutes and 5 minutes had no statistically significant difference in biaxial strength and hardness when compared to the specimens that were crystallized in a conventional furnace for 7 minutes. From this study, it was determined that microwave crystallized lithium disilicate ceramic has equivalent mechanical and color properties compared to conventional crystallized ceramic material but required about half of the crystallization processing time.