Effect of Multiple Firing Cycles on the Physical Properties of Three Pressable Lithium Silicate Glass Ceramics

Abstract

The increased esthetic demands of nowadays patients led to increase in use of lithium silicates glass ceramics in dentistry. During laboratory procedures glass ceramic dental restorations may undergo multiple firing cycles to improve the esthetic outcome by glazing, staining or surface layering. These firings influence the optical and mechanical properties of material which in turn may affect restorations clinical results. This in-vitro study was designed to evaluate the effect of multiple firing on translucency and biaxial flexural strength of three different lithium silicate glass ceramic materials. A total of 72 disc shape specimens (10mm diameter and 1mm thickness) were fabricated by heat pressing of LTA1 shaded ingots. 24 specimens of Celtra press, and the same number of IPS e.max press and Livento press lithium silicate materials. Each material group were subdivided in to three subgroups (8 samples per group) according to the number of firing cycles. One, three, and five firing cycles’ protocols were constructed, to simulate the firing protocols of the three common laboratory fabrication techniques (staining, cutback, and layering) of glass ceramic restorations. CIELAB system was introduced in 1978, to defined color in terms of 3 coordinate values (L*, a*, and b*). L* refers to the lightness coordinate and its value ranges from 0 (black) to 100 (white). The values of a* and b* represent the chromaticity coordinates, where a* represents the red (positive value) or green Chroma (negative value), while b* represents yellow (positive value) and blue Chroma (negative value). Since then CIElab color coordinates were extensively used in dental research to measure color.