The Effect Of Repeated Firings On The Color And Translucency Of The Lithium Disilicate Glass-ceramic

All-ceramic restorations have been advocated because of their outstanding esthetics,stable color,excellent compatibility in oral cavity and superb abrasive and corrosion resistance.They are more and more widely used in clinical.Glass-ceramic isa new kind of all-ceramic material.It consists of glass substrate and a variety of crystalline phase.Its color is more natural than ordinary all-ceramic material.In the process of production and modification,all-ceramic restorations often need multiple high temperature sintering.While studies have shown that repeated firings do have influence on the color of some kinds of all-ceramic materials,such as feldspathic porcelain,alumina ceramic,zirconia ceramic.However,there is lack of adequate support in the literature about the effect of repeated firings on lithium disilicate glass-ceramic.For this study we choose IPS e.max HT Press,which is one kind of lithium disilicate glass-ceramic commonly used in clinical,as the material and try to explore the effect of multiple firings on the color and translucency of it,hoping it will provide clinical reference for future work.Objective:The aim of this study was to investigate the effect of firing numbers on the color and translucency of the lithium disilicate glass-ceramic.To make a preliminary exploration of changes in the internal lithium disilicate glass-ceramic crystal structure after repeated firings.Methods:For this study,we choose IPS e.max Press from IPS e.max（?）all-ceramic system which is one kind of lithium disilicate glass-ceramic commonly used in clinical as the experimental material.Thirty wax patterns for the lithium disilicate specimens were made,10 mm in diameter and 1 mm in thickness,embedded in a phosphate-bonded investment（IPS Press VEST Speed）with a crucible provided by the manufacturer,and burned out in a furnace at 850 for 45 minutes.IPS e.max Press,high transluncency（HT）ingots were used to fabricate the disc-shaped ceramic specimens.The specimens were heat-pressed at 925℃ in the pressing furnace（IPS Empress EP5000）.The casting ring was removed from the furnace after heat-press and the investment was air cooled on theplatform.Then the specimens were divested using airborne-particle abrasion with50-μm glass beads.The tip of the micro-etcher apparatus remained about 1 mm away from the surface of each specimen and was applied for 5 seconds.A slow speed diamond grinding wheel was used to cut down the sprue and polish the edge of specimen.After that,the specimens were inlaid with self-curing acrylic resin fixed to a grinding and polishing machine,the surfaces of each side was progressively polished by aluminum sandpaper at 400 mesh,600 mesh,800 mesh,1000 mesh,2000 mesh,then by abrasive paste in 5μm,2.5μm,0.5μm.Finally the specimens are up to metallographic standards.Removed the specimen from the machine with acetone dissolved self-curing plastic.Successively,the specimens were ultrasonic cleaned in acetone and deionized water.Then dry the specimens with lens paper.The thickness of each specimen was then measured with a digital micrometer with an accuracy of 0.01 mm,and firings were performed in a dental ceramic furnace（Programat P700/G2）.The specimens were randomly divided into six groups,five each.The specimens’ color parameter（L^*a^*b^*）,△E and τ after 0,1,2,3,4,5 firings were measured and calculated by colorimeter and transmitivity test apparatus.Color difference AE were calculated in the following formulae:△E=[（△L^*）²+（△a^*）²+（△b^*）²]^1/2.One-way analysis of variance and Turkey’s multiplecomparision test were used to analyze the data（a=0.05）by SPSS 17.0 statistic software.Then two specimens from each group were selected to observe the crystal phase by XRD.After that FE-SEM was used to observe the surface morphology of specimen from each group.Results:With increasing number of firings,significant reductions in L^*（P<0.05）and increases in a^*,b^*,τ（P<0.05）were recorded.In other words,the samples became darker,redder,more yellow and more translucent.After repeated firings,the chromatic aberration（AE）ranged between 0.88-2.45.Values change incrementally,the minimumvalue appeared after first firing,while the maximum value came after five times firings.XRD test showed that the crystal phases were not changed after repeated firings,the main crystal phase was lithium disilicate,vice-crystalline phase were Lithium metasilicate and Lithium orthophosphate.The intensity of diffraction pattern of lithium disilicate was improved from 55000 to 100000.FE-SEM observation results: With the increasing number of firings,close-knit needle shaped crystals gradually changed into short virgate and even cylindrical and there emerged many spherical crystals.Conclusion:All in all,repeated firings affected the color and translucency of the lithium disilicate glass-ceramic.The effect might be related to the increased crystal content of main crystal phase and the glass matrix.After four times firings,the color of lithium disilicate glass ceramic changed obviously,therefore,we should try to control the number of firings for glazing and staining when lithium disilicate glass-ceramic are applied to the production of all-ceramic restorations in clinical.Moreover,we can adjust the color and translucency of restorations by controlling the number of firings in order to meet the individual requirements of patients.