| Glass ceramic is a kind of multiphase solid material obtained from host glass after crystallization,which is constituted by the crystal phase and glass phase.It has possessed the advantages of both ceramic and glass.Lithium disilicate ceramic(LDC)has excellent mechanical properties,such as high strength,good resistance against wear,and optical properties that are more matched with tooth tissue,which can be attributed to its unique crystalline nature and distribution characteristics.Hence,this glass ceramic occupied an important post in dental all-ceramic restoration materials.Our research group has carried out a large amount of basic research on the crystallization mechanism,computer colouring and hot pressing technique of LDC,and has successfully developed the experimental LDC(ELDC)with favorable mechanical strength and colors that can satisfy the requirement of clinical color matching,which has displayed broad application prospect.However,the ELDC developed in our previous research showed only single translucence,which can not satisfy the increasing demands for aesthetic restoration as well as the different demands for masking different abutment conditions.Additionally,prior to the clinical application,essential basic research should be carried out on its adhesive property,fatigue characteristic and surface treatments.This project is the extension of our previous research,which mainly aims to study the relationships of influencial factors,such as addition of various coloring materials,production processes,and material thicknesses,with the translucency and microstructure of ELDC,and to optimize the translucent characteristics of the novel material,thus providing experimental evidence for material selection under various clinical abutment conditions.Meanwhile,our study also attempts to investigate properties related to clinical application,such as the adhesive property and fatigue characteristic of that material,so as to lay a foundation for the early application of this novel material into the dental restoration.Part Ⅰ Optimization of the translucent characteristics of the novel ELDC Experiment 1.Effects of addition of CeO2 and TiO2 at various ratios on the translucence of the novel ELDC.Various contents of CeO2and TiO2 were added into the glass raw materials;to be specific,group A contained 0.5wt.%CeO2,group B contained 0.5wt.%TiO2,group C contained 0.5wt.%TiO2+ 0.5wt.%CeO2,group D contained 0.75wt.%TiO2+ 0.75wt.%CeO2,and group E contained 1.0wt.%TiO2+1.0wt.%CeO2.The ELDC samples were prepared using the melting method.In addition,the transmissivity was measured using visible spectrometer,phase composition was analyzed using X-ray diffraction analysis,and the effects of coloring material addition on the microstructure were observed through the scanning electron microscope(SEM).The results indicated that,glass ceramic prepared using the melting method had fine and smooth texture,with high surface gloss.Addition of various contents of CeO2 and TiO2 could change translucence of ELDC,but had no obvious influence on phase composition.Group C(0.5wt.%TiO2+ 0.5wt.%CeO2)had the highest transmissivity,while group A had the lowest transmissivity.Meanwhile,the transmissivity was gradually reduced with the increases in CeO2 and TiO2 concentrations.Experiment 2.Effects of production process and thickness on the translucence of the novel ELDC.This experiment had analyzed the relationships of production processes(sintering process and melting method)and material thickness on the translucence of ELDC.In brief,disk samples of various thicknesses(0.5mm,0.8mm,1.0mm,1.2mm and 1.5mm,n=5)were prepared using sintering process and melting method,respectively.The translucence parameter of samples was measured using the Shadepilot digital colorimeter,and the microstructure of samples was observed under SEM.The results suggested that,the TP values in both groups were remarkably reduced with the increase in thickness(melting group:36.0-23.7;sintering group:19.3-8.58).Typically,the mean TP values in melting group at all thicknesses were higher than those in sintering group(P<0.05).SEM resutls indicated that,the fracture surface in samples from melting group was even,with high density while little pores,while micropores could be observed on the fracture surface of samples from sintering group.Experiment 3.Comparisons of the translucent characteristics and strength of ELDC with different transparencyIn this experiment,the translucence parameter(TP)and biaxial flexural strength(BFS)of ELDC materials with various transparency were compared and analyzed,including LDC for hot pressing(IPS e.max Press,4 kinds of translucencies),ELDC(2 kinds of translucencies)and LDC for CAD/CAM(IPS e.max CAD,2 kinds of translucencies).Samples from each group were prepared for BFS tests,and the TP values were tested at the thicknesses of 0.5mm and 1.0mm,respectively.The crystal structure and microstructure were analyzed using X-ray diffraction and SEM.The results revealed that,the BFS values of two hot pressed LDC were markedly higher than that of CAD/CAM glass ceramic.However,the difference in the BFS value between two hot pressed glass ceramics was not statistically significant.No significant difference was observed in the crystal composition among the tested glass ceramics,but IPS e.max CAD had relatively small crystal morphology.Part Ⅱ:Research on properties of new glass ceramic related to clinical application Experiment 4.Effects of surface treatments on the bonding strength between the ELDC and resinIn this experiment,the effects of different surface treatment processes on the bonding strength between ELDC and resin were investigated.The samples were divided into 7 groups(n=10)according to the surface treatment methods,including sand blasting+ acid etching(20s)+ coupling,sand blasting+ acid etching(40s)+ coupling,sand blasting+ acid etching(60s)+ coupling,acid etching(20s)+ coupling,acid etching(40s)+ coupling,acid etching(60s)+ coupling,and sand blasting+ coupling.Moreover,the resin bond system was used in bonding the specimens for shearing strength tests.The specimens were stored in the 37℃ constant temperature water bath for 24h,followed by 10000 thermal-cold cycles at 5-55.5℃.Afterwards,shear tests were conducted on the universal testing machine,and fracture appearance was observed using SEM.The results indicated that,the sand blasting+ coupling group had the lowest bonding strength(13.04±2.06 MPa),and the difference was statistically significant(P<0.05).No differences were seen in the bonding strength among the remaining 6 groups(P>0.05).To sum up,acid etching(20s)+ coupling could serve as the recommended clinical surface treatment for ELDC.Experiment 5.Fracture resistance of ELDC crowns with different thicknesses and failure model analysis.Full crowns(6 in each group)at various thicknesses(0.5mm,0.8mm,1.0mm,1.2mm and 1.5mm)were prepared from IPS e.max press(EMAX)and ELDC,respectively.The crowns were bonded onto the PMMA resin abutment using composite resin,followed by storage in the 37℃ constant temperature water bath for 24h,and 10000 thermal-cold cycles at 5-55.50C.Afterwards,the fracturing load was tested on the universal testing machine,and fracture was analyzed using stereoscopic microscope and SEM.The results indicated that,the F(u)values of all EMAX crown at various thicknesses ranged from 685 to 1827N,while those of all ELDC crown ranged from 700 to 1791N.In addition,the F(u)value was increased with the increase in thickness.Difference in F(u)value between ELDC crown and EMAX crown of the same thickness was not statistically significant.As a result,thickness might have important influence on the fracture resistance of ELDC.Experiment 6.Preliminary study on the bonding of the novel ELDC to zircoia using hot pressing techniqueThis experiment had analyzed the shear bond strength of zirconia ceramic with 4 veneering ceramic materials as well as the failure type.The samples were divided into different groups according to different veneering ceramic materials and production processes,including ZIRP group(IPS e.max ZirPress,hot pressing technique),CAD-ON group(IPS e.max CAD,CAD-ON technique),CERAM group(IPS e.max Ceram,slip casting technique)and ELDC group(ELDC,hot pressing technique),with 8 samples in each group.All test specimens were stored in the 37℃ constant temperature water bath for 24h,followed by 10000 thermal-cold cycles at 5-55.5℃.Afterwards,shear tests were conducted on the universal testing machine,and the failure strength was recorded.In the meantime,fracture was analyzed under stereoscopic microscope and SEM.The results suggested that differences among the four groups were statistically significant,with the lowest mean being seen in CERAM group(10.03±2.06 MPa).In addition,there was no statistical difference between ELDC group(17.10±5.51 MPa)and CADON group(19.56±2.99 MPa),both of which were markedly higher than CERAM and ZIRP groups(13.68±5.06 MPa).These results had verified the possibility and potential advantage of applying ELDC on zirconia surface for veneering. |
