| Dental ceramics has been used in clinical practice for several years. Even it has natural luster and good properties, there are some disadvantages in present ceramic products: high brittleness, non-ideal mechanical properties, high cost, et al. Natural leucite has been used to reinforce dental ceramics in some foreign commercial products such as IPS-Empress castable ceramic system and this product has been accepted by clinical application for its adjustable color series and higher compressive strength. Before that, leucite was mainly used to adapt porcelain fused to metal for its high thermal expansion coefficient. As a kind of ore, leucite has several properties such as near-tooth color, glass luster, suitble hardness to be used in dental ceramics. We have researched some thermal treatment methods leading to leucite microcrystallization because the natural leucite mine is rare to exploit in China.In this paper, a thermal treatment technology which can make leucite nuclear and microcrystalize in glass matrix was studied based on some primary studies, meanwhile the factors that affected the properties of the glass ceramics were investigated. Then, this kind of K2O-Al2O3-SiO2 system dental ceramics reinforced by leucite microcrystals was fabricated by the new technology and the ceramic samples were tested. The aim of this study was to investigate the key technology of microcrystallizing leucite to reinforce dental glass ceramics, test the main properties of the glass ceramics and research the marginal adaptation of this glass ceramics. The clinical application of the glass ceramics was also studied.Part I Study on Thermal Treatment Technology to Microcrystalize Leucite in Glass MatrixIn this part, two groups of the feedstock powders were selected from a group of designated raw materials according to the ratios of the theoretical composition of leucite and mixed, ball-milled, melted at 1500 ℃ and 1600 ℃ respectively. The results by polarizing microscope showed that 1600℃ could melt the feedstock powders to glass matrix well. Three groups of the same feedstock powders were heated to 1600℃ then cooled to 600℃ and kept for one hour, then heated again to 800℃,1000℃ and 1200℃ separately and kept for one hour. The results by polarizing microscope showed that there were bud-like leucitecrystals about 0.1 ^m grew in the glass matrix from the group of 1200°C. Two groups of the feedstock powders according to the ratios of the theoretical composition of leucite were selected, mixed, ball-milled, then heated to 1500°C and 1600°C respectively and kept for one hour. The results by polarizing microscope and X-ray diffractometer showed that all the samples had leucite crystals about 0.23Hm, but leucite crystals tended to gather in the group of 1600°C. One group of the feedstock powders alike the designated raw materials in the first experiment were mixed, ball-milled for 24 hours, melted at 1600°C and then cooled to 1000 °C quickly, then sintered at 1200°C for 1 hour ,and then at 1500°C for 1 hour. The results analyzed by polarizing microscope and X-ray diffractometer showed that leucite crystals about 0.8Mm dispersed in the glass matrix evenly, the volume of leucite was about 10% (Vol%). According to above experiments, a diagrammatic sketch was drawn out to express the thermal treatment system of microcrystallizing leucite.Part II Study on Some Properties of K2O-AI2O3-S1O2 System Dental Glass Ceramics Reinforced by Leucite MicrocrystalsThe feedstock powders were selected and mixed according to the ratios of the theoretical composition of leucite as which in Part I, ball-milled, melted at 1600°C and then cooled to room temperature quickly. The cooled clinkers were ball-milled again for 24 hours to 4Mm. After cold-isostatic pressure molded and air sintered at 1200°C for 1 hour , and then at 1500 °C for 1 hour, five C 10 X 10mm dental glass ceramics were formed. The products were analyzed by polarizing microscope and X-ray diffractometer. It could be seen that the crystals (the diameter was about 0.8Mm, the volume was about 50%) scattered in the glass matrix evenly, the main powder-crystal spectrum of the crystals was as the same as that of leucite. That is to say, the new type of K2O-Al2O3-SiO2 system dental glass ceramics reinforced by leucite microcrystals was formed by an appropriate thermal treatment technology from a group of designated raw materials. Tested by electro-hydraulic servocontrolled testing machine, the mean compressive strength was about 231.4MPa (which was 206.6MPa in primary study) and the mean density was about 2.28 g/cm3 (which was 2.17 g/cm3 in primary study).Part IH Study on Marginal Adaptation of K2O-Al2O3-SiO2 System Dental Glass Ceramics Reinforced by Leucite Microcrystals on Extracted Teeth16 non-caries, non-crack, extracted premolars were cleaned and then their root canals were obturated immediately. Subsequently, class V cavities were prepared in both buccal and lingual aspects of the teeth, the incisal margins of the cavities were in enamel and the cervical margins in dentin. Two groups of KaO-AfcCb-SiCb system dental glass ceramic inlays were fabricated (non-gap was made in the specimens of groupl, specimens of group2 had 200Mm luting spaces meanly). All the inlays in each group were cemented with Variolink? II composite luting cement. After being subjected to 5 and 55°C thermal cycling 200 times, the prostheses were immersed in Ringer's solution for two months at room temperature. Then half specimens in each group were immersed in basic fuchsin solution for 24 hours, and dye penetration along the cavity walls was measured. Another half specimens were analyzed in the SEM by means of describing visible crack of adhesion at the margin of the restoration. The results showed that with regard to dye penetration at both ceramic/composite interface and composite/dentin interface, specimens without luting spaces performed better marginal adaptation significantly than specimens with 200Mm luting spaces statistically (Kruskal-Wallis test, p<0.001 at ceramic/composite interface and p<0.01 at composite/dentin interface). These findings were supported by SEM analysis. So it is assumed that polymerization stress within Variolink? II luting cement could not be completely compensated for by lager luting spaces with K2O-Al2O3-SiC>2 system dental glass ceramics.Part IV Study on Application of K2O-AI2O3-S1O2 System Dental Glass Ceramic Inlays in Some Clinical CasesFour patients with different wedge-shaped defect were treated according to their symptom, in which one root canal was obturated. All the defects were restored using K2O-Al2O3-SiC>2 system dental glass ceramic inlays and Variolink? II composite luting cement after the endodontic and periodontic symptom dissappeared. All the restoration received at least 3 months follow-up. There were no endodontic and periodontic abnormalities, no secondary caries and no dropping of prostheses were found in the follow-up.It can be concluded in the present study that leucite is able to be microcrystallized by an appropriate thermal treatment system and high K2O feedstock, microcrystallized leucite was about 0.8Hm and scattered in glass matrix evenly. The mean compressive strength of the glass ceramic was about 231.4MPa and the mean density was about 2.28 g/cm3. Large luting spaces can't compensate for polymerization stress within the luting cement when Variolink? II composite luting cement was used. Application of K2O-Al2O3-SiO2 system dental glass ceramic inlays in clinical practice was satisfatory in a short period.
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