Effect of heat treatment and finishing procedures on strength, hardness, and wear resistance of lithium disilicate glass-ceramic
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Objectives: New block materials have been developed to fabricate full-contour restorations. Typically, a partially crystallized lithium desilicate glass-ceramic (IPS e.max CAD) is milled to full contour and then subjected to a heat cycle to complete crystallization. Variations in the heating cycle and finishing procedures may affect physical and mechanical properties of the material. The purposes of this study are (1) to evaluate the flexural strength, microstructure and micro hardness of IPS e.max CAD subjected to various heating cycles and finishing procedures and (2) to evaluate wear and abrasiveness of IPS e.max CAD by toothbrush abrasion and enamel wear. Materials and methods: (1) Blocks of the IPS e.max CAD lithium disilicate were cut into 120 bars (4mm x 2mm X 15mm) using a Buehler Isomet Diamond saw and then randomly divided into 8 groups of 15 specimens each. Experimental groups are untreated, crystallized, crystallized and polished (Buehler Ecomet polisher, 45, 15, 6, 1 micron diamond grit), over-crystallized/under-crystallized ([plus or minus]50 degrees C deviation from recommended peak crystallization temperature), a glaze cycle and simulated porcelain veneer firing cycles for 3 and 5 times. A 3-point-bending test, span 10 mm, was used to measure the flexural strength of materials using an Instron machine at a crosshead speed of 0.5 mm/min with a 1kN load cell. Then three specimens from each group were arbitrarily selected, embedded in epoxy resin, and polished. Specimens were put on the Microhardness Tester (Micromet 2003, Buehler), and indented with a Knoop diamond pyramid indenter at a load of 200g. Specimens were also etched using 5% hydrofluoric acid for 45 seconds and water rinsed. Then the surfaces of specimens were examined in the SEM. (2) 72 ceramic specimens (2x12x15mm)--24 each of IPS e.max CAD glazed, IPS e.max CAD polished, and Vita Mark II polished--were prepared. Each group of 24 was divided into 2 groups of 12. One group received toothbrush abrasion and the other was for enamel wear in a wear machine under a constant 400g loads for a tota1 50,000 cycles. Enamel specimens were prepared from sound, caries-free, extracted teeth. The amount of enamel and ceramic wear was determined by measuring weight loss and then calculating volume loss. Before and after the wear test, surface roughness was measured for each of the 12 ceramic specimens using a portable surface roughness tester. Statistical analysis was conducted using ANOVA and Tukey post hoc test at p=0.05 and Weibull analysis. Results: (1) One-Way ANOVA demonstrated significant difference in flexural strength between the test groups and under-crystallized groups were significantly lower in flexural strength values than other groups (P[less than]0.05). There is no significant difference in hardness between over-crystallized, under-crystallized and crystallized groups. The SEM photo of the under-crystallized group samples demonstrated that the formed crystal structure was not ideal because of the decreased crystallization temperature. (2) Statistical analysis revealed a significantly lower volume loss for the Vita Mark II polished group than for the e.max CAD glazed and polished groups (P[less than]0.001). The greatest amount of enamel wear was produced by the e.max CAD glazed group followed by the e.max CAD polished group and the Vita Mark II polished group. However, statistical analysis showed that there is no significant difference between the 3 groups (P[greater than]0.05). Conclusions: (1) There are significant differences between the test groups. Under crystallization, over crystallization and polishing may significantly lower the flexural strength. (2) In this study, the Vita Mark II polished group was more wear resistant to toothbrush abrasion and enamel wear than both the e.max CAD polished and the glazed groups. We also found that e.max CAD polished, e.maX CAD glazed and Vita Mark II polished groups all have similar abrasiveness to enamel. All 3 groups demonstrated significant change of surface roughness after toothbrush abrasion and enamel wear.
PLEASE NOTE: This work is protected by copyright. Downloading is restricted to the BU community: please click Download and log in with a valid BU account to access. If you are the author of this work and would like to make it publicly available, please contact email@example.com.Thesis (MSD) --Boston University, Goldman School of Dental Medicine, 2009 (Department of Restorative Sciences and Biomaterials).Includes bibliographic references: leaves 91-96.
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