Dental Cad / Cam Processing Of Machinable Mica Glass-ceramics

Using ceramic restorations to restore defects of teeth and dentition is one of the most important methods in fixed prosthodontics. Porcelain-fused-to-metal (PFM) restoration is the most commonly used ceramic restorations. However, the metal substructures of the PFM restorations generally result in poor biocompatibility and poor aesthetics because of the precipitation of metal ions and low transmittance. These drawbacks can be completely overcome by the application of all-ceramic restorations, which have natural appearance and the best biocompatibility. With the increasing aesthetic demands of patients, all-ceramic restorations will become the main method of fixed prosthodontics. Usually, the restorations are fabricated by slip-casting, controlled crystallization of glass or hot-pressing of precrystallized glasses. The properties of these materials are highly dependent on the skills of the dental technicians. In addition, process-dependent microdefects reduce the fracture strength and increase the time-dependent failure probability of restorations. In recent years, the use of computer-aided design/computer-aided manufacturing (CAD/CAM) systems to process dental restorations has completely changed the traditional lost-wax method and led to a revolution in dental technology. Prosthodontics has stridden into the modem high-tech fields with the application of CAD/CAM. By this new technology, conventional process time and steps are reduced to minimum, and dental restorations ?? can be fabricated and placed in one visit. Dental CAD/CAM technology has the greatest development potential in the field of all-ceramic restorations. The key idea of this study was to make prefabricated machinable mica glass-ceramic blocks directly used for dental CAD/CAM systems to produce dental restorations simply and quickly. Considerations were given to both the mechanical properties and the aesthetic effects during the development stage. Further heat treatment was no longer needed after the fabrication and lots of time could be saved K20-MgO-MgF2-Si02 quaternary system was chosen to make machinable tetrasilicic fluormica glass-ceramics. The base glass, with main compositions of Si02, K2C03, MgO, MgF2, Zr02, and Al203, was produced by melting reagents at I 500C for 2h. Heat-treatment procedure was determined by refer to differential thermal analysis (DTA) of the base glass. The nucleation and crystallization temperature were 650 慍 and 1060-?- 1 160C, and nucleation and crystallization time were lh and 4h, respectively. After the heat treatment, tetrasilicic fluormica (KMg2.5Si4O 10F2) glass-ceramics were successfully developed. Differential thermal analysis (DTA), X-ray diffractometry (XRD), scanning electron microscopy (SEM), and mechanical tests were used in this study. By changing the content of Zr02 and A1203 and adjusting crystallization temperature, the influence of Zr02, A1203 and crystallization temperature on crystallization behavior, microstructure and physical properties of the glass-ceramics was investigated. Cerec II dental CAD/CAM system was used to perform the clinical machining test by using the prefabricated mica glass-ceramic blocks. Biocompatibility of the glass-ceramics was evaluated with acute haemolysis test, cytotoxicity test, oral mucous membrane irritation test, and short-term toxicity test. The results of this study show that: 1. Zr02 content does not effe...