cosmetic dentistry_ beauty & science International Edition

review _ ceramics I 2.2Subcategory2.2:Highleucite-containing (approximately50%)glass,glass-ceramics The microstructure of these materials consists of a glass matrix surrounding a second phase of individual crystals. The material starts out as a homogeneous glass. A secondary heat treatment nucleates and grows crystals that give this class of materials improved mechanical and physical prop- erties owing to the physical presence of the crystals and generation of compressive stress around the crystals. Glass-ceramic materials may be ideally suitedforuseasdentalrestorativematerials.Glass- ceramics generally have improved mechanical and physical properties, such as increased fracture re- sistance, improved thermal shock resistance, and resistance to erosion. Improvements in properties are dependent upon the interaction of the crystals and glass matrix, as well as on the crystal size and amount. Finer crystals generally produce stronger materials. Glass-ceramics are in widespread use for cookware,missilenosecones,andevenheatshields on space vehicles. They may be opaque or translu- cent, depending upon the chemical composition and percentage of crystallinity. A fundamental method of improving strength and fracture resist- ance is to add a second phase to a glass material, causing dispersion strengthening. The crystals may act as roadblocks to crack propagation. A crack spreadingfromadefectmustgothroughoraround thecrystal,whichtakessomeenergyawayfromthe propagating crack and may stop its progress. Thus, therestorationmaycontinuetofunctioninsteadof being cracked in half. In addition to the roadblock effect, compressive stresses around the growing crystals may help pin cracks and further enhance fracture resistance. The most widely used version is the original pressable ceramic, IPS Empress (Ivoclar Vivadent), but there are several other products in this cate- gory (Figs. 5, 6a & b). A number of pressable ma- terials with properties and microstructures similar to IPS Empress are available. This include Finesse (DENTSPLY), Authentic (Jensen), PM9 (VITA) and OPC (Pentron). A machinable version, IPS Empress CAD (Ivoclar Vivadent), designed for both the CEREC (Sirona) and E4D Technologies (Planmeca) CAD/CAM systems for high-leucite ceramics, has performed well clinically when used for posterior inlays and onlays, as well as anterior veneer and crown restorations.9–14 Machinable and pressable systems have much higher fracture resistance than powder/liquid systems, and have shown excellent clinical results for posterior inlay and onlay applications, and anterior veneer and crown restorations.2–7, 10–14 2.3Subcategory2.3: Lithiumdisilicateglass-ceramic This is a type of dental glass-ceramic originally introduced by Ivoclar Vivadent as IPS Empress II (and later in the form of IPS e.max pressable and machinable ceramics). Increasing the crystal con- tent to about 70 % and refining the crystal size achieved improvements in flexural strength. The glass matrix consists of a lithium silicate with mi- cron-size lithium disilicate crystals in between submicronlithium orthophosphate crystals (Figs. 7, 8a & b). This creates a highly filled glass matrix. A veneer porcelain consisting of fluorapatite crystals in an aluminosilicate glass may be layered on to the core to create the final morphology and shade of the restoration. The shape and the volume of crystals increase the flexural strength to about 360 MPa, or about three times that of IPS Empress.15–19 This material can be very trans- lucent even with the high crystalline content. This is due to the relatively low refractive index of the lithium disilicate crystals. This material is trans- lucent enough that it can be used for full contour restorations or for the highest aesthetics and can be veneered with special porcelain. Veneer porce- lainconsistingoffluorapatitecrystalsinanalumi- nosilicate glass may be layered on the core to create the final morphology and shade of the restoration. Fluorapatite is a fluoride containing calcium phosphate (chemical formula Ca5(PO4)3F). The fluorapatite crystals contribute to the veneer- ing porcelain’s optical properties and CTE so that it matches the lithium disilicate pressable or machinable material. Both the veneering material and the lithium disilicate material are etchable Figs. 10a–c_A VITA In-Ceram SPINELL crown. I 29cosmeticdentistry 1_2014 Fig. 10b Fig. 10cFig. 10a CDE0114_26-34_McLaren 11.06.14 14:06 Seite 4