cosmetic dentistry_ beauty & science International Edition

I review _ ceramics 1.Category1:Glass-basedsystems Glass-based systems are made from materials that contain mainly silicon dioxide (also known as silicaorquartz)andvariousamountsofalumina(or aluminium oxide, chemical formula Al2O3). Alumi- nosilicates found in nature that contain various amounts of potassium and sodium are known as feldspars. Feldspars are modified in various ways to createtheglassesusedindentistry.Syntheticforms of aluminosilicate glasses are also manufactured for dental ceramics. We could not find any doc - umented references that demonstrated that nat - urally occurring aluminosilicate glasses perform betterorworsethansyntheticglasses,eventhough therehavebeenclaimstothecontrary.Thesemate- rials were first used in dentistry to make porcelain denture teeth. The mechanical properties are low flexural strength,usuallyinthe60–70MParange.Thus,they tend to be used as veneer materials for metal or ceramic substructures, as well as for veneers using either a refractory die technique or a platinum foil. The microstructure of a glass is shown in Figure 1. This is an electron micrograph of an acid-etched glass surface. The holes indicate a second glass, whichwasremovedbytheacid.Theveneerrestora- tion uses a glassy porcelain (Figs. 2a & b). 2.Category2:Glass-basedsystems withcrystallinesecondphase,porcelain This category of materials has a very large range of glass–crystalline ratios and crystal types. So much so that we can subdivide this category into three groups. The glass composition is similar to the pure glass of category 1. The difference is that varying amounts of different types of crystals have been either added or grown in the glass matrix. The primary crystal types today are leucite, lithium disilicate and fluorapatite. Leucite is cre- ated in dental porcelain by increasing the potas- sium oxide (chemical formula K2O) content of the aluminosilicate glass. Lithium disilicate crystals are created by adding lithium oxide (chemical for- mula Li2O) to the aluminosilicate glass. It also acts a flux, lowering the melting temperature of the material. These materials have also been developed into very fine-grained machinable blocks, VITABLOCS Mark II (VITA Zahnfabrik), for use with the CEREC CAD/CAMsystem(SironaDentalSystems).Thisma- terial is the most clinically successful documented machinable glass for the fabrication of inlays and onlays, with all studies showing a less than 1 % per year failure rate, which compares favourably with metal–ceramicsurvivaldata.2–7 Thebenefitofapre- manufactured block is that there is no residual porosityinthefinishedcorethatcouldactasaweak point, which could lead to catastrophic failure. 2.1Subcategory2.1:Lowtomoderate leucite-containingfeldspathicglass Even though other categories have a feld - spathic-like glass, these materials have come to be called “feldspathic porcelains” by default. Leucite mayalterthecoefficientofthermalexpansion(CTE) ofthematerial,aswellasinhibitcrackpropagation, which improves the strength of the material. The amount of leucite may be adjusted in the glass, based on the type of core and the required CTE. Thesematerialsarethetypicalpowder/liquidmate- rials that are used to veneer core systems and are the ideal materials for porcelain veneers. The original materials had a fairly random size and distribut leucite crystals, with the average par- ticle size being around several hundred microns. Thisrandomdistributionandlargeparticlesizecon- tributed to the materials’ low fracture resistance and abrasive properties relative to enamel.8 Newer generations of materials (e.g. VITA VM 13, VITA Zahnfabrik) have been developed with much finer leucite crystals (10–20 µm) and very even particle distribution throughout the glass. These materials are less abrasive and have much higher flexural strengths.9 An electron micrograph of a typical feldspathic porcelain reveals a glass matrix sur- roundingleucitecrystals(Fig.3).The mostcommon use of these materials is as veneer porcelains for metal–ceramic restorations (Fig. 4). Figs. 8a & b_IPS e.max restorations replacing existing amalgam. Fig. 9_A scanning electron micrograph of the microstructure of VITA In-Ceram ALUMINA. 28 I cosmeticdentistry 1_2014 Fig. 8b Fig. 9Fig. 8a CDE0114_26-34_McLaren 11.06.14 14:06 Seite 3