cosmetic dentistry_ beauty & science

I 11 review _ ceramics I cosmeticdentistry 2_2013 Aestheticfactors Space requirements for workability and shade change: A minimum working thickness of 0.8 mm and0.2–0.3mmforeachshadechangeisrequired. Environmentalfactors 1. Substrate condition: There is less than 50 % of the enamel on the tooth, less than 50 % of the bonded substrate is enamel, and 30 % or more of the margin is in dentine. 2. Flexure risk assessment: The risk is medium for Empress, VITABLOCS Mark II and Authentic-type glass-ceramics, and layered IPS e.max. In cases inwhichflexureriskismediumtohigh(andfull- crown preparation is not desirable), the authors havefoundintheirclinicaltrialsthatmonolithic IPS e.max has been 100 % successful for as long as 30 months in service. All glass-ceramic restorations, including IPS e.max, were adhe- sively bonded in their samples. 3. Tensile and shear stress risk assessment: The risk is medium for Empress, VITABLOCS Mark II and Authentic-type glass-ceramics, and layered IPS e.max. It is medium to medium/high for bonded monolithic IPS e.max. 4. Bond/seal maintenance risk assessment: There is a low risk of bond/seal failure for Empress, VITABLOCS Mark II and Authentic-type glass- ceramics, and layered IPS e.max. It is medium for monolithic IPS e.max. Summary Pressed or machined glass-ceramic materials, suchasEmpress,VITABLOCSMarkIIandAuthentic, are indicated for thicker veneers, anterior crowns, and posterior inlay and onlays (Figs. 7 & 8) in which medium or less flexure, and shear and tensilestressriskisdocumented(Figs.9&10).Also, they are only indicated in clinical situations in whichlong-termbondandsealcanbemaintained. IPS e.max (Figs. 11 & 12), which is a different type of glass-ceramic that has higher toughness, is also indicated for the same clinical situations as the other glass-ceramics, but can be extended for single-teeth use in higher-stress situations (as in molar crowns). This is provided it is used in a full- contour monolithic form and cemented with a resin cement. _Category 3: High-strength crystalline ceramics Guidelines Mostly (e.g. VITA In-Ceram, VITA Zahnfabrik) all-crystalline materials are used for core systems to replace metal that would then be veneered with porcelain. Alumina-based systems, for exam- ple In-Ceram and NobelProcera (Nobel Biocare), were first on the market but are now generally being replaced with zirconia systems. Alumina systems have shown to be very clinically success- ful for single units, with a slightly increased risk in the molar region.15, 16 They can be recommended for any single-unit anterior or bicuspid crown (Figs. 13 & 14). The authors have observed a slight increase in failure with conventional cements. For example, after using alumina restorations for many years at theUCLACenterforEstheticDentistry,theauthors observed that at between eight and ten years, the failure rate doubled to approximately 2 %, with those failures being core fractures necessitating replacement (unpublished data). Their suggestion for alumina-core restorations is either a resin- modified glass ionomer luting cement (e.g. RelyX, 3M ESPE) or a resin cement. For zirconia-core systems(e.g.LAVA,3MESPE),theauthorshavenot experienced core fracture but have seen problems with chipping of porcelain. WhiteandMcLaren17 foundthataspecialslow- cool thermal cycle minimises the stress in the porcelain and at the porcelain/zirconia interface. Clinically, since the authors of this current article have been using the altered firing schedules, their replacement rate for chipping has been reduced by less than 1 %. Fig. 15_Pre-op image of old PFM. The patient was unhappy with the opacity and metal display at margin. Category 3 or 4 material is required for this case. Fig. 16_Post-op image of a LAVA coping with the new LAVA Digital Veneering System (3M ESPE). Fig. 17_Post-op image of teeth 18 and 20 in a case with subgingival margins and poor bonding. Metal ceramics with a coping (CAPTEK). Fig. 15 Fig. 16 Fig. 17