cosmetic dentistry_ beauty & science

10 I I review _ ceramics Aestheticfactors Space requirements for shade change: 0.2– 0.3 mm is required for each shade change. Environmentalfactors 1. Substrate condition: There is 50 % or more re- mainingenamelonthetooth,50%ormoreofthe bonded substrate is enamel, 70 % or more of the margin is in enamel. It is important to note that these percentages are subjective assessments based on an overall evaluation of all parameters affecting the teeth to be restored and which may influence material selection. If bonding to some dentine substrate, the dentine should be mostly unaffected and superficial, since sclerotic den- tine exhibits a very poor bond strength. 2. Flexure risk assessment: There is a higher risk and a more guarded prognosis when bonding to dentine. Owing to dentine’s flexible nature, it is recommended that restorations with low fracture resistance materials be avoided and, therefore, the presence of a higher percentage (i.e. at least 70 % in high-stress areas such as the margins) of enamel is recommended when restoring using powder/liquid (Category 1) ma- terials.Byincreasingthepresenceofenamel,the prognosis is improved and, depending on the dentine/enamel ratio, the risk can be assessed as low to moderate. 3. Tensile and shear stress risk assessment: There is a low to low/moderate risk. Large areas of unsupported porcelain, deep overbite or overlap of teeth, bonding to more-flexible substrates (e.g. dentine and composite), bruxing, and more distally placed restorations in- crease the risk of exposure to shear and tensile stresses. 4. Bond/sealmaintenanceriskas- sessment: There is an absolute low risk of bond/seal failure. Summary Porcelains are generally indi- cated for anterior teeth. Occa- sional bicuspid use and rare molar use would be acceptable only with all parameters at the least- risk level. Category 1 materials are ideal in cases with significant enamel on the tooth, and generally with low flexure and stress risk assessment. These materials require long-term bond maintenance for success. _Category 2: Glass-based pressed or machinable materials Guidelines Glass-ceramic pressable materials, for example IPS Empress (Ivoclar Vivadent) and Authentic (Jenson) and the higher-strength IPS e.max ma- terials (Ivoclar Vivadent), can be used in any of the same clinical situations as Category 1 materi- als.Machinableversionsofglass-ceramicmaterial, for example VITABLOCS Mark II (VITA Zahnfabrik), IPS Empress CAD (Ivoclar Vivadent), and IPS e.max CAD,canbeusedinterchangeablywiththepressed versions. Monolithic IPS e.max, owing to its high strength and fracture toughness, has shown promise as a full-contour, full-crown alternative, even on molars.14 Glass-ceramics can also be used in clinical situations when higher risk factors are involved. Other than certain risk factors (see below) that would limit their use, these materials can be diffi- cult to use when there is less than 0.8 mm in thick- ness, except at marginal areas. They can gradually thin to a margin of approximately 0.3 mm. All things being equal, if the restoration is still a Category 1 clinical situation and there is more than 0.8 mm of working space, glass-ceramics should be considered owing to their in- creased strength and toughness, and the presence of sufficient room to achieve the desired aesthetics. Fig. 11_Pre-op image of a case in which the patient refused surgery and orthodontics. The treatment goal was to perform minimal preparation and use a tough material due to general medium to high risk in every area, other than seal maintenance. Fig. 12_Post-op image with bonded full-contour restorations on posterior teeth and incisally layered anterior teeth. All performed with IPS e.max HT. Fig. 13_Pre-op image of old un-aesthetic PFM. Fig. 14_Post-op of high-alumina crown system, veneered with VITA VM 7 (SPINELL, VITA Zahnfabrik). cosmeticdentistry 2_2013 Fig. 11 Fig. 12 Fig. 13 Fig. 14