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Dental Tribune Indian Edition

18 Dental Tribune Indian Edition - July 2013Cosmetic high risk for bond and seal failure is based on the evaluated parameters. Category 1: Powder/liquid porcelains Guidelines Bonded pure-porcelain restorations are ideal as the most-conservative choice but are the weakest mate- rial and require specific clinical pa- rameters to be successful.13 Many good materials and techniques are available for bonded porcelain (e.g., Creation, Jensen Dental; Ceramco 3, DENTSPLY; EX-3, Noritake ). Howe- ver, VITA VM 13 (VITA Zahnfabrik) is recommended by the authors when 3D-Master shades are taken, and Vin- tage Halo (SHOFU) when classic sha- des are taken. When following clinical parame- ters and guidelines at the University of California, Los Angeles’s Center for Esthetic Dentistry (UCLA Center for Esthetic Dentistry), these materials have been used with similar success rates compared with porcelain fused to metal (i.e., less than a 1% fracture rate if all parameters are followed, unpubli- shed data; Figs. 5 & 6). Aesthetic factors Space requirements for shade change: 0.2– 0.3 mm is required for each sha- de change. Environmental factors 1. Substrate condition: There is 50% or more remaining enamel on the tooth, 50% or more of the bonded substrate is enamel, 70% or more of the margin is in enamel. It is impor- tant to note that these percentages are subjective assessments based on an overall evaluation of all pa- rameters 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 dentine 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 mate- rials be avoided and, therefore, the presence of a higher percentage (i.e., at least 70% in high-stress are- as such as the margins) of enamel is recommended when restoring using powder/liquid (Category 1) materials. By increasing the pre- sence of enamel, 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 asses- sment: There is a low to low/mo- derate risk. Large areas of unsup- ported porcelain, deep overbite or overlap of teeth, bonding to more- flexible substrates (e.g., dentine and composite), bruxing, and more di- stally placed restorations increase the risk of exposure to shear and tensile stresses. 4. Bond/seal maintenance risk asses- sment: There is an absolute low risk of bond/seal failure. Summary Porcelains are generally indicated for anterior teeth. Occasional bicuspid use and rare molar use would be ac- ceptable only with all parameters at the leastrisk level. Category 1 materials are ideal in cases with significant enamel on the tooth, and generally with low flexure and stress risk assessment. These ma- terials require long-term bond main- tenance 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 - te- rials (Ivoclar Vivadent), can be used in any of the same clinical situations as Category 1 materials. Machinable versions of glass-ceramic material, for example VITABLOCS Mark II (VITA Zahnfabrik), IPS Empress CAD (Ivoclar Vivadent), and IPS e.max CAD, can be used interchan- geably with the pressed versions. Mo- nolithic 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 cer- tain risk factors (see below) that would limit their use, these materials can be difficult to use when there is less than 0.8 mm in thickness, except at margi- nal areas. They can gradually thin to a margin of approximately 0.3 mm. All things being equal, if the re- storation 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 increased strength and toughness, and the presence of sufficient room to achieve the desired aesthetics. Aesthetic factors Space requirements for workability and shade change: A minimum wor- king thickness of 0.8 mm and 0.2–0.3 mm for each shade change is required. Environmental factors 1. Substrate condition: There is less than 50% of the enamel on the to- oth, 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, VITA- BLOCS Mark II and Authentic- type glass-ceramics, and layered IPS e.max. In cases in which flexu- re risk is medium to high (and full- crown preparation is not desirable), the authors have found in their clinical trials that monolithic IPS e.max has been 100% successful for as long as 30 months in service. All glass-ceramic restorations, in- cluding IPS e.max, were adhesively bonded in their samples. 3. Tensile and shear stress risk asses- sment: The risk is medium for Em- press, VITABLOCS Mark II and Authentic-type glass-ceramics, and layered IPS e.max. It is medium to medium/high for bonded mono- lithic IPS e.max. 4. Bond/seal maintenance risk asses- sment: There is a low risk of bond/ seal failure for Empress, VITA- BLOCS Mark II and Authentic- type glassceramics, and layered IPS e.max. It is medium for mono- lithic IPS e.max. Summary Pressed or machined glass-ceramic materials, such as Empress, VITA- BLOCS Mark II, and Authentic are indicated for thicker veneers, anterior crowns, and posterior inlay and on- lays (Figs. 7 & 8) in which medium or less flexure, and shear and tensile stress risk is documented (Figs. 9 & 10). Also, they are only indicated in clinical situations in which long-term bond and seal can be maintained. IPS e.max (Figs. 11 & 12), which is a different type of glass-ceramic that has higher toughness, is also indica- ted 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 fullcontour monolithic form and ce- mented 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 sy- stems, for example In-Ceram and No- belProcera (Nobel Biocare), were first on the market but are now generally being replaced with zirconia systems. Alumina systems have shown to be very clinically successful 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 con- ventional cements. For example, after using alumina restorations for many years at the UCLA Center for Esthetic Dentistry, the authors 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 resinmodified glass ionomer luting cement (e.g., RelyX, 3M ESPE) or a re- sin cement. For zirconia-core systems (e.g., LAVA, 3M ESPE), the authors have not experienced core fracture but have seen problems with chipping of porcelain. White and McLaren17 found that a special slowcool thermal cycle minimises the stress in the por- celain and at the porcelain/zirconia interface. Clinically, since the authors of this current article have been using the altered firing schedules, their re- placement rate for chipping has been reduced by less than 1%. Aesthetic factors Space requirements for workability and maximum aesthetics: A mini- mum working thickness of 1.2 mm is required, and 1.5 mm is ideal if ma- sking. Environmental factors 1. Substrate condition: Substrate is not critical, since a high-strength core supports veneering material. 2. Flexure risk assessment: The risk is high or below. For high-risk si- tuations, core design and structural support for porcelain become more critical. 3. Tensile and shear stress risk asses- sment: The risk is high or below. Note that for high-risk situations, core design and structural support for porcelain become more critical. Preparations should allow for a 0.5 mm core plus 1 mm of porcelain to ensure the best aesthetic results. Additionally, there should not be more than 2 mm of unsupported occlusal or incisal porcelain; the restoration core should be built out to support marginal ridges. For higher-risk molar regions, it is bet- ter to use zirconia cores rather than alumina cores, provided the cur- rent firing parameters are followed. Full-contour zirconia restorations (e.g., BruxZir, Glidewell Labora- tories) have been recommended for highrisk molar situations. Failure of these restorations is not likely to be an issue; some preliminary concern involves wear of the op- posing dentition with full-contour zirconia.18 No clinical data could be found to confirm or refute this. Clinically, only full-contour zirco- nia against full-contour zirconia in the molar region should be conside- red when no other clinical option is viable. 4. Bond/seal maintenance risk asses- sment: If the risk of obtaining or losing the bond or seal is high, then zirconia is the ideal all-ceramic to use. Summary A high-strength ceramic (specifically zirconia) is indicated when significant tooth structure is missing, unfavoura- ble risk for flexure and stress distri- Figure 4: Image of preparation with poor substrate and subgingival margins whe- re maintaining seal would be difficult. High-strength ceramics or metal ceramics would be indicated. Figure 5: Image of minimal preparations prior to receiving bonded porcelain. Figure 6: Two-year post-op image of very conservative Category 1 bonded porcelain restorations, using VITA VM porcelains. Figure 7: Pre-op image of a case for an inlay in tooth 18 and onlay in tooth 19. Figure 8: Post-op image using non-layer IPS e.max HT. Figure 12: Post-op image with bonded full-contour restorations on posterior te- eth and incisally layered anterior teeth. All performed with IPS e.max HT. Figure 13: Pre-op image of old unaesthe- tic PFM. Figure 14: Post-op of high-alumina crown system, veneered with VITA VM 7 (SPI- NELL, VITA Zahnfabrik). Figure 11: Pre-op image of a case in which the patient refused surgery and orthodon- tics. The treatment goal was to perform minimal preparation and use a tough ma- terial due to general medium to high risk in every area, other than seal maintenance. Figure 9: Pre-op image of case requiring significant lengthening. There is at least a medium risk of flexure and unfavourable stress, and some of the substrate would be dentine. Thus, Category 1 materials were eliminated as a choice. Figure 10: Post-op image of the same case using Category 2 materials, in this case VITABLOCS Mark II with minimal porcelain layering in the incisal one- third.