cosmetic dentistry_ beauty & science

12 I I review _ ceramics Aestheticfactors Space requirements for workability and maxi- mum aesthetics: A minimum working thickness of 1.2 mm is required, and 1.5 mm is ideal if masking. Environmentalfactors 1. Substratecondition:Substrateisnotcritical,since a high-strength core supports veneering material. 2. Flexureriskassessment:Theriskishighorbelow. For high-risk situations, core design and struc- turalsupportforporcelainbecomemorecritical. 3. Tensile and shear stress risk assessment: The risk ishighorbelow.Notethatforhigh-risksituations, 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 en- sure the best aesthetic results. Additionally, there should not be more than 2 mm of unsupported occlusal or incisal porcelain; the restoration core shouldbebuiltouttosupportmarginalridges.For higher-riskmolarregions,itisbettertousezirco- nia cores rather than alumina cores, provided the current firing parameters are followed. Full-con- tour zirconia restorations (e.g. BruxZir, Glidewell Laboratories) have been recommended for high- riskmolarsituations.Failureoftheserestorations is not likely to be an issue; some preliminary con- cerninvolveswearoftheopposingdentitionwith full-contour zirconia.18 No clinical data could be found to confirm or refute this. Clinically, only full-contour zirconia against full-contour zirco- nia in the molar region should be considered when no other clinical option is viable. 4. Bond/seal maintenance risk assessment: If the riskofobtainingorlosingthebondorsealishigh, then zirconia is the ideal all-ceramic to use. Summary A high-strength ceramic (specifically zirconia) is indicatedwhensignificanttoothstructureismissing, unfavourable risk for flexure and stress distribution is present, and it is impossible to obtain and main- tain bond and seal (e.g. most posterior full-crown situations with subgingival margins; Figs. 15 & 16). _Category 4: Metal ceramics Guidelines For almost half a century, metal ceramics have been the standard for aesthetic full-crown resto- rations. Generally, they have the same indications as Category 3 zirconia-based restorations. With metal ceramics, manufacturers have eliminated the complications throughout the years; these materialsdonothavethesamethermalfiringsensi- tivityaszirconiadoes.However,anteriorteethmetal ceramics need to be approximately 0.3 mm thicker to have the same aesthetics as properly designed zirconia/porcelain crowns. When porcelain-fused- to-metal restorations are indicated, the CAPTEK (Precious Chemicals USA) system has been the material of choice at the UCLA Center for Esthetic Dentistry owing to its superior aesthetic properties. Aestheticfactors 1. Spacerequirementsforworkability:1.5–1.7mm is required for maximum aesthetics. 2. Substrate condition: The substrate is not as crit- ical, since the metal core supports the veneering material. 3. Flexureriskassessment:Theriskishighorbelow. For high-risk situations, core design and struc- turalsupportforporcelainbecomemorecritical. 4. Tensile and shear stress risk assessment: The risk is high or below. For high-risk situations, core design and structural support for porcelain become more critical. 5. Bond/seal maintenance risk assessment: If the riskofobtainingorlosingthebondorsealishigh, then metal ceramics are an ideal choice for a full-crown restoration. Summary Metalceramicsareindicatedinallfull-crownsit- uations, esp. when all risk factors are high (Fig. 17). _Conclusion Thisarticlehaspresentedasystematicprocessof clinical evaluation and rationale for material selec- tion.Themostimportantpointisthatthemost-con- servative restoration should be done if the clinical criteria are met; for example, a full-coverage crown or deep-cut glass-ceramic restoration should not be performed when a more conservative porcelain restoration is indicated._ Editorial note: A complete list of references is available fromthepublisher. cosmeticdentistry 2_2013 Prof.Edward A.McLaren,DDS,MDC, is the founder and director of UCLA postgraduate aesthetics,and Director of the UCLA Center for Esthetic Dentistry in LosAngeles,California. Yair Y.Whiteman,DMD,is a full-time faculty member at the UCLA Center for Esthetic Dentistry. _about the authors cosmeticdentistry