cosmetic dentistry_ beauty & science

I 09 review _ ceramics I cosmeticdentistry 2_2013 Excessiveshearandtensilestressriskassessment The third parameter is the risk (or amount) of ongoing shear and tensile stresses that the resto- ration will undergo, since the prognosis is more guarded for specific materials. All types of ceramics (especially porcelains) are weak in tensile and shear stresses.9 Ceramic materials perform best under compressivestress.Ifthestressescanbecontrolled, then weaker ceramics can be used, for example bondedporcelaintothetooth.Thesameparameters are evaluated, similar to flexure risk, for example deep overbites and potentially large areas where the ceramic would be cantilevered (Fig. 3). Ifahigh-stressfieldisanticipated,strongerand tougher ceramics are needed; if porcelain is used as the aesthetic material, the restoration design should be engineered with such support (usually a high-strength core system) that it will redirect shear and tensile stress patterns to compression. In order to achieve that, the substructure should reinforce the veneering porcelain by utilising the reinforced-porcelain system technique, which is generally accepted in the literature as a metal– ceramic concept.10 The practitioner can assess andcategoriselow,medium,orhighriskfortensile and shear stresses based on the parameters and symptoms mentioned above. Bond/sealmaintenanceriskassessment The fourth parameter is the risk of losing the bond or seal of the restoration to the tooth over time. Glass-matrix materials, which consist of the weaker powder/liquid porcelains, and the tougher pressed or machined glass-ceramics, require maintenanceofthebondandsealforclinicaldura- bility.11, 12 Owing to the nature of the glass-matrix materials and the absence of a core material, the veneeringporcelainsaremuchmoresusceptibleto fracture under mechanical stresses and, therefore, a good bond in combination with a stiffer tooth substructure(e.g.enamel)isessentialforreinforc- ing the restoration. If the bond and seal cannot be maintained, then high-strength ceramics or metal ceramicsarethemostsuitable,sincethesemateri- als can be placed using conventional cementation techniques. Clinical situations in which the risk of bond failure is higher are _moisture control problems; _higher shear and tensile stresses on bonded in- terfaces; _variable bonding interfaces (e.g. different types of dentine); _material and technique selection of bonding agents (i.e. as dictated by such clinical situations as inability to achieve proper isolation for mois- ture control to enable use of adhesive tech- nology); and _the experience of the operator (Fig. 4). An assignment of low, medium, or 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 material and require specific clinical parameters to besuccessful.13 Manygoodmaterialsandtechniques are available for bonded porcelain (e.g. Creation, JensenDental;Ceramco3,DENTSPLY;EX-3,Noritake). However, VITA VM 13 (VITA Zahnfabrik) is recom- mended by the authors when 3D-Master shades are taken, and Vintage Halo (SHOFU) when classic shades are taken. When following clinical parameters and guide- lines at the University of California, Los Angeles’s Center for Esthetic Dentistry (UCLA Center for EstheticDentistry),thesematerialshavebeenused withsimilarsuccessratescomparedwithporcelain fused to metal (i.e. less than a 1 % fracture rate if all parameters are followed, unpublished data; Figs. 5 & 6). Fig. 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. Fig. 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. Fig. 9 Fig. 10