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cosmetic dentistry_ beauty & science

26 I I clinical technique _ bonding cosmeticdentistry 1_2012 Figs. 14a–c_Applying a silane- coupling agent onto the fitting surface of a silica-based ceramic restoration forms silica–silane bonds, resulting in chemical adhesion at the cement–restoration interface. Fig. 15_Poor enamel-etching pattern with a seventh-generation DBA. Fig. 16_Profound enamel-etching pattern with OptiBond XTR. Figs. 17a & b_OptiBond XTR has deeper penetration into the dentine tubules with a reduced film thickness of only 5 µm, compared with 35 µm of other self-etch bonding agents. porcelain (Figs. 9–11). This substructure can be either metal or a dense, high strength ceramic core, and these restorations can be either bonded with a resin cement or luted with RMGI.5 Dental ceramics can arbitrarily be categorised as silica, alumina or zirconia based. Silica-based materials are weaker materials with a high glass content and excellent optical properties, making them the most aesthetic type of ceramic, e.g. feldspathic, leucite-reinforced, lithium disilicate and synthetic porcelains (Fig. 12). Alumina and zirconia have reduced glass content, reduced translucency and poorer light transmission, making them less aesthetic but offering greater strength, e.g. alumina (flexural strength of 700 MPa) and zirconia (flexural strength of >1000 MPa). However, owing to their hardness and in- ferior optical properties, uni-layered alumina and zirconia restorations are impractical. Hence, these high strength ceramics are ideal for bi- layer prostheses, acting as an underlying dense core for supporting weaker silica-based aesthe- tic porcelains for both single and multiple-unit FPDs. Clinical scenario The final aspect that determines the choice of cement is the clinical scenario. If the resistance and retention form of the tooth abutment is less than the ideal of 6º axial tapers (12º convergence angle), a resin cement is a prudent choice for reinforcing and im- proving the fracture strength of the abutment/cement/restoration complex.6 Similarly, when a remake of a restoration with poor marginal integrity is not immediately possi- ble, it may be possible to seal open margins using resin cements. Finally, if a dry environment is challenging, e.g. deep sub-gingival margins, RMGI is a better choice since it is less sensitive to moisture. _Bonding indirect aesthetic restorations RED bonding indirect aesthetic restorations is demanding and technique sensitive. Failure to fol- low meticulous clinical protocols, Fig. 14b Fig. 14cFig. 14a Fig. 17a Fig. 17b Fig. 15 Fig. 16