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cosmetic dentistry_ beauty & science

be used in combination with a separate DBA, either a total-etch or self-etch system. Conven- tional resins have numerous shades and try-in pastes for precise shade matching. In addition, light-cured CR can be used for restorations with thicknesses of 1.5 to 2 mm or thinner and dual- cured CR for thicknesses of >2 mm or opaque cores, thereby increasing their versatility and clinical applications. NX3 Nexus (Kerr) is a CR cement available in a large selection of tooth-coloured shades, en- abling accurate shade matching. Its try-in pastes precisely correspond to the definitive cement shades, allowing colour assessment and alter- ation before final cementation. The defining fea- tures of NX3 are chromatic stability over time and compatibility with most seventh-generation DBAs. Amajorconcernwithresincements,especially associated with dual-cured resin cements, is ageing colour shift causing unsightly yellowing below translucent, aesthetic restorations. This is attributed to the amine-initiated setting reaction of the luting agents. To mitigate the latter, NX3 Nexus incorporates an amine-free redox initiator system that guarantees chromatic stability over time. It is also essential that the CR and DBA be compatible with each other. Compatibility is particularly an issue with self-etch DBA agents owing to the residual acidic inhibition layer that retards or impedes setting of dual- or dark-cured resin cements. NX3 has excellent bond compati- bility with seventh-generation total-etch and self-etch DBA without requiring an activator for dual-cured adhesives. This simplifies clinical pro- tocols and ensures predictable bonding at the cement–toothinterface,andincombinationwith a DBA has a shear bond strength (SBS) of ap- proximately 34 MPa for dentine and 30 MPa for enamel. At the cement–restoration interface, NX3 chemically adheres to most restorative ma- terials, including resin-based composites, porce- lain CAD/CAM blocks, alumina, zirconia and cast metal, achieving a maximum SBS of over 30 MPa. Finally, NX3 offers the choice of light or dual cur- ing,allowingrestorationswithreducedlightpen- etration, i.e. thicker than 2 mm or highly opaque (e.g. alumina or zirconia cores), to be predictably cemented. Dentine bonding agent Achieving RED bonding with CR cements re- quires use of a DBA. The adhesion mechanism of resin cements and DBA at the cement–tooth interface is both micromechanical, by forming a hybrid layer, and chemical, by bonding with calcium ions from the hydroxyapatite of the tooth substrate. In order to resist the polymerisa- tion stresses of the overlying resin cement, the bond strength of the DBA should be greater than 25 MPa. OptiBondXTR(Kerr)isthelatestself-etch,uni- versally compatible DBA for direct and indirect restorations. The XTR is a retro-step to the sixth- generation bonding agents, eliminating many of the drawbacks of existing single-component Fig. 22_Plaster cast of inlay cavity showing clearly defined margins. Fig. 23_Completed silica-based ceramic inlay on plaster cast. Fig. 24_Careful handling of the delicate inlay is essential during pre-treatment of the fitting (or intaglio) surface. Fig. 25_Pre-treatment of intaglio surface: etching with HF acid. Fig. 26_Pre-treatment of intaglio surface: rinsing off HF acid and drying with warm air until the surface appears frosty. Fig. 27_Pre-treatment of intaglio surface: application of OptiBond XTR adhesive, dry, light-cure and store in a light-sealed container while the pre-treatment of the intra-oral abutment is carried out. NB: Application of silane onto the fitting surface of the porcelain is unnecessary when using OptiBond XTR. Fig. 23 Fig. 24Fig. 22 Fig. 26 Fig. 27Fig. 25 I 29 clinical technique _ bonding I cosmeticdentistry 1_2012