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cosmetic dentistry_ beauty & science

24 I I clinical technique _ bonding cosmeticdentistry 1_2012 fer chemical adhesion at both interfaces. How- ever, a vast number of interfaces are possible depending on the substrate on the tooth and restoration sides. These interfaces are the weak- estlinkandaccountforadhesivefailure.Cohesive failureisthebreakdownofthecementorfracture of the tooth or the restoration. A tight and secure seal is essential for pre- ventingmicro-leakagebetweentheconcealedin- terfaces beneath the bulk of the restoration and at the “open” margins exposed to the oral cavity. Furthermore, exposed margins are also vulnera- ble to occlusal stresses transmitted from the coronal part of the restoration to the cervical aspect, and the cement should be resilient to these forces in order to maintain a long-lasting hermetic seal. _Contemporary cements At present, there is no single cement that can ubiquitously be used for all indirect restorations. The choice of cement depends on the type of restoration, the restorative material and prevail- ing clinical scenarios. Judicial selection is imper- ative for efficacious cementation and longevity of a prosthesis. Contemporary permanent cements fordefinitiverestorationsarebroadlycategorised as resin-modified glass ionomers (RMGI) and resins (Table I). The latter are further divided into conventional resins (CR) and adhesive resins (AR).2 True AR are only those that contain the monomers MDP (10-methacryloyloxydecyl dihy- drogen phosphate) or 4-META (methacryloxy- ethyl trimellitate anhydride),3,4 e.g. Maxcem Elite (Kerr), RelyX Unicem (3M ESPE), and Panavia 21, Panavia F2.0, Clearfil SA (Kuraray Dental). _Selecting a permanent cement Thechoiceofcementforanindirectprosthesis depends on the type of restoration, the restora- tive material from which the restoration is made, andtheclinicalsituation.(TableIIsummarisesthe ideal choice of cement depending on the type of restoration and restorative material.) Type of restoration Indirect restorations are categorised as intra- coronalorextra-coronal.Inaddition,therestora- tion can be retentive or non-retentive (Table III). Retentive restorations gain retention and re- sistance from of the geometry of the tooth pre- paration (e.g. crown preparation), and therefore adhesive cementation is not obligatory. Conse- quently, these restorations can be luted with traditional cements such as zinc phosphate or glass-ionomer varieties, which are less technique sensitive. Conversely, non-retentive restorations have limited retentive tooth preparation features and are predominantly, or totally reliant on RED bonding to the tooth substrate, e.g. Maryland/ Rochette, fibre-reinforced fixed partial dentures (FPD), porcelain laminate veneers (PLV) and in- lays/onlays. This paradigm shift from retentive to non- retentive restorations has been possible owing to advances in dental material technology and adhesive clinical techniques, placing a greater Fig. 2_Defective amalgam restorations requiring replacement. Fig. 3_After removing the amalgam fillings, no attempt is made to extend the cavity to create undercuts, thereby maintaining the structural integrity of the tooth. Also, soft carious dentine is excavated, but hard, discoloured infected dentine is left in situ to preserve tooth substrate. Fig. 4_An impression is taken for fabricating indirect ceramic inlays. Fig. 5_Plaster cast showing undercuts in the cavity preparations, which will eventually be filled with the permanent resin-based cement. Fig. 6_The cavity undercuts are blocked on the plaster cast to facilitate fabrication of the ceramic inlays. Fig. 7_Post-cementation of ceramic inlays with a resin-based cement. Fig. 3 Fig. 4Fig. 2 Fig. 6 Fig. 7Fig. 5