cosmetic dentistry_ beauty & science

is worth noting that 50 % of clinical performance of dental cements is influenced by operator vari- ables,1 including an exacting clinical technique together with mixing, dispensing and loading the cement. The remaining risk factors are tooth preparation design (ideal 12º convergence angle for adequate resistance form), material proper- ties, location of tooth in the mouth and patient factors, such as oral hygiene. _Interfaces The primary function of dental cement is re- taining an indirect restoration on an intra-oral abutment, which can be natural tooth substrate or an artificial restorative material. The mecha- nisms by which cements achieve retention can broadly be termed “luting” or “bonding”. Luting is non-adhesive retention, and bonding implies a closer attachment of the cement to the restora- tion and tooth, which includes micromechanical and chemical adhesion. The cementation mechanism of cements is classified as: 1. non-adhesive or mechanical interlocking re- tention by engaging tooth surface and restora- tion intaglio surface irregularities, measuring 20 to 100 µm (this mechanism is applicable to all dental cements); 2. micromechanical “adhesion” by engaging finer surfaceirregularities<2µmcreatedbyetching, air abrasion, and usually in combination with a DBA by formation of a hybrid layer (0.5 to 10 µm); 3. chemical (molecular) adhesion by bipolar, Van der Waals forces and chemical bonds, which is the ideal that contemporary cements strive to achieve. Inordertounderstandthecementationmech- anism, two interfaces between the cement and the tooth/restoration complex require consider- ation. On the tooth side, the substrate is den- tine, enamel or cementum, and this is called the “cement–tooth interface”. On the opposing side is the artificial restoration, termed the “cement– restoration interface” (Fig. 1). Some cements of- Table I_Properties of contemporary permanent dental cements and luting mechanisms at cement–tooth and cement–restoration interfaces. Table II_Choice of cement depending on type of restoration and restorative material. Table III_Retentive and non-retentive restorations. I 23 clinical technique _ bonding I cosmeticdentistry 1_2012 Type of restoration Restorative material Ideal cement Possible cement Cast-metal crowns and inlays, intra-radicular posts, High gold and semi-precious alloys AR, RMGI ZP, PC, CR PFM crowns and FPDs (bridges) Maryland/Rochette bridges and splints Semi-precious alloys AR CRD Fibre-reinforced composite bridges and splints Composite, fibre AR CRD Light-transmitting intra-radicular posts Fibre, zirconia AR CRD Orthodontic fixed brackets Metal alloy AR CRD Inlays and onlays Composite or silica-based ceramic AR CRD PLVs (feldspathic) Silica-based ceramics CRL AR All-ceramic crowns, e.g. feldspathic, leucite-reinforced silica-based ceramics AR, CRD RMGI pressed glass, lithium disilicate All-ceramic crowns and FPDs of glass-infiltrated alumina, Alumina- and zirconia-based ceramics AR, RMGI densely sintered alumina, zirconia substructures Implant-supported crowns or FPDs PFM, or alumina- and zirconia-based ceramics AR, RMGI ZOE Key AR: adhesive resin; CR: conventional resin; CRL: conventional resin, light-cured; CRD: conventional resin, dual-cured; FPD: fixed partial denture; PC: polycarboxylate; PLV: porcelain laminate veneer; RMGI: resin-modified glass ionomer; ZOE: zinc/oxide eugenol; ZP: zinc phosphate. Table II Type of restoration Intra-coronal Extra-coronal Inlay Non-retentive Onlay Non-retentive PLV Non-retentive Maryland/Rochette bridges and splints Non-retentive Fibre-reinforced composite bridges and splints Non-retentive Orthodontic brackets Non-retentive Full-coverage crown Retentive FPD Retentive Implant-supported crowns and FPD Retentive Table III