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cosmetic dentistry_ beauty & science

I 25 clinical technique _ bonding I cosmeticdentistry 1_2012 Fig. 8_Delamination of the veneering porcelain on the distal abutment of a FPD. Fig. 9_Plaster cast of tooth preparations for a full-coverage crown. Fig. 10_Uni-layer restorations are entirely fabricated of a single ceramic, and gain support from the underlying tooth. Fig. 11_Bi-layer restorations are fabricated from a dense core (metal or ceramic), which supports an overlying aesthetic veneering porcelain. Fig. 12_All-ceramic crowns fabricated from silica-based ceramics, which are the most aesthetic type of indirect restorations. Fig. 13_Porcelain laminate veneers are delicate restorations requiring careful handling to prevent inadvertent breakage during the cementation procedure. emphasis on preserving natural tooth substrate. Whereas in the past, preparation design was geo- metric and extensive (dictated by the properties of the restorative material), it is now amorphous and minimalist (dictated by the extent of disease; Figs. 2–7). Aesthetic restorations Essentially, any restoration that achieves health and function can also be aesthetic. How- ever, the term “aesthetic restorations” usually refers to tooth-coloured restorations or prosthe- ses. Aesthetic restorations can be direct, using resin-based composites, or indirect, fabricated exclusively from a single ceramic material or with a strong substructure (ceramic or metal) that is subsequently veneered with a weaker overlying porcelain. This is the basis for the extremely suc- cessful porcelain-fused-to-metal (PFM) crowns and FPD. The major disadvantage of PFM restorations is poor aesthetics at the cervical margins, pre- senting as greying owing to visibility of the metal substructure or “shine through” thin periodontal biotype gingivae. Therefore, a concerted effort has been made to seek alternatives, using dense, high strength ceramic cores to support aesthetic weaker porcelains. Although ceramics are capa- ble of mimicking the appearance of natural teeth, they are plagued with fracturing in an aqueous anddynamicoralenvironment.Waterimbibitions and occlusal stresses propagate crack formation of any exposed surface irregularities within the ceramic,leadingtochippingorcatastrophicfrac- tures. Furthermore, even if the surface is highly polished or glazed, the tenet for using ceramics in the oral cavity is that they must be supported by either the natural tooth substrate or an under- lying high strength substructure. Ceramics are inherently brittle materials (high modulus of elasticity) and therefore susceptible to fractures. Microscopic imperfections within the material are termed “Griffith flaws”, which grow into cracks and, if unimpeded, lead to ca- tastrophic fracture of the ceramic. The cracks are propagated by the hostile oral environment: dynamics (occlusal forces) and humidity (stress corrosion). Furthermore, static fatigue is time dependent, which eventually results in breakage (Fig. 8). Many strengthening mechanisms are used for halting fracture propagation, including re- inforcement and infiltration with glasses, and phase transformation toughening. Preventing fractures also depends on the clinical scenario, method of fabrication of the restoration, and the manufacturing technique and strengthening process of the ceramic. In order for ceramics to survive in the oral cav- ity, they must be supported by either the natural tooth substrate or a substructure. Two types of ceramic restorations are possible: first, a uni- layer restoration that is entirely composed of a singleceramic,gainingsupportthroughanadhe- sive bond to the underlying tooth substrate; and, second, a bi-layer restoration that has a support- ing substructure for the aesthetic veneering Fig. 9 Fig. 10Fig. 8 Fig. 12 Fig. 13Fig. 11