10: Metal–ceramic crowns

Chapter 10 Metal–ceramic crowns

Metal–ceramic full coverage crowns

Metal–ceramic crowns consist of:

The rationale for this restoration is to combine the strength of a metal substructure with the aesthetic qualities of dental porcelain. However, tooth preparation for a full coverage metal–ceramic restoration is destructive of tooth substance (Figure 10.3) as clearance needs to be created not only for the metal but also for the opaceous ceramic and the dentine and enamel effect veneering ceramics. To provide sufficient strength the metal substructure should normally be between 0.3 and 0.7 mm thick; ideally this should be at least 0.5 mm thick with a greater thickness on the occlusal surface.

The metal–ceramic bond

The opaceous ceramic layer masks the colour of the underlying metal and should be 0.2–0.3 mm in thickness. Once the metal substructure has been cast, several processes prepare the metal surface to enable reliable bonding of the ceramic layers:

The mechanism(s) by which the ceramic bonds to the metal coping has been a matter for debate. However, it is now generally accepted that the total bond is created by different mechanisms, with the chemical bonding thought to be most important. The total bond between the ceramic and metal substructure is thought to consist of:

The ceramic

The types of dental porcelain available are discussed in Chapter 11 and are used to provide the aesthetic component of these restorations. Usually glass ceramics are used for metal–ceramic restorations; these ceramics must be thermally compatible with the specific alloy being used. The difference in coefficient of thermal expansion between the ceramic and metal is important; if the difference is too great, stresses will be introduced into the ceramic, leading to crazing, cracking and ceramic fracture. Ideally, the coefficient of thermal expansion of the metal should be slightly higher than that of the ceramic (see compression bonding above). The thickness of the veneering ceramic should allow the dental technician to match the colour of the restoration to the desired shade (Figure 10.4). However, it should not be more than about 1 mm thick as the ceramic has a relatively low flexural strength and may fracture if not adequately supported by the metal framework, although newer ceramics have improved physical properties.

Advantages of metal–ceramic full coverage crowns

Disadvantages of metal–ceramic full coverage crowns

Extensive tooth preparation. All full coverage preparations and their restorations have the potential to be the final insult to a stressed pulp, resulting in loss of vitality. The rate at which this occurs is a matter for debate (see Chapter 3). Systematic reviews of the literature have demonstrated the single crowns have a lower incidence of pulp necrosis than fixed bridgework. It must be stressed that many of these te/>

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Jan 9, 2015 | Posted by in Operative Dentistry | Comments Off on 10: Metal–ceramic crowns

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