Metal ceramic

All-ceramic anterior restorations can appear very natural. Unfortunately, the ceramics used in these restorations are brittle and subject to fracture from high tensile stresses. Conversely, all-metal restorations are strong and tough but, from an esthetic viewpoint, acceptable only for posterior restorations. Fortunately the esthetic qualities of ceramic materials can be combined with the strength and toughness of metals to produce restorations that have both a natural toothlike appearance and very good mechanical properties. As a result they are more successful as posterior restorations than all-ceramic crowns. A cross section of a ceramic-metal anterior crown is shown in Fig. The cast metal coping provides a substrate on which a ceramic coating is fused. The ceramics used for these restora­tions are porcelains, hence the common name, porcelain-fused-to-metal restorations. These ceramic-metal restorations are highly popular and are used for most of the crown and bridge restorations made today.

 Cross section of a ceramic-metal crown showing a sold alloy or base metal copins, the opaque body (dentin), and enamel ceramic layers.

High fusing temperature of the alloy. The fusing temperature must be substantially higher (>100° C) than the firing tem­perature of the ceramic and solders used to join segments of a bridge.

Low fusing temperature of the ceramic. The fusing temperature must be lower than ceramic used for all-ceramic restora­tions so no distortion of the coping takes place during fabrication.

The ceramic must wet the alloy readily when applied as a slurry in order to pre­vent voids forming at the interface. In general, the contact angle should be 60 degrees or less.

A good bond between the ceramic and metal is essential and is achieved by the interactions of the ceramic with metal ox­ides on the surface of metal  and by the roughness of the metal coping.

Compatible coefficients of thermal expan­sion of the ceramic and metal so the ceramic does not crack during fabrica­tion. The system is designed so the value for the metal is slightly higher than for the ceramic, thus putting the ceramic in compression (where it is stronger) during cooling  

Adequate stiffness and strength of the alloy core. This requirement is especially impor­tant for fixed bridges and posterior crowns. High stiffness in the alloy reduces stresses in the ceramic by reducing deflection and strain. High strength is essential in the interproximal regions in fixed bridges.

High sag resistance is essential. The alloy copings are relatively thin; no distortion should occur during firing of the ceramic or the fit of the restoration will be compromised.

An accurate casting of the metal coping is required even with the higher fusing tem­perature of the alloy.