10: Metal–ceramic crowns

Chapter 10 Metal–ceramic crowns

Brian Stevenson

Indications for metal–ceramic full coverage crowns

If a tooth requires a full coverage indirect restoration and there is a need for an aesthetic restoration, a metal–ceramic crown can be considered. However, all-ceramic restorations may be more appropriate in patients with extremely high aesthetic demands and these are increasingly being used by some clinicians. There are also special circumstances that require specific modifications to the standard metal–ceramic crown detailed throughout this chapter. Metal–ceramic restorations can be waxed-up or milled to provide space for a movable component of a fixed prosthesis (Figure 10.1; see also Chapter 19) or a rest for a removable partial denture (see Chapter 8). These must not be confined to ceramic as this can lead to catastrophic fracture and failure. In addition, guide planes and undercuts to aid the retention of a removable prosthesis can be incorporated into the restoration.

Figure 10.1 Female component of movable connector that is incorporated into a fixed prosthesis. The component is confined to metal within the restoration.

Metal–ceramic full coverage crowns

Metal–ceramic crowns consist of:

• A metal substructure

• An opaceous ceramic layer

• A veneering ceramic layer (Figure 10.2).

Figure 10.2 The making of a metal–ceramic crown, showing the metal substructure (cast using the lost wax technique) and the opaceous ceramic layer (left) and the veneering ceramic layer in its unglazed state (right).

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.

Figure 10.3 Metal–ceramic crown preparation with shoulder buccally merging into a chamfer lingually. Reduction of the buccal cusp has significantly reduced the amount of remaining tooth tissue.

The metals

Metal alloys used in dentistry fall into three groups:

• Gold alloys. These have been described in the previous chapter.

• Base metal alloys. These are usually alloys of nickel and chromium, some with small amounts of other metals (e.g. molybdenum). Some patients may be allergic to nickel and these alloys should be avoided in such patients.

• Titanium alloys.

Any of these metals can be used for metal–ceramic restorations, providing their melting point is above the sintering temperature of the ceramic. If the two are too close, thin sections of the metal might melt and the metal framework may warp or sag when it is placed in the ceramic furnace.

The price of metals fluctuates and therefore prescribing patterns often vary depending on the cost of the constituents of these metals. Precious metal alloys have a long clinical track record with good longevity in longitudinal studies. Titanium alloys have recently been used with some short-term clinical success; however, their long-term effectiveness remains unproven.

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:

• Smoothing is completed in one direction, usually using a brown stone.

• The coping is sandblasted (usually with 50 μm aluminium oxide particles) and cleaned using a steam cleaner.

• Formation of an oxide layer. The exact process is often specific to the alloy used due to the varying amounts of base metal alloy present. The oxide layer can usually be formed by placing the substructure back into the furnace to complete a de-gassing (oxidization) cycle.

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:

• Van der Waals forces. The forces that attract (or repulse) molecules, but where no electrons are exchanged between the molecules (covalent bonding).

• Mechanical interlocking. The coping surface will have small irregularities on its surface. These provide a surface in which micromechanical interlocking, or retention can occur. The surface of the metal is often treated (e.g. sandblasted with Al₂O₃) to increase the surface area available for bonding.

• Compression bonding. The metal coping should expand slightly more than the overlying ceramic when heated to place the ceramic interface under compression when cooled down. In addition, the ceramic will shrink more than the metal once sintered and cooled, thereby allowing the ceramic to ‘wrap-round’ the metal coping.

• Chemical bonding. Thought to be created by the partial dissolution of the porcelain at the metal–ceramic interface causing saturation of the porcelain with metal oxides and the subsequent intimate atomic contact between the metal and porcelain.

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.

Figure 10.4 Shade match of a metal–ceramic restoration placed on the upper left lateral incisor.

Advantages of metal–ceramic full coverage crowns

• Strength. Laboratory studies have shown metal–ceramic restorations to be stronger than conventional all-ceramic restorations. However, it must be borne in mind that bespoke all-ceramic crown materials (e.g. zirconia-based restorations) have improved physical properties compared to the conventional porcelains.

• Fit. Different materials produce crowns with different marginal gaps. In laboratory studies, high gold alloys often produce restorations with the smallest gaps.

• Metal lingual and occlusal surfaces. This reduces the amount of tooth preparation required. It is important, where possible, that occlusal surfaces are covered in metal only, not only to reduce the amount of tooth reduction required but also to reduce the potential adverse effects of having a ceramic occlusal surface: ceramic that has not been adequately polished or glazed, or subsequently been ground/worn, has been shown to accelerate the wear of the dentition opposing the ceramic. Additionally, increased plaque accumulation and a reduction in the strength of the restoration have been found following wear/grinding of the ceramic. For some patients the appearance of a metal occlusal surface may be unacceptable; for example, a lower premolar crown may be very visible and a pragmatic approach is required provided the patient is fully informed of the potential pitfalls. Metal occlusal surfaces are generally better accepted in the maxilla than the mandible where they tend to be visible.

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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