Non-Metal Restorative Material Options for Full Crowns

Source: Supra-Gingival Minimally Invasive Dentistry: A Healthier Approach to Esthetic Restorations, published by Wiley-Blackwell

There are several unique options for restorative materials, specifically for full crowns. But using one type for all cases would be a mistake. The right choice depends on factors such as

• the condition of the tooth after removal of the old crown
• the margin location
• the cement to be used
• the tooth’s location in the mouth (anterior or posterior). 

Anterior and posterior teeth also have different thresholds for esthetic requirements, as well as occlusal loads and forces, which means that the choice of dental crown materials will vary based on those requirements. 

Non-metal restorative material options for full crowns

Leucite-reinforced feldspathic porcelain

Layered or pressed leucite-reinforced feldspathic porcelain is an option in the anterior region when margins are supra-gingival, there is a good amount of remaining enamel for adhesive luting, the adjacent teeth are being treated with feldspathic veneers and there are great esthetic demands (Figure 1). The use of leucite-reinforced feldspathic porcelain for full crowns in the posterior area is questionable, although it has been popular with computer-aided designed and manufactured (CAD-CAM) crowns.
This material depends on adhesion for its strength because of its intrinsic weakness. Strong enamel substrate is also very desirable and subgingival margins are undesirable because they complicate adhesion. Generally speaking, this material is better used for bonded partial coverage veneers and onlays in rare situations [1,2]. They are not a primary choice because the crowns made from this material’s mode of failure is fracture of the restorative material [3,4].

Figure 1: (a) Existing unesthetic porcelain-fused-to-metal crown on a right central incisor. (b) Leucite-reinforced feldspathic crown preparation (right central incisor) used along several feldspathic veneers. (c) Close-up of final feldspathic crown and veneers. (d) Improved smile.

Lithium disilicate

Lithium disilicate is unique because it can be used in two different modalities. First, bonded lithium disilicate may be used when margins are supragingival to allow for predictable adhesive luting. Because lithium disilicate is much stronger when bonded to the tooth, space requirements are less, 1 mm axial, 1.5-2 mm occlusal.

Lithium disilicate (e-Max, Ivoclar) is much stronger when bonded than when cemented. When bonded to mostly rigid tooth structures such as enamel, it can be close to the strength of zirconia, though it is less strong when bonded to a less rigid structure like dentin [5].

It comes in high translucency and low-translucency versions. High translucency is preferable for most cases of parietal coverage and supragingival margins where the cavosurface margin is considerably coronal to the gingival line and good blending is very important. Low translucency may be preferable if the color of the tooth needs to be hidden, but the disadvantage of opacious materials is that the margin line of the tooth and restoration will be very visible and will often need to be hidden subgingivally. E-Max can also be layered, but its strength decreases considerably [6].

Second, cemented (not-bonded) lithium disilicate can be used when margins are subgingival and isolation will be difficult, requiring the use of more contamination resistant traditional crown and bridge cement (Figures 2). Because e-Max is considerably weaker when it is cemented, the material must be thicker to increase strength, thus the space-reduction requirement will be 1.5 mm axial and 2 mm occlusal, requiring more aggressive tooth removal.

Figure 2: In a situation in which isolation and bleeding control may be compromised, cemented lithium disilicate would be a desirable choice.

Laboratory-quality composite

Laboratory-quality composite may be used when margins are supragingival, to allow for adhesive luting. Because of its low flexural strength, it requires more space-reduction even if bonded, 1.5 axial and 2 mm occlusal. Composite is also a popular material for CAD/CAM fabricated full crowns, and has shown good strength in this situation [7]. Composite crowns require aggressive preparation because they must be thick. Composite lacks rigidity and when the crown is thin, bedding forces may lead to an increased chance of debonding, and possibly a shorter life [8, 9]. Composite crowns also should be thick because of occlusal wear, and this can be considered a disadvantage in the durability of a full crown [10,11]. Finally, composite crowns should be cemented with adhesive and resin cement to increase strength, which means that subgingival margins would be contraindicated.

Zirconia

There are also two options for zirconia: monolithic and layered (PFZ). Layered zirconia can be used in the anterior and posterior area as it is esthetically pleasing and is the only non-metal material that can be used predictably for fixed partial dentures or bridges. Zirconia can also be used when the margins are subgingival  and adhesive cementation is very complicated. Because of its great strength, space and preparation requirements are low, at 1 mm axial and 1.5-2 mm occlusal, allowing for tooth preservation. Layered zirconia has a long history of success in dentistry [12,13,14]. Its intrinsic strength is remarkable, exceeding 1000 MPa. It can be milled into a thin coping, then a layered or pressed ceramics veneer can cover the coping, creating a layered restoration PFZ, similar to the porcelain-fused-to-metal (PFM) restorations of old.

Zirconia is very opacious unless it is used thinly. Thus, a correctly design thin facial coping and translucent layer of porcelain thin facial coping and translucent layer of porcelain allows PFZ to be far more translucent than a PFM or full zirconia crown; this is not possible with full zirconia. In the anterior area, a very thin coping of 0.3 mm can be requested to allow for a restoration with some translucency and better marginal blendging. In the posterior, a default 0.5 mm coping is recommended, and the design of the coping is essential for success. In a long study conducted by Rella Christensen (the author is part of this study) the importance of a proper coping design was established, as well as the importance of correctly calibrated porcelain ovens and the use of the ideal veneers porcelain, such as Cerabien ZR Press (Kuraray) [12,15]. 

Too often, laboratories leave too much unsupported porcelain, as shown in Figure 3a,b, compromising the results. The coping must be designed using an anatomical design (Figure 4a-c). Figure 4b displays red wax, showing that there is 0.5 mm coping for maximum translucency on the facial and occlusal surfaces of the coping. The marginal ridges are reinforced with thicker zirconia, supporting the veneer porcelain that will be placed on top.  

Figure 3: (a) Fractured veneer porcelain on marginal ridge of a layered zirconia crown. (b) X-ray shows the reason for the previous veneer porcelain fracture: an excessive amount of unsupported layering porcelain.

Figure 4: (a, b and c) Anatomical design of coping.

Monolithic zirconia may also be used when margins are subgingival, when esthetics are not important or a very thin restoration is desired, because of strength an axial reductions of 0.5 mm and 1-1.5 mm occlusal is all that is needed, allowing for tooth preservation. 

Monolithic or full zirconia with no external veneers has become extremely popular in recent years. Full zirconia is very opacious and unesthetic. Marginal blending is very poor and, for that reason, margins usually have to be placed subgingivally when used in areas that patients can see (Figures 5), usually forward from the molars. Outside the second and sometimes the first molars, full zirconia is usually contraindicated as the goal. This material may be indicated for full crowns on second molars with limited occlusal space, where there is a high risk of fracture and in teeth that have lost their rigidity, such as endodontically treated teeth with lower esthetic requirements. One serious disadvantage of full zirconia crowns is that removal can be an extremely time-consuming task, often requiring the use of several diamonds. This can be further compromised if the zirconia crown is bonded.

Figure 5: (a, b) Unesthetic full zirconia crowns.

Choice of material and tooth condition

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Sources

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