|Year : 2011 | Volume
| Issue : 1 | Page : 10-13
Metal-free restorations: Clinical considerations
Chethan Hegde, Anita Nitin, S Vijai, SR Anil, D Ramya
Vikram Perfect, Mysore, Karnataka, India
|Date of Web Publication||4-Mar-2011|
Vikram Perfect, Mysore, Karnataka
Source of Support: None, Conflict of Interest: None
| Abstract|| |
Porcelain is known to be the most aesthetic material for dental restorations. Dental porcelains have had to be reinforced with metal substructures in the past due to their unpredictable strength. However, clinicians have often faced an aesthetic challenge when restoring anterior teeth with porcelain-fused to metal restorations. A demand for a more aesthetic alternative has led to the evolution of metal-free restorations. This article discusses the various metal-free ceramic systems available, their applications, including their margin configurations and cementation protocols and specific indications and advantages of each system. Clinical evidence and experimental studies have been reviewed to provide an evidence-based application of these materials in different situations.
Keywords: All-ceramic, aesthetic restorations, metal-free ceramics
|How to cite this article:|
Hegde C, Nitin A, Vijai S, Anil S R, Ramya D. Metal-free restorations: Clinical considerations. J Interdiscip Dentistry 2011;1:10-3
Restoration of anterior teeth with full coverage crown is a great challenge for a dental surgeon. Predictable strength and reasonable aesthetics have continued to make traditional metal ceramic restorations popular, but patients' demand for improved aesthetics has led to the development of metal-free restorations in highly challenging anterior teeth restorations.
Metal-free restorations have a strong ceramic core onto which layering ceramic is applied to achieve a natural appearance. These all-ceramic systems can be categorized broadly into two groups, based on the ceramic core they have.
All-ceramic crowns with translucent core are superior in aesthetics, but weaker in strength. These crowns can be used to cover acceptably colored dentin and at times for anterior fixed partial denture prostheses (FPD), where the masticatory load is less. Crowns with an opaque core are indicated for teeth with heavily discolored dentin or over metal posts  and can be used for posterior crowns and some can be used for posterior FPD as they have superior strength.
- Translucent core
For example, Leucite Core - IPS Empress Esthetic
Lithium disilicate Core - IPS e.max
- Opaque core
For example, Lithium disilicate Core - IPS e.max Alumina core - In ceram Alumina, Procera AllCeram
Zirconica core - Lava, Cercon, Procera Zirconia, Cerona
| Leucite Core|| |
The IPS Empress Esthetic is a glass ceramic with a leucite core. The highly homogenous quality and density of the leucite crystals provide excellent aesthetics, by blending well into the natural surroundings, producing esthetically superior crowns with a chameleon effect. These crowns can be fabricated using the press and layering or staining technique producing good aesthetics and an occlusal fit. These crowns with a flexural strength of 160 ± 8 MPa are indicated for anterior crowns and veneers where aesthetics is highly demanding. As the leucite core is translucent they can be used only where the dentin is of acceptable color. These crowns have to be adhesively cemented to the tooth using resin cements. The color of the cement may affect the color of the restoration, as the leucite core is translucent [Figure 1].
|Figure 1: PFM crowns with 11, 21 replaced with IPS Empress Esthetic crowns|
Click here to view
| Lithium Disilicate Core|| |
IPS e.max is a lithium disilicate glass ceramic. They can be produced either by the press technique, where an ingot of the desired shade is plasticized by heating and pressed into an investment mold under vacuum and pressure, or it can be processed using CAD / CAM technology. The high crystal density of lithium disilicate results in improved mechanical properties and a strength value of 360 - 450 MPa.  IPS e.max press ingots are available as high, medium translucency and medium, and high opaque. Translucent ingots can be used where the dentin is not discolored and aesthetics is the prime requisite. Opaque ingots can be used where teeth are highly discolored. When using opaque ingots, the strength of the crown allows the conventional cementation method to use resin modified Glass Ionomer Cements, as in these crowns the color of the cement is not made visible due to the opacity of the core [Figure 2] and [Figure 3].
Anterior and posterior single crowns
Veneers, Inlays, and onlays
Three unit anterior fixed partial denture prostheses
Three unit fixed partial denture prostheses replacing the Premolars
| Alumina Core|| |
In-ceram Alumina uses high temperature sintered alumina glass infiltrated copings that can be used for single and three unit anterior FPDs.  This high strength alumina core is fabricated via the slip-casting technique.  A slurry of densely packed Al 2 O 3 (70 - 80 wt.%) is applied and sintered to a refractory die at 1120°C for 10 hours. This produces a porous skeleton of alumina particles, which is infiltrated with Lanthanum glass in a second firing at 1100°C for 4 hours to eliminate porosity, increase strength, and limit potential sites for crack propagation. This coping is opaque and is veneered with feldspathic porcelain. The flexural strength of the framework material ranges from 236 MPa to 600 MPa, and fracture toughness ranges between 3.1 and 4.61 MPa/m 1/2 .  This is indicated for anterior crown, anterior and posterior three unit fixed partial denture prostheses.
Procera Alumina uses 99.9% high purity aluminium oxide coping, combined with low fusing veneering porcelain. Procera has the highest strength of 699 MPa  among all alumina based materials. Procera Alumina is indicated for anteriors, premolar, and molar crowns.
| Zirconia Core|| |
Crowns with zirconia core materials are the strongest among all the metal-free restorations, Yttrium-oxide is added to pure zirconia to control the volume expansion and to stabilize it in the tetragonal phase, at room temperature.  Yttrium-oxide partially stabilized Zirconia (Y-TZP) has mechanical properties with a flexural strength of 900 - 1200 MPa  and high fracture toughness, making it suitable for anterior and posterior crowns as well as for long-span fixed partial dentures. Lava (3M, ESPE), Cercon (Dentsply), and Procera Zirconia (Nobel Biocare) Cerona are the various Zirconia crowns available [Figure 4].
|Figures 4: Zirconia core crowns to restore anterior and posterior discolored hypoplastic teeth|
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| Discussion|| |
With the availability of a variety of metal-free restorations, clinicians have to be aware of their various properties in order to ensure that they select the right restoration for a given case. For anterior full coverage restorations, where aesthetics is the prime concern, all ceramic crowns with a translucent core are an excellent choice. Although crowns with translucent core are good in aesthetics, they are poor in strength, so they are recommended for anterior teeth with dentin that are not heavily discolored. In heavily discolored teeth a temporary crown is a useful guide,  to know if its shade is influenced by the underlying substrate, in which case a crown with an opaque core can be used. While using translucent crowns tooth preparation with an equigingival margin is recommended.  They will require resin cement for cementation, which will enhance the strength of the crown through bonding. It is advisable to use light cured resin luting agents, as they are more color stable compared to dual cure resin cements, which may discolor due to the release of amines, over a period of time.
Crowns with opaque cores are superior in strength, with good aesthetics, and can be used for anterior and posterior teeth. Crowns with the Zirconia core are recommended for fixed partial dentures. One can use resin or conventional luting agents for cementation as the color of the cement will not affect the shade of the crown. When restoring anterior teeth with these crowns it is advisable to end the margin subgingivally, as there could be a mismatch in shade between the tooth margin and the restoration. It is advisable to use an opaque core in teeth with heavy discoloration. The strength of these restorations is dependent on the ceramic material used, the Core-Veneer bond strength, the crown thickness, and the design of restoration.  Metal-free restorations are not recommended in subjects who have heavy bruxism.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4]