by Robert Margeas, DDS
When it comes tometal-free restorations, what criteria is used to decide which material to use? There are a number of metal-free restorations available today, from feldspathic porcelain jackets to CAD/CAM restorations. This article will attempt to help clinicians decide when to use one restoration over another.
The Richmond crown was introduced in 1885 as an all-ceramic restoration, and not until more than 50 years later were there any other major developments. These crowns were made with a powder/liquid of feldspathic porcelain. Today, the porcelain jacket is still a viable and useful restoration. Many of the early failures were because they could not be bonded to tooth structure with zinc phosphate cement. Although they were usually successful, resin bonding increased the success rate substantially.
The traditional powder/liquid feldspathic jacket is useful when matching a single tooth with a good substrate of color. The preparation can be as minimal as 1 mm and the restoration can be bonded using traditional etching and bonding. In fact, these are some of the most esthetic restorations available. The downside is their inherently low strength. They should be used in the anterior and in patients who do not exhibit any occlusal problems. If the underlying tooth structure is dark, this type of restoration may not be the best choice. Opacious dentin must be used to block out the underlying tooth color so the value will not be low once it is bonded. These restorations work well in combination with porcelain veneers, as the same material can be used for both types of restorations.
Pressable ceramics were introduced to allow for better margins and increased strength. These types of restorations were made with a CAD/CAM or by using a lost wax technique and ingots pressed to form the crown. The crowns could be surface stained or cut back and incisal porcelains added to create a natural translucence. An example of these crowns is Empress (Ivoclar Vivadent). These are bonded using traditional bonding techniques. These are leucite-reinforced glass ceramics, which gain their strength from a leucite crystalline filler. The downside of this material is its natural translucence, which may not be ideal when trying to cover a discolored tooth. The internal aspect is etched with hydrofluoric acid and bonded. This material can be used for porcelain veneers or crowns. The preparation is more aggressive when used for porcelain veneers, due to the minimal thickness of the core material. The strength is derived from the core. The flexural strength of the core material is approximately 112 MPa. This is about double the strength of feldspathic powder/liquid. This material can be used for anterior and posterior restorations, but in my clinical experience, posterior restorations have had a higher failure rate.
Empress II was developed for three-unit bridges and posterior restorations. This material is about three to four times stronger than Empress I. It is made of lithium dislicate and gains its strength from bonding. The material is slightly more opacious than Empress I. This material is most ideal for highly esthetic anterior fixed partial dentures. The strength is still a compromise, but the esthetics achievable is high. The connectors can pose a problem if there is not enough room for adequate strength.
Glass-infiltrated alumina (In-Ceram Alumina) was developed for anterior and posterior restorations, including three-unit bridges. Clinically, these restorations have had extremely high success rates. The downside is the core material. It is very opacious, and tranclucency is difficult to achieve. The flexural strength is approximately 446 MPa. These types of restorations can be use where traditional porcelain fused to metal would be used.
Glass-infiltrated magnesium alumina (In-Ceram Spinell) was created to produce a core that was more translucent. This was achieved, but the strength was compromised. A highly translucent restoration can be fabricated and anterior single crowns are where this material would best be used. The advantage to using alumina is that the material can be cemented using traditional cements. The strength is from the core, not the bonding.
Densely sintered high-purity aluminum oxide (Procera, Nobel Biocare) has a high flexural strength of more than 600 MPa and can be used for anterior and posterior crowns. These restorations can be used where traditional porcelain-to-metal restorations would be used. They are opacious in nature, but have great strength and marginal fit. These restorations are made with CAD/CAM technology. Traditional cementation methods can be used.
The most recent introduction of all-ceramic materials is Yttrium Tetragonal Zirconia polycrystals (LAVA, 3M ESPE). This material can be used for anterior and posterior single crowns and bridge frameworks. It is the strongest of the all-ceramic materials available. Transformation toughening provides the strength, and 900 MPa to 1200 MPa is achievable with this material. The opacious core can block out dark teeth, and the restorations can be cemented with traditional materials. CAD/CAM or wax and milling are employed to fabricate the restorations. Overlay porcelain is used over the core or frameworks. The weak point is with the layering porcelains. During the last three years of using LAVA restorations, I have not had any frameworks fracture. The material is still opacious, so matching single restorations can be challenging. The ability to cement the restorations and the high strength of the material makes this an ideal material where porcelain-to-metal restorations may be indicated. Cercon Zirconia (DENTSPLY) is another option in the high-strength esthetic category. A Yttrium Stabilized Zirconium Oxide, Cercon restorations may be used for single units or bridges up to 48mm, and may be placed with conventional luting cement or bonding adhesive.
When using any of the materials listed, it is important that the preparation does not have any sharp areas. There must be sufficient occlusal space for the materials to prevent fractures. For darker substrates, an aluminus or zirconia restoration is the treatment of choice. The ability to cement with traditional techniques makes the aluminus and zirconia restorations attractive to the everyday practicing dentist.
