Achieving ultimate esthetics

by William G. Dickerson, DDS

Due to the improvements in ceramic materials, all porcelain restorations are being placed where conventional porcelain fused to metal restorations have been placed in the past. Sacrificing the strength of the PFM restoration was determined to be worth the esthetic benefit of the all ceramic restorations, especially in the anterior teeth. Elimination of the need to hide the margins subgingivally was also a benefit, thereby maintaining the gingival integrity of the restored tooth. Since metal prevented light from entering the tooth, a dark line always appeared around the margins of the PFM restorations. With an all-porcelain restoration, the light is allowed to pass through to the underlying tooth, eliminating the dark line that most patients rightly find so objectionable. If properly done, the margins of the all-porcelain restoration can be made virtually invisible. The natural translucency of most ceramic restorations allow for almost perfect imitations of natural teeth.

Pioneers in the area of all-porcelain crowns are Cerestore and the Dicor systems. Cerestore suffered the fate of being discovered before adequate dentin-bonding agents were available; hence, most all Cerestore crowns were cemented. Fracturing of the restorations were commonplace and led to the ultimate demise of these restorations. Dicor is a castable ceramic that is still available, with its drawback being that the color is on the surface. Other systems came on the market utilizing an investment model, such as Hi-Ceram, Mirage, Optec, and Fortune.

One of the problems with ceramic materials is the propagation of microscopic defects and fractures and their brittleness that lead to failure of the restorations. IPS Empress (Ivoclar, Amherst, N.Y.), utilizes pressing of a softened ceramic ingot, producing a more fracture-resistant material. It also utilizes a controlled crystallization and leucite formation, where the fracture propagation is minimized. Conventional porcelains have a leucite concentration of 30-35 percent, Dicor has a leucite concentration of 50-60 percent, and IPS Empress has a leucite concentration of 80-85 percent. This increase in leucite formation, along with the placement of compressive stress in the material, produces a flexural strength higher than both conventional porcelain and Dicor.

The basis glass is melted in a continuous crucible furnace at 1450 C. To assure homogeneity, the material is quenched after the first firing, dried, ground, and remelted. Subsequently, the material undergoes a special tempering process which transforms the amorphous glass into a glass ceramic. Microscopic compressive zones are created internally, which also increases the volume strength of the material. The addition of small quantities of additives to the basis glass produces the final Empress glass ceramic.

This leucite-reinforced ceramic (the leucite crystals are only a few microns in diameter) is pressed into ingots that are refired for the restorations construction.

Since the crystallization is controlled (not accidentally formed) in the Empress ceramic material, the crystallized foci are created at the same rate throughout the glass matrix, are uniform in size, and are very small in dimension. In other words, the crystals are significantly smaller and more densely dispersed. This micromorphology limits fracture progression by interrupting the fracture progression when the fracture tip enters a microfracture within the leucite crystal. These fractures occur during the final phase of contraction in the ceramic's development, when the tensile stress becomes higher than the cohesive strength of the leucite. The elimination of this fracture progression plays a central role for the strength of this ceramic system. Since the microfractures within the leucite crystals are rounded, prevention — or at least limitation — of the fracture progression occurs. This complex, manufacturally controlled process, produces an importantly high flexural strength of 220 Mpa for the IPS Empress ceramic material.

The Empress utilizes a lost wax technique, with the pre-heated ingot being pressed into the investment ring. The pressing process occurs at approximately 1100 C, depending on whether using the colored or uncolored ingots. When the ceramic ingots become plasticized, they are pressed into the cavity at 3.5 bar. After pressing, there is a programmable holding period during which the ceramic accurately reproduces the fine detail of the investment cavity, even thin margins. It is this technique that allows for optimum fit of porcelain, with margins of around 20 microns. This, along with the enamel-like wear and kindness to the opposing tooth structure, is a major benefit of this ceramic material over conventional porcelain and other ceramic materials.

From the laboratory standpoint, and for the dentist who wishes to characterize and repair porcelain in-office, the beauty of this material is its ability to be refired numerous times without a refractory die. Shading characterizations and ceramic additions can be done many times without worrying about the rolling of the ceramic margins and slumping of the material. For the uncolored ingots, which are usually used in the formation of veneers and inlay/onlays, the colorization is done via the shading technique. For full coverage restorations, colored ingots with a layering technique is used for final esthetic development.

An impression and bite registration is taken and a shade for the final restoration is determined. A shade of the remaining tooth structure is also taken in a stump shade guide. This unique concept allows the laboratory technician to determine the adequate color to place to achieve the final desired result, as well as the ability to blend the cervical margins to the remaining tooth.