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.Clinical techniqueWith the Empress restoration, the preparation is important and almost critical. Proper reduction is necessary to allow for adequate pressing of the material. Overwaxing of the restoration and cutting back after pressing can be used, but is more difficult and can result in problems. Most importantly is the 1 mm shoulder or deep champher necessary. Avoid tapered margins, feathered edges, or beveled margins. Anterior and posterior teeth should be reduced 1.5 mm for full coverage and inlay/onlay formation, with 2 mm reduction at the incisal and cusp tip of the posterior teeth. Proximal boxes should have 60-80 degree cavosurface angles. Avoid thin walls or flared preparations. Veneers should be reduced at least .6 mm at the cervical third, with .8 to 1 mm at the incisal. Adequate thickness is not only necessary for proper pressing, but to allow adequate strength. On all types of restorations, avoid sharp internal line angles and avoid undercuts.
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.Dr. Dickerson's 21-step system for successWhen the Empress restorations come back from the laboratory, it is very important to put in place a system to assure the proper cementation of the restorations. Here is the system that I have developed for my practice and for the clinicians attending the Las Vegas Institute.1. Seat the restorations on the models and check them for fit and length, then anesthetize your patient and remove the temporaries.2. Try-in each restoration dry to check for the fit and marginal integrity.3. Place all restorations with try-in past for fit of contacts and color. Be sure to use a paste that does not alter the color of the restoration.4. Seat the patient upright and verify the midline and cant of the restorations. If necessary, make changes at this point. If everything is satisfactory, remove the restorations and then clean with phosphoric acid and rinse.5. Apply fresh silane and dry.6. Place primer/adhesive on restorations and dry.7. Place a rubber dam on the patient to isolate the prepared teeth. Etch three to four teeth at a time for 15 to 20 seconds using Ultraetch phosphoric acid from Ultradent and rinse with water. Lightly dry but do not desicate the teeth.8. Apply Tublicid or Ultracid to teeth with a microbrush and blot off the excess.9. Apply primer/adhesive to teeth for 20 seconds, dry for five seconds (gently blowing air in a gingival direction) and light cure for five seconds.10. Fill restorations with light-cured luting cement (base only).11. Place the restorations on teeth #8 and #9 only, remove excess with a rubber tip or a microbrush, and tack for 10 seconds at the gingival margin seating the restoration firmly apically and gingivally.12. Be certain that there is no flash of base material on the adjacent teeth that will prevent the next restoration from seating completely.13. Place restorations on teeth #6 and #7 only, remove excess with a rubber tip or a microbrush, and tack for 10 seconds at the gingival margin seating the restoration firmly apically and gingivally.14. Place restorations on teeth #4 and #5, remove excess with a rubber tip or a microbrush, and tack for 10 seconds at the gingival margin seating the restoration firmly apically and gingivally. 15. Place restorations on teeth #10 and #11, remove excess with a rubber tip or a microbrush, and tack for 10 seconds at the gingival margin seating the restoration firmly apically and gingivally. 16. Place restorations on teeth #12 and #13, remove excess with a rubber tip or a microbrush, and tack for 10 seconds at the gingival margin seating the restoration firmly apically and gingivally. 17. Now, remove any excess base material (rubber tip, brush, and floss) and cure each restoration for 10 seconds from the lingual and then for 20 seconds from the facial.18. Remove any cured excess with a scaler and/or Bard Parker, then polish with Caulk Enhance cups and Epitex strips.19. Adjust occlusion and polish.20. Seat the patient upright and check for anterior guidance and excursive movements.21. Take post-op photos and have a celebration with your patient! Using systems and checklists can greatly simplify how you treat your patients!