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Pressed ceramics Thanks to advanced metal-free systems, all-ceramic restorations—with proper tooth preparation—offer esthetics and strength.
To address some of the esthetic drawbacks of PFM, porcelain jacket crowns were developed. No metal foundation was used and the ceramic crowns were cemented to place. Esthetics improved, but porcelain fracture was common. A cast ceramic was developed using a process similar to cast metal. The restoration was waxed to full contour, sprued, invested and melted out, just as in a metal restoration. A glass ingot then was melted and cast into the mold. These restorations then were cemented to place using conventional cements. Even though cast ceramic fit was better compared to conventional porcelain jacket crowns, my experience was a higher than acceptable rate of fracture. The core material was transparent and color had to be achieved by adding stains to the surface. Any adjustments resulted in loss of esthetics. The lamination process When it was discovered that conventional ceramic materials could be bonded to natural tooth structure, it became possible to use ceramics for inlays, onlays, and veneers as well as for full crowns. The lamination process of bonding the porcelain to natural tooth structure strengthens the materials in a fashion comparable to that of fusing to metal. Pressed ceramics A pressing process was developed as an alternative to casting the ceramic. The process results in forcing a molten glass pellet into the mold created. In turn, this results in a very dense ceramic material that fits with the accuracy of a gold restoration and has high flexural and compressive strengths. Pressed ceramics exhibit higher filler particle content, resulting in increased toughness (resistance to crack propagation). Because the outside surface of the porcelain cools before the inside, it develops a lower coefficient of thermal expansion, causing it to expand more. The resulting compressive forces cause further strengthening of the porcelain. In addition, a higher leucite content helps increase the strength of today’s pressed ceramics. They are bonded to tooth structure, adding the lamination effect, which further strengthens the materials. All these advantages make pressed ceramics useful for anterior and posterior restorations, where higher strength is needed. Even though the ceramic pellets can be made in various shades, they are pressed in one piece. Restorations are monochromatic unless the surface is stained or room is left to cut back the surface to add special effect porcelains. See “General Prep Guidelines,” page 100, for some basic tips on preparing teeth to receive pressed ceramic restorations. Case study Figure 1 shows the smile of a patient with PFM crowns on the maxillary right canine, lateral incisor and both central incisors. A close-up view is shown in Figure 2. She also had a PFM crown on her maxillary left first premolar. There were amalgam restorations in her maxillary right premolar teeth, as shown in the occlusal view in Figure 3. The patient wished to have her smile revitalized with more esthetic restorations. Pressed ceramic restorations were treatment planned to replace the previous PFM crowns. The amalgam restorations were to be replaced with pressed ceramic onlay/veneers, and a veneer and onlay/veneer were planned for the left canine second premolar. A facebow transfer record (Denar, Waterpik Technologies Inc., www.waterpik.com) was taken and impressions were taken for preoperative models. The models were mounted on a semi-adjustable articulator (Denar, Waterpik Technologies Inc.), and a wax-up was performed to simulate the desired end result. At the preparation appointment, the patient was anesthetized and the PFM crowns were removed. Figure 4 shows the patient’s maxillary right canine, lateral and central incisors, and left first premolar were prepared for pressed ceramic crowns. Her left canine was prepared for a veneer and her left second premolar and both right premolars were prepared for onlay/veneers, as can be seen in the occlusal view of the prepared teeth in Figure 5. Final impressions were taken with a polyvinyl siloxane impression material (Aquasil Ultra, Dentsply Caulk, www.caulk.com) and an occlusal registration was taken (Virtual, Ivoclar Vivadent, www.ivoclarvivadent.us). Provisional restorations were fabricated using a bisacrylic material (Luxatemp, Zenith Dental/DMG, www.zenithdental.com). The bisacrylic material was injected into a polyvinyl siloxane form made over the wax-up and placed over the prepared teeth. Provisional restorations are shown in place in Figure 6. Pressed ceramic restorations (IPS Empress, Ivoclar Vivadent, www.ivoclarvivadent.us) were fabricated in the dental laboratory. They are shown on the working models from the facial view in Figure 7. They can be seen from the incisal view in Figure 8. Note the esthetics and characterization in the close-up views of three of the pressed ceramic restorations are shown in Figure 9 photographed on a mirrored surface. At the placement appointment, the patient was anesthetized and provisional restorations were removed. The pressed restorations were tried-in and approved. Silane was applied to the internal etched surfaces of the restorations and dried. They were bonded to place using a total etch technique with a dentin and enamel bonding agent (Prime and Bond NT, Dentsply Caulk, www.caulk.com) and dual curing luting composite (Calibra, Dentsply Caulk, www.caulk.com) Final restorations are shown from the facial view in Figure 10 and from the incisal view in Figure 11. Her new smile can be seen in Figure 12. Conclusion The development of pressed ceramics has given the profession and our patients one more alternative for both functional and esthetic reconstruction of broken down or damaged natural tooth structure. The lamination effect of the bonding process helps strengthen the ceramic materials to a point where they can be used in higher stress areas. This same lamination process can also strengthen the remaining natural tooth structure. The lost wax technique used to fabricate these remarkably well-fitting restorations was taken from the casting procedure for metal restorations. The process of pressing the molten glass material under pressure causes it to be more dense than conventional stacked porcelain and adds to its higher flexural and compressive strengths. Preparation design must allow room for the pressed core (that is inherently monochromatic) and layering porcelain to create natural color variation, if true mimicking of the appearance of natural tooth structure is to be achieved. Precise preparation design, which allows for prescribed ceramic thickness, rounded line angles and butt margins with no bevels, is needed for clinical success. When the proper parameters are followed and the restorations are bonded to place properly, they can provide excellence both in esthetics and function. GENERAL PREP GUIDELINES Crowns Anterior crown preparation requires facial or lingual axial reduction of 1.0 to 1.5 mm and lingual contact clearance of 1.5 mm. Incisal reduction of at least 1.5 to 2.0 mm is desired. Rounded shoulders with 1.0 mm of depth should be prepared for the margins. All internal angles should be rounded. Posterior crowns require 1.5 mm of axial reduction, 1.5 to 2.0 mm of occlusal clearance, a 1.0 mm rounded shoulder margin and rounded line angles. Veneers Veneers with stained surfaces need only 0.6 to 0.8 mm of facial reduction, 1.0 to 1.5 mm of incisal thickness and rounded chamfer margins. Should the clinician desire to use layered ceramics for color vitality instead of surface stains, a full millimeter of facial reduction is advocated so there is room for the framework material and at least 0.2 mm of layering ceramic. Inlays/onlays Preparation for inlays and onlays requires 1.5 to 2.0 mm of occlusal reduction to allow for proper thickness of material. The gingival floor of a proximal box should be 1.0 to 1.5 mm wide and the occlusal isthmus should be 1.5 to 2.0 mm in width. All internal angles should be rounded and butt margins with no bevels are prescribed. A veneer could be added for esthetics. I call this type of restoration an onlay/veneer.Ross W. Nash, DDS, is co-founder of the Nash Institute for Dental Learning in Charlotte, N.C. He can be reached at 888-442-0242 or rosswnashdds@aol.com. Acknowledgment The author would like to thank Robert Passaro of the Passaro Center for Ceramics at the Nash Institute for fabrication of the restorations in this article.
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