The ceramic symposium of the Society for Dental Ceramics (Arbeitsgemeinschaft für Keramik in der Zahnheilkunde e.V.) spotlighted treatment procedures involving all-ceramics in conjunction with the CAD/CAM technique. The moderator, Prof. Jörg Strub of FreiburgUniversity, announced that all-ceramics in dentistry are set to go ahead into a promising future. While in the recent past the range of indications for all-ceramic materials has broadened considerably and increased in volume, Strub maintains that longer-term experience must be gathered – particularly for oxide ceramics in wide-span posterior bridges, in implant prosthetics and telescope crowns – to offer practices and dental labs the highest possible clinical certainty.
Professor Ralf Janda of the University of Düsseldorf gave an overview of the current all-ceramic systems and their practice value. As a chemist, he participated in the development of castable ceramics in the 1980s, and for that reason was able to portray the challenges accompanying the early years of all-ceramics. Only the introduction of the adhesive technique qualified laboratory-pressed silicate ceramics for the construction of durable inlays and onlays. Then, the subsequent leucite-reinforced silicate ceramics made it possible to construct crowns for the anterior dentition and premolars. Starting in the 1990s, computer-controlled milling machines processed grindable silicate-ceramic blanks, which, thanks to their industrial manufacture, possessed a homogeneous granular structure and strength. This made it possible to construct high-quality restorations according to reproducible standards (Fig. 1). Since then, clinical studies have proven the superior survival rate of CAD/CAM-manufactured silicate-ceramic restorations over laboratory-laminated inlays of sintered ceramic [1, 2, 3, 4, 5, 6].
Customized esthetics
In the crown-and-bridge technique, framework ceramics of aluminum oxide (Al2O3) and zirconium oxide (ZrO2) have successfully established themselves; for esthetic reasons, the frameworks receive fuse-on ceramic veneers. Al2O3, whether glass infiltrated (In-Ceram) or press sintered (Procera), has translucent properties and is thus also suitable for the higher esthetic demands of anterior teeth and premolars. Zirconium oxide (ZrO2) – whether in its green state (Cercon, Everest, In-Ceram YZ, inCoris, Lava, Procera, Zeno, Zerion i.a.) or milled from a hot isostatic pressed (HIP) blank – has proven itself for crowns and bridges in the posterior dental arch [7]. Without sacrificing strength, the pure white-colored framework can be stained in dentin hues to deepen the ultimate, natural overall color impression. In this way, the veneering can be kept thin. As a consequence, also given the thin crown walls, more tooth substance is conserved on the crown stump (Fig. 2). The literature shows that the interplay of ZrO2, substance-conserving crown caps, and thin veneers makes it possible to reduce the preparation depth below what is required for porcelain-fused-to-metal crowns [7]. New, cost-saving procedures such as the press-on technique meanwhile allow pre-formed veneers of fluorapatite sintered glass ceramic to be pressed on (Fig. 3) or, alternatively, subtractively milled and then sintered onto the framework [8]. Chipping (veneer fractures) can be avoided if the crown frameworks are shaped to support cusps, thus obviating tensile forces in the veneer [9, 10].
The most recent development for highly esthetic crowns is lithium disilicate ceramic (e.max CAD, Fig. 4). In terms of optical quality and flexural strength, this material occupies a place between feldspathic and oxide ceramics; with increasing wall thicknesses, the translucence gradient exhibits rapidly increasing opacity compared to glass ceramic. In their pre-crystallized state, the 150-MPa-strong lithium disilicate blocks are CAM ground. After grinding, sintering is performed for 25 minutes at 840°C, in which the lithium disilicate crystals initiate a structural transformation while the strength simultaneously increases to 360 MPa. In contrast to other CAD/CAM ceramics, lithium disilicate is largely shrinkage-free; the restoration can be checked for fit immediately. The 0.2% density increase associated with sintering is stored in the respective CAD software and is taken into account during the milling process.
Lithium disilicate was the response to the need for a high-strength glass ceramic which could also be
CAM milled quickly and with relatively little tool wear. Further, it was intended to stop uncontrolled crack propagation by virtue of a needle-shaped, matted crystalline structure. For lithium disilicate, a sintering procedure has been created with a two-step crystallization process, in which first, lithium metasilicate crystals (Li2SiO3) precipitate out of the glass matrix, and second, the metasilicate phase dissolves and lithium disilicate (Li2Si2O5) crystallizes. The process is virtually shrinkage-free, and the fracture strength increases markedly [11]. Prior to sintering, the metasilicate glass is initially blue (Figs 5, 6); during sintering, a color change occurs via coloring ions, resulting in exactly the tooth color selected. The blocks can be milled with the CAD/CAM systems Cerec, inLab, and Everest.
Using lithium disilicate, crowns can be milled without a framework, i.e., completely anatomically, then polished and glazed; veneering is not necessary – which helps cut costs. For very high esthetic demands, the cut-back method is used (Figs 7, 8): following anatomic grinding-to-shape, an enamel-thick layer is ground off and a veneer fused on. With this method, the restoration can be individually characterized (Fig. 9). The indications approved by the manufacturer are veneers, partial crowns, crowns in the anterior and posterior regions. Due to their strength, lithium disilicate crowns can be conventionally luted, i.e., with glass-ionomer cement (Ketac, Vivaglass). For clinically short crowns or small retention surfaces, adhesive luting is indicated (Syntac, Multilink Automix).
As a pressable product option (e.max Press), lithium disilicate is available for the pressing technique for crowns and 3-unit anterior bridges up to the 2nd premolar. Due to the fact that the ingots are solid cast, they prevent pore formation during processing (Fig. 10).
Silicate, lithium disilicate, and oxide ceramics present the dental profession with materials suited to meet the multiple demands of restorative and prosthetic dentistry (Fig. 11). Whereas all-ceramic crowns used to more or less belong to the “premium price class”, today they can satisfy the desire for both “customized esthetics“ and economy by using different refinement techniques for surface and color characterization (polishing, glazing, staining, individual shadings, cut-back procedures with veneers, conventional framework veneering).
Manfred Kern, Society for Dental Ceramics, Secretary - e-mail:
[1] Isidor, F., Brondum, K.: A clinical evaluation of porcelain inlays. J Prosthet Dent 1995; 74: 140-149.
[2] van Dijken, J.W.V., Hasselrot, L., Örmin, A., Olofsson, A.L.: Restorations with extensive dentin/enamel bonded ceramic coverage. A 5-year follow-up. Eur J oral Sci 2001; 109: 222-229.
[3] Molin, M.K., Karlsson, S.L.: A randomized 5-year clinical evaluation of 3 ceramic inlay systems. Int J Prosthodont 2000; 13: 194-200.
[4] Qualtrough, A.J.E., Wilson, N.H.F.: A 3-year clinical evaluation of a porcelain inlay system. J Dent 1996; 24: 317-323.
[5] Schulz, P., Johansson, A., Arvidson, K.: A restrospective study of Mirage ceramic inlays over up to 9 years. Int J Prosthdont 2003; 16: 510-514.
[6] Thordrup, M., Isidor, F., Hörstedt-Bindsley, P.: A 5-year clinical study of indirect and direct resin composite and ceramic inlays. Quintessence Int 2001; 32: 199-205.
[7] Sailer, I., Fehér, A., Filser, F., Gauckler, L.J., Lüthy, H., Hämmerle, C.H.F.: Klinische 5-Jahres-Ergebnisse für Seitenzahnbrücken mit Zirkoniumdioxidgerüst, hergestellt mit einem Prototyp-CAM Verfahren. Quintessenz ZT 2008; 34: 86-95.
[8] Schweiger, J., Beuer, F., Eichberger, M.: Sinterverbundkronen und –Brücken, neue Wege zur Herstellung von computergefertigtem Zahnersatz. Digital Dental News 2007, 5: 14-21.
[9] Tinschert, J., Natt, G., Latzke, P., Schulze, N., Heussen, N., Spiekermann, H.: Vollkeramische Brücken aus DC-Zirkon; ein klinisches Konzept mit Erfolg? Deutsch Zahnärztl Z 2005; 60:435-445.
[11] Bürke, H.: TEM-Grundlagenuntersuchungen zur Gefügeentwicklung in aushärtbaren Glimmerglaskeramiken. Dissertation, Universität Würzburg, 2004.
Figures:
Fig. 1: Adhesively luted inlays of leucite-reinforced silicate ceramic have achieved a survival rate of 90% after 18 years.
Fig. 2: Thin-walled (0.3-0.5 mm) ground anterior crown caps of ZrO2(LAVA) save hard tooth substance.
Fig. 3: Pressed-on veneering of fluorapatite sintered glass ceramic on a lithium disilicate framework, individualized fissures.
Fig. 4: Implant crown of lithium disilicate, region 36, milled framework-free, upon insertion on the titanium abutment.
Fig. 5: CAD/CAM-milled crown of lithium disilicate with initial blue color before sintering.
Fig. 6: Lithium disilicate crown after sintering with final tooth color. Different stains allow individualization.
Fig. 7: Anatomically reduced lithium disilicate framework for anterior crowns.
Fig. 8: Application of veneers.
Fig. 9: Framework-free lithium disilicate crown, enamel thickness ground back in cut-back method, manually veneered and individualized.
Fig. 10: Lithium disilicate crowns made with the pressing technique, glazed.
Fig. 11: This vademecum of all-ceramic restoration consolidates international specialist knowledge, and — thanks to the authors’ years of experience — gives the clinical methodology a common denominator.
Kunzelmann, K.-H., Kern, M., Pospiech, P., Raigrodski, A. J., Strassler, H.E., Mehl, A., Frankenberger, R., Reiss, B., Wiedhahn, K.: All-Ceramics at a Glance. 1st English Edition. 98 pages, size 8.26x8.26”, Hardcover, Sales Price 49.00
US-$, ISBN 978-3-00-021677-0.
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