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Written by David A. Little, DDS; Justin J. Crocker, DDS Sunday, 01 December 2002 00:00
Unquestionably, one of the most intensely satisfying aspects of cosmetic dentistry is surpassing patient hopes by delivering more than is expected. Through the evolution of products and processes in the field, our chief goal, restoring teeth to their original, natural beauty is both achievable and routine. Made possible though advanced technology, an innovative technique now offers the option of all-ceramic single- and multiple-unit restorations.1 Previously, dentists had no choice but to rely on porcelain-fused-to-metal (PFM) restorations for strength and reliability in single- and multiple-unit restorations, and using a metal substructure made creating lifelike restorations extremely challenging and sometimes unpredictable.Today, however, dentists can provide patients with very predictable, natural-looking, metal-free restorations in both the anterior and posterior areas of the mouth, retaining needed strength and intensifying aesthetics.2
This article presents four cases using a new technology the Cercon System (DENTSPLY Ceramco)that provides patients with all-ceramic bridges, even with multiple pontics with a span up to 38 mm.3,4
MATERIALS AND TECHNIQUE
The Cercon System can be used with the Illuminessence Shade Guide, which provides eight shades brighter than B2 from which to choose, thus providing a wide range of shades to match teeth that have been professionally whitened. The system utilizes the translucent ceramic Zirconia and an innovative computer-aided manufacturing process.5 Further, this new technology utilizes conventional crown and bridge preparation and cementation guidelines that have been in practice for years, an integration of the old with the new.6
Cercon Zirconia exhibits high strength (900 MPa) and fracture toughness (9 MPam, 0.5)4 (Figure 1). Zirconia, modified with a small addition of yttria (yttrium oxide, Y203) exhibits a unique property called transformation toughening. Ceramic restorations typically fail through microcrack propagation, initiated and driven by repeated mastication stresses. The high stress at a microcrack tip causes Cercon Zirconia to change instantaneously from a tetragonal crystal structure into a monoclonic form that is slightly (approximately 4%) higher in volume.4 At the microcrack tip, the expansion acts to clamp the crack shut, resisting crack propagation (Figure 2), thus enhancing strength and toughness.7,8
The dentist sends an impression of the prepared restoration area to the laboratory. Standard all-ceramic preparation guidelines apply (Figure 3). The laboratory duplicates the preparation and designs a restoration in wax. The wax model is placed in the Cercon brain and scanned by a precision confocal laser. The pattern is then machined out of a Cercon base blank of presintered Zirconia in an enlarged size. Zirconia, like most high-strength ceramics, shrinks on sintering. The enlargement factor compensates for the sintering shrinkage and is contained in a barcode for each blank.4 A combination of coarse and fine diameter milling tools is used, and the milled coping, or framework, is sintered in the Cercon heat furnace. After sintering, the dimensions of the Cercon framework match the dimensions of the wax pattern. The sintered Cercon framework is veneered with the aesthetic, low-fusing Cercon Ceram S veneering porcelain (DENTSPLY Ceramco), which is engineered to be compatible with Zirconia. The cementation surfaces of the restoration are lightly sandblasted with alumina by the laboratory. The restoration is ready to be placed using conventional cementation or adhesive bonding.
The Cercon preparation follows principles common to most all-ceramic preparations.9 The Cercon Smart Ceramics Prep Kit (Axis Dental) offers the appropriate selection of instruments to achieve the desired prep design for Zirconia copings (Figure 4). Breaking contact is achieved with the C858-014 bur. Occlusal and lingual reduction of at least 2 mm, with smooth contours, is done quickly with the egg-shaped SC379-023 diamond instrument. A modified shoulder diamond (SC847Kr-016) is ideal for the preparation of tooth margins for Cercon restorations. This instrument produces smooth, rounded internal angles with a 1.5-mm shoulder margin. A 6 to 10 taper is provided naturally by the taper of the rotary instrument. Finishing of the margins and axial walls is accomplished by using a matching 12-bladed carbide (H336-016) or matching diamond (F847Kr-016). Figure 5 illustrates Cercon preparation guidelines for anterior teeth.
Figure 6 shows a preoperative smile of a 51-year-old male patient who presented with a PFM cantilever bridge involving teeth Nos. 6 and 7 (replacing No. 7), and a PFM crown on tooth No. 9 with gingival recession exposing the margins. In order to satisfy the patient's demand for an aesthetic metal-free bridge, Cercon was treatment planned (three-unit bridge Nos. 6 through 8, single-unit Cercon Nos. 9 and 10). The Cercon System can be cemented or bonded with the luting material of the clinician's choice. In this case, the bridge and single-unit crowns were cemented with Rely X Luting Cement (3M ESPE). Figures 7 and 8 are final retracted views of the Cercon restorations.
|Figure 1. Comparison of flexural strength of Cercon to other all-ceramic materials.||Figure 11. Cercon crowns on a duplicate model poured with a soft tissue index to aid laboratory technicians in visualizing gingival architecture in order to fabricate proper gingival embrasures.|
|Figure 2. Even when enough stress is added to crack zirconium oxide it actively resists crack propagation through transformational toughening.||Figure 12. Occlusal view of restored Cercon crowns.|
|Figure 3. The Cercon System.||Figure 13. Facial view of restored Cercon crowns.|
|Figure 4. Cercon Smart Ceramics preparation set (Axis Dental LS-7529).||Figure 14. Pretreatment view of four-unit anterior bridge.|
|Figure 5. Cercon anterior tooth preparation guidelines.||Figure 15. Cercon Zirconia anterior framework.|
|Figure 6. Pretreatment photograph of anterior teeth. Discolored cantilever bridge.||Figure 16. Final view of Cercon four-unit bridge.|
|Figure 7. Postoperative treatment of anterior teeth restored with Cercon Ceramics.||Figure 17. Pretreatment view of failing cantilever bridge.|
|Figure 8. Postoperative treatment of anterior teeth restored with Cercon Ceramics.||Figure 18. Cercon Zirconia framework.|
|Figure 9. Facial view of upper premolar teeth with failing amalgam restorations.||Figure 19. Cercon crowns, teeth Nos. 6 through 10.|
|Figure 10. Occlusal pretreatment view.||Figure 20. Cercon restoration. Note incisal translucency.|
In the final analysis, patient satisfaction and product effectiveness are the benchmarks for all innovations in a professional environment filled with dynamic changes. All-ceramic restorations can withstand functional tests,10 and metal-free crowns and bridges can be prescribed with confidence in their strength, longevity, and marginal integrity.11 Both in clinical and in-vitro tests, Cercon Zirconia has proven to be a strong and reliable dental ceramic. It successfully resists the thermo-mechanical stresses of the oral environment to offer a desirable aesthetic and biocompatible restoration for crowns and bridges.12,13
With appreciation, the authors wish to acknowledge the ceramic talent and laboratory support of Peter Finke, MDT.
1. Shurzenegger B, Feher A, Luthy H, et al. Klinische studie von Zirkonoxidbrucken im Seitenzahngebiet herge-stelt mit dem DCM-system. Acta Med Dent Helv. 2000;5:131-139.
Dr. Crocker graduated from and is currently a part-time clinical professor in the Department of General Dentistry at The University of Texas Health Science Center at San Antonio. He maintains a full-time practice in San Antonio concentrating on cosmetics and implant dentistry.
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