• Outline of the Development of Cercon Zirconia and the Latest Processing Techniques

• High Strength Ceramic in Theory and Practice

• Strength and Reliability of 4-unit All Ceramic Bridges

• Temporary Cementation of Zirconia Based Single Crowns and Bridges – Results of a Clinical Trial

• Clinical Performance of All Ceramic Cantilever Bridges – Initial Report

• Prospective Clinical Trial With Conventionally Luted Zirconia Based Restorations –18 Month Results

• Applying Porcelain Facings to an Au Alloy and CAM Zirconia Ceramic

Outline of the development of Cercon Zirconia and the latest processing techniques

Zirconia – more precisely: Y-TZP (yttrium-stabilized tetragonal zirconia polycrystals) – has undeniably the highest flexural strength and greatest fracture toughness of dental ceramics currently available. Zirconia is suitable even for long-span restorations in the posterior region.

Y-TZP is a well-established material in medicine for the manufacture of hip joints, but a few technical hurdles in the procedure had to be overcome before this high-strength porcelain could be used in the dental sector. Attempts were first made to prepare zirconia in the sintered state. This required high expenditure for equipment and a lengthy processing time, as the material is exceptionally hard.

A team headed by Professor Dr Ludwig Gauckler at the ETH Zürich (Swiss Federal Institute of Technology) developed a procedure for preparing pre-sintered zirconia. In cooperation with the dental team headed by Professor Dr Peter Schärer at the University of Zürich, this led to the development of the DCM system (Direct Ceramic Machining) for dentistry, which allowed for cost-effective fabrication of crown and bridge frameworks in the dental laboratory.

The Dental School of the University of Zürich has been fitting long-span bridges within the framework of a controlled, prospective, clinical study using the DCM system since 1998. The zirconia frameworks exhibited exceptional breaking strength. This study also indicated, however, that the DCM system still required improvement with regard to the accuracy of fit of the frameworks and the facing porcelain (7).

Degussa Dental GmbH (now DeguDent GmbH) purchased the rights to the DCM procedure and, in close cooperation with the two Zürich teams, developed the Cercon smart ceramics system based on the DCM system (Cercon base).

The accuracy of fit was greatly improved with DeguDent’s development of the hardware and software for the combined scanning and milling unit (Cercon brain), and a new facing porcelain (Cercon Ceram S) especially for Y-TZP frameworks was developed ready for the market. Further in vitro tests as well as long-term clinical studies were initiated for clinically testing the Cercon system.
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High-strength ceramic in theory and in practice

Restorations fabricated from high-strength zirconia ceramic are becoming increasingly popular in dentistry, as more companies market their systems and initial findings of clinical studies become available. An effective method of fabricating zirconia restorations is to mill presintered blanks and compact them after milling by sintering. This method reduces milling time and increases the service life of the unit and instruments. Shrinkage during sintering is compensated for by increasing the size of the framework by the shrinkage factor. The Cercon system can currently be used for milling bridges of up to four units, and it is planned to extend this to an anatomical length of 47 mm. (Note: This type of restoration is now possible using Cercon base 47.)

These restorations can be cemented with conventional cements such as zinc-phosphate cements or using adhesives. The interface between cement and tooth has been well researched, but very little has been published regarding the cement/zirconia interface.

Various cements were tested in a test set-up according to Fig. 1+ 2 (zirconia-cement-zirconia) to obtain information about this type of interface (1).

In a sandwich set-up two specimens fabricated from zirconia (cone and disc with a conical hole) were bonded with cement.
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Strength and reliability of 4-unit all-ceramic bridges

The purpose of this study was to determine the in vitro failure load of 4-unit dental bridges made of glass ceramic with lithium-disilicate crystals (Empress2, Ivoclar) and made of zirkonia-reinforced glass-infiltrated alumina (In-Ceram Zirkonia, Vita Zahnfabrik) and to compare the data with those of zirkonia stabilized with 3 mol % yttria (ZrO2-TZP bridges, Cercon smart ceramics, DeguDent).

Per material 15 specimens were fabricated according to the manufacturer’s instructions. All specimens possessed exactly the same dimensions with connectors of 7mm2 cross section area, and all fitted one master base metal model with two posterior abutments. The specimens were placed uncemented on the master model, and the two middle pontics were loaded perpendicularly on their occlusion until failure. The tests were made on a universal testing machine at a crosshead speed of 0.5 mm/min. The failure data were recorded and analyzed with Weibull statistics.
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Temporary Cementation of Zirconia-based Single Crowns and Fixed-partial-dentures Results from a Clinical Trial

This study evaluates the clinical performance of temporarily luted zirconia-based single crowns and fixed partial dentures (FPDs) manufactured with a CAM-system (Cercon system, DeguDent, Germany) in a private practice.

From January 2000 to July 2002, 125 Cercon restorations (44 anterior crowns, 65 posterior crowns and 16 three-unit posterior FPDs) were placed in 53 patients (average age: 43,8 years). All restorations had a minimum framework thickness of 0,4 mm and connector areas ranging from 7– 9 mm2. 20 anterior crowns and 28 posterior crowns as well as 14 FPDabutements were fabricated with a circular ceramic shoulder. All restorations were luted temporarily by using a eugnol-free temporary cement (Temp-Bond NE, Kerr GmbH, Germany) for a period of 2 to 4 weeks. Evaluation parameters were loss of retention, fracture of the framework or chipping of the veneering material during removal of the restoration prior to permanent cementation.
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Clinical Performance of All-ceramic Cantilever Fixed Partial Dentures

This study evaluates the initial clinical performance of all-ceramic cantilever fixed partial dentures manufactured with a CAM-system (Cercon system, DeguDent GmbH, Germany) in a private practice.

From June to December 2002, 11 three-unit cantilever bridges (with one mesial or distal premolar extension) were placed in 9 patients. All restorations had a minimum framework thickness of 0,4 mm and connector areas ranging from 7–9 mm2. All restorations were luted temporarily by using a eugnol-free luting agent (Temp-Bond NE, Kerr GmbH, Germany) for a period of 2 to 4 weeks. A glass-ionomer cement (Ketac-cem µ, 3M Espe, Germany) was used for permanent luting. Evaluation parameters were loss of retention during temporary cementation, fracture of the framework or chipping of the veneering material during removal of the restoration prior to permanent cementation. Moreove, the color match, marginal integrity, and postoperative sensitivity were recorded at baseline (7 to 21 days after permanent cementation) according to modified Ryge criteria.
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Prospective Clinical Trial with Conventionally Luted Zirconia-based Fixed Partial Dentures – 18-month Results

The aim of this prospective clinical was to evaluate the clinical performance of posterior zirconia-based allceramic fixed-partial dentures (FPDs) fabricated with a CAM-System (Cercon, DeguDent GmbH, Germany). Apart from the clinical stability of the framework the effect of different types of veneering porcelain should be evaluated.

From April 2001 to January 2003, 59 all-ceramic posterior FPDs (44 three-unit and 15 four-unit posterior bridges) were fabricated for 46 patients. Two types of veneering materials were used:
A = an experimental veneering porcelain (46 restorations)
B = a ready-to-market veneering porcelain (Cercon Ceram S, DeguDent GmbH, Germany), (13 restorations).
All bridges were luted with a zinc-phosphate cement and evaluated according to modified CDA-criteria every 6 months.

During the evaluation period no fracture of a framework could be detected, neither any sign of secondary caries nor a loss of retention. Two chippings of the veneering porcelain were recorded. Both failures occurred with the veneering material A within the first six months in-situ. The mean clinical observation period for the experimental veneering material (A) was 453 days (success rate according to Kaplan- Meier: 96%) compared to 237 days for the now commercially available porcelain (B), (success rate according to Kaplan-Meier: 100%).
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Ceramic Veneering of Au Alloy and CAM Zirconia Ceramic

The aim of this study was to compare the fracture resistance and marginal adaptation of ceramic veneered computer manufactured Y-TZP zirconia core with ceramic veneered Au-alloy framework.

Human molar teeth were inserted in a PMMA resin with the roots surrounded by a polyether layer simulation the periodontium. 24 three-unit FPDs were fabricated of I) the CAM ZrO2 all-ceramic Cercon (Veneering porcelain: Cercon Ceram S) and II) the Au-alloy framework Degudent H (Veneering porcelain: Duceram Plus; all materials by DeguDent GmbH, Germany). All restorations were adhesively fixed using the resin bonding system Variolink II /Syntac Classic (Ivoclar Vivadent, FL). Marginal adaptation was evaluated at the transitions cement-tooth (�gCT�h) and cement-FPD �gC/FPD�h using scanning microscopy (Stereoscan 240, Cambridge Instr., GB) before and after thermal cycling and mechanical loading (TCML: 6000 x 5�‹/55�‹C, H2O, 2 min each cycle, 1.2* 106 x 50N). The criterion was �gmarginal gap�h with a smooth transition without interruptions. After TCML the fracture resistance was determined (Zwick 1446, v=1 mm/min G). As a control one all-ceramic group was investigated without TCML. Medians and percentiles were calculated and statistical analysis was performed using the Mann-Whitney-U-tests durchgefuhrt (p.0.05).
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