• From Metal Ceramics To All Ceramics	• Technology And Medical Applications
• System Description	• Zirconia As A Prosthodontic Material
• Preparation	• Impression
• Luting	• Special Aspects in Fabricating
• Special Clinical Recommendations	• Access cavities and removal of restorations
• Clinical experience

From Metal Ceramics To All Ceramics

Resin Veneering
The introduction of the metal-ceramic restoration more than 40 years ago has led to major advances in the fabrication of tooth-coloured prosthetic restorations. The disadvantages of resin-veneered crowns and bridges (e.g. inadequate resistance to discoloration and insufficient wear resistance) were eliminated by the use of dental ceramics as a veneering material.

Progress Through Metal Ceramic Restorations
Metal-ceramic restorations are now the standard in fixed prosthodontics. Consistent improvements in materials have resulted in optimized aesthetics, especially in the cervical region.

Improved Aesthetics In The Cervical Region
The following materials and techniques are used to optimize aesthetics:
• Gold-coloured alloys
• Modified framework designs
• Ceramic shoulder

Improved Bonding
Today, the risk of insufficient bonding between the metal framework and veneer material is no longer an issue, at least not in high-noble alloys. Considering all possible sources for failure, we can assume that the failure rate of a metal-ceramic restoration is 1–1.5% p.a. After many years of clinical application, metal-ceramic restorations can be considered safe and clinically proven. They are now the standard against which to evaluate innovative restorative methods in dental prosthetics. However, the metallic framework’s lack of transparency has a limiting effect on the aesthetic success of a metal-ceramic restoration. Moreover, there is a risk of metal incompatibility (e.g. nickel, cobalt, chrome) especially with non-precious metalceramic alloys.
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Technology And Medical Applications

Proven in Medicine for 30 Years
Zirconia (more precisely: Y-TZP, yttria-stabilized tetragonal zirconia polycrystals) has been used successfully in orthopaedics for artificial hip joint heads since 1969. More than 400,000 applications have been documented worldwide. Numerous aging experiments and mechanical stress tests have shown that the mechanical properties of Y-TZP are sufficient for clinical applications in orthopaedic or dental implants even after 50 years of use. In various in-vitro and in-vivo experiments, Y-TZP showed neither mutagenic (chromosome aberration test) nor carcinogenic (Ames test) effects. It was reported across the board that no local toxic effects were to be expected when zirconia ceramics made contact with bones or soft tissue.

Until now, zirconia has been used in dentistry predominantly as prefabricated root posts, orthodontic brackets or implant abutments. Without exception, zirconia has been processed in a densely sintered state and is associated with heavy wear on tools and a considerable expenditure of time. Based on processing the material in a presintered state, the Cercon system is a new efficient and economical technique for manufacturing crowns and bridges in the dental laboratory.

Significantly Stronger
The static fracture strength of milled three-unit posterior FPDs on a zirconia base is 2–3 times higher compared to three-unit bridges made of pressable ceramics (Empress II) or infiltration ceramics (In-Ceram Alumina). The fatigue strength of Y-TZP is about three times higher than that of In-Ceram Alumina. When comparing the fracture strength of posterior allceramic crowns, zirconia restorations produced with the Cercon system show a significantly higher fracture strength than current commercial all-ceramic systems (In-Ceram Zirconia, Procera, Empress II). In-vitro studies on the fracture strength of anterior crowns revealed that the fracture strength of Cercon restorations is comparable with the fracture strength of metalceramic crowns based on precious metal alloys.
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System Description

The Intelligent Way of Processing Zirconia:
The Cercon system is based on a development by Swiss researchers at the renowned Federal Institute of Technology in Zurich (ETH), in cooperation with the Zurich Dental School. The procedure was described in the literature several times under the name of DCM (direct ceramic machining). In cooperation with Degu- Dent, it was finally optimized and launched. In this process, the die or the wax model of a crown or FPD framework is optically scanned with a laser. In a computer-aided process, an expanded structure is milled from a presintered Y-TZP blank and then sintered densely.

Scanning and cutting take place in the Cercon Brain unit. The cutting process is two-staged. While the rough shape is carved in an initial pre-milling process, a second step involves fine-trimming of the framework.

Shorter Processing Time, Lower Tool Wear
The milled object is sintered in the Cercon Heat unit for six hours in a thermal process, at a final temperature of 1,350 °C. The entire sintering process takes about six hours. Because the sintering process is associated with volumetric shrinkage of the milled object, the framework cut from the blank shows expansion by about 30 %. This expansion is also duplicated in the Cercon Brain unit component using a computer calculation after the scanning process. This is possible because the shrinking behaviour of the Cercon base zirconia blanks can be determined very precisely during production and is therefore predictable. The duration of the scanning and milling process will depend on the size of the object, the number of objects processed and the procedural steps selected (CAM or CAD/CAM). This process will take no longer than 95 minutes for the most extensive objects.

The densely sintered zirconia framework with its precise fit is then veneered with the Cercon Ceram kiss veneering ceramic developed especially for this purpose.You will find detailed information about the Cercon all-ceramic system in separate publications by DeguDent or on the Internet at www.cercon-smartceramics.de.
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Zirconia As A Prosthodontic Material

Translucency
Partially yttria-stabilized zirconia not only has the advantage of being extremely strong, but the material is also translucent. Its translucency of approximately 50% of the incident light allows the fabrication of naturallooking restorations. At the same time, this semi-opacity allows the use of its material even on discoloured tooth substance.

Tooth Coloured Framework Material:
To further optimize the esthetic properties of the restoration, pre-coloured zirconia blanks may be used. Cercon base colored is an ivory-coloured variant of this framework material suited especially for anterior restorations.

Biocompatibility
Thanks to its aesthetic properties, its proven excellent biocompatibility and its low coefficient of thermal conductivity, partially yttria-stabilized zirconia is an ideal material for FPDs. It has numerous clinical handling advantages over previously known all-ceramic systems.

Indications
All-ceramic restorations made of partially yttria-stabilized zirconia and produced with the Cercon system have been approved for the following indications in the anterior and posterior regions:
• Anterior or posterior single crowns
• End-abutment bridges of up to 47 mm in anatomical length in the anterior and posterior regions. The number of teeth replaced must be limited to two molars per bridge span.
• Inlay bridges for replacing a single tooth with a maximum pontic width of 10 mm.
• All-ceramic primary crowns.

Contraindications (patient-related)
Bruxism and refractory parafunctions are general contraincidations.
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Preparation

Not Suitable: Tangential Preparation And Shoulder Preparation With Bevelled Edge
The preparation guidelines for Cercon restorations are geared towards the known recommendations for all-ceramic systems. Accordingly, the tangential preparation and preparation of a shoulder with a bevel are not suitable for zirconia restorations, because both preparation forms result in margins that drain off too thinly and hold the risk of fracture.

Recommended: Chamfer, Shoulder With Rounded Axio-Cervical Line Angle
The following preparation shapes are suitable:
• Pronounced chamfer (90°)
• Shoulder preparation with rounded axio-cervical line angle
Cylindrical diamond instruments with rounded tips are suitable for chamfer preparations. Conical diamond instruments with rounded edge are especially suitable for shoulder preparations with rounded axio-cervical line angles.

Circular Cutting Depth 1.0mm
The use of rotary instruments with an average grit size of 30 µm is recommended for finishing the preparation. The minimum wall thickness of the zirconia frameworks is 0.4 mm, while the minimum amount of space in the cervical region required for veneering is 0.6 mm. A circular marginal cutting depth of 1.0 mm should therefore be the objective.
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Impression

Recommended: The Double Cord Technique
A good reproduction of the preparation margin is achieved by using braided retraction cords (e.g. Ultrapak, Ultradent Products, USA). Good and reproducible impressions can be achieved with the double-cord technique.With this technique, an initial thin cord is placed in the sulcus, where it remains throughout the impression-taking process. A second, thicker cord is then placed over the first, but removed shortly before the impression is taken. The first cord is to block bleeding from the sulcus. At the same time, it keeps the gingiva from folding back over the preparation limit.

Custom Impression Tray
With the one-step putty-wash technique, the use of laboratory-manufactured custom trays – or at least the customization of an impression tray (Rimlock tray) – is recommended, along with a distal dam made of lightcuring plastic or thermoplastic material.

Impression Materials
The impression can be taken with any impression materials used in fixed prosthetics (hydrocolloid, polysiloxanes, polyethers). Both the one-step and two-step putty-wash techniques can be used with polysiloxanes. Only the one-step putty-wash technique can be used with polyether materials.

Tip
With both the one-step and two-step putty-wash techniques, we recommend to spread the impression material in the air stream after application of the lowviscosity component. This ensures good wetting of the die surface.
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Luting

Preparation
The luting surfaces of Cercon restorations should be air-abraded before insertion (alumina, 110 ìm, 3–4 bar). Improved bonding strength is produced by roughing the surface in conventional and adhesive luting.

Conventional or Adhesive?
Both zinc phosphate and glass ionomer cements as well as compomer cements (such as Dyract cem plus, Dentsply, Konstanz, Germany) are suitable for conventional luting. Special conditioning of the die surface is not necessary in conventional cementing.

Tip
Based on current knowledge, Panavia 21 and Panavia F can be recommended for adhesive luting of Cercon restorations. Provisional luting of Cercon restorations can be done with any temporary cement (e.g. Temp Bond, Kerr GmbH, Karlsruhe, Germany). If adhesive luting is planned for a later stage, the use of a noneugenol temporary cement is recommended.

Adjustment
Fine-grained diamond instruments (mean grain size: 15 µm) are recommended for adjusting the occlusal contact points. Subsequent intraoral polishing can be performed using diamondized rubber cups and sometimes ceramic polisher.
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Special Aspects In Fabricating

Cercon FPDs have beenclinically proven for more than five years
Thanks to its excellent mechanical properties, zirconia is especially well-suited for fabricating all-ceramic posterior FPDs.Based on more than five years of clinical experience, the use of three- and four-unit bridges fabricated with the Cercon system can now be considered sufficiently proven.

The connector areas in FPD frameworks must have a minimum cross-sectional area of 9 mm2. The minimum framework thickness must be at least 0.4 mm.

Conventional FPD Cementing
Cercon bridges can be cemented in the conventional way. In the present clinical study, the risk of framework fracture was not increased. To avoid premature loss of retention, the axial abutment height must be at least 0.4 mm. For larger mandibular FPDs spanning more than two premolars to be replaced, adhesive retention is generally recommended.

Temporary FPD Cementing
If no ceramic shoulders are present, the FPD can, in principle, be cemented in place temporarily. For easier removal of the restoration, temporary cements should be used with 20 percent modifier added. If the FPD includes ceramic shoulders, immediate definitive insertion (conventional or adhesive) is recommended, since the ceramic shoulders are more prone to fracture when removing temporarily cemented
FPDs.
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Special Clinical Recommendations For

• All-ceramic inlay FPDs 18
• All-ceramic implant superstructures 19–20
• All-ceramic primary crowns 21–22
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Access Cavities And Removal of Restorations

Trephination In Two Stages
Access cavities for endodontic treatment can be prepared in reasonable time without additional clinical problems if appropriate tools are selected. A two-stage procedure is recommended for the preparation of the access cavity. To begin with, the veneering ceramics should be abraded with a diamond instrument without perforating the framework.

In a second step, the framework ceramic material should be perforated with a diamond-coated instrument of the required size. A distance of 0.5 mm from the veneer ceramics should be maintained. Chipping of the veneer material can thus be prevented while the framework ceramics is being cut.

Tip
The structural strength of Cercon crowns is preserved even after endodontic treatment, so that the restoration can remain in place. The easiest way to close the access cavity is to use a composite filling. Incidentally: The radiopacity of Cercon restorations is similar to that of metal-ceramic restorations.

Cylindrical diamond instruments with rounded tips and diamond grit sizes of 125–150 µm in an angled handpiece (4:1 speed transformation at maximum watercooling) are especially suitable for the preparation of an access cavity and for crown removal.

Removing A Restoration
To remove a Cercon restoration, it must be cut along the axial wall near the midline of the occlusal surface or the incisal edge. The restoration is then bent back with a suitable instrument and fractured. In the case of adhesively luted restorations, residual cement on the surface can be removed with an ultrasonic instrument.
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Clinical Experience

Since April 1998, all-ceramic crown and bridge restorations using the Cercon method have been clinically tested. Already in an early phase of the system’s development, a prospective clinical longterm study was being started at the Zurich University Dental School. So when the system was launched on the market in 2002, there were already more than three years of experience to refer to.

In the meantime, other projects to test the clinical safety of Cercon restorations were initiated. The following table shows the current projects:

1. Prof. P. Schärer, Zurich, Switzerland
Prospective clinical study of 3 to 5-unit all-ceramic bridges in the posterior region
Start of study: April 1998
Number of restorations inserted: 84
Results: No framework fracture after 3 years.

2. Prof. A. Hüls, Göttingen, Germany
Prospective clinical study of conventionally cemented Cercon bridges in the posterior region
Start of study: September 2000
Number of restorations inserted: 73
Results: No framework fracture or failure of the Cercon Ceram S veneering ceramic after
two yeers; two four-unit mandibular FPDs had to be recemented.

3. Prof. C. Hämmerle, Zurich, Switzerland
Prospective randomised study to compare all-ceramic and metal-ceramic posterior bridges
Start of study: May 2002
Number of restorations inserted: 60
Results: No framework fracture and no loss of retention after one year.

4. Dr. S. Rinke, Hanau, Germany
Observation of application with conventionally cemented Cercon single crowns
Start of study: January 2000
Number of restorations inserted: 214
Results: No framework fracture. After an observation period of three years, veneer chipping occurred in four crowns. The restorations did not have to be replaced.
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