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SCIENTIFICALLY SECURE.   1 LONG-TERM PERFORMANCE OF CEREC RESTORATIONS   2 PRECISION   3 MARGINAL GAP   4 OCCLUSAL DESIGN   5 AESTHETICS   6 CERAMIC MATERIALS

3 MARGINAL GAP

The performance of the luting materials and the chosen bonding technique have a decisive impact on the success of all-ceramic restorations.

3.1.1 MATERIALS

Metal restorations rely principally on macroretention. By contrast etchable all-ceramic materials (silicates/disilicates) are luted directly to the hard dental tissues and rely on microretention. The bond-ing of CEREC restorations (VITA Mark II, Ivoclar Empress CAD) does not differ from the bonding of laboratory-fabricated inlays, onlays and veneers made of comparable materials. This procedure has remained virtually unchanged since the introduction of dentin adhesives in 1991.

The first step is the CONDITIONING (e.g. etching) of the enamel, dentin and ceramic with the goal of creating a clean micro-roughened surface.

This is followed by the application of a PRIMER, the function of which is to make the clean surface wettable for the hydrophobic bonding material. The third logical step is BONDING – i.e. the application of an unfilled bonding resin, which forms an intermediate layer between the tooth surface, the luting composite and the ceramic material.

Older adhesive systems consist of separate products for each of these steps. The newer systems try to reduce the number of bottles needed.

CONCLUSION The adhesive bonding of silicate ceramics has been proved over a period of many years. The various materials must be carefully matched.

High-strength oxide ceramics such as inCeram, aluminium oxide and zirconium oxide do not lend themselves to etching and hence can be conventionally cemented. Self-adhesive luting materials have meanwhile become available.

Luting composites fall into three different categories: chemically cured, light-cured and dual-cured.

This ten-year study compared CEREC 2 inlays which had been luted either with chemically cured or dual-cured composites.

The success rate after ten years was 77% in the case of dual-cured composite and 100% in the case of chemically cured composite.

CONCLUSION Dual-cured composites should be used only in situations in which chemically cured or light-cured composites are un-suitable.

3.1.2 MARGINAL SEAL

Shortly after the introduction of CEREC there were naturally no long-term studies to draw upon. It was therefore necessary to establish whether the width of the luting interface (i.e. the thickness of the luting composite layer) had any influence on the marginal seal.

All the investigations showed that it was advantageous to locate the restoration margin in the enamel. This in vitro study (which involved dye penetration tests) showed that the thickness of the luting composite layer did not have any influence on the marginal seal. Subsequent long-term studies of CEREC 1 and CEREC 2 restorations confirmed these findings.

CONCLUSION The thickness of the luting composite layer does not have any influence on the marginal seal.

In relation to deep cavities the question is whether a liner plays a beneficial role for the protection of the pulp.

According to a study carried out by N. Krämer/Erlangen the occurrence of initial hypersensitivity doubled in cases where a liner was laid. The failure rate of ceramic inlays (in this case Empress) trebled when a liner was deployed.

CONCLUSION The placement of liners under ceramic inlays/onlays is contra-indicated.

3.1.3 WEAR OF THE ADHESIVE INTERFACE

Various Empress inlays placed using Variolink low (low viscosity) and Tetric (high viscosity) were measured in order to determine the wear of the luting composite in highly loaded areas.

After 8 years the mean interfacial width had increased from 176 μm to 207 μm.

The two different composites did not exhibit any significant statistical differences.

CONCLUSION Low-viscosity and high-viscosity composites are suitablefor the placement of CEREC inlays and onlays.

3.2 COMPARISON OF OTHER RESTORATION TYPES

Temporaries and methods of bonding labside and chairside produced ceramic inlays have a decisive influence on enamel defects and the margin quality.

3.2.1 ENAMEL INTEGRITY
Inlay cavities treated with temporaries demonstrate, after simulated temporary wear in a chewing simulator, a deterioration of the enamel integrity. In particular in both oral and vestibular surfaces cracks developed. By cavities treated immediately with chairside produced ceramic inlays no such enamel defects arose.

CONCLUSION Chairside produced CEREC inlays offer the treated tooth a reduced risk of enamel cracks, due to there being no provisional.

3.2.2 MARGIN QUALITY
The analysis of bonding systems demonstrated that conventional bonding is still superior to self-adhesive systems. Selective enamel etching as used with CEREC inlays enhances the bond with the hard tooth tissue and improves the quality of the enamel margin. In contrast to the general assumption a broader adhesive gap does not result in inferior margin quality.

Measurement results of damage to the enamel marginal edge (with or without temporaries). Chairside ceramic treated cavities show fewer enamel cracks [TML: thermocycling and mechanical loading].

CONCLUSION Immediate treatment without temporaries has a stabilizing effect on the tooth substance. Attainable bonding technology takes into account a broader adhesive gap.