Fixation of thin glass ceramic restorations

The influence of heating highly filled composites as luting materials on their properties

Composites are among the most frequently used tooth-colored materials in dentistry. They are an elementary component of minimally invasive treatment concepts. In addition to the indication “direct restoration”, high-viscosity composites can be used for the adhesive integration of glass-ceramic restorations. To make handling easier, the composites are heated before use, for example. But what influence does this process have on its mechanical properties?

For the overall stability and thus the survival rate of a restoration, the strength of the restoration, the strength of the luting material and the optimal bond between tooth structure, restoration and luting material are essential. For ceramics with lower strengths (below 350 MPa), adhesive bonding (bonding the restoration to the tooth surface) is necessary [1]. It is important to note that the higher the strength of the luting material, the higher the overall stability and survival rate of the restoration.

The mechanical properties of composites can be improved by adding higher inorganic components (ceramic filler particles) and the targeted selection of the monomers they contain [2, 3]. However, increasing filler proportions cause a higher viscosity of the composite. This in turn can complicate clinical application. Direct high-viscosity composites show higher strengths. Therefore, in practice they are often used to cement thin glass-ceramic restorations.

Glass ceramic inlays on the model (Image: Carsten Fischer)

However, it is necessary for the treating team to be able to influence the rheological properties (viscosity) of the solid but highly filled composites and thus ensure easy handling. This is possible using ultrasound [4] or prior heating [5-7]. Special small preheating ovens (e.g. AdDent Inc. or Calset heater, ms dental) are now commercially available for practice. These enable the use of heated composites. Studies on the flowability of preheated nanohybrid composites showed a decrease in viscosity of 25%. An even greater decrease was observed with microhybrid composites (reduction of 70%) [5-7].

Simplified handling

The reduction in viscosity through heating is advantageous for the adhesive bonding of indirectly manufactured ceramic inlays, onlays or veneers. The decrease in viscosity also makes it easy to use composites with a high filler content for adhesive bonding and increases the overall stability of the restoration.

Heating several composite capsules (left) and an applicator with a fixed composite capsule (middle) in a special preheating oven (AdDent Inc.). Right picture: Highly viscous composite (Tetric Ceram A3) in cool (top) and warmed states (bottom).

Reduction of hypoallergenic reactions and improved edge density

Further advantages of this technology were seen in a higher polymerization efficiency: the conversion rate increases [8, 9] while the required light intensity decreases. For example, it was observed that when the composite is heated to 40 °C, a 50% shorter polymerization time results in the same conversion rate [10, 11]. The same polymerization time, however, results in less residual monomer content, which can minimize the risk of hypoallergenic reactions [12-15]. Furthermore, less leakage was observed in Class II restorations when using a preheated composite [16].

Influence on strength

In another study, the influence of preheated composites on the strength and the bond strength to glass ceramic and dentin was tested [17]. Three different direct composites were compared (Tetric Evo Ceram (Ivoclar Vivadent), Filtek SupremeXT (3M) and Venus (Heraeus Kulzer)). Only the preheated Tetric Evo Ceram (37 to 68 °C) showed higher strength values ​​than the unpreheated control group (25 °C). For the other composites, heating had no influence on the strength values.

Influence on bond strength

Likewise, no negative influence on the bond strength to dentin was observed. The bond strength to glass ceramic showed a positive influence at a temperature between 25 to 68 °C for Tetric Evo Ceram. Improved wettability was observed for all heated composites.

Heating with the heater leads to lower viscosities of the composites (improved handling), whereas the mechanical properties were not negatively affected. With a heating oven like the Calset heater, you can choose between heating temperatures of 37°C, 54°C and 68°C. Both composite capsules and composite syringes can be heated. The authors can definitely recommend the prior heating of a composite to make it easier to attach glass-ceramic restorations.

Authors: Annett Kieschnick, Bogna Stawarczyk, Anja Liebermann

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