Glass-ceramic composites are a group of materials in the field of dental technology. Due to their material properties, they are suitable for the indirect fabrication (fabrication in the dental laboratory after taking an impression) of framework-free restorations (restorations) such as esthetic-looking tooth-colored inlays (inlay fillings), onlays (inlay fillings incorporating the cusps of the tooth) and metal-free full crowns as well as for tooth-colored veneering, e.g. of telescopic crowns (telescopically interlocking double crowns). Composites are plastics based on methyl methacrylate or its chemical derivatives, which have been further developed to improve the material properties, and fillers embedded in them, such as minute glass-ceramic particles. Chemical curing of the resin base material can be initiated by the addition of appropriate initiators (triggers of the chemical reaction) both chemically and by light; the latter makes processing very user-friendly. Composites are also frequently used in the direct restorative technique in dental practice; however, for indirect production in the laboratory they have a particularly high glass-ceramic filler content of approx. 75 %. Together with the better finishing possibilities under laboratory conditions and the lower residual monomer content (monomers: individual components from which the larger macromolecular compounds, the polymers, are formed by agglomeration) due to a higher degree of polymerization, they thus possess clearly superior material properties compared to fillings produced directly in the mouth. These are explained in more detail below:
Glass-ceramic composite materials are used for the fabrication of indirect fillings such as inlays and onlays, as well as crowns or veneers. These potential applications are a result of the following material properties, which should be seen in particular in comparison to ceramic materials:
- Excellent impact strength, which is about twice the value of veneering ceramics; impact strength says something about the resistance of a material to a sudden force;
- Extra high modulus of elasticity (inherent strength), resulting in a high load capacity;
- Excellent flexural strength, significantly higher than that of veneering ceramics; this property has a positive effect, for example, in a – in itself to be avoided – cusp early contact;
- The natural tooth similar abrasion (abrasion), thereby antagonists (in contact tooth of the opposing jaw) more gentle than ceramic;
- Faster and therefore cheaper processing in the dental laboratory than ceramics;
- Color stability, however, inferior to ceramics;
- High aesthetics; this is still slightly inferior to ceramics, but reaches a very high level due to tiny, sometimes nano-fine ceramic fillers;
- Chameleon effect in inlays, onlays and crowns, i.e. the material passes on the color of the neighboring teeth and the tooth supplied with it and thus fits aesthetically into the tooth row.
Indications (areas of application)
- Use for bruxism (grinding and clenching of teeth).
- For the treatment of patients in whom it is to be feared in advance that the antagonists (teeth of the opposing jaw with which the tooth to be restored comes into contact when closing the mouth) could react sensitively to the harder ceramic due to the bite.
- Slight cost savings in the dental field due to time savings, with no cost differences found in the dental calculation to the ceramic.
- The veneering of telescope crowns is generally due to the manufacturing process not ceramic, but with composites.
Contraindications
The key contraindication to composite restorations of any kind are very rare allergic intolerance reactions to the unavoidable residual monomer, which must be ruled out in advance by an allergist if suspected. If the results are positive, preference should be given to restorations and crowns made of biologically inert ceramics. In this case, however, the inlays, onlays or crowns should not be cemented with thin-flowing luting composite, as is usually the case, but with conventional cements, which has a negative effect on the marginal seal and the adhesion of the restoration to the tooth.
The process
The explanation of the procedure is based on the example of inlay production:
- Preparation of the tooth to be restored by preparation (grinding) is carried out by the dentist; this is done according to aspects that allow the finished inlay to fit into the tooth in a specific direction of insertion and take into account a minimum layer thickness of the inlay for reasons of material stability;
- After the dental treatment, an impression of both the affected jaw and the opposing jaw is taken in order to be able to relate the tooth to be restored to its antagonists for the surface design of the inlay;
- The impression (“negative”) is poured with special plaster in the dental laboratory: The plaster model (“positive”) serves the dental technician as a basis on which, after model preparation and isolation of the tooth stump (model of the impression tooth to be restored).
- The glass-ceramic composite material is applied layer by layer with special modeling instruments; the so-called modeling takes place. In this process, according to the natural tooth, for the imitation of dentin and enamel materials of different color, opacity and translucency (translucency) are available, which are applied in layers and in between light polymerized (cross-linked by light as a trigger of the chemical reaction and thus cured). Effect materials, which are incorporated into the enamel layer, for special color characterization are also available.
- Particularly translucent (translucent) effect materials, which are applied at the contact points to the adjacent teeth, provide the chameleon effect.
- Final polymerization (final curing).
- Finishing with special sets matched to each other in the components.
Possible complications
These may arise from the biocompatibility (biological compatibility) of acrylics and adhesive (adherent) luting technique to the dentin. The crucial role is played by the unavoidable content of residual monomer in the final polymerized material, which poses the following risks:
- Diffusion of residual monomer into the pulp (tooth pulp), thereby triggering pulpitis (tooth pulp inflammation);
- Very rare allergic reactions (see contraindications);
- A not clinically confirmed potential carcinogenicity (carcinogenic effect).
- Also an estrogen-stimulating effect is suspected, without this being experimentally proven.
