yield point

Composites also composite (from the Latin componere = to compose) are tooth-coloured filling materials with plastic properties used in dental treatment. In lay terms they are often referred to as plastic fillings, also erroneously sometimes confused with ceramic fillings due to their tooth colour. After being placed in a cavity they cure chemically or by irradiating with light or a combination of the two (dual-curing). Nowadays, composites are also used as luting materials. The working time can be regulated with light-curing systems, which is a great advantage both when placing fillings and during adhesive luting of restorations. Dual-curing luting materials are paste/paste systems with chemical and photosensitive initiators, which enable adequate curing, even in areas in which light curing is not guaranteed or controllable. Composites were manufactured in 1962 by mixing dimethacrylate (epoxy resin and methacrylic acid) with silanized quartz powder (Bowen 1963). Due to their characteristics (aesthetics and advantages of the adhesive technique) composite restorations are now used instead of amalgam fillings.

History of composite development

The essential development stages of composite materials can be illustrated as follows:

• 1933 discovery of PMMA
• 1951 PMMA-based composite
• 1955 acid-etch technique (Buonocore)
• 1962 Bowen monomer
• 1963 first composite with quartz fillers
• 1970 light-curing as an idea
• 1974 microfilled composites
• 1977 light-curing composites commercially available
• 1980 hybrid composites (literature)
• 1985 hybrid composites commercially available
• 1990 total-etch technique as a standard procedure
• 1993 "compomers"
• 1998 "ormocers"
• 2000 nanofillers

The material consists of three constituents: the resin matrix (organic component), the fillers (inorganic component) and the composite phase. The resin matrix mainly consists of Bis-GMA (bisphenol-A-glycidyldimethacrylate). As Bis-GMA is highly viscous, it is mixed in a different composition with shorter-chain monomers such as, e.g. TEGDMA (triethylene glycol dimethacrylate). The lower the proportion of Bis-GMA and the higher the proportion of TEGDMA, the higher the polymerisation shrinkage (Gonçalves et al. 2008). The use of Bis-GMA with TEGDMA increases the tensile strength but reduces the flexural strength (Asmussen & Peutzfeldt 1998). Monomers can be released from the filling material. Longer light-curing results in a better conversion rate (linking of the individual monomers) and therefore to reduced monomer release (Sideriou & Achilias 2005) The fillers are made of quartz, ceramic and/ or silicon dioxide. An increase in the amount of filler materials results in decreases in polymerisation shrinkage, coefficient of linear expansion and water absorption. In contrast, with an increase in the filler proportion there is a general rise in the compressive and tensile strengths, modulus of elasticity and wear resistance (Kim et al. 2002). The filler content in a composite is also determined by the shape of the fillers.

Classification based on Lutz is still included in virtually every textbook and based on the differentiation according to macro, micro and hybrid composites and also describes the microfiller composites which include pre-polymers.

There are now basically three different classifications:

1. according to the consistency
2. according to the filler content
3. according to the base chemistry (matrix)

All three versions are correct but only give a clear evaluation of the materials in their interplay as, for example different matrix components can be combined with various filler particles.

   Minimally-invasive preparation and     indiscernible composite restoration

Composite restorations

Conclusion

Development of high-performance filling materials is crucial for successful dental treatment. It is important to note that, apart from the filling materials, there are also additional aspects that are crucial for successful treatment. These are reliable and correct use of the adhesive system, a patient who performs good oral hygiene and, last but not least, a dentist who processes the material carefully and correctly. (Hickel & Manhart 2001). As in all areas of dentistry the saying "Practice makes perfect" applies.