The composite resin, as its name implies, consists of a mixture of two or more materials. Each of them contributes to the overall properties of the composite resin.
This material is probably the most used in dentistry. We use it in a huge variety of clinical applications. Ranging from fillings and indirect restorations to the endodontic treatment.
The composite resin used in dentistry has three basic components: organic resin, an inorganic filler and a coupling agent.
The resin is a chemically active component of the composite. It is originally a monomer liquid, but turns into a rigid polymer by an addition reaction. It is this ability to be transformed from a plastic mass in a rigid solid which allows this material to be used for the restoration of teeth.
The fillers are added to the composite materials to improve their properties.In addition, fillers offer significant benefits:
- Shrinkage is reduced because the amount of resin used is reduced and the filler does not take part in the polymerization process.
- The ceramic fillers have a coefficient of thermal expansion similar to the tooth tissue.
- They can also improve the mechanical properties such as hardness and strength
- They provide radiopacity, easy to spot in radiography
- The fillers are an ideal instrument for controlling various sensory characteristics such as color and clarity.
In order for the composite to have acceptable mechanical properties, it is very important that the filler and the resin is strongly bonded together.If this bond is not strong enough voltages developed will not be distributed efficiently throughout the material and there is a high probability for it to break.
1) handling characteristics
Composite resins are not inherently adhesive to enamel and dentin, and therefore require etching with phosphoric acid and the application of the binder dentin.
Proper placement of the dental restorative material and careful oral hygiene of the patient is even more important because, if secondary caries occurs ,it tends to evolve much faster faster than with other restorative materials.
Composite resins are complex structures such as the various components and the products released by these materials.
However, this should not be interpreted as an indication that those materials present a risk for the patient.The amount released is very small and hypersensitivity reactions related to composites are rare.
3) Water absorption
The absorption of water should be kept low because the excessive absorption of water has a detrimental influence on the color stability and resistance of the material to wear.
If the composite resin can absorb water,it is also able to absorb other fluids from the oral cavity, which can lead to discoloration.
4) thermal expansion coefficient
To minimize the stress that is developed due to differential expansion and contraction,thermal coefficient of the composite material should be as near as possible to that of the tooth tissue. The fillers have a low coefficient of expansion, while the resins have a high rate, so that the greater the content of the inorganic filler, the lower the expansion coefficient.
When composites are used as a restorative material in posterior teeth their radiopacity is paramount. Detection of caries beneath a non-radiopaque material is weak and will allow the process of dental caries to continue undetected for a long time.