Dental Composite Resin :: In Details ...

Definition

Dental composite resins are types of synthetic resins which are used in dentistry as restorative material or adhesives. Synthetic resins evolved as restorative materials since they were insoluble, aesthetic, and insensitive to dehydration and were inexpensive. It is easy to manipulate them as well. Composite resins are most commonly composed of Bis-GMA monomers or some Bis-GMA analog, a filler material such as silica and in most current applications, a photoinitiator. Dimethacrylates are also commonly added to achieve certain physical properties such as flowability. Further tailoring of physical properties is achieved by formulating unique concentrations of each constituent.Unlike Amalgam which essentially just fills a hole, composite cavity restorations when used with dentin and enamel bonding techniques restore the tooth back to near its original physical integrity.

Composition

Dental composite resin.
As with other composite materials, a dental composite typically consists of a resin-based oligomer matrix, such as a bisphenol A-glycidyl methacrylate (BISMA) or urethane dimethacrylate(UDMA), and an inorganic filler such as silicon dioxide (silica). Compositions vary widely, with proprietary mixes of resins forming the matrix, as well as engineered filler glasses and glass ceramics. The filler gives the composite wear resistance and translucency. A coupling agent such as silane is used to enhance the bond between these two components. An initiator package (such as: camphorquinone (CQ), phenylpropanedione (PPD) or lucirin (TPO)) begins the polymerizationreaction of the resins when external energy (light/heat, etc.) is applied. A catalyst package can control its speed.

Advantages

The main advantage of a direct dental composite over traditional materials such as amalgam is improved aesthetics. Composites can be made in a wide range of tooth colours allowing near invisible restoration of teeth. Composites are glued into teeth and this strengthens the tooth's structure. The discovery of acid etching (producing enamel irregularities ranging from 5-30 micrometers in depth) of teeth to allow a micromechanical bond to the tooth allows good adhesion of the restoration to the tooth. This means that unlike silver filling there is no need for the dentist to create retentive features destroying healthy tooth. The acid-etch adhesion prevents microleakage; however, all white fillings will eventually leak slightly. Very high bond strengths to tooth structure, both enamel and dentine, can be achieved with the current generation of dentine bonding agents. The downside[vague] to composite when compared to amalgam is a shorter lifespan of the filling, and the high likelihood of requiring root canal therapy if the failure of the filling is not caught quickly. Amalgam fillings may crack a portion of the tooth off, but otherwise tend to fail at a much slower rate.

Disadvantages

Composite resin restorations have several disadvantages: They are technique-sensitive meaning that without meticulous placement they may fail prematurely. They take up to 50% longer to place than amalgam fillings and are thus more expensive. In addition clinical survival of composite restorations placed in posterior teeth has been shown to be significantly lower than amalgam restorations.[1].
Survival and reasons for failure of amalgam versus composite posterior restorations placed in a randomized clinical trial.

Direct dental composites

Direct dental composites are placed by the dentist in a clinical setting. Polymerization is accomplished typically with a handheld curing lightthat emits specific wavelengths keyed to the initiator and catalyst packages involved. When using a curing light, remember that the light should be held as close to the resin surface as possible, a shield should be placed between the light tip and the operator's eyes, and that curing time should be increased for darker resin shades. Light cured resins provide denser restorations than self-cured resins because no mixing is required that might introduce air bubble porosity.
Direct dental composites can be used for:

• Filling gaps (diastemas) between teeth using a shell-like veneer or
• Minor reshaping of teeth
• Partial crowns on single teeth

Indirect dental composites

This type of composite is cured outside the mouth, in a processing unit that is capable of delivering higher intensities and levels of energy than handheld lights can. Indirect composites can have higher filler levels, and are cured for longer times. As a result, they have higher levels and depths of cure than direct composites. For example, an entire crown can be cured in a single process cycle in an extra-oral curing unit, compared to a millimeter layer of a filling.
As a result, full crowns and even bridges (replacing multiple teeth) can be fabricated with these systems. A stronger, tougher and more durable product is likely.
Indirect dental composites can be used for:

• Filling cavities in teeth, as fillings, inlays and/or onlays
• Filling gaps (diastemas) between teeth using a shell-like veneer or
• Reshaping of teeth
• Full or partial crowns on single teeth
• And even bridges spanning 2-3 teeth

Composite shrinkage

Composite resins have a notorious reputation for shrinking upon curing, however, uses as a dental restorative material focus on low shrinkage composites. Composite shrinkage can be reduced by altering the molecular and bulk composition of the resin. For example, UltraSeal XT Plus uses Bis-GMA without dimethacrylate and was found to have a shrinkage of 5.63%, 30 minutes after curing. On the other hand, this same study found that Heliomolar, which uses Bis-GMA, UDMA and decandiol dimethacrylate, had a shrinkage of 2.00%, 30 minutes after curing.[3] In the field of dental restorative materials, reduction of composite shrinkage is a "hot topic". Soon to be introduced are patent pending, is a safe, non-leaching antimicrobial agent which minimizes recurrent decay of the tooth and reduces the harmful effects of micro-organisms and which some may cause gingivitis and periodonttitis (periodontal disease).

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