Dental composite
Dental composite resins are types of synthetic resins
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...

 which are used in dentistry as restorative material or adhesives. Synthetic resins evolved as restorative materials since they were insoluble, aesthetic, insensitive to dehydration, easy to manipulate and reasonably inexpensive. Composite resins are most commonly composed of Bis-GMA
Glycidyl methacrylate
Glycidyl methacrylate is an ester of methacrylic acid and a common monomer used in the creation of epoxy resins. While typical home epoxies contain diglycidyl ether of bisphenol A , glycidyl methacrylate is instead used to provide epoxy functionalization to polyolefins and other acrylate resins...

 monomers or some Bis-GMA analog, a filler material such as silica and in most current applications, a photoinitiator
A photoinitiator is any chemical compound that decomposes into free radicals when exposed to light. Photoinitiators are found both in nature and in industry ....

. 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
Amalgam may refer to:* Amalgam , mercury alloy* Amalgam , material of "silver" tooth fillings* Amalgam Comics, publisher* Amalgam, Gauteng, South Africa* Amalgam , fictional character in the Marvel Universe...

 which essentially just fills a hole and requires retention features to hold the filling, composite cavity restorations when used with dentin and enamel bonding techniques restore the tooth back to near its original physical integrity. Nevertheless, time to failure is still longer for amalgam, and it has remained a superior restorative material over resin-base composites, but with poor aesthetic qualities.

History of use

Initially, composite restorations in dentistry were very prone to leakage and breakage due to weak compressive strength. In the 1990s and 2000s, composites were greatly improved and are said to have a compression strength sufficient for use in posterior teeth. Today's composite resins have low polymerization shrinkage and low coefficients of thermal shrinkage, which allows them to be placed in bulk while maintaining good adaptation to cavity walls. The placement of composite requires meticulous attention to procedure or it may fail prematurely. The tooth must be kept perfectly dry during placement or the resin will likely fail to adhere to the tooth. Composites are placed while still in a soft, dough-like state, but when exposed to light of a certain blue wavelength (typically 470 nm, with traces of UV), they polymerize and harden into the solid filling. It is challenging to harden all of the composite, since the light often does not penetrate more than 2–3 mm into the composite. If too thick an amount of composite is placed in the tooth, the composite will remain partially soft, and this soft unpolymerized composite could ultimately irritate or kill the tooth's nerve. The dentist should place composite in a deep filling in numerous increments, curing each 2–3 mm section fully before adding the next. In addition, the clinician must be careful to adjust the bite of the composite filling, which can be tricky to do. If the filling is too high, even by a subtle amount, that could lead to chewing sensitivity on the tooth. A properly placed composite is comfortable, aesthetically pleasing, strong and durable, and could last 10 years or more. (By most North American insurance companies 2 years minimum)

The most desirable finish surface for a composite resin can be provided by aluminum oxide disks. Classically, Class III composite preparations were required to have retention points placed entirely in dentin. A syringe was used for placing composite resin because the possibility of trapping air in a restoration was minimized. Modern techniques vary, but conventional wisdom states that because there have been great increases in bonding strength due to the use of dentin primers in the late 1990s, physical retention is not needed except for the most extreme of cases. Primers allow the dentin's collagen fibers to be "sandwiched" into the resin, resulting in a superior physical and chemical bond of the filling to the tooth. Indeed, composite usage was highly controversial in the dental field until primer technology was standardized in the mid to late 1990s. The enamel margin of a composite resin preparation should be beveled in order to improve aesthetics and expose the ends of the enamel rods for acid attack. The correct technique of enamel etching prior to placement of a composite resin restoration includes etching with 30%-50% phosphoric acid
Phosphoric acid
Phosphoric acid, also known as orthophosphoric acid or phosphoric acid, is a mineral acid having the chemical formula H3PO4. Orthophosphoric acid molecules can combine with themselves to form a variety of compounds which are also referred to as phosphoric acids, but in a more general way...

 and rinsing thoroughly with water and drying with air only. In preparing a cavity for restoration with composite resin combined with an acid etch technique, all enamel cavosurface angles should be obtuse angles. Contraindications for composite include varnish and zinc oxide-eugenol
Eugenol is a phenylpropene, an allyl chain-substituted guaiacol. Eugenol is a member of the phenylpropanoids class of chemical compounds. It is a clear to pale yellow oily liquid extracted from certain essential oils especially from clove oil, nutmeg, cinnamon, basil and bay leaf. It is slightly...

. Composite resins for Class IIs were not indicated because of excessive occlusal wear in the 1980s and early 1990s. Modern bonding techniques and the increasing unpopularity of amalgam filling material have made composites more attractive for Class II restorations. Opinions vary, but composite is regarded as having adequate longevity and wear characteristics to be used for permanent Class II restorations (although amalgam has proved to last considerably longer and have reduced leakage and sensitivity when compared to Class II composite restorations).


As with other composite material
Composite material
Composite materials, often shortened to composites or called composition materials, are engineered or naturally occurring materials made from two or more constituent materials with significantly different physical or chemical properties which remain separate and distinct at the macroscopic or...

s, a dental composite typically consists of a resin
Resin in the most specific use of the term is a hydrocarbon secretion of many plants, particularly coniferous trees. Resins are valued for their chemical properties and associated uses, such as the production of varnishes, adhesives, and food glazing agents; as an important source of raw materials...

-based oligomer matrix, such as a bisphenol A-glycidyl methacrylate (BISGMA) 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
Glasses, also known as eyeglasses , spectacles or simply specs , are frames bearing lenses worn in front of the eyes. They are normally used for vision correction or eye protection. Safety glasses are a kind of eye protection against flying debris or against visible and near visible light or...

 and glass ceramics. The filler
Filler (materials)
Fillers are particles added to material to lower the consumption of more expensive binder material or to better some properties of the mixtured material...

 gives the composite wear resistance and translucency. A coupling agent such as silane
Silane is a toxic, extremely flammable chemical compound with chemical formula SiH4. In 1857, the German chemists and Friedrich Woehler discovered silane among the products formed by the action of hydrochloric acid on aluminum silicide, which they had previously prepared...

 is used to enhance the bond between these two components. An initiator package (such as: camphorquinone (CQ), phenylpropanedione (PPD) or lucirin (TPO)) begins the polymerization
In polymer chemistry, polymerization is a process of reacting monomer molecules together in a chemical reaction to form three-dimensional networks or polymer chains...

 reaction of the resins when external energy
In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...

 (light/heat, etc.) is applied. A catalyst package can control its speed.


The main advantage of a direct dental composite over traditional materials such as amalgam
Amalgam (dentistry)
Amalgam is an alloy containing mercury. The term is commonly used for the amalgam employed as material for dental fillings, which consists of mercury , silver , tin , copper , and other trace metals...

 is improved aesthetics
Aesthetics is a branch of philosophy dealing with the nature of beauty, art, and taste, and with the creation and appreciation of beauty. It is more scientifically defined as the study of sensory or sensori-emotional values, sometimes called judgments of sentiment and taste...

. Composites can be made in a wide range of tooth colors 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 micro leakage; however, all white fillings will eventually leak slightly. Very high bond strengths to tooth structure, both enamel and dentin, can be achieved with the current generation of dentin bonding agents.


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.

Direct dental composites

Direct dental composites are placed by the dentist in a clinical setting. Polymerization is accomplished typically with a hand held curing light that emits specific wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...

s keyed to the initiator
An initiator can refer to:* A person that takes an initiative in making something happen.* Modulated neutron initiator, a neutron source used in some nuclear weapons...

 and catalyst packages involved. When using a curing light, 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
Porosity or void fraction is a measure of the void spaces in a material, and is a fraction of the volume of voids over the total volume, between 0–1, or as a percentage between 0–100%...


Direct dental composites can be used for:
  • Filling gaps (diastemas
    Diastema (dentistry)
    Diastema is a space or gap between two teeth. Many species of mammals have diastemata as a normal feature, most commonly between the incisors and molars.-In humans:...

    ) 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 crown
    Crown (dentistry)
    A crown is a type of dental restoration which completely caps or encircles a tooth or dental implant. Crowns are often needed when a large cavity threatens the ongoing health of a tooth. They are typically bonded to the tooth using a dental cement. Crowns can be made from many materials, which...

    s on single teeth
  • And even bridges spanning 2-3 teeth

Composite shrinkage

Composite resins are notorious for shrinking upon curing. However, their use as dental restorative materials focuses 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. In the field of dental restorative materials, reduction of composite shrinkage is a "hot topic".
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