Dental composite
Dental composite resins are dental cements made of synthetic resins. Synthetic resins evolved as restorative materials since they were insoluble, of good tooth-like appearance, insensitive to dehydration, easy to manipulate and inexpensive. Composite resins are most commonly composed of Bis-GMA and other dimethacrylate monomers, a filler material such as silica and in most applications, a photoinitiator. Dimethylglyoxime is also commonly added to achieve certain physical properties such as flow-ability. Further tailoring of physical properties is achieved by formulating unique concentrations of each constituent.
Many studies have compared the lesser longevity of resin-based composite restorations to the longevity of silver-mercury amalgam restorations. Depending on the skill of the dentist, patient characteristics and the type and location of damage, composite restorations can have similar longevity to amalgam restorations. In comparison to amalgam, the appearance of resin-based composite restorations is far superior.
Resin-based composites are on the World Health Organization's List of Essential Medicines.
History of use
Traditionally resin-based composites set by a chemical setting reaction through polymerization between two pastes. One paste containing an activator and the other containing an initiator. To overcome the disadvantages of this method, such as a short working time, light-curing resin composites were introduced in the 1970s. The first light-curing units used ultra-violet light to set the material, however this method had a limited curing depth and was a high risk to patients and clinicians. Therefore, UV light-curing units were later replaced by visible light-curing systems employing camphorquinone as the photoinitiator.Traditional period
In the late 1960s, composite resins were introduced as an alternative to silicates and unfulfilled resins, which were frequently used by clinicians at the time. Composite resins displayed superior qualities, in that they had better mechanical properties than silicates and unfulfilled resins. Composite resins were also seen to be beneficial in that the resin would be presented in paste form and, with convenient pressure or bulk insertion technique, would facilitate clinical handling. The faults with composite resins at this time were that they had poor appearance, poor marginal adaptation, difficulties with polishing, difficulty with adhesion to the tooth surface, and occasionally, loss of anatomical form.Microfilled period
In 1978, various microfilled systems were introduced into the European market. These composite resins were appealing, in that they were capable of having an extremely smooth surface when finished. These microfilled composite resins also showed a better clinical colour stability and higher resistance to wear than conventional composites, which favoured their tooth tissue-like appearance as well as clinical effectiveness. However, further research showed a progressive weakness in the material over time, leading to micro-cracks and step-like material loss around the composite margin. In 1981, microfilled composites were improved remarkably with regard to marginal retention and adaptation. It was decided, after further research, that this type of composite could be used for most restorations provided the acid etch technique was used and a bonding agent was applied.Hybrid period
Hybrid composites were introduced in the 1980s and are more commonly known as resin-modified glass ionomer cements. The material consists of a powder containing a radio-opaque fluoroaluminosilicate glass and a photoactive liquid contained in a dark bottle or capsule. The material was introduced, as resin composites on their own were not suitable for Class II cavities. RMGICs can be used instead. This mixture or resin and glass ionomer allows the material to be set by light activation, allowing a longer working time. It also has the benefit of the glass ionomer component releasing fluoride and has superior adhesive properties. RMGICs are now recommended over traditional GICs for basing cavities. There is a great difference between the early and new hybrid composites.Initially, resin-based composite restorations in dentistry were very prone to leakage and breakage due to weak compressive strength. In the 1990s and 2000s, such composites were greatly improved and have a compression strength sufficient for use in posterior teeth.
Image:MethmethacrylateBPA-glyc.png|class=skin-invert-image|thumb|Chemical structure of bis-GMA, bearing two polymerizable groups, it is prone to form a crosslinked polymer that is used in dental restorations.
Method and clinical application
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, 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 lead to leaching of free monomers with potential toxicity and/or leakage of the bonded joint leading to recurring dental pathology. 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, of good appearance, strong and durable, and could last 10 years or more.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 the appearance 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 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. Composite resins for Class II restorations 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. Whether composite materials last as long or have similar leakage and sensitivity properties when compared to Class II amalgam restorations was described as a matter of debate in 2008.
Composition
As with other composite materials, a dental composite typically consists of a resin-based oligomer matrix, such as a bisphenol A-glycidyl methacrylate, urethane dimethacrylate or semi-crystalline polyceram, and an inorganic filler such as silicon dioxide. Without a filler the resin wears easily, exhibits high shrinkage and is exothermic. 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 greater strength, wear resistance, decreased polymerisation shrinkage, improved translucency, fluorescence and colour, and a reduced exothermic reaction on polymerisation. It also however causes the resin composite to become more brittle with an increased elastic modulus. Glass fillers are found in multiple different compositions allowing an improvement on the optical and mechanical properties of the material. Ceramic fillers include zirconia-silica and zirconium oxide.Matrices such as BisHPPP and BBP, contained in the universal adhesive BiSGMA, have been demonstrated to increase the cariogenicity of bacteria leading to the occurrence of secondary caries at the composite-dentin interface. BisHPPP and BBP cause an increase of glycosyltransferase in S. mutans bacteria, which results in increased production of sticky glucans that allow S.mutans' adherence to the tooth. This results in a cariogenic biofilms at the interface of composite and tooth. The cariogenic activity of bacteria increases with concentration of the matrix materials. BisHPPP has furthermore been shown to regulate bacterial genes, making bacteria more cariogenic, thus compromising the longevity of composite restorations. Researchers are highlighting the need for new composite materials to be developed which eliminate the cariogenic products contained in composite resin and universal adhesives.
A coupling agent such as silane is used to enhance the bond between these two components. An initiator package, phenylpropanedione or Lucirin ) begins the polymerization reaction of the resins when blue light is applied. Various additives can control the rate of reaction.