Dental composite resins (better referred to as "resin-based composites" or simply "filled resins") are
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.[1]
Many studies have compared the lesser longevity of resin-based composite restorations to the longevity of silver-mercuryamalgam 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. (See Longevity and clinical performance.) In comparison to amalgam, the appearance of resin-based composite restorations is far superior.
Traditionally resin-based composites set by a chemical setting reaction through polymerization between two pastes. One paste containing an activator (not a tertiary amine, as these cause discolouration) and the other containing an initiator (benzoyl peroxide).[3] To overcome the disadvantages of this method, such as a short working time, light-curing resin composites were introduced in the 1970s.[4] 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.[4] Therefore, UV light-curing units were later replaced by visible light-curing systems employing camphorquinone as the photoinitiator.[4]
The 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.[5]
The Microfilled Period
In 1978, various microfilled systems were introduced into the European market.[6] 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.[5]
The Hybrid Period
Hybrid composites were introduced in the 1980s and are more commonly known as resin-modified glass ionomer cements (RMGICs).[3] The material consists of a powder containing a radio-opaque fluoroaluminosilicate glass and a photoactive liquid contained in a dark bottle or capsule.[3] The material was introduced, as resin composites on their own were not suitable for Class II cavities.[5] RMGICs can be used instead. This mixture or resin and glass ionomer allows the material to be set by light activation (resin), allowing a longer working time.[3] It also has the benefit of the glass ionomer component releasing fluoride and has superior adhesive properties.[3] RMGICs are now recommended over traditional GICs for basing cavities.[6] There is a great difference between the early and new hybrid composites.[5]
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.
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 (typically 470 nm
Light activated resin). 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.[8]
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.[9]
Composition
As with other
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.[10]
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.[11]
A coupling agent such as silane is used to enhance the bond between these two components.[citation needed] An initiator package (such as: camphorquinone (CQ), phenylpropanedione (PPD) or lucirin (TPO)) begins the polymerization reaction of the resins when blue light is applied. Various additives can control the rate of reaction.
Filler types and particle size
Resin filler can be made of glasses or ceramics. Glass fillers are usually made of crystalline silica, silicone dioxide, lithium/barium-aluminium glass, and borosilicate glass containing zinc/strontium/lithium. Ceramic fillers are made of zirconia-silica, or zirconium oxide.[12]
Fillers can be further subdivided based on their particle size and shapes such as:
Macrofilled filler
Macrofilled fillers have a particle size ranging from 5 - 10 µm. They have good mechanical strength but poor wear resistance. Final restoration is difficult to polish adequately leaving rough surfaces, and therefore this type of resin is plaque retentive.[12]
Microfilled filler
Microfilled fillers are made of colloidal silica with a particle size of 0.4 µm. Resin with this type of filler is easier to polish compared to macrofilled. However, its mechanical properties are compromised as filler load is lower than in conventional (only 40-45% by weight). Therefore, it is contraindicated for load-bearing situations, and has poor wear resistance.[12]
Hybrid filler
Hybrid filler contains particles of various sizes with filler load of 75-85% by weight. It was designed to get the benefits of both macrofilled and microfilled fillers. Resins with hybrid filler have reduced thermal expansion and higher mechanical strength. However, it has higher polymerisation shrinkage due to a larger volume of diluent monomer which controls viscosity of resin.[12]
Nanofilled filler
Nanofilled composite has a filler particle size of 20-70 nm Nanoparticles form nanocluster units and act as a single unit.[13] They have high mechanical strength similar to hybrid material, high wear resistance, and are easily polished.[14][15] However, nanofilled resins are difficult to adapt to the cavity margins due to high volume of filler.[12]
Bulk filler
Bulk filler is composed of non-agglomerated silica and zirconia particles. It has nanohybrid particles and filler load of 77% by weight. Designed to decrease clinical steps with possibility of light curing through 4-5mm incremental depth, and reduce stress within remaining tooth tissue. Unfortunately, it is not as strong in compression and has decreased wear resistance compared to conventional material. [16]
Recently, nanohybrid fillers have seen wide interest.[17]
Advantages
Advantages of composites:
Appearance: The main advantage of a direct dental composite over traditional materials such as amalgam is improved tooth tissue-mimicry. Composites can be in a wide range of tooth colors allowing near invisible restoration of teeth. Composite fillings can be closely matched to the color of existing teeth. Aesthetics are especially critical in anterior teeth region - see Aesthetic anterior composite restorations.
Bonding to tooth structure: Composite fillings micro-mechanically bond to tooth structure. This strengthens the tooth's structure and restores its original physical integrity. The discovery of acid etching (producing enamel irregularities ranging from 5-30 micrometers in depth) of teeth to allow a micro-mechanical bond to the tooth allows good adhesion of the restoration to the tooth. Very high bond strengths to tooth structure, both enamel and dentin, can be achieved with dentin bonding agents.
Tooth-sparing preparation: The fact that composite fillings are glued (bonded) to the tooth means that unlike amalgam fillings, there is no need for the dentist to create retentive features destroying healthy tooth. Unlike amalgam, which just fills a hole and relies on the geometry of the hole to retain the filling, composite materials are bonded to the tooth. In order to achieve the necessary geometry to retain an amalgam filling, the dentist may need to drill out a significant amount of healthy tooth material. In the case of a composite restoration, the geometry of the hole (or "box") is less important because a composite filling bonds to the tooth. Therefore less healthy tooth needs to be removed for a composite restoration.
Less-costly and more conservative alternative to
dental crowns
: In some situations, a composite restoration may be offered as a less-expensive (though possibly less durable) alternative to a dental crown, which can be a very expensive treatment. Installation of a dental crown usually requires removal of significant healthy tooth material so the crown can fit over or into the natural tooth. Composite restoration conserves more of the natural tooth.
Alternative to tooth removal: As a composite restoration bonds to the tooth and can restore the original physical integrity of a damaged or decayed tooth, in some cases composite restoration can preserve a tooth that might not be salvageable with amalgam restoration. For example, depending on the location and extent of decay, it might not be possible to create a void (a "box") of the geometry necessary to retain an amalgam filling.
Versatility: Composite fillings can be used to repair chipped, broken or worn teeth[18] which would not be repairable using amalgam fillings.
Repairability: In many cases of minor damage to a composite filling, the damage can be easily repaired by adding additional composite. An amalgam filling might require complete replacement.
Longer working time: The light-curing composite allows the on-demand setting and longer working time to some degree for the operator compared to amalgam restoration.
Reduced quantity of mercury released to the environment: Composites avoid mercury environmental contamination associated with dentistry. When amalgam fillings are drilled for height adjustment, repair or replacement, some mercury-containing amalgam is inevitably washed down drains. (See Dental amalgam controversy - Environmental impact) When amalgam fillings are prepared by dentists, improperly disposed excess material may enter landfills or be incinerated. Cremation of bodies containing amalgam fillings releases mercury into the environment. (See Dental amalgam controversy - Cremation)
Reduced mercury exposure for dentists: Preparing new amalgam fillings and drilling into existing amalgam fillings exposes dentists to mercury vapor. Use of composite fillings avoids this risk, unless the procedure also involves removing an existing amalgam filling. A review article found studies indicating that dental work involving mercury may be an occupational hazard with respect to reproductive processes, glioblastoma (brain cancer), renal function changes, allergies and immunotoxicological effects.[19] (See Dental amalgam controversy - Health effects for dentists)
Lack of corrosion: Although corrosion is no longer a major problem with amalgam fillings, resin composites do not corrode at all. (Low-copper amalgams, prevalent before 1963, were more subject to corrosion than modern high-copper amalgams.[20] )
Disadvantages
Composite shrinkage and secondary caries: In the past, composite resins suffered significant shrinkage during curing, which led to inferior bonding interface.
dental restorative materials, reduction of composite shrinkage has been achieved with some success.[9] Among the newest materials, silorane resin exhibits lower polymerization shrinkage, compared to the dimethacrylates.[9]
Durability: In some situations, composite fillings may not last as long as amalgam fillings under the pressure of chewing, particularly if used for large cavities. (See Longevity and clinical performance, below.)
Chipping: Composite materials can chip off the tooth.
Skill and training required: Successful outcomes in direct composite fillings is related to the skills of the practitioner and technique of placement.[9] For example, a rubber dam is rated as being important for achieving longevity and low fracture rates similar to amalgam in the more demanding proximal Class II cavities.[24]
Need to keep working area in mouth completely dry: The prepared tooth must be completely dry (free of saliva and blood) when the resin material is being applied and cured. Posterior teeth (molars) are difficult to keep dry. Keeping the prepared tooth completely dry can also be difficult for any work involving treatment of cavities at or below the gumline,[25] though techniques have been described to facilitate this.[26]
Time and expense: Due to the sometimes complicated application procedures and the need to keep the prepared tooth absolutely dry, composite restorations may take up to 20 minutes longer than equivalent amalgam restorations.[25] Longer time in the dental chair may test the patience of children, making the procedure more difficult for the dentist. Due to the longer time involved, the fee charged by a dentist for a composite restoration may be higher than for an amalgam restoration.[18]
Costs: Composite restoration cases generally have limited insurance coverage. Some dental insurance plans may provide reimbursement for composite restoration only on front teeth where amalgam restorations would be particularly objectionable on cosmetic grounds. Thus, patients may be required to pay the entire charge for composite restorations on posterior teeth. For example one dental insurer states that most of their plans will pay for resin (i.e. composite) fillings only "on the teeth where their cosmetic benefit is critical: the six front teeth (incisors and cuspids) and on the facial (cheek side) surfaces of the next two teeth (bicuspids)."[25] Even if charges are paid by private insurance or government programs, the higher cost is incorporated in dental insurance premiums or tax rates. In the UK, dental composites are not covered by NHS for the restoration of posterior teeth. Patients, therefore, may require to pay the entire charge of the treatment or have to pay according to the private charge rate.[27]