COMPOSITES

Dr. Kalyan Reddy

INTRODUCTION: An era in dental restorative materials began in 1955, when Buonocore found that acrylic resin formed acceptable micromechanical adhesion with dry enamel that had been etched with phosphoric acid. Many generations of restorative materials have existed in the last five decades, and the modern clinician may be overwhelmed when attempting to make decisions as which material or technique must be most appropriate in varying clinical situations.

When the best course of treatment, many factors must be considered. These include the mechanics of tooth preparation, including Blacks principles, the physical properties of materials, esthetics of and functional demands on the completed restoration, and factors such as patient health, oral hygiene, diet, quality and quantity of saliva and motivation. In the current age of adhesive dentistry or microdentistry, conservation of tooth structure is paramount. Rather than using extension for prevention as a treatment guideline, emphasis is now placed on restriction with conviction. The advent of composite resin restorative materials has led the way towards achieving this goal

DEFINITION: Oxford dictionary Composite n. a thing made up of several parts

Origin Latin componere put together Skinner: Skinner defined the composite as a compound of two or more distinctly different materials with properties that are superior to or intermediate to those of individual constituents.Philips and lutz: Resin based restorative materials are defined as 3 dimensional combinations of at least two chemically different materials with a distinct interface.John F. Mc Cabe: A composite materials is a product, which consists of at least two distinct phases normally formed by blending together components having different structures and properties.

Karl Leinfelder:

The term composite originated in the field of material science. From a physical or mechanical point of view a composite is a material consisting of two or more components that are chemically bonded together to provide overall properties superior to those of either constituent. The number of natural and man made composites is unlimited. Bone, which is an example of a natural composite consists of collagen and calcium appetite. The Collagen component is soft but strong. Calcium appetite on the other hand is hard and brittle. As a composite bone can withstand many types of mechanical stresses. Fibre glass is an example of man made composite of Glass fibres in a resin matrix.

Mosbys dental dictionary :

Defines a dental composite a resin used for restorative purposes and usually formed by reaction of an ether of Bisphenol-A (an epoxy molecule ) with acrylic resin monomers , initiated by a benzoyl peroxideamine system , to which is added as much as 75% inorganic filler ( Glass beads or rods , lithium aluminium silicate , quartz and tricalcium phosphate)

HISTORY: Tooth coloured restorative materials have increasingly been used to replace missing tooth structure and to modify tooth colour and contour, thus enhancing facial esthetics. During the first half of the 20th century, silicates were the only tooth coloured esthetic materials available. Although silicates release fluoride, they are no longer used to restore permanent teeth because they severely Erode and discolour within a few years (

Unfortunately, these acrylic resins also have poor wear resistance and they shrink severely during curing, which causes them to pull away from the cavity walls and produce leakage along margins. Their excessive thermal expansion and contraction cause further stresses to develop at the cavity margins when hot or cold beverages and foods are consumed. These problems were overcome by the introduction of composites by addingan inert filler to the unfilled acrylic resins. In 1955, MICHAEL BUANOCORE introduced the concept of bonding acrylic to teeth by acid etching.

In 1962, Dr.RAPHAEL BOWEN of ADA research unit at the national Bureau of standards began experiments on reinforcing epoxy resin with filler particles.deficiencies in epoxy resin such as slow cure and tendency to discolour stimulated him to work on combining the advantages of epoxy resin and acrylates i.e. the reaction product of bisphenol A and a glycidyl methacrylate, which has been abbreviated as Bis-GMA.These two discoveries revolutionized the application of composites in restorative dentistry.

COMPOSITION: These are the structural components in dental resin-based composites: Matrix A plastic resin material that forms a continuous phase and binds the filler particles. Filler reinforcing particles and / or fibres that are dispersed in the matrix. Coupling agent bonding agent that promotes adhesion between filler and resin matrix.

Composites contain other components in addition to these primary constituents. An activator-initiator system required to convert resin paste from a soft moldable filling material to a hard durable restoration.Pigments-help to match colour of tooth structure. Ultra violet (UV) absorbers and other additives improve colour stability.Polymerization inhibitors extend storage life and provide increased working time for chemically activated resins.

RESIN MATRIX: (continuous phase) Most dental composites use a blend of aromatic and/or aliphatic dimethacrylate monomers such as Bis-GMA, one of the most widely used ingredients, Triethylene gycol dimethacrylate (TEGDMA) and Urethane dimethacrylate (UDMA). UDMA, Bis-GMA, and TEGDMA are widely used resin matrix ingredients that form highly cross-linked polymer structures in composites and sealant materials.

Because Bis-GMA and UDMA have almost five times the molecular weight of methyl methacrylate, the density of methacrylate double-bond groups is approximately one-fifth as high in these monomers, which reduces polymerization shrinkage proportionately. The use of a dimethacrylate also results in extensive cross-linking, which increases the strength and rigidity of the polymer.

FILLER PARTICLES: (dispersed phase) The primary purposes of filler particles are to strengthen a composite and to reduce the amount of matrix material. Several important properties of dental composites are improved by increased filler loading (volume fraction): Reinforcement of the matrix resin, resulting inincreased hardness, strength, and decreased wear; reduction in polymerization shrinkage; reduction in thermal expansion and contraction;improved workability by increasing viscosity (liquid monomer plus filler yields a paste consistency);

reduction in water sorption, softening, and staining; and increased radiopacity and diagnostic sensitivity through the incorporation of strontium (Sr) and barium (Ba) glass and other heavy compounds that absorb x-rays. Filler particles are most commonly produced by grinding or milling quartz or glasses to produce particles ranging in size from 0.1 to 100m. Submicron silica particles of colloidal size (~0.04m), referred to collectively as microfiller or individually as microfillers, are obtained by a pyrolytic or precipitation process.

In these processes, a silicon compound (e.g,SiCl4) is burned in an O2 and H2 atmosphere to form macromolecule chains of SiO2. These macromolecules are of a colloidal size and constitute the inorganic filler particles. Composites are classified on the basis of the average size of the major filler component. In addition to filler volume level, other important factors that determine the properties and the clinical application of the resultant composites include the filler size, size distribution, index of refraction, radiopacity, and hardness of the filler. Quartz has been used extensively as a reinforcing filler, particularly in the early versions of dental composites. It has the advantage of being chemically inert and yet also very hard, making it abrasive as well as difficult to grind into very fine particles.

However, this hardness also makes quartz composites difficult to polish and potentially abrasive to opposing teeth or restorations. So-called amorphous silica has the same composition and refractive index as quartz, but it is not crystalline and not as hard, thus greatly reducing the abrasiveness of the composite surface structure.

Types of fillers used -QuartzFused silicaAluminum silicatesBarium glassesBoro silicatesGlassesLithium aluminum silicate, pyrogenic silica The latest filler used is zirconiumZinc and yttrium glassesHydroxyapatite

COUPLING AGENTS: (interfacial phase) It is essential that filler particles be bonded to the resin matrix.This allows the more flexible polymer matrix to transfer stresses to thehigher modulus (more rigid and stiffer) filler particles. The bond between the two phases of the composite is provided by a coupling agent. A properly applied coupling agent can impart improved physical and mechanical properties and inhibit leaching by preventing water from penetrating along the filler-resin interface.

Although titanates and zirconates can be used as coupling agents,organosilanes such as -methacryloxypropyl trimethoxysilane are used most commonly. In the presence of water, the methoxy groups (-OCH3)are hydrolyzed to silanol (-Si-OH) groups that can bond with other silanols on the filler surfaces by formation of a siloxane bond (Si-O-Si). The organosilane methacrylate groups form covalent bonds with the resin when it is polymerized, thereby completing the coupling process.

ACTIVATOR-INITIATOR SYSTEM: Both monomethacrylate and dimethacrylate monomers polymerize by the addition polymerization mechanism initiated by free radicals. The free radicals can be generated by chemical activation or by external energy activation (heat, light, or microwave).

CHEMICALLY ACTIVATED RESINS: Chemically activated products are supplied as two pastes, one of which contains the benzoyl peroxide (BP) initiator and the other an aromatic tertiary amine activator (e.g, N, N-dimethyl--toluidine).When the two pastes are mixed together, the amine reacts with the BP to form free radicals, and additional polymerization is initiated. Today, these materials are mainly used for restorations and large foundation structures (buildups) that are not readily cured with a light source.

LIGHT-ACTIVATED RESINS: Light-curable dental composites are supplied as a single paste contained in a light-proof syringe. The free radical initiating system, consisting of a photosensitizer and an amine initiator, is contained in this paste. As long as these two components are not exposed to light, they do not interact. However, exposure to light in the blue region (wavelength of ~468nm) produces an excited state of the photosensitizer, which then interacts with the amine to form free radicals that initiate addition polymerization.

Camphorquinone (CQ) is a commonly used photosensitizer that absorbs blue light with wavelengths between 400 and 500nm. Only small quantities of CQ are required (0.2 wt% or less in the paste). A number of amine initiators are suitable for interaction with CQ, such as dimethylaminoethyl methacrylate (DMAEMA), which is also present at low levels, that is approximately 0.15 wt%.

INHIBITORS: Inhibitors are added to the resin system to minimize or prevent spontaneous or accidental polymerization of monomers. Inhibitors have a strong reactivity potential with free radicals. If a free radical is formed, for example, by brief exposure to room lighting when the material is dispensed, the inhibitor reacts with the free radical faster than the free radical can react with the monomer.

This prevents chain propagation by terminating the reaction before the free radical is able to initiate polymerization. After all of the inhibitor is consumed, chain propagation begins. A typical inhibitor is butylated hydroxytoluene (BHT), which is used in concentrations on the order of 0.01 wt%. Thus inhibitors have two functions: they extend the storage lifetime for all resins and they ensure sufficient working time.

DUAL-CURE RESINS: One way to overcome limits on curing depth and some of the other problemsassociated with light curing is to combine chemical curing and visible-light curing components in the same resin. So-called dual-cure resins are commercially available and consist of two light-curable pastes, one containing benzoyl peroxide (BP) and the other containing an aromatic tertiary amine. When these two pastes are mixed and then exposed to light, light curing is promoted by the amine/CQ combination and chemical curing is promoted by the amine/BP interaction. Dual-cure materials are intended for any situation that does not allow sufficient light penetration to produce adequate monomer conversion, for example, cementation of bulky ceramic inlays.

Curing lamps Most curing lamps are hand held devices that contain the light source and are equipped with a relatively short, rigid light guide made up of fused optical fibres.Four types of lamps may be used for photoinitiation process. QTH lamps. QTH lamps have a quartz bulb with a tungsten filament that irradiates both UV and white light that must be filtered to remove heat and all wavelengths except those in the violet blue range (~450to500 nm).the intensity of the bulb diminishes with use, so a calibration meter is required to measure the output intensity.

LED lamps. Using a solid-state ,electronic process, these light sources emit radiation only in the blue part of the visible spectrum between 440 and 480 nm,and they do not require filters.LEDs require low wattage ,can be battery powered ,generate no heat ,and are quiet because a cooling fan is not needed.Althougfh they produce the lowest intensity radiation, new technology is rapidly overcoming this limitation.

PAC lamps. They are high intensity light curing units. PAC lamps use xenon gas that is ionized to produce a plasma . the high intensity light is filtered to remove heat and to allow blue light (~400to500nm)to be emitted.Argon laser lamps. Argon laser lamps have the highest intensity and emit at a single wavelength. lamps currently available emit ~490nm.

Precautions of using curing lamps The light emitted by curing units can cause retinal damage if a person looks directly at the beam for an extended period of time or even for short periods in case of lasers. To avoid such damage ,never look directly into the light tip and minimize observation of reflected light for longer periods. Protective eyeglasses and various types of shields that filter the light are available for increased protection for both clinical personnel and patients.

CLASSIFICATION OF COMPOSITES:I. The commonly used is the simplest classification given by Skinner: Traditional or conventional composites 8-12 .m Small particle filled composites 1-5 . m Microfilled composites 0-04 0.9 . m. Hybrid composites 0.6-1 . m

II Philips and Lutz classification: According to the mean particles size of the major fillers - Traditional composite resins: (5.30 m earlier, 1.5 m current)Hybrid composite resins: (1.5 m. earlier, 0.05-0.1 m. current)Homogeneous microfilled composites: 0.05-0.1 .mHeterogeneous micro filled composites: 0.05-01, 1-25 .m

III Classifications based on inorganic loading: a. Heavy filled materials 75% of inorganic loading by wt. b .Lightly filled material 66% of inorganic loading by wt.

IV. Based on method of curing 1. Chemical cured 2. Light cured 3. Heat cured 4. Dual cured V Classification based on area used Anterior composites Posterior composites

VI.GENERATIONS OF COMPOSITE RESTORATION (Marzouk)A. First Generation compositesConsist of macro-ceramic reinforcing phase.Has good mechanical properties.Highest surface roughnessB. Second Generation compositesConsists of colloidal and micro-ceramic silica.Low strengthUnfavourable coefficient of thermal expansionWear resistance better than first generationBest surface texture.

C. Third Generation compositesHybrid composite [combination of macro and micro (colloidal) ceramics]Ratio of75:25Good surface smoothness and reasonable strengthD. Fourth Generation composites Hybrid composite (heat-cured, irregularly shaped, highly reinforced composite macro-particles with micro (colloidal) ceramics].Comparatively better surface characteristics and mechanical properties

E. Fifth Generation compositesHybrid composite (heat-cured, spherical, highly reinforced composite macro. particles with micro (colloidal) ceramics].Improved workabilitySurface texture and wear is similar to second generation compositesPhysical and mechanical properties similar to fourth generation compositesF. Sixth Generation composites Hybrid composite [agglomerates of sintered micro (colloidal) ceramics and micro-ceramics]Highest percentage of reinforcing particlesBest mechanical propertiesWear and surface texture similar to fourth generationLeast polymerization shrinkage

VII. Classification according to Bayne and Heyman:

CategoryParticle sizeMegafill- 1-2 mmMacrofill- 10-100 .mMidifill- 1-10 .mMinifill- .01-.1. m.Microfill - 0.04-0.4 Nanofill- .005-.01 .m.

Traditional Composites The traditional composites have comparatively large filler particles. This category was developed during the 1970s and modified slightly over the years. These composites are also referred to as conventional or macrofilled composites. Because these materials are no longer widely used, the term traditional is more meaningful than is conventional. The most commonly used filler for these materials is finely ground amorphous silica and quartz. Although the average size is 8 to 12 m, particles as large as 50 m may also be present. Filler loading generally is 70 to 80 wt% or 60 to 70 vol% exposed filler particles, some quite large, are surrounded by appreciable amounts of the resin matrix.

A major clinical disadvantage of traditional composites is the rough surface that develops during abrasive were of the soft resin matrix, thus exposing the more wear resistant filler particles, which protrude from the surface. Finishing of the restoration produces a roughened surface, as does tooth brushing and masticatory wear over time. These restorations also tend toward discoloration, undoubtedly caused in part by the susceptibility of the rough textured surface to retain stain.

Small-Particle-Filled Composites To improve surface smoothness and retain or improve the physical and mechanical properties of traditional composites, inorganic fillers are ground to a size range of 0.5 to 3m, but with a fairly broad size range distribution. This broad particle size distribution facilitates a high filler loading, and small-particle-filled (SPF) composites generally contain more inorganic filler (80 to 90 wt% and 65 to 77 vol%) than traditional composites. This is particularly true for those composites designed for posterior restorations. This category of composite generally exhibits superior physical and mechanical properties.

Microfilled Composites The problems of surface roughening and low translucency associated with traditional and small particle composites can be over come through the use of colloidal silica particles as the inorganic filler. The individual particles are approximately 0.04 m (40 nm) in size. This value is one tenth of the wavelength of visible light and 200 to 300 times smaller than the average particle in traditional composites. The concept of the microfilled composite entails the reinforcement of the resin by means of the filler; yet these composites exhibit a smooth surface, similar to that obtained with the unfilled direct filling acrylic resins.

Hybrid Composites This category of composite materials was developed in an effort to obtain even better surface smoothness than that provided by the small particle composites, while still maintaining the desirable properties of the latter. As the name implies, hybrid composites contain two kinds of filler particles. Most modern hybrid fillers consist of colloidal silica and ground particles of glasses containing heavy metals, constituting a filler content of approximately 75 to 80 wt%. The glasses have an average particle size of about 0.4 to 1.0 m. Colloidal silica represents 10 to 20 wt% of the total filler content. In this case, the microfillers also contribute significantly to the properties.

The smaller filler particle size, as well as the greater amount of micro fillers, increase the surface area. Thus the overall filler loading is not as high as it is for some of the SPF composites. Because of their surface smoothness and reasonably good strength, these composites are widely used for anterior restorations, including Class IV sites. Although the mechanical properties of hybrid composites generally are somewhat inferior to those SPF composites, the hybrid composites are widely employed for stress bearing, posterior restorations.

PHYSICAL PROPERTIES Working and Setting times For light cured composites, initiation of polymerization is related specifically to the application of the light beam to the material; about 75% of the polymerization takes place during the first 10 minutes. The curing reaction continues for a period of 24 hours. Not all of the available unsaturated carbon double bonds react; studies report that about 25% remain unreacted in the bulk of the restorations. If the surface of the restoration is not protected from air by a transparent matrix, polymerization is inhibited, the number of unreacted carbon double bonds may be as high as 75% in the tacky surface layer. Although the restoration can be finished with abrasives and is functional after 10 minutes, the optimum physical properties are not reached until about 24 hours after the reaction is initiated.

For most composites that are initiated by visible light, there is a critical time period after dispensing of the paste onto a paper pad during which fresh composite flows against tooth structure at an optimum level. Within 60 to 90 seconds after exposure to ambient light, the surface of the composite may lose its capability to flow readily against tooth structure, and further work with the material becomes difficult. Florescent lights labeled gold can be substituted to provide unlimited working time for light cured composites. The setting times for chemically activated composites range from 3 to 5 minutes. These short setting times have been accomplished by controlling the concentration of initiator and accelerator.

Polymerization shrinkage Free volumetric polymerization shrinkage is a direct function of the amount of oligomer and diluent, and thus micro hybrid composites shrink only 0.6% to 1.4%, compared with shrinkage of microfilled composites of 2% to 3%. This shrinkage creates polymerization stresses as high as 13 MPa between the composite and tooth structure. These stresses severely strain the interfacial bond between the composite and the tooth, leading to a very small gap that can allow marginal leakage of saliva. This stress can exceed the tensile strength of enamel and result in stress cracking and enamel fractures along the interfaces. The potential for this type of failure is even greater with microfilled composites, in which there is a much higher volume percent of polymer present, and polymerization shrinkage is greater. The net effect of polymerization shrinkage can be reduced by incrementally adding a light cured composite and polymerizing each increment independently, which allows for some contraction within each increment before successive additions.

Thermal Properties The thermal expansion coefficient of composites ranges from 25 to 38 X 10-6/oC for composites with fine particles to 55 to 68 x 10-6/o C for composites with microfine particles. Thermal stresses place an additional strain on the bond to tooth structure, which further compounds the detrimental effect of the polymerization shrinkage. Thermal changes are also cyclic in nature, and although the entire restoration may never reach thermal equilibrium during the application of either hot or cold stimuli, the cyclic effect can lead to material fatigue and early bond failure. If a gap were formed, the difference between the thermal coefficient of expansion of composites and teeth could allow for the percolation of oral fluids.

Water sorption The water sorption of composites with fine particles (0.3 to 0.6 mg/cm2) is greater than that of composites with micro fine particles (1.2 to 2.2 mg/cm2), because of the lower volume fraction of polymer in the composite with fine particles. The quality and stability of the silane coupling agent are important in minimizing the deterioration of the bond between the filler and polymer and the amount of water sorption. It has been postulated that the corresponding expansion associated with the uptake of water from oral fluids could relieve polymerization stresses.

Solubility The water solubility of composites varies from 0.01 to 0.06 mg/cm2. Adequate exposure to the light source is critical in light cured composites. Inadequate polymerization can readily occur at a depth from the surface if insufficient light penetrates. Inadequately polymerized resin has greater water sorption and solubility, possibility manifested clinically with early color instability.

Color and Color stability Change of color and loss of shade match with surrounding tooth structure are reasons for replacing restorations. Stress cracks within the polymer matrix and partial debonding of the filler to the resin as a result of hydrolysis tend to increase opacity and alter appearance. Discoloration can also occur by oxidation and result from water exchange within the polymer matrix and its interaction with unreacted polymer sites and unused initiator or accelerator. Color stability of current composites has been studied by artificial aging in a weathering chamber (exposure to UV light and elevated temperature of 70oC) and by immersion in various stains (coffee/tea, cranberry/grape juice, red wine, sesame oil). Composites are resistant to color changes caused by oxidation but are susceptible to staining.

Clinical Properties Depth of Cure (light-cured Composites) Maximum intensity of the light radiation beam is concentrated near the surface of a light cured composite. As the light penetrates the material, if is scattered and reflected and loses intensity. A number of factors influence the degree of polymerization at given depths from the surface after light curing. The concentration of photo-initiator or light absorber in the composite must be such that it will react at the proper wavelength and be present in sufficient concentration. Both filler content and particle size are critical to dispersion of the light beam. For this reason, microfilled composites with smaller and more numerous particles scatter more light than micro hybrid composites with larger and fewer glass particles. Longer exposure times are needed to obtain adequate depth of cure of microfilled composites.

The light intensity at the resin surface is a critical factor in completeness of cure at the surface and within the material. The tip of the light source must be held within 1 mm of the surface to gain optimum penetration. More opaque shades reduce light transmission and cure only to minimal depths (1 mmm). A standard exposure time using most visible light is 20 seconds. In general, this is sufficient to cure a light shade of resin to a depth of 2 or 2.5 mm. A 40 second exposure improves the degree of cure at all depths, but it is required to obtain sufficient cure with the darker shades. Application of the light beam through 1 mm or less thickness of tooth structure produces a sufficient cure at shallower depths, but the hardness values obtained are not consistent.

Because the light beam does not spread sufficiently beyond the diameter of the tip at the emitting surface, it is necessary to Step the light across the surface of large restorations so that entire surface receives a complete exposure. Larger tips have been manufactured for placement on most light curing units However, as the light beam is distributed over a larger surface area, the intensity at a given point is reduced. Use a longer exposure time of up to 60 seconds when larger emitting tips are used.

Radiopacity Modern composites include glasses having atoms with high atomic numbers, such as barium, strontium, and zirconium. Some fillers, such as quartiz, lithium-aluminum glasses, and silica, are not radiopaque and must be blended with other fillers to produce a radio opaque composite. Even at their highest volume friction of filler, the amount of radiopacity seen in composites in noticeably less than the exhibited by a metallic restorative like amalgam. The microhybrid composites achieve some radiopacity by incorporating very finely divided heavy metal glass particles.

Aluminum is used as a standard reference for radiopacity. A 2 mm thickness of dentin is equivalent in radio opacity to 2.5 mm of aluminum, and enamel is equivalent to 4 mm of aluminum. To be effective, a composite should exceed the radio opacity of enamel, but international standards accept radiopacity equivalent to 2 mm of aluminum. Amalgam has a radiopacity greater than 10 mm of aluminum, which exceeds all the composite material available.

Wear Rates One problem with composites is the loss of surface contour of composite restorations in the mouth, which results from a combination of abrasive wear from chewing and toothbrushing and erosive wear from degradation of the composite in the oral environment. Wear of posterior composite restorations is observed at the contact area, where stresses are the highest. Interproximal wear has also been observed.

Ditching at the margins within the composite is observed for posterior composites, probably resulting from inadequate bonding and polymerization stresses. Currently accepted composites for posterior applications require clinical studies that demonstrate, over a 5 year period, a loss of surface contour less than 250 m or an average of 50 m per year of clinical service. Products developed as packable or laboratory composites usually have better wear resistance than micro filled or flowable composites

Biocompatibility of Composites The chemical insult to the pulp from composites is possible if components leach out or diffuse from the material and subsequently reach the pulp. Adequately polymerized composites are relatively biocompatible because they exhibit minimal solubility, and unreacted species are leached in very small quantities. From a toxicological point of view, these amounts should be too small to cause toxic reactions. However, from an immunological point of view, under extremely rare conditions, some patients and dental personnel can develop an allergic response to these materials.

Inadequately cured composite materials at the floor of a cavity can serve as a reservoir of diffusible components that can induce long term pulp inflammation. This situations is of particular concern for light activated materials. If a clinician attempts to polymerize too thick a layer of resin or if the exposure time to the light is inadequate (as discussed previously), the uncured or poorly cured material can release leachable constituents adjacent to the pulp.

The second biological concern is associated with shrinkage of the composite during polymerization and the subsequent marginal leakage. The marginal leakage might allow bacterial in growth, and these micro organisms may cause secondary caries or pulp reactions. Therefore the restorative procedures must be designed to minimize polymerization shrinkage and marginal leakage.

Bisphenol A, a precursor of Bis-GMA has been shown to be a xenoestrogen .BPA and other endocrine disrupting chemicals (EDCS) have been shown to cause reproductive anomalies ,especially in the developmental stages of fetal wildlife. Although the effect on humans are still unclear, testicular cancer, decreased sperm count, hypospadias (displacement of urethral meatus)have been seen as a result of exposure to EDCs.Controversy surrounds this issue because it is unclear how much BPA or BPA-DM is released to the oral cavity and what dosage is enough to affect human health.

Composites for posterior restorations Direct Posterior Composites Amalgam has long been the direct filling material of choice for restoration of posterior teeth. Its attributes are ease of placement, good mechanical properties, excellent wear resistance, and the unique characteristic of being self-sealing (i.e. reducing leakage within marginal gaps as the restorations ages.) however, the increasing demand for aesthetic dentistry and the concern of some individuals regarding the potential toxicity of mercury has resulted in an increased interest and frequency in use of composites for Class I and Class II restorations.

Requirement of Posterior composites:Aesthetics: the colour match to the natural tooth should be as close as possibleOptical properties: The R.I of composite should be similar to that of the R.I of the enamel (1.5) Hardness value of the filler particles must not be higher than that of hydroxy appetite crystal which in 3.39 GpaYoungs modulus - the value of Youngs modulus should be equal to or less than that of dentin

Compressive strength should be more than that of enamel (348 Mpa) & dentin (297 Mpa)Occlusal wear should be comparable to a attritional wear rate which is 39 m/yearRadiopacity should be greater than that of enamel is 198% One of the most commonly faced problem with posterior composite in that of occlusal wear and poor fracture resistance to occlusal loading and polymerization shrinkage.

Advantages:Hg freeThermally non-conduciveBonding to the tooth structureTooth coloured Disadvantages:Technique sensitiveContouring and wedging for proximal contact is difficult in case of direct restorationGap formation at interface due to shrinkageE.g. Solitaire (3 M), Surefil (Densply), Enamel pyramid, Alert, dentin pyramid etc.

Indication for Posterior composites:All Class I and Class II cavities.Patients allergic or sensitive to mercuryPatients who are afraid of possible mercury toxicityPatients who demand aesthetics even in posterior teeth

Contraindications for Posterior Composites:Where moisture control is not possible Proximal step of Class II cavities below gingival margin. In this situation, not only does placement of composites become difficult, but moisture control from gingival tissues is difficult, if not impossible to control.If lack of time for placement is a factor.If cost is a factor.

Packable Composites Compared with amalgam, the technique of composite placement is far more time consuming and demanding. Because of the highly plastic, pastelike consistency in the procured state, composites cannot be packed vertically into a cavity in such a way that the material flows laterally as well as vertically to ensure intimate contact with the cavity walls. A solution to this problem is offered by resin composites with filler characteristics that increase the strength and stiffness of the uncured material and that provide a consistency similar to that of lathe-cut amalgams. The so called packable and condensable composites form two special categories of hybrid composites. These materials were introduced in the late 1990s to provide resin composites that enable clinicians to apply techniques similar to those used for amalgam restorations.

The packable /condensable characteristics derive from inclusion of elongated, fibrous, filler particles of about 100 m in length, and / or textured surfaces that tend to inter lock and resist flow. This causes the uncured resin to be stiff and resistant to slumping, yet moldable under this causes the uncured resin to be stiff and resistant to slumping, yet moldable under the force of amalgam condensing instruments (Plugger). At the present time these materials have not demonstrated any advantageous properties or characteristics over the hybrid resins, other than being somewhat similar to amalgam in their placement technique.

Indirect Posterior Composites Indirect composites for fabrication of onlays are polymerized outside the oral environment and luted to the tooth with a compatible resin cement. Indirect composite inlays or onlays reduce wear and leakage and overcome some of the limitations of resin composites. The potential advantage of these materials is that a somewhat higher degree of polymerization is attained, which improves physical properties and resistance to wear. The polymerization shrinkage does not occur in the prepared teeth, so induced stresses and bond failures are reduced, which reduces the potential for leakage. Further more, these resins are repairable in the mouth and they are not as abrasive to opposing tooth structure as ceramic inlays.

Flowable Composites A modifications of the SPF and hybrid composites results in the so called flowable composites. These resins have a reduced filler level so as to provide a consistency that enables the material to flow readily, spread uniformly, and intimately adapt to a cavity form to produce a desired dental anatomy. The reduced filler makes them more susceptible to wear, but improves the clinicians ability to form a well adapted cavity base or liner, especially in Class II posterior preparations and other situations in which access is difficult. Because of their greater ease of adaptation and greater flexibility as a cured material, flowable composites are useful in Class I restorations in gingival areas.

Another application is in minimal Class I restorations to prevent caries, when used in a manner similar to the use of fissure sealants. Flowable composites are also indicated for applications in which there is poor accessibility and little or no exposure to wear and for applications in which excellent adaptation is needed.

Indications:Sealing gingival floor of the proximal box of Class II restorations.Class V cavitiesSmall Class III cavitiesFirst increment of all deep restorations to prevent voids and porosities and to get good seal.Small Class I cavities frequently referred to as Preventive Resin RestorationsBlocking out cavity undercuts during inlay, onlay and crown preparations. Contraindications:Avoid on the surface of moderate to large restorations because of its less wear resistance, compared to viscous composites and compomers.

COMPOMERS They are polyacid- modified resin composites.They are similar to resin-modified glass ionomers in that they contain all the major components of both polymer based composites and glass ionomers,with the exception of water. Water is excluded to prevent premature setting of the material and also to ensure that setting occurs only through a polymerization reaction. limited acid-base reactions are believed to occur once the material is exposed to ,and absorbs ,water. Although the name implies that the material possesses a combination of characteristics of both composites and glass ionomers,

These materials are essentially polymer-based composites that have been slightly modified to take advantage of the potential fluoride releasing behavior of glass ionomers.The properties of compomers are superior to those of traditional glass ionomers and resin-modified glass ionomers,and in some cases, are equivalent to those of contemporary polymer based composites.Although compomers are capable of releasing fluoride,the release sustained at a constant rate and anticariogenicity is questionable.

REVIEW OF LITERATURE:Ralph W Phillips , David R Avery , Rita Mehra , Marjorie L Swartz and Robert J Mc Cune (1973)

Did a three year evaluation of class II amalgam and composite restorations. Except wear, composite performed well in terms of marginal adaptation, recurrence of secondary caries and discoloration.Felix Lutz and Ralph W Phillips, (1983) Classified the composites resins based on the manufacturing technique, the average size and chemical composition of filler and analyzed the performance of the different types. They concluded that hybrid composite resins can be considered in optimal combination of the well tried traditional and the new microfiller composite resin technology. If esthetics is the prime concern and virtually undetectable restorations are desired, microfilled resin systems, particularly light cured are the materials of choice.

R.B. Joynt , G . Wiezkowski et al (1987) studied the effect of composite restorations on resistance to cuspal fracture in posterior teeth. They found prepared unrestored teeth were weaker than restored. No significant difference was noted in fracture resistance between teeth restored with amalgam and with composite resin. Hideaki Shintani , Naoki Satou et al (1989) studied two brands of the hybrid type posterior composite resin placed in extensive occlusal cavities after removal of only the caries detector stainable tissue , and application of a bonding agent showed no adverse pulp reaction after four years and may be considered suitable for posterior restorations whne indicated.

Frank Cougman , Fredrick Rueggrberg et al (1995) reviewed the literature to arrive at guidelines for optimal curing conditions ;Attention should be paid to the signs of degradation of the bulb , reflector and curing tip of the curing lamp Restoration to be light cured no greater than 2 mm increments, when using darker shades 1 mm is optimal .Distance of the light tip to the restorations surface should be kept within 6 mm to ensure sufficient depth of cure .Exposure time of 60 seconds is optimal if light intensity id greater than 280 mW/ cm2

A. Versluis , W H Douglas et al (1996) evaluated the popular belief that incremental technique reduces residual shrinkage stress . in there study of various layering techniques they concluded that incremental filling techniques increase the deformation of restored tooth. The incremental deformation decreases the amount of composite needed to fill a cavity. but incremental filling methods may need to be retained for reasons such as densification , adaptation , thoroughness of cure and bond formation .

K D Jandt , R W Mills et al.(2000) studied the depth of cure and compressive strength of dental composites cured with blue light emitting diode and compared with the QTH lamps. The conventional lamps cured significantly deeper, but both cured deeper than required by ISO 4049 standards. The compressive strengths were similar.With its inherent advantages, such a constant power out over the life time of the diodes, LEDs have great potential to achieve clinically consistent quality of composite cure.

Noboru Ebi , Satoshi Imazato et al (2001) studied inhibitory effects of resin composites containing bactericide immobilized filler on plaque accumulation. In experimental composite containing immobilized bactericide at 2.83% was prepared by the incorporation of antibacterial monomer 12 methacryloloxydodecylpyridinium bromide (MDPB) into a prepolymerised resin filler, and elution of antibacterial components and inhibition of invitro plaque accumulation by streptococcus mutans was determined.The experimental composite had reproducible inhibitory effects against plaque all though it showed no elution of unpolymerised MDBB. The plaque inhibitory effect was found to depend upon the ability to inhibit attachment , glucan synthesis and growth of bacteria on its surface .

Raul W. Arcis , Manuel Toledano et al (2002) studied the mechanical properties of visible light cured resins reinforced with hydroxyapetite . The surface of the hydroxyapetite particles was modified using a coupling agent (citric, maleic acrylic or methacrylic acid).The addition of 50 to 60 wt % of hydroxyapetite particles to the unfilled monomer lead to the increase of both Youngs modulus and surface hardness of the material., while the flexural strength decreased.

H H Xu , G E Schumacher et al (2003) studied the effect of continuous fiber performed reinforcement dental resin composite restorations . Glass fibers were sialinzed , impregnated with a resin cured and cut to form inserts for tooth cavity restorations . Substational improvement in flexural strength toughness and stiffness were achieved.

Sanjukta Deb , Harminder Shemi et al (2003) conducted a study to investigate the effect of plasma arc light using a 3 s and a step cure regime on the properties of 4 dental restorative materials and compared it with properties resulting form halogen light curing of the same materials. Plasma step cure and halogen curing were found to yield composites with superior properties in comparison to 3 s plasma cure suggesting that a step cure resin is preferred method, when a plasma light unit is used

CONCLUSION Today composites have come a long way from the early used unfilled resins .New research and development in these materials have made it possible to overcome their initial drawbacks such as poor color stability, excessive wear, microleakage strength of these materials Composites with their various applications have encroached into all fields of dentistry, for prevention as pit and fissure sealants and block out resins , for strengthening of teeth in periodontal splints , as orthodontic retainers, in fixed prosthodontic and restorative dentistry.

Esthetic dentistry has emerged taking the inherent advantage of composites which can produce outstanding results. Diastema closures, cosmetic mockups, cover up of stains can all be done in a single sitting. With the development in indirect resin systems they have come to challenge ceramics , the most popular of the esthetics restorative materials till date.Packable composites have literally replaced amalgam as esthetic posterior restorations. With all this inherent advantages and vast applications composites have become the restorative material to look out for the present millennium.

REFERENCES:1.Inhibitory effects of resin composite containing bactericide immobilized on plaque accumulation. Noboru Ebi , Satoshi Imazato, Yuichiro Noiri , Shigeeyuki Ebisu . Dental mat. 17 (2001) 485 4912. Mechanical properties of visible light cured resins reinforced with hydroxyapetite for dental restorations . R W Arcis , A Lopz , Macipe , M Toledano , E Osorio , R Rodriguez Clemente , J Murtra , M A Fanovich , C D Pascual Dental mat. 18 (2002) 49-573. A comparative studies of the properties of the dental resin composites polymerized with plasma and halogen light. S Deb , H Shemi Denatl mat. 19 (2003) 517 5224 Continuous fiber reinforcement of dental resin composite restorations . H H Xu , G E Schumacher , F C Eichmiller , R C Peterson, J M Antonucci , H J MuvllerDental mat. 19 (2003)

5. Depth of cure and compressive strength of dental composites cured with blue light emitting diodes. K D Jantt , R W Milles , G B Blackwell , S H Ashworth , Dental mat. 16 (2000) 41 47 6. Does an incremental filling technique reduce polymerization shrinkage stress? A Versluis , W H Dougles , M Cross , R L Sakaguchi J Dent. Res. 75 ; 871 878 7. Classification and evaluation of composite resin systems . Felix Lutz and R W PhilipsJ Prosthet. Dent. 50 , 4, 480-488 (1983)8 . Effects of composites resin restorations on resistance to cuspal fracture in posterior teeth. R B Joynt , G Wieczkowski Jr, R Klockwski , E L Davis J Prosthet. Dent 57 , 4 , 431- 435 (1987)10. Clinical significance of polymerization shrinkage of composite resins . J R Boaush , K D Lange , C L Davidson , A Peters , A J DeGeee J Prosthet. Dent 48 ,1 , (1992)

11. Clinical evaluation of two posterior composite resins retained with bonding agents.Hideaki Shintani , Naoki Satou , Jumko Satou, J Prosthet. Dent 62 , 627 632 (1989)12. Philips Science of dental materials 11 th edition K J Anusavise13. Restorative dental materials 11 th edition - Robert G. Craig 14. Sturdevants art and science of operative dentistry 4 th edition T M Roberson15. Dental materials properties and selection - William J OBrien 16. Adhesive metal free restorations D Dietschi, R Specafico17. Esthetics dentistry B G Dale , K W Aschheim18. Clinical restorative materials and techniques K F Leinfelder, J E Lemons

19 . The dental clinics of North America Oct 198320 The dental clinics of North America July 199321 The dental clinics of North America Jan 200122 Oxford concise dictionary 23. Mosbys dental dictionary

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