Composites can also be defined as a three-dimensional combination of two or more chemically dissimilar materials with a distinct interface between them. Its properties are superior or intermediate to those of the individual components.
It is the chemically active component of the composites, which undergoes polymerization to convert the carbon double bonds in the monomer into single bonds of polymers during addition polymerization reaction. Thus, the fluid monomer converts to the rigid polymer. It is this ability of the composites to convert from a plastic mass into a rigid solid that allows it to be used for restoration of the teeth.
Most composite resins use monomers that are aromatic or aliphatic diacrylates. The most commonly used monomer is the bis-GMA, a dimethacrylate derived from the reaction of bisphenol A and glycidyl methacrylate. This resin is commonly referred to as “Bowen’s resin” after its inventor R.L. Bowen (1962). Since it has a higher molecular weight than MMA, the density of methacrylate double bond groups is lower in the bis-GMA monomer, which results in lower polymerization shrinkage. The use of a dimethacrylate also results in extensive cross-linking that consequently enhances resistance to degradation by solvents and also improves the properties of the polymer.
Other difunctional molecules used in composites are urethane dimethacrylate (UDMA) and Triethylene glycol dimethacrylate (TEGDMA). UDMA replaces the bisphenol A backbone with a linear isocyanate one. Both bis-GMA and UDMA, because of their high molecular weight, are highly viscous at room temperature. For practical reasons, they are diluted with another diluent monomer with an aliphatic backbone, TEGDMA, which has a much lower viscosity.
A 50/50 blend of bis-GMA and TEGDMA has a very low viscosity of 200 cP (centipoises), whereas a blend of 25% TEGDMA and 75% bis- GMA has a viscosity of 4300 cP. Both bis-GMA and TEGDMA contain two reactive double bonds, which during polymerization form covalent bonds between the polymer chains in a cross-linked network. This cross-linking improves the properties of the composite produced, such as increase in modulus and reduction in solubility.
Other diluent monomers used are methylmethacrylate (MMA) and ethylene glycol dimethacrylate (EGDMA).
Incorporation of filler particles into the resin matrix significantly improves the properties of the composite if the filler particles are well bonded to the matrix. Filler particles are of inorganic composition. The first filler particles used were quartz (silicon dioxide/silica). Filler particles may also be composed of borosilicate or lithium aluminum silicate glasses, or barium, strontium or zinc glasses, which are radiopaque. Colloidal silica and elements of high atomic weight such as barium, strontium, zirconium, or ytterbium are also incorporated into the filler particles.
Inorganic quartz has been used extensively as a filler in the first generation of composites. It is chemically inert and extremely hard and difficult to grind into finer particles. Hence, composites containing quartz fillers are more difficult to polish and may cause more abrasion of the opposing teeth.
The volume of fillers particles, their size, size distribution, index of refraction, radiopacity, and hardness are the important factors that determine the properties and clinical applications of the resulting composite resin. Because silica filler particles are approximately three times as dense as the resin monomer, 75 wt% of the filler is equivalent to approximately 50 vol% of the filler.
The amount of fillers that can be incorporated into the resin matrix is determined by the surface area of the filler particles. When comparing equivalent weight percentages of fine (0.4–3 mm) and microfine (0.04–0.2 mm) fillers, the surface area of the microfillers is much larger. This large surface area can form polar bonds with the monomer molecules and thicken the resin.
The properties of the composite resin depend on the bond between the resin matrix and the filler particles. The filler particles are not soluble in the resin matrix since the resins are hydrophobic, whereas the silica-based glass particles are hydrophilic due to a surface layer of hydroxyl groups bound to the silica. Hence, the glass particles do not bond to the resin matrix naturally. This problem is addressed by the incorporation of coupling agents such as an organic silicone compound called silane. The silane coupling agent most commonly used is g-methacryloxypropyl trimethoxysilane
The silanes have a hydroxyl group and a methacrylate group on either ends. The hydroxyl group reacts with the hydroxyl group on the hydrophilic fillers by a condensation reaction at the interface between the glass particles and the silane. This creates covalent bonds at the glass–silane interface. On the other hand, the methacrylate group (similar to conventional resin composites) reacts with the resin matrix forming a bond from the filler through the silane coupling agent to the polymer matrix.
Composite resins can be either chemically activated or light activated. Chemical cure composites are available as two-paste systems—universal paste which contains the benzoyl peroxide initiator and the catalyst paste which contains a tertiary amine activator, N,N-dimethyl-p-toluidine. When the two pastes are mixed together with a plastic spatula, the benzoyl peroxide reacts with the amine to form free radicals which initiate the process of polymerization.
Initial light-activated systems used ultraviolet (UV) light to initiate polymerization. The potential hazards of the UV light prompted its replacement with visible light activating systems in the blue spectrum of the wavelength (470 nm). The photosensitizer used is camphoroquinone, which has an absorption range between 400 nm and 500 nm (blue range of the visible light spectrum). Light-cured composite resins are available as a single paste containing the photosensitizer camphoroquinone (0.2 wt%) and an amine activator. When the paste is left unexposed to light, these two components do not react. However, exposure to the blue light of a correct wavelength produces an excited state of the camphoroquinone which then interacts with the amine to form free radicals that initiate polymerization.
Some composites such as core and provisional materials and luting cements are dual cured. These materials contain initiators and activators that allow both light and chemical activation. Available as a two-paste system, when mixed, chemical activation starts; the process can also be expedited by light activation.
To prevent a spontaneous start of polymerization of monomers, a very small number of inhibitors are added to the resin. These inhibitors have a strong reactivity potential with free radicals. In situations where the composite is exposed to ambient light briefly when the material is dispensed, free radicals may be formed. The inhibitor present in the composite resin reacts with these free radicals, thus inhibiting the ability of the radicals to initiate the polymerization process. It is only after all the inhibitor molecules are consumed that the polymerization process can be initiated. The inhibitor used is butylated hydroxytoluene in concentrations of 0.01%.
To achieve the various shades of dentin and enamel, dental composites must have visual coloration and translucency that can simulate the tooth structure. Shading can be achieved by adding different pigments such as various metal oxides in minute quantities. Titanium dioxide and aluminum oxide are added in minute amounts (0.001–0.007 wt%) as effective opacifiers. All color modifiers and opacifiers affect the light transmitting ability of the composite resin. Darker shades of composites transmit less light than lighter shades, suggesting that different shades and opacities of composites have different depth of cure when cured with light. Studies have suggested that darker shades of composites be placed in thinner layers or cured for a longer period of time to ensure optimal polymerization.
- Esthetics: Tooth-colored restorative material with different shades and translucency makes composite resins an ideal direct restorative material over silver amalgam.
- Conservative cavity preparations: A major advantage of composite resins over amalgam is the ability to prepare very conservative cavity designs, resulting in preservation of tooth structure.
- Reparability: Composite resin can be repaired by the addition of another composite layer, even though the oxygen-inhibiting layer has been removed, if the surface is first etched, silanated, and then a compatible bonding agent applied.
- Low coefficient of thermal conductivity: When compared to silver amalgam (30 times more conductive than dentin) and gold (500 times more conductive) that require an insulating base, composite resins offer greater insulation than enamel and hence do not require insulating bases.
- Technique sensitive: Posterior composite restorations are more technique sensitive than silver amalgam restorations and require almost twice the time for completion.
- Proper contact and contour: Accomplishing proper contacts and contours of the proximal surface with composite resins is quite challenging due to its plastic consistency.
- Isolation: Isolation is critical in composite restorations, especially in gingival areas of deep class II and class V lesions, when compared to silver amalgam and glass ionomer cements, since moisture contamination of composite resins results in reduced physical properties.
- Nonantimicrobial: Unlike glass ionomer cements, composite resins cannot arrest the growth of microorganisms. Hence, the incidence of secondary and recurrent caries is more with composite restorations.
- Polymerization shrinkage stress: During polymerization shrinkage of the composites, stresses are induced within the material. When this stress is greater than the bond strength of the adhesive used, debonding can occur, leading to marginal gap formation, postoperative sensitivity, and irritation
to the pulp due to bacterial ingress.
- Class I, II, III, IV, V, and VI restorations
- Foundations or core buildups
- Sealants and preventive resin restorations (conservative composite restorations)
- Esthetic enhancement procedures:
- Partial veneers
- Full veneers
- Tooth contour modifications
- Diastema closures
- Cements (for indirect restorations)
- Temporary restorations
- Periodontal splinting
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