COMPOSITE RESINS
INTRODUCTION
The search for the ideal esthetics material for restoring teeth has resulted in significant improvements in esthetic materials and techniques for using them.
Composite and acid-etch-technique represents 2 major advances. Composites have largely replaced other types of tooth-colored materials used for esthetic restorations. They now enjoy universal clinical application. They can be used almost anywhere in the mouth for any kind of restorative procedure.
The reason for such expanded usage of these materials relate to improvements in their ability to bond to tooth structure and their physical properties.
DENTAL COMPOSITES
Dental composites are highly cross-linked polymeric materials reinforced by dispersion of glass, crystalline or resin filler particles bound to matrix by silane coupling agents.
HISTORY OF COMPOSITES
Late 1930s – First mention of the methyl methacrylates.
Kramer & Mclean (1949) – Published several papers on a number of materials in this category.
Bronocore 1954- Development of micro-mechanical adhesion to enamel simply by acid etching the enamel for upto 1 minute rather than applying a thin coat of a very low viscosity resin.
Bowen (1960) – Proved the value of including a variety of fillers. Also modified the resin formula from simple methyl methacrylate to bisphenol – A diglycidyl dimethacrylate & modern concept of “filled/composite resin was generated.
CLASSIFICATION OF COMPOSITES
Composites are usually divided into 3 types based on size, amount and distribution of inorganic fillers.
Type 1 – Conventional composites
Conventional composites generally contain 75% to 80% inorganic filler approximately by weight.
Average particle size of conventional composites is 8 to 12µm. Also known as traditional composites or macrofilled composites.
Because of relatively large particle size and extreme hardness of filler particles, they exhibits a rough surface texture and a pattern of unacceptable wear of both itself and the opposing tooth.
Type 2 – Microfilled composites
Microfilled composites have an inorganic filler content of approximately 35% to 60% by weight. Average particle size is 0.01 to 0.04µm.
They exhibit a smooth lustrous surface similar to tooth enamel. They are clinically very wear resistant.
Type 3 – Hybrid composites
These are also known as small particle composites.
They contain a combination of macrofilled particles with a proportion of microfiller particles. It’s the most commonly used composite resin.
They generally have an inorganic filler content of approximately 75% to 85% by weight. Average particle size of hybrid composites is 0.4 to 1µm.
They exhibit superior physical and mechanical characteristics and have a smooth surface texture in the finished restoration.
Hybrid composites have sufficient strength to restore fractured incisal edges.
COMPOSITION AND FUNCTION OF EACH COMPONENTS
The components are discussed below:-
1. Resin matrix
A plastic resin material that forms a continuous phase and binds the filler particles.
These are generally aromatic and / or aliphatic dimethacrylate monomers such as bis-GMA, TEGDMA ie, triethylene glycol dimethacrylate.
These form highly cross linked polymer structures that results in a rigid resin matrix that is highly resistant to softening/degradation by heat and solvents.
2. Filler particles
These are reinforcing particles and /or fibers that are displaced in the matrix. These may be :-
A). Macrofillers with aparticle size of about 5 – 30mm.
e.g. glass, quartz, ceramic, etc.
B). Microfillers with aparticle size of 0.04mm.
e.g. amorphous silica.
The primary purpose of filler particles is to strengthen composite and to reduce the amount of matrix material, resulting in increased hardness, strength and decreased wear, reduction in polymerization shrinkage.
3. Coupling agent
Bonding agent that promotes adhesion between filler and resin matrix.
e.g. Titanates and Zirconates Organosilanes.
4. Activated initiator system
The polymerization mechanism in dental composites is initiated by free radicals.
Free radicals can be generated by chemical activation or by visible light activation.
The chemically activated systems are supplied as 2 pastes which contains Initiator as Benzoyl peroxide
Activator as tertiary acromatic amine.
The visible high activated systems are supplied as single paste which contains –
- Photosensitizer as camphor, quinine
- Iinitiator as a tertiary amine.
5. Inhibitors (stabilizers)
Inhibitors are added to prevent spontaneous/accidental polymerization of monomers.
e.g. Butylated hydroxytoluence.
They have2 functions –
- They extend the storage lifetime
- They ensure sufficient working time.
SUPPLIED AS –
Composites are usually supplied in a kit containing the following
- Syringes of composite resin pastes in various shade.
- Etching liquid (37% phosphoric acid)
- Enamel dentin bonding agent.
- Shade guide.
CHEMICALLY ACTIVATED COMPOSITES ARE AVAILABLE AS
2 paste system –
- Base and catalyst paste supplied in jars or syringes.
- Powder liquid system – Powder in jars and liquid in bottles.
LIGHT ACTIVATED COMPOSITES ARE AVAILABLE AS
Single paste system supplied in light tight syringe.
CURING OF RESIN BASIN COMPOSITES
Chemical activated:-
Chemically activated resins are supplied as 2 paste, one contains Benoyl peroxide initiator and other an aromatic tertiary amine activator. When 2 pastes are mixed together, the amine reacts with Benzoyl peroxide to form free radicals, and polymerization is initiated.
These materials are mainly used for restorations and large foundation structure (buildups).
Light activation:-
Light activated resins are supplied as a single paste containing a photosensitizer and an amine initiator. On exposure to light in the blue region, the photosensitizer interact with the amine to form free radicals that initiate polymerization.
ADVANTAGES OF USING LIGHT CURE RESINS
The light activated resins have overcome many of the deficiencies of chemically activated resins
(1) They provide control over working time
(2) Exhibit less internal porosity and
(3) Colour stability.
DRAWBACKS OF LIGHT CURED RESINS:-
There are also several drawbacks of light cured resins –
- Limited curing depth, requiring buildup layers of 2mm or less.
- Relatively poor accessibility in certain posterior and interproximal locations.
- Variable exposure times because of shade differences.
PROPERTIES OF COMPOSITES
1. Polymerization
Chemical reaction in which monomers of a low molecules weight are converted into chains of polymers with a high molecular weight.
In a chemically activated composite resin, the reaction takes place almost uniformly throughout the bulk of the material.
In the light activated systems, much of the resin not activated initially by the light at the time of curing will remain unset.
Most of the chemistry of the setting reaction and the consequent construction will take place within the first few seconds during light activation and the remainder will be complete within 2 days.
2. Water sorption and solubility
Water sorption is higher for microfilled resins and for hybrid and macrofilled resins.
Variation in the water sorption and solubility of different composite resin is associated with the type and amount of monomers.
Composite materials do not show any clinically relevant solubility.
3. Polymerization contraction
Composite materials shrink while hardening. This is referred to aspolymerization shrinkage.This phenomenon can not be avoided. The problems and risks are less in restoring deciduous teeth because the cavities are relatively small and therefore is easier to get light activation to penetrate the relatively short distance to the base of the restoration.
Polymerization shrinkage usually does not cause significant problems with restorations having all enamel margins. When a tooth preparation has extended upto root surface, the polymerization shrinkage can cause agap formationat the junction of composite and root surface. This V-shaped gap is composed of composite in the restoration side and hybridized dentin on the root side. The clinical significance of this gap is not fully known.
Contraction gap (exaggerated). A, V-shaped gap on root surface.
B,Restoration-side vector is composite; root-side vector is hybridized dentin.
4. Wear
Wear resistance refers to a material’s ability to resist surface loss as a result of abrasive contact with opposing tooth structure, restorative material, food boli, and items as tooth brush bristles and tooth picks.
Clinical wear of the composite resin remains one of the main weakness. In its use as a restoration on load bearing surfaces.
5. Biocompatibility
The biocompatibility of restorative materials usually relate to the effects on the pulp from 2 aspects:-
a) The inherent chemical toxicity of the material and
b) The marginal leakage of oval fluids.
Adequately polymerized composite are relatively biocompatible because they exhibit minimum solubility, and unreacted species are leached in very small quantities. Inadequately cured composite materials at the floor of cavity can serve as a reservoir of diffusible components that can induce long term pulp inflammation. This situation is of particular concern for light activated materials.
The second biological concern is associated with shrinkage of the composite during polymerization and the subsequent marginal leakage. The marginal leakage may allow bacterial in growth, and these organisms may cause secondary caries pulp reaction.
6. Marginal leakage
When the gingival margins of the cavity preparation are located in dentin, Cementum or both, and the resin is firmly anchored to the etched enamel at the other margins, the material tends to pull away from the gingival margins during curing because of polymerization shrinkage. This leads to formation of gap at that interface. Hence the risk for marginal leakage and its ensuring problems of marginal staining and secondary caries is enhanced.
7. Radiopacity
Resins are inherently radiolucent. However, leaking margins, secondary caries, poor proximal contacts, wear of proximal surfaces, and other problems can not be detected unless adequate radiographic contrast can be achieved. Thus radiopacity is an especially important property for any posterior restorative material. Radiopacity is imparted by certain glass filler particles containing heavy metal atoms.
8. Repair
Composite can be repaired by placing new materials over the oral (material) composite. This is a useful procedure for converting defects or altering contours on existing restorations.
The procedures for adding new material differ, depending on whether the restoration is freshly polymerized or whether it is an older restoration.
When a restoration has just been placed and polymerized, it may still have an oxygen inhibited layer of resin on the surface, so the addition of new composite can be made directly to this layer because this represents an excellent bonding substrate.
As the restoration ages, fewer and fewer unreacted methacrylate groups remain, and greater cross linking reduces the ability of fresh monomer to penetrate into the matrix.
The strength of the bond between the original material and the new resin decreases in direct preparation to the time that has elapsed between polymerization and addition of the new resin.
9. Survival probability
The clinical performance of dental restoration is best judged on the basis of long- term clinical trials.
The most consistent survival levels are exhibited by amalgam restorations.
The variability is much larger for the composite restorations as compared with amalgam restorations.
The survival rate overall for composite in permanent teeth after 7 years was 67.4% compared with 94.5% for amalgam restorations.
10. Linear coefficient of Thermal expansion (LCTE)
LCTE is the rate of dimensional change of a material per unit change in temperature.
The closer the LCTE of the material is to the LCTE of enamel, the less chance there is for creating voids or openings at the junction of the material and the tooth when temperature changes occur.
The LCTE of improved composites is approximately 3 times that of tooth structure.
11. Surface texture
It is the smoothness of the surface of the restorative materials. Restorations in close approximation to gingival tissues require surface smoothness for optimal gingival health.
The size and composition of the filler particles primarily determine the smoothness of s restoration, as does the materials ability to be finished and polished.
Microfill composites – smoothest restorative surface
Hybrid composites – provide surface texture that are esthetic and compatible with soft tissues.
12. Modulus of elasticity
It is the stiffness of a material. A material having a higher modulus is more rigid and material with a lower modules is more flexible.
Microfill composite – increase flexibility perform better in certain class V restorations
Hybrid composite – more rigid
Properties of Composite Restorative Materials
MANIPULATION – CLINICAL STEPS
The steps are as follows-
1) The fault is removed from the tooth
2) The enamel should be etched with 37% orthophosphoric acid to demineralise the enamel to a depth of 20 to 30µm and render it porous.
3) A fluid resin adhesive material is bonded to etched surface and allowed to soak in the porosities for about 30 seconds.
4) Composite resin is then bonded to the resin, contoured and polished.
PRE REQUISITES FOR ETCHING
- First, the enamel margins must be fully mineralized and soundly based on healthy dentin.
- Also there must be no microcracks present on the tooth.
- The best union will be developed at the ends of enamel rods so it’s desirable to develop a reasonably long level at the cavo-surface margin.
GOAL OF RESIN DENTIN BONDING AGENT
It is to attach the composite resin to healthy dentin and to seal dentin tubules against the entry of bacteria and their toxins.
This will avoid post restoration sensitivity, caries and loss of restoration
PRINCIPLES TO SUCCESSFUL RESIN – DENTIN BONDING
- Dentin should be etched to remove smear layer and dentin tubules plugs.
- Surface should be thoroughly washed to remove all remaining etchant.
- Surface should remain wet but not flooded.
- Apply a hydrophilic primer containing acetone to facilitate penetration of resin adhesive around the exposed collagen fibers. Finally apply resin adhesive and cure before applying composite resin.
DELIVERY AND PLACEMENT
Place the freshly mixed material into disposable syringe and then tamp the material into the cavity with a small plastic sponge.
Placement must be undertaken with care and attention to the depth of cure available through a curing light.
Ensure that the lower layers are also cured adequately.
INCREMENTAL BUILD UP
It is essential to undertake incremental build up of any restoration deeper than about 2mm. Incremental placement means placement of the composite in small quantities in selected areas of the cavity and then directing the light activating unit.
A, First incremental layer of resin composite (gray area) has been placed and cured. B, Second increment being cured with a light source. C, Third composite increment during curing.
DEPTH OF CURE
In a child patient the depth of cure of a composite resin is quite significant. It is imperative that the activator light be placed within 1-2mm of the surface of the newly placed restoration otherwise the depth of cure will be limited.
FACTORS TO BE CONSIDERED WHILE CURING
- The degree of cure will decrease with increasing depth.
- Increased time of exposure to the light will increase depth of cure.
- The move heavily filled the resin and the larger the particle size, the greater depth of cure. Microfilled resins will cure to a depth of 2-3mm only while hybrid resins may cure to a depth of 4-5mm.
- Lighter the shade of the material the greater the depth of cure and the greater the translucency the deeper the cure.
- The tip of the light source should be placed as close as possible to the restoration and should never be move than 4mm away.
INDICATIONS
Composite can be used for most clinical applications. Generally the indications for use are as follows:
1) Class I, II, III, IV, V and VI restorations
2) Formations or core buildups
3) Sealants and preventive resin restorations
4) Esthetic enhancement procedures
- Partial veneers
- Full veneers
- Tooth contour modifications
- Diastema closures
Teeth restored esthetically by veneering
Esthetic restoration for localised enamel hypoplasia
5) Cements (for indirect restoration)
6) Temporary restorations
7) Periodontal splinting
8) Composite for posterior restoration
Composite allows conservative tooth preparation in the posterior area of a second premolar tooth. See also color plate.
Use of a segmental matrix band is best for restoring proximal contacts with composite. See also color plate.
Class II composite restoration (DO in second premolar tooth). See also color plate.
LIMITATIONS OF COMPOSITE RESINS
Constituents such as HEMA have been identified as allergens.
Any unreacted polymer chains may be irritant to the pulp and lead to post – insertion sensitivity.
The tissue cells respond less favourably to composite resin.
ADVANTAGES
1) Esthetics
2) Conservation of tooth structure removal
3) Less complex when preparing the tooth
4) Insulative, having low thermal conductivity.
5) used mostly universally
6) Bonded to tooth structure, resulting in good retention, low microleakage, minimal interfacial staining, and increased strength of remaining tooth structure
7) Repairable
DISADVANTAGES
1) May have a gap formation, usually occurring on the root surface as a result of forces of polymerization shrinkage being greater than initial early bond strength.
2) More difficult, time consuming and costly because tooth treatment of bonding usually requires multiple steps.
3) Are more technique sensitive because the operating site must be appropriately isolated, and the placement of etchant, primer and adhesive on the tooth structure is demanding of proper technique.
4) May exhibit greater occlusal wear in areas of high occlusal stress or when all of the tooth’s occlusal contact are on the composite material.
5) Have a higher LCTE, resulting in potential marginal percolation if an inadequate bonding technique is used.
CLINICAL CONSIDERATIONS
In pediatric dentistry a restoration will not be expected to last for more that a few years so a limited amount of wear can be tolerated. However a relatively large setting shrinkage will be undesirable. Aesthetic and fracture resistance will not be of great importance but the ability to bond effectively to both enamel and dentin will be relevant. Simplicity of placement techniques will be important.
