Composite Resin – From their initial introduction in the late 1960s to the present, composites have undergone dramatic evolution, especially with respect to their adaptability, handling, retention, and esthetics. A composite is composed of a resin polymer with a filler of glass, silica, and quartz. Similar to a wall composed of mortar and bricks, a composites resin matrix holds the filler particles together.
Most composite resin bases are usually BIS-GMA (bisphenol A— glycidyl methacrylate) or urethane dimethacrylate. The filler particles are coated with a silane coupling agent that bonds to the particles and allows the particles to bond to the resin matrix. Composites can be classified according to their process of polymerization initiation and filler type.
POLYMERIZATION of composite resin
The composite resin matrix chemically takes its tInal form through a process called polymerization. In simple terms, we can call it hardening or curing. Actually, it is a chemical process in which monomers (simple chemical compounds with a single unit) and oligomers (chemical compounds with a few structural units) form long-chain polymers (compounds with repeating structural units). There are three methods by which this polymerization process is initiated.
The first composites that were introduced in the field of restorative dentistry, were self-cure composites. In this variety of composite, the polymerization cycle starts by mixing two components (labelled base and catalyst, or A and B) and an initiator (a chemical component that begins polymerization, usually a benzoyl peroxide). After the mixing this material sets on its own in 2 to 4 minutes, so the dentist has that much time to place the material before the composite hardens to its final form.
Because of its self-curing nature this variety of composites requires quick placement. Considerable time is necessary to finish and polish the very hard fmal form of the composite resin. Working time is approximately ito 1½ minutes, with setting time about 4 to 5 minutes from the start of the mix.
Light Cure Composites
To overcome the shortcomings of the self-cure composite resin and to give the control of setting time in the hands of the user of the material, the researchers have developed composite resin that polymerize quickly only upon exposure to light of a certain wavelength. These light cure (or photo cure) composites provide the operator more time for placement and require less time for finishing.
Unlike the self-cure composite resin (which have two components that must be mixed), the light cure composites are produced as single-paste formulas with the initiator (usually camphorquinone) included in the paste. A light cure composite resin can be manipulated until the operator exposes it to a curing light, giving the option to the operator to satisfactorily finish the filling before initiating the polymerization or setting of the material.
A curing light is an instrument designed to emit the type of light required to initiate polymerization. When the initiator absorbs the light, polymerization occurs. This hardening can often be achieved with a 20 second exposure to the curing light. When this light curing takes place, be sure to wear your shield and protective glasses and not look directly at the light, as it may harm the eyes.
Dual Cure Composites
In certain applications, such as the placement of composite resin or porcelain inlays, onlays, and veneers or the case of bonded amalgams, it is impossible for the curing light to penetrate through the restorative to polymerize the composite resin cement. A light cured composite resin cement used in these situations would only polymerize at the exposed margins.
Thus, researchers developed the dual cure composite resin. The dual cure materials contain two initiators—a light cure initiator and a self-cure initiator. The light cure component allows for rapid polymerization of the composite material at the critical margins on exposure to light curing light, while the self-cure initiator ensures that the composite underneath the restorative completes polymerization, without exposure to light source.
There are three categories of filler types based on the size of the particles and their distribution.
The composite resin introduced initially used to contain relatively large filler particles of irregularly shaped quartz with a range of 1 to 38 microns in size. These composites were self- cure in nature. These macrofihled composites used to have higher wear and tear resistance because of higher compressive strength. But they used to carry some shortcomings also.
Once placed, the large particle size of the macrofill would leave a microscopically rough surface when finished. This surface tended to accumulate stain and plaque or suffer from plucking. Plucking is the process through which the filler particles are removed or lost from the composite resin matrix. The factors such as expansion, contraction, and the various strains placed by biting and chewing will loosen or knock out the particles. Macrofills are available in self-cure and light cure versions.
The microfilled composites represent the composite resin with the smallest filler particles. Pure microfills contain filler particles under 1 micron in size, often as small as 0.04 micron. These filler particles are primarily colloidal silica. Smaller filler particles create a smoother composite surface. First introduced as self-cure materials, they are an improvement over the macrofills due to their highly polishable surface.
However, they do not have as strong a compressive or shear strength and are subject to fracture or chipping under shear strains. So, there use was generally restricted to anterior teeth or non-stress bearing restorations. Microfills are available in self-cure and light cure versions.
Hybrid composite resin get their name because they contain more than one type of filler particle. They are the mixture of microfilled and macrofilled composites. Most commonly, they would be a glass particle with a range of 1—3 microns in size and a colloidal silica particle of 0.04 micron in size. When these two particles are combined, the composite reaches optimum strength. Other changes in chemical composition have also improved the handling properties of the hybrids:
• Minimizing the tackiness, or tendency to stick to the placement instrument and pull the material away when the instrument is withdrawn.
• Increasing the viscosity for easier packing and shaping.
• Eliminating slump and flow, once placed, and maintaining the desired shape.
• Changing some formulations so fluoride is released.
Hybrid composite resin match more closely the refractive and reflective light properties of tooth structure.
Therefore, they blend in and match the existing tooth’s colour and characteristics better. Hybrid composites polish much like microfills, resulting in a shiny, smooth surface. The addition of the glass particles make hybrid composite resin less likely to fracture and chip because of the increased compressive and shear strengths. Today’s most popular hybrid composite resin are all light cure in nature.
• Do not use composites in the presence of eugenol-containing cements or copal varnishes, or with chloroform present. Aromatic medicaments such as eugenol and chloroform will affect the composite’s ability to polymerize correctly.
• When using a light cure composite resin, be sure to keep the fresh composite in its light-protective syringe or compule until its used. Although normal overhead lighting won’t properly polymerize the light cure composite resin, it can affect the handling properties—making the material more difficult to place and manipulate. If a patient’s gingival tissue is unhealthy, the dentist may postpone treatment until the tissue has improved. Bleeding and moisture contamination will interfere with the setting reaction.
The first composites were placed in a similar fashion as the traditional amalgams. The cavity preparation was made with a box form for retention; a calcium hydroxide liner was placed, and the composite resin restoration was completed. With no direct bonding to the surrounding tooth structure, these composite restorations were susceptible to microleakage. Failure would frequently occur, which lead to the concept of different ways of bonding composites to enamel
Enamel is primarily (96%) inorganic, containing mostly hydroxyapatite (a calcium compound). On the other hand, dentin has a higher percentage of organic components. Its 70% inorganic and 30% organic matter (e.g., collagen) and water. This results in drastically different surfaces for an adhesive to bond.
Michael Buonocore of the Eastman Dental Center in Rochester, New York, was the first researcher who brought the concept of Enamel Bonding. His research demonstrated that the enamel rod structure could be opened up with a phosphoric acid etchant. The etchant created a very rough microscopic surface into which the unfilled composite resin would flow. Upon hardening, the composite resin provides a mechanical attachment to the tooth. On the basis of his research study the first bonding systems for composites bonded to the enamel of the tooth, was introduced.
In this enamel bonding technique 35—50% solution (liquid or gel) of phosphoric acid is used, which is carefully painted over the involved enamel margins of the prepared cavity and it is allowed to remain in contact with enamel surface for 30—60 seconds. In this process, called as acid etching, the acid penetrates into the enamel rods, opening up their prism structures.
After that, the acid is rinsed away with water and then the tooth is thoroughly dried, giving a dull or chalky- white appearance to the etched enamel. The enamel bond composite resin (basically the same resin polymers as in the composite to be used but without any filler particles) is painted over the etched enamel and dentin surfaces, so that the resin flows into the opened prisms and, upon polymerization, forms composite resin tags. After that, the composite restorative material is placed, which bonds to the enamel bond composite resin, completing the restoration.
Enamel bonding is, therefore, a mechanical system consisting of the composite resin tags locked into the etched enamel. The initial types of enamel bonds were low-viscosity, self-cure variety with limited working time. They consisted of two liquid components that were mixed and painted onto the etched enamel.
Subsequently, light cure versions of enamel bond systems were introduced, which consist of a one-component liquid whose polymerization is initiated by a curing light.
The third kind of enamel bonds are the dual cure versions, which consist of two components that can be light cured or will self-cure if the light can not reach the material.
After the success of enamel bonding systems, the dentin bonding material were introduced for the use in composite restorative procedures. It gave the additional bonding and the more reliability to composite resin restoration procedures.
Because the dentin of the tooth is so close to the vital pulpal tissues, care must be taken as to what we treat the dentin with. Modem dentin etchant/conditioners prepare the dentin surface for bonding by removing or modifying the smear layer. The smear layer is the layer of the tooth that remains after the drilling or debridement is completed. It consists of attached and loose organic and inorganic matter. Modem dentin primer/adhesives attempt to mechanically lock to the prepared dentin and chemically adhere to the collagen in the dentin.
1. The preparation is cleaned with a commercially available cavity cleanser! disinfectant, pumice, or chiorhexidine. It’s then rinsed thoroughly with a water spray.
2. The entire cavity preparation is then etched or conditioned with the supplied etchant (35% phosphoric acid solution). The enamel is coated first, and then the sensitive dentin is coated. Dentin needs only 15—20 seconds of exposure, while the enamel can be etched for up to 60 seconds.
3. All surfaces are thoroughly rinsed to remove any trace of the etchant. The enamel should be dried to ensure that excess moisture is removed from the newly created prisms. Most new dentin bonding systems have been shown to bond better to moist or damp dentin. Thus, the dentin should be dried only enough to remove excess standing “puddles” of water.
4. The primer (the solution used in dentin bonding) is then placed over the entire cavity preparation. Multiple coats are required by some systems, for which the manufacturer’s instructions must be followed. Dentin-bonding primers are typically hydrophilic (water-attractive) monomers that wet the tooth structure and prepare the surfaces for the bonding resin. In almost all systems, the dentin primer is dried from the tooth.
5. The bonding composite resin is then applied over the primer. Many dentin-bonding resins contain adhesion promoters in addition to their base composition of unfilled resin. These adhesion promoters enhance the resins ability to adhere to the tooth structure. The bonding composite resin penetrates into the intertubular and peritubular dentin, forming a complex mixture of tooth structure and composite resin, which is called the hybrid layer. Its this hybrid layer that makes dentin bonding nearly as strong as enamel bonding. Even more importantly, the enhanced seal of the hybrid layer reduces or eliminates microleakage, which has been implicated in causing sensitivity, restorative failure, and recurrent decay. After the application of the bonding and its curing the tooth is restored as usual with the selected composite resin.
Composite Resin Cements
Composite resin cements are unfilled or slightly filled versions of dental composites. That is, they contain little or no filler particles. As with their composite cousins, the composite resin is primarily BISGIVIA (bisphenol A—glycidyl methacrylate) or urethane dimethacrylate. Resin cements are available in the three polymerization types that composites are available: self-cure, light cure, and dual cure..
Self-cure composite resin cements are used in cases where a light would not reach to initiate polymerization, such as under crowns, bridges, and resin-bonded bridges. Other uses for self- cure resin cements include cementing endodontic posts and bonding orthodontic brackets.
Light cure composite resin cements can be used in applications where the light will reach the material to initiate the polymerization. For thin anterior veneers and certain porcelain crowns, these resin cements feature special colouration to esthetically match the existing tooth structures.
Dual cure composite resin cements are the best to use with inlays and onlays, although they could be used in any application resin cement might be used. Dual cure cements are also used in the bonded amalgam technique.
Adhesive dentistry has evolved to the point that dentin bonding provides the tightest marginal seal available today. For this reason, newer techniques have combined the advantages of this adhesive technology with the long-popular dental amalgam. These new techniques create the bonded amalgam composite resin, which is essentially an amalgam restoration placed with the dentin bonding system, It can even be used to repair or add to an older amalgam restoration.
Varnishes are used to protect the tooth from sensitivity due to microleakage and irritants. They are “painted” over all exposed dentin. Upon hardening, they form a seal over the dentinal tubules.
Copal composite resin
The original varnish formulations are copal (gum) composite resin based products and are used under dental amalgams being placed in the traditional technique. Some copal varnishes also include fluoride or antimicrobial agents for enhanced protection if microleakage occur. The copal resin is carried in a solvent of ether or chloroform. The solvent rapidly evaporates, leaving the resin behind.
When dental composite resin became popular, it was found that the copal varnishes interfered with the composite’s cure. Because of this interference, a new generation of varnishes based on polyamide composite resin was developed. This polyamide layer provides the same properties as the copal composite resin layer, while maintaining full compatibility with the composite resins. However, varnishes are no longer used with composite resins.