Use of Composite Resins in Dentistry
The terms "white filling" or "synthetic porcelain" are often used to refer to the group of materials known as dental resin composites. These materials usually come in the form of a paste or a viscous liquid that can be manipulated and shaped in a variety of ways and then set (polymerized) with a special light. Dental resin composites are mainly used in the fabrication of tooth-coloured fillings and veneers, cementation of crowns and the placement of pit and fissure sealants. A dental resin composite is a plastic resin to which filler particles have been added.
The filler particles are composed of finely ground quartz, borosilicate, lithium-aluminum-silicate glass and/or amorphous silica. Oxide glasses with barium, strontium zinc or other metals are usually added to make the material radiopaque. The filler particles are necessary to give the material appropriate physical properties and to minimize the shrinkage that occurs with polymerization. Some or all of the elements fluorine, sodium, silicon, calcium, aluminum and strontium are released by the filler particles.
The resin matrix is comprised of a monomer system, an initiator system for polymerization of the monomer(s) and stabilizers for maximizing both storage of the uncured resin and stability of the cured resin composite. An organo-silane is usually added to promote chemical bonding between the filler particles and the resin matrix. Impurities from the manufacturing process are present in the unreacted dental resin composite. By-products from the reaction of polymerization occur. The degradation of the resin matrix allows for the release of all of these chemicals and the creation and release of other substances as well.
The monomer 2,2-bis[4-(2-hydroxy-3-methacryloxypropoxy)phenyl]propane is the mainstay of modern dental resin systems. It was first synthesized in 1956 by the reaction of bisphenol A with glycidal methacrylate. It is customarily referred to by its acronym, BisGMA or as the Bowen monomer. The co-monomers ethylene glycol dimethacrylate (EGDMA) and/or triethylene glycol dimethacrylate (TEGDMA) are usually added to the BisGMA monomer to reduce viscosity. Another group of monomers are the urethane dimethacrylates which are synthesized from hydroxyalkyl methacrylates and diisocyanates. The most common of these, 1,6-bis(methacryloxy-2-ethoxycarbonylamino)-2,4,4-trimethylhexan (UDMA) is used alone or in combination with BisGMA and/or TEGDMA. Recent studies of current dental composite resins identified the following monomers and co-monomers:
Bis-GMA Bowen Monomer Bis-PMA Propoxylated bisphenol-A-dimethacrylate Bis-EMA Ethoxylated bisphenol-A-dimethacrylate Bis-MA Bisphenol-A-dimethacrylate UDMA 1,6-bis(methacryloxy-2-ethoxycarbonylamino)-2,4,4-trimethylhexan UPGMA Urethane bisphenol-A-dimethacrylate TEGDMA Triethylene glycol dimethacrylate TEGMMA Triethylene glycol monomethacrylate TEEGDMA Tetraethylene glycol dimethacrylate DEGDMA Diethylene glycol dimethacrylate EGDMA Ethylene glycol dimethacrylate DDDMA 1,10-Decanediol dimethacrylate HDDMA 1,6-Hexanediol dimethacrylate PDDMA 1,5-Pentanediol dimethacrylate BDDMA 1,4-Butanediol dimethacrylate MBDDMA ½ BDDMA-methanol-adduct ½ DBDDMA ½ BDDMA-auto-adduct ½ PRDMA 1,2-Propanediol dimethacrylate DMTCDDA Bis(acryloxymethyl) triclodecane BEMA Benzyl methacrylate SIMA 3-Trimethoxysilane propylmethacrylate SYHEMA ½ ½-Cyclohexene methacrylate TYMPTMA Trimethylolpropane trimethacrylate MMA Methyl methacrylate MAA Methacrylic acid
These same studies have also identified, from the same group of dental resin composites, the following reaction and degradation by-products, additives, and contaminants:
CQ Camphoroquinone BL Benzil DMBZ Dimethoxybenzoin CEMA N-(2-Cyanoethyl)N-methylanilin DMABEE 4-N,N-Diethylaminobenzoic acid ethyl ester DMABBEE 4-N,N-Diethylaminobenzoic acid butyl ethoxy ester DMABEHE 4-N,N-Diethylaminobenzoic acid 2-ethylhexyl ester DMAEMA N,N-Diethyl aminoethyl methacrylate DEMAEEA N,N-(Bisethylmetacrylate)-2-ethoxyethylamine HMBP 2-Hydroxy-4-methoxy benzophenone TINP 2(2'-Hydroxy-5'-methylphenyl) benzotriazol TIN326 Tinuvin 326 TIN350 Tinuvin 350 Tin328 Tinuvin 328 HQME Hydroxyquinone monomethyl ester BHT 2,6-Di-t-butyl-4-methyl phenol MBP 2,2-Methylenebis(6-t-butylphenol) MBEP 2,2-Methylenebis(6-t-butyl-4-ethylphenol) BPE Benzoic acid phenylester MMMA Methyl methacrylate methanol adduct CA Camphoric anhydride HC ½ 2(3)-endo-Hydroxyepicamphor TPP Triphenyl phosphane TPSb Triphenyl stibane DMDDA Dimethyl dodecylamine DMTDA Dimethyl tetradecylamine DCHP Dicyclohexyl phthalate DEHP Bis(2-ethylheexyl) phthalate FormaldehydeCuring of dental resin composites does not result in complete polymerization; 25 to 50% of the bond sites on the molecules remain unreacted. Included in this is about 10% residual monomer. Reaction products other than the desired polymer are produced during polymerization. Photo, thermal, mechanical and chemical factors result in degradation of the cured resin whereby the polymer is cleaved into various oligomers. Leaching of these substances from the set dental resin composite has been demonstrated.
Biologically active compounds have been identified in the substances leached from the cured resin matrix. It has been suggested that formaldehyde which is a by-product of polymerization may be responsible for oral lichenoid reactions. Formaldehyde is considered to be moderately carcinogenic. Cytotoxicity of the monomers bis-GMA, bis-MA, UDMA, DEGDMA, and TEGDMA has been demonstrated under laboratory conditions. The inhibitor BHT has been found to be severely cytotoxic, as have the initiators DMBZ and DMTDA and the contaminants TPP and TPSb. TEGDMA and GMA have been found to be moderately mutagenic and TPSb has been shown to be genotoxic. The estrogenic compound bisphenol-A can be cleaved from the resins and oligomers of some dental resin composites and sealants.
Dental resin composites wear more rapidly than alternative dental materials and appear to be less effective than metallic restorations in areas of high stress such as large cusps or marginal ridges. Failing resin composite restorations often exhibit active caries under the pulpal floor of the restoration. The polymerization shrinkage (2 - 3% in current commercial systems) that pulls the restoration away from the tooth and towards the light source during curing may account for this observation. A resin composite restoration in the posterior area of the mouth can be expected to last between 5 and 8 years, roughly half the lifespan of a dental amalgam restoration.
There are many factors to be considered in the decision to use dental resin composite as a filling material. That many of the resin constituents have not been assessed exhaustively for their possible biological hazards is a concern but perhaps more worrisome is the presence of contaminants introduced during the manufacturing process as well as the by-products created during polymerization and then degradation of the material over its lifespan. There is still much research to be done on the toxicity of the breakdown products, the reaction by-products and the contaminants. Total surface area of resin composite exposed, the amount of dental resin composite placed in a patient's mouth and exposure of the material to the forces of mastication are three factors that would be, at this time, prudent to limit.
John G. Evans
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L. Stephen Buchanan, Diplomat of the American Board of Endodontics: "Posterior composites are tombstones."
Gordon Christensen, Clinical Research Associates: "Although 'OK' direct resins are available now, no 'excellent' resins are available.... (all) direct resins currently available shrink two to three percent when they polymerize....expand and contract significantly with temperature changes....wear too much."
Chester Douglass, Professor of Oral Health Policy and Epidemiology, Harvard School of Dental Medicine and School of Public Health: "....mercury amalgam is safe and remains the most cost-effective dental material for posterior tooth restoration."