Advances in Dental Research

Biodegradation of Dental Composites/ Glass-Ionomer Cements G. Øilo ADR 1992 6: 50 DOI: 10.1177/08959374920060011701 The online version of this article can be found at:

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NIOM, Scandinavian Institute of Dental Materials N-1344 Haslum, Norway Adv Dent Res 6:50^54, September, 1992

Abstract—Studies of the degradation processes, types of tests, and measurements and analyses of substances leaching out from resin-based composite materials and glass-ionomer cements are reviewed. For both types of materials, the initial release rate rapidly decreases to a low, but nearly constant, level. For composites, various types of degradation processes have been demonstrated. Elements from filler particles and degradation products from the resin (e.g., formaldehyde) leak out. Many substances afe not properly identified. It is, however, difficult for in vitro and in vivo degradation to be compared. For glass ionomers, a total disintegration of a surface layer is observed, together with a slow release of elements from the bulk. Of the elements released, fluoride is the most interesting. Marked differences have been shown between in vitro and in vivo solubility tests.

This manuscript is published as part of the proceedings of the NIH Technology Assessment Conference on Effects and Sideeffects of Dental Restorative Materials, August 26-28,1991, National Institutes of Health, Bethesda, Maryland, and did not undergo the customary journal peer-review process.



iodegradation is defined as the gradual breakdown of a material mediated by specific biological activity (Williams, 1987). In the mouth, this is a complex process, including disintegration and dissolution in saliva and other types of chemical/physical degradation, wear, and erosion caused by food, chewing, and bacterial activity. No in vitro test is capable of reproducing this complex process. In vitro tests are, therefore, limited to a quality control of material properties. Such properties may indicate what might happen in vivo, but the correlation between in vitro data and clinical performance is in many cases unknown. COMPOSITES The quality of composites has constantly improved, and the expected lifetime of a single-surface restoration is from six to seven years. A multi-surface restoration has an expected lifetime of four years (Mjor et ai, 1990). There are many reasons for failure, but degradation (including discoloration), loss of substance, and probably also fracture are still important factors limiting the lifetime (Mjor, 1989). One major problem is that the polymerization of dental composites is far from complete. A degree of conversion in the range between 60% and 75% has been shown (Ruyter and Svendsen, 1978; Ferracane and Greener, 1984; Ruyter and 0ysaed, 1987). Light-initiated polymerization, which is probably the dominating polymerization method for products currently on the market, has not improved the situation. On the contrary, light-curing has increased the possibilities for incomplete conversion during clinical work, since new sources forfailurehavebeenintroduced(Fig. 1). Incomplete conversion may leave unreacted monomers which might dissolve from the material in a wet environment. Another possiblity is that reactive sites (e.g., double bonds) are susceptible to hydrolization or oxidation and, thereby, degradation of the material. Inhibition of polymerization in surface layers exposed to oxygen is an additional problem with the use of dental composites. If not removed, such layers will release an amount of monomers or degradation products from the composite corresponding to the thickness of the unpolymerized layer (0ysaed^a/., 1988). Elution of unused/unreacted components from dental composites is a diffusion rate-dependent process. Diffusion is dependent upon polymer type, surface treatment of the filler particles, and the type of solvent. Tests have shown that approximately 50% of the leachable species are eluted within three hours in water. When soaked in an ethanol/water mixture, 75% of leachable molecules were eluted during the same length of time, explained by the differences in the amount of solvent absorbed (Ferracane and Condon, 1990). Elution of nearly all leachable components was complete within 24 hours in both solvents.

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VOL. 6



Arsenic content, Glass ionomer cements


Solubility Tests

Solubility is tested in a quality control of composites according to ISO 4049 (1988), but not in the corresponding ANSI/ADA (1989) specification. This newly revised ISO standard for anterior composite materials has a solubility test based on a gravimetric method. A maximum weight loss of a specimen after one week in water is set at 5 pg/mm3, based on a 'roundrobin' test on chemically curing composites performed from 1978 to 1980. The new test, however, has excluded several of the later marketed light-curing materials with acceptable clinical performance. This illustrates how difficult it is to design simple quality-control tests with clinical relevance. It also probably indicates a difference in quality between chemically cured and some light-cured composites. The time after curing before immersion in water is important in a solubility test, since many materials will have a post-curing period with increased polymerization (Ferracane and Condon, 1990). A time for immersion similar to the time for contact with saliva, i.e., 10 min after polymerization, seems relevant. The time in water may also be important. An increased storage time will increase the amount of reaction products between water and polymer, as well as between water and filler, and will reduce the measured amount of solubility (Ruyter, 1991). It has also been suggested that an ethyl alcohol/water solution be used for an accelerated solubility test (Ferracane and Condon, 1990). Previous studies have shown, however, that such solvents extract/elute the various types of species in different proportions than does water (Thompson etal, 1982). It is, therefore, questionable that alcohol solutions will give clinically relevant data. Clinically, restorations will be covered within a short time by an organic film, a pellicle (S0nju^ al, 1974). This film will probably change the diffusion on the surface of the composite. A pellicle formation is difficult or impossible to simulate in vitro.

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Solubility in water or other solvents is expressed in either percent weight reduction, weight of eluted substance in relation to surface area, or volume of the specimen. If volume is used, sufficient time must be allowed for total saturation of the specimen. It is often problematic for values to be compared and amounts of chemical species to be calculated from these results, and the relation to the clinical situation is also unknown. The maximum accepted weight loss of 5 pg/mm3, according to ISO 4049 (1988), is equivalent to 0.2-0.5% M/M, depending on the density of the composite. Previous studies have shown solubility in the range of from 2 to 9 pg/mm3 (0ysaed and Ruyter, 1986) and 1.5-2.0% of the original weight (Ferracane and Condon, 1990). Calculation of realistic clinical values from these data is not possible, but it is clear that the major part of the dissolution occurs within the first hours or days. Residual oligomers and monomers are identified in the material. Inoue and Hayashi (1982) found 0.4-1.2% residual BIS-GMA monomer in several composites, but identified considerably less leaching out in water. Thus, it might be that mainly degradation products leach out in water. Several studies have also shown that elements from the filler particles are dissolved in water, dependent upon the type of filler. Boron and silicon are the main elements (Soderholm, 1983; 0ysaed and Ruyter, 1986). Boron is obviously from the glass filler, whereas silicon may also be a result of degradation of the surface treatment of the filler. Elements from filler particles are in the range from 0.0 to 180 pmol/g. Traces of Al are observed from only a few products (0ysaed and Ruyter, 1986).

Degradation Polymers and residual monomers can degrade either by oxidation or hydrolysis. It has been shown that one of the degradation products from dental composites is formaldehyde. The total elution has been quantified to be in the range of 0.10.5 pm/cm2. The release decreased with time, but was still detectable after 115 days (0ysaed et al., 1988). Munksgaard and Freund (1990) have also shown that enzymatic hydrolysis of dimethacrylate polymers gave rise to

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liberation of methacrylic acid. Benzoic acid has been identified as an eluted species and is expected to be a product from hydrolization of the initiator benzoylperoxide(Ruyterand0ysaed, 1987). Otherunidentified species observed are suspected to be degradation products of the other additives to a composite, i.e., the catalyst, inhibitor, accelerator, and UV-stabilizer (Ruyter, 1991). It has also been shown that the enzymatic degradation of composites results in a decreased surface hardness and increased wear (Freund and Munksgaard, 1990).

Wear Wear of composites has been identified as being due mainly to fatigue. Micro-particles fracture from the surface and are probably swallowed. Abrasive wear is also possible, especially after degradation by food or enzymes. This tribochemical process is corroborated by several clinical observations and by the experiments by Freund and Munksgaard (1990). A large filling, e.g., an occlusal surface of 5 mm x 5 mm, with a vertical wear of 50 pm per year might give 1.25 mm3 of particles which could be decomposed in the body. GLASS-IONOMER CEMENT

Glass-ionomer cement is used mainly for single-surface restorations, as liners under other materials, or for luting of crowns and bridges. Its brittle properties prevent its use in large and multi-surface restorations. The expected lifetime of a single-surface restoration is good, with a retention rate of more than 90% after five years (Matis et al, 1991). A slow substance loss is, however, one of the possible limiting factors. Biodegradation of glass-ionomer cements is a complex process of absorption, disintegration, and outward transportation of ions. Kuhn and Wilson (1985) indicate three different mechanisms for dissolution: surface wash-off, diffusion through pores and cracks in the cement, and diffusion from the bulk. The absorption of water at an early stage will prevent or change the setting reaction. It is difficult to prevent this early contact with water completely, and therefore a surface layer of more or less disintegrated cement can often be observed. Clinically, this is mainly a problem of esthetics. The question is whether or how such surface layers should be removed and added to the dissolved material in a solubility test. Solubility Tests

One of the most important factors for the rate and type of dissolution is the time of immersion in water after the start of mixing. Several studies have shown that a continued setting reaction in glass-ionomer cements drastically reduces solubility during the first hours and days (Finger, 1983; Um and 0ilo, 1992). Another important factor is the type of test, i.e., either a static situation or a test involving a change of or movement of the solvent to simulate the mechanical factors in the mouth, such as the water jet applied during an erosion test. The type of solvent, i.e., water or acid solution, the type of acid, molarity, andpH are also of importance (Crisp etal, 1980; Mesu, 1982; Finger, 1983; Walls et al., 1988). These studies have shown that glass-ionomer cements have a much higher resistance to


dissolution in an acid environment, which is more realistic for oral conditions, than do other dental cements. The present standards for glass-ionomer cements [ISO 7489 (1986); ANSI/ADA Specification No. 66 (1989)] contain a gravimetric method similar to the one used for other dental cements. The method is simple and seldom presents problems for a quality control. Total erosion after seven days in water is below 1% M/M (Crisp et al, 1980). There is some question, however, as to whether or not the drying process to remove water before the residuals are weighed also removes some of the dissolved organic material. The new common ISO standard for all water-based cements includes a jet-erosion test [ISO 9917—Dental water-based cements (1991)]. The cements placed in standardized cavities are exposed to a continuous jet of a 0.02-M lactic acid solution. Solubility will be expressed as depth of erosion in mm per hour. It should solve problems with regard to destroyed surface layers and supposedly simulates the clinical situation better than passive storage in distilled water (Setchell et al., 1985; Walls et al, 1988). A conductometric method has also been used as a more simple method for measurement of the solubility of various cements (Prosser et al, 1982). This method is included in the standard ISO 4104—Dental zinc polycarboxylate cements (1984), and has also been used to rank solubility of glassionomer cements (0ilo, 1988). It is not, however, applicable for more specific studies of dissolution.

Clinical Solubility Tests Various in vivo tests have shown large variations in substance loss from the various types of dental cements. A common finding is that glass-ionomer cements have a much higher resistance to disintegration than do other dental cements. As much as a 40-fold difference in substance loss over time has been reported (Pluim et al, 1984; Phillips et al, 1987). Eluted Species and Quantification

Several studies have shown that materials leach out in proportions different from those found in the parent material. It contains both water-soluble and water-insoluble fractions (Crisp etal, 1980). It also differs with time. In an initial phase, the glass-ionomer cement absorbs water, and disintegration of a surface layer is the main problem (Um and 0ilo, 1992). Glass particles, ions, and some of the organic material can be found in the solvent. After a short time, minimal amounts of structurally important elements leach out (Crisp et al, 1980). The standard solubility test shows that residuals in water are well below the maximum limit of 1% M/M. The new lightcuring glass-ionomer cements have improved properties in the initial phase of setting, including a reduced early solubility (Um and 0ilo, 1991). Of special interest is the release of fluoride ions from the glass-ionomer cement. Various studies have shown that the release rate is high in an initial period but stabilizes at near-toconstant and is practically similar to that of the silicate cement after some weeks (Schwartzes al, 1984). Fluoride release may be reported as content (ppm) of the solvent or in relation to surface area or weight of cement. A quantification of released

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fluoride from a mean-sized filling, based on in vitro tests, has been performed, and results indicated that approximately 0.075 mg is released during the first one to seven days. After a year, this value is reduced to 0.005 mg per week (Forsten, 1990). This has been shown to correspond to a higher uptake of fluoride in the enamel and dentin surrounding a glass-ionomer filling, to prevent demineralization of adjacent enamel, and to change the type and amount of bacteria living on the glassionomer filling surface (Retief et aL, 1984; Forss and Seppa, 1990; Svanberg et aL, 1990). There are also observations indicating that some glass-ionomer cements may absorb fluoride from the surrounding media, such as a fluoride-containing toothpaste (Forsten, 1991). The glass-ionomer filling can then act as a fluoride depot which constantly establishes a highfluoride environment in its immediate surroundings. The release of aluminum ions has also been a matter of public concern. Approximately 5-10% of the total weight of a glass-ionomer filling is aluminum, but only a fraction of that is soluble. Studies have also shown that after the initial period involving disintegration, little aluminum is released from the glass-ionomer material (Crisp et aL, 1980). The quality control of glass-ionomer cements involves tests for acid-soluble arsenic and lead. A limit of 2 mg/kg for arsenic content and 50 mg/kg for lead has been established (ISO 7489, 1986). Continous testing in recent years has shown that this varies from batch to batch, probably due to impurities of the raw materials used for the production of the glass. A few products have exceeded the limits set for both arsenic and lead (Fig. 2).


• • •

Clinical evidence shows that degradation is a slow process for both composites and glass-ionomer cements. In vitro tests tell us that certain substances or elements are constantly eluted. During a short initial period, the elution rate is high and thereafter stabilizes at a low, but constant, value. The correlation between in vitro measurements and the in vivo situation is not established. The substances eluted, especially from composites, are not sufficiently identified. There is a strong need for both development of standardized in vitro and in vivo tests and for more research into identification and quantification of what is leaching out of composites and glass-ionomer cements under clinical conditions.

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glass-ionomer cements.

Studies of the degradation processes, types of tests, and measurements and analyses of substances leaching out from resin-based composite materials an...
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