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The following article describes and classifies glass ionomers and resin-based fluoride-containing systems.
An increasing number of dental materials are claiming the advantages of both composites and glass ionomers. These products lie along a spectrum that includes traditional glass ionomer cements at one end and traditional composites at the other end.
The differences among materials result from their different components and different chemistries that determine their setting reaction.
Glass ionomers set by an acid-base reaction in the presence of water in which the acidic polyacrylic acid reacts with the surface of an acid-soluble fluoro-aluminosilicate basic glass. The remaining unreacted glass reinforces the material and results in a cement in a soluble environment where fluoride ions can diffuse between the glass, the polyacid matrix and the tooth. This property is essential for fluoride release and to maintain the attachment between the matrix and filler.
Composites set by a polymerization reaction where monomers are converted into cross-linked polymers. The composite resin matrix is reinforced by glass particles and is usually hydrophobic. Water absorbed into the resin matrix increases the separation of the bond between the resin matrix and the filler particles. This process is called hydrolysis. As these materials absorb more water they become less durable and less fracture resistant.
Materials on the glass ionomer end of the spectrum tend to have low thermal expansion, high fluoride release, and adhere chemically to tooth structure (by an ionic- and diffusion-based bond). Materials on the composite end tend to have increased thermal expansion, lower fluoride release, and require a bonding system to attach to tooth structure.
These systems have multiple names, which makes distinctions between specific products and product classes difficult. The marketing arms of some manufacturers have invented terms to imply the superiority of their materials.
As of this writing, all the manufacturers we have consulted have accepted the following terms for the classification of these products and they should be considered standard definitions.
Glass Ionomers are most commonly referred to as glass ionomer cements (GIC's). These are materials which consist of an aqueous polyacid liquid and an acid-soluble fluoroaluminosilicate glass. These set by an acid-base reaction in the presence of water.
The word "cement" implies a luting material to most practitioners and using the term "glass ionomer" without the term cement, although incomplete, is an accepted shorthand.
Resin-Modified Glass Ionomers (RMGI's) are also referred to as Reinforced Glass Ionomers (RGI's) or Resin-Ionomers.
These are glass ionomer materials which consist of a matrix of acidic and polymerizable polymers which set by both acid/base and polymerization reactions.
Polyacid-Modified Resin Composites (PMRC's) are commonly referred to as Compomers, a name first used by Dentsply for their material Dyract. These are polyacid-modified resin composites which consist of glass ionomer components and a polymerizable resin matrix. They may or may not be hydrophilic. These materials are anhydrous and set by a polymerization reaction. Once set, water absorption may cause a glass ionomer chemical reaction.
Ionomer-Resin Suspensions (IRS's) are also referred to as Fluoride Releasing Resins (FRR's). These usually contain a fluoroaluminodsilicate glass glass suspended in a resin matrix which sets by a polymerization reaction. Once set, these may or may not absorb water. Even if they absorb water they cannot undergo an acid/base reaction with glass ionomer fillers since there is no acid to cause a chemical reaction.
Composite Resins are also referred to as Composites or Filled Resins. Composite resins consist of an inert glass or quartz filler in a resin matrix. These set by a polymerization reaction. Once set, they absorb minimal amounts of water.
III. A Closer Look at the Different Materials
A. Glass Ionomers
Conventional glass-ionomer cements consist of a basic glass and an acidic water-soluble polymer, and set by an acid-base reaction in the presence of water. They are used as luting agents, liners, bases, or restorative materials. They originated in Europe and have never received the acceptance in the United States that they have throughout the rest of the world, perhaps because they require new clinical skills to make optimal use of them, and they are less tough, and less esthetic than the newer macrofilled composite resins and the microfilled composites introduced at the same time.
The early conventional glass ionomer materials were technique-sensitive, slow setting, considerably opaque when set, and sensitive to both desiccation and hydration during setting. This led to early loss of material from the surface.
All of these problems have been alleviated in newer materials. Modern materials are fast setting, more esthetic and hydration and sensitivity problems have been greatly reduced. However, unlike composites, they still should not be used as stress bearing restorations.
Newer glass ionomer materials have been developed which are highly filled, packable and quick setting (also called HFGI's) (Fuji IX GP [called Fuji IX in Europe], Ketac Molar and Shofu Hi-Dense). These heavily filled, radiopaque, tougher GIC's are useful for nonstress bearing buildups, root caries, tunnel restorations, and long term provisional restorations in primary and adult dentitions. Their opacity makes them less desirable in esthetic areas.
Characteristics of Traditional Glass-Ionomer Materials:
- Form a hard substance upon setting
- Low exothermic reaction-self cure
- Less shrinkage than polymerizing resins
- Coefficient of thermal expansion similar to tooth structure
- No free monomer present
- Dimensional stability at high humidity
- Filler-matrix chemical bonding
- Resistant to microleakage
- Marginal integrity
- Adhere chemically to enamel and dentin in the presence of moisture
- Fluoride release discourages microbial infiltration
- Rechargeable fluoride
- Early moisture sensitivity requires protection (e.g., with varnish) immediately after placement
- Poor abrasion resistance
- Average esthetics
B. Recharging GIC's
Recharging glass ionomers has been referred to as the "reservoir effect". Glass ionomers release fluoride from a reservoir (contained in the unreacted glass ionomer filler of the matrix). Once it has been depleted from a constant fluoride release the reservoir can be replenished.
The fluoride content in glass ionomers is much higher than in the tooth. With ion exchange over time, fluoride ions diffuse from their area of high concentration (in the GIC) to the area of lower concentration (in the tooth). In the process, some of the hydroxyapatite in the tooth is permanently transformed into fluoroapatite. In time, an equilibrium of fluoride between the glass ionomer and the tooth is established.
Glass ionomers generally release fluoride from the surface into the saliva. Since an equilibrium of fluoride between the GIC surface and the oral fluids is not possible, most of the fluoride from the ionomer's surface is lost to the oral fluids. Only some of this fluoride is available to the 1 to 3 mm periphery around the restoration. This means caries inhibition at the restoration margins by the fluoride released from glass ionomers decreases over time. This is of particular concern in patients with high caries susceptibility such as the elderly and patients who have undergone radiation treatment.
Fluoride can be replenished in glass ionomers and with a topical application of fluoride in a gel, rinse or toothpaste, so fluoride protection can continue. See Figure 2.
C. Resin-Modified GI's
Resin-Modified Glass Ionomers (RMGI's) are also called resin-ionomers. But "resin-ionomer" is technically incorrect since they started as glass ionomers and then were modified. However, since the term resin-ionomer is widely used we will use it in this text.
Resin ionomers are materials in which a polymerizing resin is added to the glass ionomer matrix. Introduced in 1991, they represent an attempt to overcome some of the problems with traditional glass ionomers. These materials have improved initial esthetics, improved physical properties (such as tensile strength and fracture toughness), set on demand through light-curing, and have fewer desiccation and hydration problems. Since glass ionomer materials usually fail cohesively (the bond is stronger than the material), the stronger resin-ionomers provide a higher bond strength to tooth structure when application is preceded by conventional etching. In addition, the resin can form a chemical bond with tooth structure. It is unknown whether this has any clinical significance.
These hybrid materials set partly through a GIC acid-base reaction and a polymerization of the resin component of the matrix. The resin component can be light-cured (Fuji II LC), dual-cured (Vitremer Restorative), or chemically cured (Vitremer Luting and Fuji Plus). Setting is achieved by adding a water soluble monomer resin, such as HEMA, into the liquid of the water-soluble polyacrylic acid. A photo and/or chemical initiator determines the setting reaction. A concern is that phase separation can occur between the hydrophilic and hydrophobic portions of these materials.
The term "light-cured or dual -cured" does not mean that the entire setting reaction is photoinitiated. A portion of the setting process can also involve the typical acid-base process between the filler and the polyacid matrix.
Understanding the timing of the various setting reactions is of critical importance. The light-curing reaction is first, and cures the surface layer closest to the light. As these materials are somewhat opaque, the initial layer, which can be considered to be the bonding layer, should be no more than a thin wash on the tooth surface. This layer needs to be completely light-cured before adding more material. Light-curing the outermost layer gives the restoration improved resistance to hydration/dissection. Air inhibition of the surface requires overfilling. A thin coating of a resin adhesive can also have this effect. Although this material appears set after light curing, the material becomes noticeably harder over time from the acid/base setting reaction between the acidic polyacids in the material and the basic glass ionomer filler particles. In these materials this reaction can be slower than in GIC's. Early finishing can damage the immature bonds to the tooth structure as well as weaken the material so unreacted GIC components can wash out.
Resin ionomers have poor color stability because some components remain unset and can leach out, which allows water-soluble agents to penetrate the surface so they darken more over time than conventional glass ionomers.
The structural integrity of resin ionomers may be weakened by the osmotic swelling which occurs due to retained unreacted resin (mostly HEMA). This occurs mostly in the deeper areas of the restoration which are further away from the light source. Thus, these materials can be greatly compromised if a thin layer is not applied over the preparation, immediately cured, and followed by cured increments.
The classification of Compomers are more correctly termed Polyacrylic acid Modified Composite Resins or PMCR's. These materials are soft, non-sticky, do not need to be mixed, and are easy to place. They are an attempt to combine the best properties of glass ionomers and composite resins. Introduced in 1993, they now account for 15% of all dental materials sold worldwide. Their use in the United States is considerably less. A major reason for their success is that they are very user friendly. They are easy to inject into a cavity, simple to shape, quick to cure, and are carveable and polishable after curing. They are replacing composite resins in anterior proximal restorations and glass ionomers in cervical restorations. In almost all other areas, composites and glass ionomers are preferred.
Compomers have nominal adhesion to tooth structure and are therefore always attached with resin-dentin bonding agents.
They are made of an acid functionalized dimethacrylate resin that can undergo an acid-base reaction with a glass ionomer powder which may be mixed with a conventional composite glass filler. They work by absorbing water which expands the restoration over time. This aborbed water can then cause an acid-base reaction between the polyacid side chains of the resin matrix and the glass ionomer filler. This results in fluoride release which is about 20% of a conventional glass ionomer. Unlike glass ionomers the physical properties of the compomers decreases as water is absorbed. With some materials this decrease can be 50% leaving an inferior material in terms of strength.
The more acidic carboxy groups polyacrylic acid modified composite (compomer) contains the more hydrophilic and ionic the matrix becomes which results in greater the water absorption. Unlike resin modified glass ionomers when a compomer absorbs water its physical properties go down. In some cases as much as 30 to 50%.
Unlike glass ionomers, these materials provide less fluoride release. The small amount of fluoride released from compomers may be of limited value, however, since the resin bonded interface prevents the fluoride of the restorative from entering the tooth. Nevertheless, surface fluoride release from compomers can affect the surrounding tooth structure.
Compared to Other Materials. On the spectrum between glass ionomers and composites these are a variety of blends, employing different proportions of acid-base and free-radical reactions to bring about cure. A number of commercial products are in the generic class of polyacid-modified resins. Composites that use a glass ionomer fluoride-containing filler and polyacid components are compomers. Compomers (e.g. Dyract by Caulk, Compoglass by Vivadent, Hytac from ESPE, and the luting material Resinomer by Bisco) are materials that have some acidic ionomer components in a resin matrix. Compomers do not contain water, yet react in the presence of water, by volumetric expansion. Unlike what the name implies, compomers exhibit few traditional glass ionomer-like properties. These materials must be light-cured.
Although some manufacturers do not recommend enamel etching, studies show approximately a threefold increase in retention when the tooth is first etched.
History. The first compomer introduced, in 1993, was Dyract (Dentsply). It was first introduced in Europe, then Canada, and then in the United States. Although many European educators feel positive about these materials, in the United States researchers and educators in the academic community are less optimistic despite their rapid acceptance by practitioners. Many researchers haven't had time to formulate significant opinions.
Dyract has undergone a few formulation changes. One was the addition of fluoride and the other was using a smaller particle size and increasing the conversion. It currently enjoys considerable success in the marketplace. Dentsply, to its credit, started clinical trials prior to its introduction, a practice which is becoming less common, with most manufacturers starting clinical trials after the product is released.
The next such material introduced was Compoglass (Vivadent). After this, many manufacturers followed suit, including Hytac (ESPE) and others. Each of these materials is completely different and none have yet undergone clinical trials. What they have in common is their water absorption after placement, which results in expansion. They are also softer than composites, and can be more easily flexed. Almost all other physical properties of these materials are less desirable than those of conventional composites.
Chemistry. The compomers, which are generally hydrophobic resins, contain polyacid side chains which are attached to one or more of their methacrylate monomers. For example, Dyract is TCB, the reaction product of butane tetracarboxylic acid and hydroxy–methyl–methacrylate, and contains two methacrylate groups and two carboxylate groups per molecule.
PMCR relies primarily on the light initiated free radical polymerization mechanism. These materials can be thought of as low fluoride releasing composite resins. The fillers include a reactive fluoroaluminosilcate glass (used in glass ionomers) which release fluoride.
The filler is a barium and reactive silicate glass (72%w/w) containing fluoride, which is also used in DeTrey Dentsply glass ionomer cement (BaseLine).
The monomer ionizes by uptaking water during the days/weeks after it is light cured. The hydrogen ions that are released then react with the glass filler to initiate an acid-base reaction. Ionic cross-linking also occurs and fluoride is released.
What they do and do not offer? It is important to compare any new material such as compomers to existing materials that have been clinically successful for a number of years.
Advantages of Compomers
- Ease of placement
- No mixing
- Easy to polish
- Good esthetics
- Excellent handling
- Less susceptible to dehydration
Disadvantages of Compomers
- Limited clinical experience and few long term clinical trials
- Require a bonding agent like composites
- More marginal staining and chipping
- Wears more than composites
- Enormous variation of products makes longevity difficult to predict
- Weaker physical properties than composites that decrease over time.
- Clinical significance of fluoride release undetermined
E. Resin-Ionomer Suspensions
Ionomer-modified resins or fluoride-containing composite resins are materials that contain glass ionomer fillers but no polyacids. They represent the first attempt to give the beneficial effects of glass ionomers to composite resins. They suspend ionomer glasses or reacted glass ionomer components in a resin cured system.
Unfortunately, while compomers absorb water, which gives them the potential to allow glass ionomer reactions to occur and fluoride to be released, suspension systems have no such potential. The only potential for fluoride release from an ionomer suspension system comes from diffusion of ionomer particles entrapped in voids which fill with water after placement. They have been on the dental market for a long time.
The distinction between compomers and ionomer-in-resin suspensions is very fine. Many materials marketed as compomers are mainly suspension systems. There is no definitive test to differentiate the two materials. There is no proof that compomers will do better clinically than suspension systems or composite resins since little clinical testing has been done comparing these restorative systems.
Ionomer-in-suspension systems exhibit almost no traditional glass ionomer-like properties. Hence materials called compomers (a term that presently sells materials in the marketplace) that are really suspension systems may turn out to be clinically disappointing. In fact, the clinical superiority of either compomers or suspension systems has yet to be determined.
An example of a suspension system is Geristore (Den-Mat). These materials must be light-cured.
F. Composite Resins
These materials have been in dentistry for a long time and their properties and characteristics are well known and appreciated. In recent years these materials have been tremendously improved with the use smaller filler particles and higher filler loading. These materials have served dentistry very well. They are still the material of choice for tooth colored direct restoratives. These materials are tough, durable, highly esthetic, highly polishable, color stable, bondable, easy to shape, wear resistance, fracture resistant, and available in many shades. Their handling properties are the most versatile of all restorative systems. They come packable, sculptable, flowable, and as a fluid.
They do have disadvantages in that they shrink on setting (as do almost all restoratives) and they must be placed in layers when used as a restorative. For these reasons, placement is technique sensitive. However most practitioners have learned to work with these materials and once the learning curve is overcome they can be used with considerable ease.
The clinical performance of composite resins has been excellent, especially when compared to previous direct restorative systems. Bonded to enamel it is the best dentistry has to offer. However, dentin bonding is "all or nothing". If the dentin seal is not properly established or is broken there can be a rapid ingress of recurrent decay since these materials have no caries inhibiting properties. Even though dentin bonding can be successful in the laboratory, in clinical use many uncontrollable factors can result in less than optimal results.
It is because of these limitations of composite there have been so many efforts to develop alternative systems which have beneficial biotherapeutic and anticaries effects.
The combined use of GIC's as liners and bases under composites resins has long been used to solve clinical problems. Although this combination of materials has long been useful, it is time consuming and technically demanding.
IV. A Comparison of Materials
A. Fluoride Release
If a primary goal of a restorative material is enough fluoride release for immediate and long term caries protection, select a material with a significant glass ionomer acid/base reaction. During this reaction a large amount or "burst" of fluoride is released followed by a more diffuse long term fluoride release. Fluoride transforms enamel hydroxyapatite into a considerably less soluble fluoroapatite. Fluoride also creates a zone of protection which extends to adjacent surfaces. These include, but are not limited to, a change in microflora which reduces strep mutans (the bacteria associated with caries) and a more acid resistant surface. More is absorbed by the enamel than the dentin.
Glass ionomers also have an advantage in that they are in direct contact with the tooth structure; whereas a resin based system requires a resin bonding layer which bars fluoride diffusion into the tooth, even if it is present in the restorative material.
The fluoride in glass ionomers can also be replenished by external sources such as fluoride rinses, gels, and toothpastes.
One can look at the fluoride release curve of any material to determine if it has GIC properties. With a GIC there is an initial spike followed by a nearly flat but steadily declining release. A fluoride release curve which is flat and then declines indicates a material that contains fluoride available only by diffusion. The further a material moves along the spectrum from GIC acid-base to resin polymerization, the less fluoride is available.
Composite resins have by far the best esthetics. Resin-ionomers can be as good initially but discolor over time. Compomers and ionomer suspension systems have good initial esthetics, but it is unlikely their long term stability will compare favorably to composite resins.
All of these technologies have the potential of developing a bond to both enamel and dentin. With resin systems a linkage to the tooth requires acid etching and the use of a bonding agent. With glass ionomers, an inherent ionic bond develops from the restorative material to the tooth structure. This can be enhanced by pretreating the tooth surface with polyacrylic acid cleansing agents.
When glass ionomers are used correctly they have excellent long term retention rates, including systems which have relatively low bond strength measurements.
Glass ionomers can be carved when the setting reaction has neared completion. Any of the light cured resin based systems set hard on curing. In general, the hardness and toughness of a material increases as one moves from GIC to resin systems. The compomers, being somewhat softer than composites, can be carved with a sharp instrument. Contouring and finishing procedures are easier with softer materials. However, the final finish of any system is ultimately controlled by the size of the filler particles.
Surface smoothness does not necessarily correlate with plaque retention; the chemical composition of the material is also important. Resin based systems have residual components which seem to attract plaque, while glass ionomers release fluoride which is toxic to plaque.
It is not possible to make general recommendations for use of one class of material over another for all uses. Each has their own use.
However some observations can and should be made. Ionomer-in-resin suspensions seem to have the least potential. Compomers offer yet unrealized promise. GIC's and composites are well understood and their clinical performance can be predicted with some certainly.
VI. Closing Comments on Making Changes
It is easy for practitioners to leave time tested materials for something that is marketed as a better replacement, on the cutting edge, a breakthrough, totally improved. However, newer systems rarely have controlled clinical studies to prove their claims. New systems usually have potential but very few new dental materials stay in the marketplace for long. The average life span of an introduced product has become shorter and shorter over the years. This is not because products are improving that fast, but more because practitioners are willing to accept newer materials in hopes that they will one day find a material that will compensate for the difficultly of having to learn a new technique.
It is tempting to look for a material that will place itself, be effortless to use and require almost no additional knowledge. The tidal wave of "new and improved" materials can be expected to continue, at least as long as marketing skills are more important than research and clinical integrity. The clinician who can evaluate and use materials effectively and who is prepared to acquire the training to use them properly, serve themselves, their patients and their profession best.
In contrast to these systems, many believe that dental adhesives may provide the most protection against microleakage of all materials . This has been shown in many laboratory studies but clinical experience has not duplicated this. If desired, resin bonding systems can be used with all of the systems discussed except the conventional glass ionomers. However, many feel that dentin-resin systems have yet to prove themselves clinically.
New materials are only as good as the knowledge and the skills of the dentist who uses them.
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