The Role of Glass Ionomers in Minimally Invasive Restorative Dentistry

Dental caries is the result of an infection. An infectious disease requires a microbial agent, a susceptible host, and substrate that promotes growth of the infectious agent. The infectious agents are Streptococcus mutans followed by an increase in lactobacilli species. Loss of tooth structure results from metabolism of carbohydrates by bacteria and the release of lactic acid as a byproduct.^1,2

The traditional approach to treatment of even a small carious lesion is wide surgical excision. This results in removal of sound tooth structure both to gain access to the lesion and to obtain retention for the restoration. The margins of the cavity were placed in so-called caries-free zones that were more accessible to plaque removal by brushing.³ Limited areas of demineralization required the same amount of tooth structure removal as more advanced lesions. Tooth preparation further increased the risk of subsequent pulpal involvement in a pulp already affected by the carious process.⁴ The available restorative materials were not adhesive and required the cavity preparation to have a shape that promoted retention.

The advent of adhesive restorative materials that were also capable of sealing the interface between the restoration and the tooth changed traditional concepts regarding tooth preparation. Further, bioactive materials capable of promoting the repair of tooth structure have changed how a carious lesion is treated.⁵ It has now been recognized that tooth structure can remineralize and heal. Research has indicated that a carious lesion can be reversed if it has not progressed to cavitation.⁵ Further, fluoride decreases susceptibility of the tooth to the development of caries.

Today, restorative dentistry emphasizes minimally invasive approaches. This encompasses prevention, remineralization, and when needed, adhesive res-torations. These approaches les-sen the chance for subsequent adverse outcomes, including pulpal involvement and tooth fracture.⁴

MINIMALLY INVASIVE DENTISTRY AND GLASS IONOMERS

The outer surface of teeth is constantly under biochemical assault. When the pH at the tooth surface falls below 5.5, calcium and phosphate are released. A demineralized collagen framework remains. When the pH rises above 5.5, calcium and phosphate ions in saliva participate in remineralization.⁵ Long-term exposure to low pH results in loss of surface integrity and cavitation. Fragile, demineralized enamel on occlusal surfaces may fracture due to stress on the teeth. Interprox-imal lesions are not exposed to the same occlusal forces and are more amenable to remineralization.

Fluoride plays an important role in this process. In the presence of free fluoride ions, remineralization involves the formation of fluorapatite. Fluorapatite is more resistant to demineralization, and the pH must fall to 4.5 before this occurs.⁶ Fluoride has the additional benefit of being mildly bacteriostatic. It also modifies the surface energy of enamel and impedes the attachment of plaque to the tooth surface. Fluoride inhibits bacterial metabolism by diffusing hydrogen fluoride from the plaque into the bacteria. Fluoride in the bacteria interferes with bacterial metabolism and decreases the adherence of bacteria to hydroxyapatite.⁷

Fluoride in the water supply and toothpaste has been shown to reduce the prevalence of caries. The formation of fluorhydroxyapatite can occur both during tooth development and following eruption.⁶ A restorative material that releases fluoride ions can protect against demineralization of tooth structure adjacent to the restoration.⁶ Silicate cements, despite being inadequate as a restorative material, released a sufficient quantity of fluoride to reduce the amount of demineralization that occurred with failure of the restoration. Glass ionomers also release fluoride and have demonstrated a similar anticaries property. The fluoride released from the glass ionomer aids in the formation of fluorhydroxyapatite on adjacent tooth structure. This will not prevent new carious lesions, but renders the adjacent tooth structure more resistant to demineralization.⁶

COMPOSITION AND PHYSICAL PROPERTIES OF GLASS IONOMERS

Conventional glass ionomers were developed by Wilson and Kent in 1974. They combined ion-leaching glasses used in silicate cements with polycarboxylic acid polymers.8 Through improvements in the composition of the glasses and carboxylic acids and the incorporation of tartaric acid, these materials became suitable for different clinical situations. The incorporation of methacrylate with glass ionomer technology has led to the development of materials that possess the desirable properties of both.9 Silver particles have been added to some glass ionomers to increase their physical strength, and these materials are considered glass ionomer cements (eg, Ketac-Silver Aplicap/ Maxicap Reinforced Glass Ionomer Restorative [3M ESPE]). The addition of small quantities of light-polymerizable resin groups has improved physical properties, as translucency and water balance are more readily maintained.^10,11

	Figure 1. A chemical bond is formed between carboxyl ions in the glass ionomer and calcium in the tooth.
	Figure 2. In addition to the chemical bond, the glass ionomer and the tooth maintain a physical union.
	Figure 3. A hypermineralized, acid-resistant surface develops on the dentin surface.

The basis of glass ionomers’ adhesion to teeth has been thought to involve a chemical interaction with tooth structure by means of ion exchange (Figure 1). In addition, micromechanical pen-etration of the glass ionomer into tooth structure has been shown to occur¹² (Figure 2). This results in a hypermineralized, acid-resistant surface (Figure 3). An understanding of the glass ionomer setting reaction is critical in obtaining the best clinical results and utilizing these materials in appropriate clinical situations.

The 3 essential constituents of glass ionomers are silica, alumina, and calcium fluoride. The setting reaction is an acid-base reaction between polymeric carboxylic acid and basic fluoroaluminosilicate glass, and requires an aqueous medium in order for the ions leached from the glass to react with the polyacid moiety. The Al₂O₃/SiO₂ ratio of glass is crucial and is required to be 1:2 or more (by mass) for cement formation. Only then is there sufficient replacement of silica by aluminum to render the network susceptible to acid attack, and this results in the release of simple and complex metal ions. This is the ion-leaching phase (H+ attacks the glass particles releasing Ca+2, Al+3, and F-). The glass ionomer sets and hardens by means of a transfer of the metal ions from the glass to the polyacrylic acid. This results in gelation in the aqueous phase. A silica gel layer is then formed at the interface between the cement matrix and the glass particles, as most of the metallic ions are lost.¹³

During this period of time, the material must be protected from dehydration because loss of water disrupts the cement structure. Fluoride, calcium, and aluminum are all soluble. The calcium and aluminum ions diffuse into the polyacrylic liquid, forming polyacrylate (a cross-linked metallic salt). When this salt begins to precipitate, gelation begins and continues until the cement is hard. Fluoride ion is also released from the glass particles, becoming available to nearby tooth surfaces and being released into saliva. Since fluoride does not play a role in the setting reaction of glass ionomer and does not become part of the structure when setting occurs, the glass ionomer is not appreciably destabilized by fluoride release.¹¹

Resin ionomers combine glass ionomer and composite resin materials to improve the physical properties of conventional glass ionomers.¹¹ In addition to the typical acid-base reaction, photoinitiation occurs through methacrylate groups joined to the polyacrylic acid chain and methacrylate groups of HEMA (2-hydroxyethyl-meth-acrylate). Some materials also have a third reaction, specifically a chemically initiated reaction between free radical methacrylate and the polymer structure.¹¹ There is a continuum of materials between conventional glass ionomers and light-cured composite resins. McLean¹⁴ proposed a classification for these new materials:

(1) The unqualified term glass ionomer cement should be reserved exclusively for a material consisting of an acid-decomposable glass and a water-soluble acid that sets by a neutralization reaction.

(2) Materials that retain a significant acid-base reaction as part of their overall curing process will cure in the dark and are classified as resin-modified glass ionomers.

(3) Materials that contain either or both of the essential components of a glass ionomer cement but at levels insufficient to promote the acid-base curing reaction in the dark should be referred to as polyacid-modified resin composites.

The properties of these materials vary widely. Some are very similar to traditional glass ionomers and cure by a traditional acid-base reaction. At the other end of the spectrum are materials that cure mainly as a result of light-activated polymerization. Materials that are more composite in nature have increased thermal expansion and demonstrate less fluoride release. They also require a dentin bonding agent for adhesion. The materials that are more glass ionomer in nature have low thermal expansion and high fluoride release. They require a conditioner for maximum adhesion that is rinsed off before the restoration is placed.¹⁵

Figure 4. Tooth No. 14 will have an MODL onlay placed after the ML cusp fractured. Fuji IX is placed in the interim.

Another type of glass ionomer is the high-viscosity, high-strength autocure variety (eg, Fuji IX GP Fast [GC America] or Ketac-Molar Quick Aplicap Glass Ionomer Restor-ative [3M ESPE]). These materials were originally developed for use in regions of the world where dental services are limited.^11,16 They are utilized following spoon excavation of a tooth. The area is then isolated as best as possible, and the restorative material is condensed into the tooth with finger pressure. A sharp instrument is used to remove excess. The higher viscosity and improved physical properties are achieved by the addition of polyacrylic acid to the powder, a finer grain-size distribution, and modifications to the chemical treatment of the glass powder to allow the incorporation of more powder into the liquid. These materials are adhesive and demonstrate fast setting times, high compressive and tensile strength, surface hardness, and fluoride release. These characteristics allow their use in patients with a high caries rate as bases and for emergency repair of fractured cusps (to cover exposed dentin and sharp margins; Figure 4), as interim restorations, and as final restorations in nonstress-bearing areas.

The surgical approach to cavity preparation requires removal of all carious tooth structure. Excessive removal of tooth structure could result in the need for endodontic therapy if the pulp chamber was breached. If, however, glass ionomers are used as the transitional restoration after removal of infected dentin, microleakage can be eliminated because of glass ionomers’ adherence to enamel and dentin via ion exchange. Ion exchange between the glass ionomer and the de-mineralized dentin allows the dentin to remineralize. After 3 months, part of the glass ionomer is removed and the final restoration is placed. If there is uncertainty that the dentin has remineralized, the entire glass ionomer restoration can be removed and the area examined prior to placing the final restoration.

It should be noted that “compomers,” or polyacid-modified resin composites, unlike true glass ionomers, do not contain water, and the setting reaction does not involve an acid-base reaction. They contain ion-leachable glass particles and anhydrous polyalkenoic acid. Eventually, water uptake from the oral environment will initiate an acid-base reaction. The amount of fluoride released is lower than traditional and resin-modified glass ionomers.¹⁰ In addition, glass ionomers have a greater capacity to be recharged. Exposure of fluoride-containing dental materials to fluoride rinses or fluoridated toothpastes provides a source that will replenish the fluoride in the dental material.¹⁰ The resin bonding agent that is utilized in adhesive restorations prevents fluoride from the compomer from being released. The weaker mechanical properties of these materials restrict their use to areas of minimal stress, such as class I, III, and V restorations in adult and primary dentition and small class II restorations in primary teeth.

CLINICAL APPLICATIONS OF GLASS IONOMERS

In addition to the use of glass ionomers as restorations, clinical applications of these materials include use as a bonding agent, liner, base, core material, sealant, and for luting.

Bonding agent. Some materials such as Fuji Bond LC (GC America) are true glass iono-mers and can be used to bond direct placement composite res-ins. Materials such as Scotch-bond Multipurpose (3M ESPE) incorporate glass ionomer technology that includes carboxylic acid groups to aid in the attachment to dentin.

	Figure 5. Tooth No. 31 was prepared for a mesial-occlusal restoration.
	Figure 6. Fuji lining was placed in the preparation.
	Figure 7. An amalgam restoration was placed.

Liner. Glass ionomers have been utilized as a cavity liner (eg, Vitrebond [3M ESPE] and Fuji Lining [GC America], Figures 5 to 7). They provide a chemical bond to tooth structure, good pulpal response, and fluoride release. The chemical bond, the initial low pH, and fluoride release help to minimize bacterial invasion.¹⁷

Figure 8. Tooth No. 14 has a large occlusal amalgam. Cracks in the enamel are evident on the mesial surface of the tooth.

Figure 9. The amalgam is removed, demonstrating the depth of the cavity.

Figure 10. UniFil Primer is placed in the distal fossa.

Figure 11. UnFil Adhesive is placed in the distal fossa.

Figure 12. UniFil Flow is placed in the distal fossa and light-cured for 20 seconds.

Figure 13. Fuji IX is placed in the mesial fossa to reduce the bulk of composite and the resulting contraction stress.

Figure 14. Gradia composite is placed as the final restoration.

Figure 15. The tooth is prepared for a class II restoration.

Figure 16. Glass ionomer is placed as a base.

Figure 17. Composite is placed as the final restoration in an open sandwich configuration.

Figure 18. When enamel remains at the gingival margin, this area is usually sealed with a composite resin.

Base. The term “sandwich technique” refers to the use of a glass ionomer to replace the dentin and composite resin to replace the enamel (Figures 8 to 18). Both materials provide advantages to the final restoration. The glass ionomer provides caries resistance, chemical ad-hesion to dentin, and remineralization capability, and by reducing the amount of composite that is needed, it reduces shrinkage. Shrinkage stress can break the bond between the restorative material and the cavity walls. Glass ionomers have a stiffness that is much less than composite filling materials, especially hybrids. Ideally, the glass ionomer should expand to compensate for the shrinkage of the composite. In addition, water absorption increases the volume of the base, which also serves to compensate for shrinkage. If failure occurs in the glass ionomer, it is usually a cohesive failure. This is helpful, as the dentin remains protected. However, due to the ongoing reaction of glass ionomers, internal cracks can be repaired, and over time the glass ionomer actually increases in strength due to this continuous reaction, especially when it is in contact with water.¹⁸

The presence of a resin-rich, nonparticulate, amorphous interfacial transition zone exists in dentin bonded with resin-modified glass ionomer cement (RMGIC) and has been called the absorption layer. This is because of the movement of water in the maturing RMGIC when placed in contact with deep, moist dentin. The RMGIC may serve to act as a stress-breaking layer similar to that of a dentin adhesive layer used to relieve polymerization stress.^19,20 The overlying composite will bond well to enamel and provides enhanced aesthetics and durability. The major drawback associated with composite is the high shrinkage that occurs after light curing, which results in stress on the adhesive interface.¹¹ In class II lesions, the glass ionomer can be placed as either an open (Figure 17) or closed sandwich (Figure 18). Glass ionomer used instead of composite in the cervical portion of the proximal box is an open sandwich. When enamel is present at the cervical margin, composite is bonded to this surface, and the glass ionomer is placed internally as a dentin replacement. The open sandwich is useful where there is no enamel at the cervical margin. Less micro-leakage has been demonstrated utilizing this technique as op-posed to conventional bonding in deep class II proximal boxes. The restoration will continually re-lease fluoride. Glass ionomer, being bioactive, releases calcium, phosphate, and fluoride in the presence of water and allows remineralization of carious areas.¹¹

The recent introduction of self-etching adhesives has reduced or eliminated postoperative sensitivity that may have been caused by the bonding agent not penetrating the entire depth of the demineralized layer (eg, UniFil Light-Cured Bonding System and UniFil Flow Light-Cured Flowable Composite Resin [GC America] and Adper Single Bond Plus Adhesive [3M ESPE], Figures 8 to 14).

Core material. Glass iono-mers that have been de-signed for use as core build-ups are available (eg, Fuji II LC Core Material [GC America] and Vitremer Glass Ionomer Core Buildup/Restorative [3M ESPE]). However, these materials are strong under compressive forces but weak under tension and shear.17 If glass ionomers are used as core materials, a majority of the tooth should be present. The glass ionomer is more ideally used as a block-out material to eliminate any undercuts in a crown or inlay/onlay preparation.²¹

Luting material. Conventional glass ionomer luting cements offer fluoride release, will bond to tooth structure, and have a low coefficient of thermal expansion.²² The addition of resin components has improved the physical and mechanical properties23 (eg, RelyX Luting Plus Cement [3M ESPE] and GC FujiCEM).

Figure 19. Fissure that isn’t wide enough for glass ionomer cement.	Figure 20. Fissure suitable for sealing with glass ionomer cement. A sharp explorer tip should enter the fissure.

Sealant material. The ideal sealant should have a long-term bond to enamel, be cariostatic, be easy to apply, be free flowing so that it can penetrate narrow fissures, and have low solubility in the oral environment. Glass ionomers must be placed in fissures with an orifice wide enough to accommodate an explorer tip (excess of 100 µm, Figures 19 and 20). If this is not done, the sealant will be lost through abrasion and erosion. If the fissures are patent, then the sealant can be used successfully.²⁴

CONCLUSION

The original concept of caries management involved the removal of all affected tooth structure and replacement by restorative material. With this philosophy, clinicians were only treating the symptoms and not the disease. Ideally, methods to prevent the disease or approaches to biological repair of a lesion should be developed.²⁵ Prevention includes antimicrobial therapy, modification of the substrate via reduction in fermentable carbohydrates (decreasing sugar intake), and making the teeth more resistant to acid dissolution (use of fluoride).

Glass ionomer materials can provide a valuable addition to the dentist’s restorative armamentarium. They are part of the profession’s transition to minimally invasive dentistry. The ion exchange and diffusion-based union of material tooth structure inhibits microleakage and maintains the seal between the tooth and the restoration. Fluoride release aids in remineralization and reduces microbial accumulation. Calcium and phosphate release and uptake help maintain remineralization potential. Glass ionomers have been modified to improve their physical properties for different restorative applications.

References

1. Featherstone JD. The continuum of dental caries: evidence for a dynamic disease process. J Dent Res. 2004;84(spec issue C):C39-C42.

2. Winston AE, Bhaskar SN. Caries prevention in the 21st century. J Am Dent Assoc. 1998;129:1579-1587.

3. Mount GJ, Ngo H. Minimal intervention: a new concept for operative dentistry. Quintessence Int. 2000;31:527-533.

4. Mount GJ, Ngo H. Minimal intervention: early lesions. Quintessence Int. 2000;31:535-546.

5. Mount GJ, Ngo H. Minimal intervention: advanced lesions. Quintessence Int. 2000;31:621-629.

6. Mount GJ. Minimal intervention dentistry: rationale of cavity design. Oper Dent. 2003;28:92-99.

7. de Jong HP, van Pelt AW, Busscher HJ, et al. The effect of topical fluoride applications on the surface free energy of human enamel: an in vitro study. J Dent Res. 1984;63:635-641.

8. Crispin BJ. Contemporary Esthetic Dentistry: Practice Fundamentals. Carol Stream, Ill: Quintessence Publishing Co; 1994:72-80.

9. Vrijhoef MMA, Mitra SM. Present and future of glass ionomers. Transactions of the Second International Congress on Dental Materials. 1993:62-70.

10. Hicks J, Garcia-Godoy F, Donly K, et al. Fluoride-releasing restorative materials and secondary caries. J Calif Dent Assoc. 2003;31:229-245.

11. Hewlett ER, Mount GJ. Glass ionomers in contemporary restorative dentistry: a clinical update. J Calif Dent Assoc. 2003;31:483-492.

12. Lin A, McIntyre NS, Davidson RD. Studies on the adhesion of glass-ionomer cements to dentin. J Dent Res. 1992;71:1836-1841.

13. Saito S, Tosaki S, Hirota K. Characteristics of glass-ionomer cements. In: Davidson CL, Mjor IA, eds. Advances in Glass-Ionomer Cements. Carol Stream, Ill: Quintessence Publishing Co; 1999:15-50.

14. McLean JW. Dentinal bonding agents versus glass-ionomer cements. Quintessence Int. 1996;27:659-667.

15. Burgess J, Norling B, Summitt J. Resin ionomer restorative materials – the new generation. In: Hunt Pr, ed. Glass Ionomers: The Next Generation. Proceedings of the Second International Symposium on Glass Ionomers. 1994:75-86.

16. Frankenberger R, Sindel J, Kramer N. Viscous glass-ionomer cements: a new alternative to amalgam in the primary dentition? Quintessence Int. 1997;28:667-676.

17. Hilton TJ. Cavity sealers, liners, and bases: current philosophies and indications for use. Oper Dent. 1996;21:134-146.

18. Davidson CL. Glass-ionomer bases under posterior composites. J Esthet Dent. 1994;6:223-224.

19. Tay FR, Sidhu SK, Watson TF, et al. Water-dependent interfacial transition zone in resin-modified glass-ionomer cement/dentin interfaces. J Dent Res. 2004;83:644-649.

20. Trushkowsky RD, Gwinnett AJ. Microleakage of class V composite, resin sandwich, and resin-modified glass ionomers. Am J Dent. 1996;9:96-99.

21. Sahmali SM, Saygili G. Compressive shear strength of core materials and restoring techniques. Int J Periodontics Restorative Dent. 2000;20:277-283.

22. Dumitru G, Hunt P. Posts and cores. In: Hunt Pr, ed. Glass Ionomers: The Next Generation. Proceedings of the Second International Symposium on Glass Ionomers. 1994:75-86.

23. Attar N, Tam LE, McComb D. Mechanical and physical properties of contemporary dental luting agents. J Prosthet Dent. 2003;89:127-134.

24. Wilson AD, McLean JW. Glass-Ionomer Cement. Carol Stream, Ill: Quintessence Publishing Co; 1988:179-189.

25. Roulet J-F, Zimmer S. Can technology cure disease? In: Roulet J-F, Vanherle G, eds. Adhesive Technology for Restorative Dentistry. Carol Stream, Ill: Quintessence Publishing Co; 2004:181-193.

Dr. Trushkowsky is a fellow in the Academy of General Dentistry, the Academy of Dental Materials, the International College of Dentists, the American College of Dentists, and the Pierre Fauchard Academy. He is a senior consultant for the Dental Advisor and a CRA evaluator. He wrote a chapter on direct composites in Esthetic Dentistry published by CV Mosby and a chapter on complex single-tooth restorations in Dental Clinics of North America. He has published more than 80 articles and abstracts in a variety of journals and magazines.Additionally, he is on the editorial boards of Contemporary Esthetics and Restorative Practice and Collaborative Techniques. He can be reached at (718) 948-5808 or composidoc@aol.com. Disclosure: Dr. Trushkowsky has received financial remuneration for providing consulting services to GC America.

 Continuing Education Test No. 64.2

After reading this article, the individual will learn:

• how fluoride release from glass ionomers contributes to the process of remineralization; and

 • the clinical applications of glass ionomers.

1. An infectious disease requires:

a. an altered immune system.

b. a susceptible host.

c. substrate to maintain the flora.

d. b and c.

2. Minimally invasive dentistry

encompasses: