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The Use of Glass Ionomers as a Chemical Treatment for Caries

The use of glass ionomers (ie, both conventional auto-curing and resin-modified) serves as a formidable means to managing dental caries. As fluoride-releasing glass ionomers can effectively seal and chemically treat areas of teeth damaged by infection, they may be used as a chemical caries treatment rather than solely as a restorative material. They have also successfully sealed occlusal pits and fissures, especially on newly erupted teeth, where potential further mineralization is desired or where fissure preparation may not be desired.

Historically, dentists have been trained to evaluate restorative materials in narrow terms (eg, strength, shear bond strength, retention, aesthetics). Alternately, one may assert that aesthetics and bond strength are not relevant if that same material fails to prevent recurrent caries from developing at the margins, especially on the root. The clinician may consider stabilizing the tooth with a dental material that has good marginal seal and caries inhibition, while at the same time focusing on controlling the bacterial infection through chemical and behavioral means.1

 

Caries Treatment and Surface Sealant

Imagine a dental material that can mimic tooth mineral. That material will be biocompatible, have thermal expansion similar to tooth mineral, be compatible with a moist environment, have a chemical bond to tooth,2 have the ability to dynamically exchange ions with the tooth, have excellent marginal seal and no contraction gap formation, and release and uptake fluoride3 to inhibit caries formation.4 Glass ionomers are known to have demonstrated these qualities.5,6 Some have suggested clinicians start thinking of this material as a chemical delivery device to prevent and treat the infection of dental caries,7 rather than view it solely as a restorative material.

Glass ionomers have been used successfully in underdeveloped countries as part of an atraumatic restorative technique (ART) to arrest gross decay when conventional restorative dentistry is not available.8,9 The surface release of fluoride has been shown to protect the teeth adjacent to the glass-ionomer restoration from demineralization.10 In addition, fluoride released from glass ionomers has the ability to recharge or be replenished from other topical sources of fluoride.11  

These studies support the ability of glass ionomers to exchange ions with tooth mineral, mimicking the mineral exchange that happens in natural teeth, thereby creating a marginal seal and inhibiting caries formation. Hence, glass ionomers have a role as a chemical caries treatment rather than solely a restorative material.

The glass ionomer can be used as a surface protectant with long-term fluoride delivery3 and recharging abilities3,12 on any surface without the need for mechanical preparation. It is ideal for root protection in the elderly as well as coronal protection on a newly erupted tooth in a child.13 The glass ionomer also has been used to successfully seal occlusal pits and fissures,5,6 particularly on newly erupted teeth, where potential further mineralization is desired or where fissure preparation is not indicated. It is also an excellent base material for large or deep restorations.13 Many clinicians prefer to use it in tunnel preparations where beveling of the external proximal margins is not possible.13 It should be noted that unlike resin-based materials, glass ionomers will chemically bond equally well to both prismatic and aprismatic enamel.2 It is advisable, however, that the surface be clean and free from plaque or debris.

 

Dynamic Nature of Glass Ionomers

The transfer of ions between glass ionomers and the tooth is a well-understood part of the chemical bond to tooth mineral,2 forming an interface some call the chemically fused interface or zone.15 Ngo demonstrated that this bond is stronger than the material itself, as demonstrated by cohesive fracture of the material when stressed.15 Studies suggest there continues to be permeability between the tooth and clinically set glass ionomers. Yiu demonstrated that water can diffuse from dentin across the chemically fused zone and into the glass ionomer itself.16 If water can freely diffuse in and out of the glass ionomer, it is speculated that this semipermeable characteristic of this unique material can be leveraged to enhance mineralization underneath and around clinically placed glass ionomer restorations.15

Even after 2 to 3 years in the mouth, integration with the adjacent enamel remains, and an increase of calcium and phosphate in the surface layer has been detected, suggesting there is additional mineralization of the glass ionomer with time.17 This supports studies demonstrating that saliva has the effect of increasing surface hardness of conventionally set glass ionomers with time.18 These studies suggest that the glass ionomer gets stronger and harder with increasing time after placement in the mouth.

Studies showing continued surface mineralization, ionic diffusion, and water transfer across the tooth-material interface have led researchers to question whether mineralization can occur beneath the restoration at the tooth-glass ionomer interface. Along with the reorganization of phosphate and other minerals that describe the chemistry of remineralization, which naturally occurs in the mouth, the glass ionomer will deliver fluoride and calcium (or strontium) to the tooth. The chemistry described has led researchers to investigate the internal remineralization,5 or an acid-resistant chemically fused zone beneath the glass-ionomer material.15 Milicich presents evidence of this phenomenon using a novel resin-impression scanning electron microscopy (SEM) technique (Figure 1).19  

Longevity

Properly placed glass-ionomer restorations have been clinically demonstrated to last as long as 8 to 15 years in many cases, dispelling the myth that glass ionomers dissolve in the mouth (Figure 2).6 Glass ionomers are ideal in the replacement of dentin and cementum--chemically sealing when used as a base or liner, and chemically fusing to roots when conventional materials (ie, amalgam) require retentive preparations or seem to create an inadequate seal when placed on roots (ie, composite). Proper placement is critical to the success of glass ionomers because their chemical bond relies on proper treatment of the tooth surface prior to material placement. The use of polyacrylic acid for 10 seconds and subsequent rinsing will remove the unwanted smear layer and prepare the surface for an ionic exchange to occur at the tooth-glass ionomer interface. Phosphoric acid should not be used as a replacement for polyacrylic acid as it will remove too much of the mineral and leave collagen exposed, even if used for only a few seconds.

The glass ionomer must be placed on a moist surface (ie, not desiccated) and must be left undisturbed while setting. Knowing the material’s limitations is just as important as proper placement techniques. Glass ionomers should not be used in high stress-bearing areas and should be used with caution in high acid environments. Acid from the stomach or even from acidic fluoride applications have been shown to erode the surface of glass ionomers.20

 

Restorative Failure or Therapeutic Success

There is ample evidence to suggest that glass ionomers could be an effective adjunct to treating dental caries rather than used solely as a restorative material. In fact, there may be times when the longevity of the dental material may be secondary to its therapeutic benefit. If patients are properly educated of the goals and objectives in material selection, they can better understand the importance of viewing restorative procedures as part of the chemotherapeutic approach to treating dental caries. Thus, if a glass ionomer treatment must later be replaced, this is less likely to be viewed as a restorative failure by either patient or clinician. In many high caries risk patients, longevity and aesthetics should be secondary to the caries treatment aspect. Once the disease process is under control, other restorative treatment options can be re-evaluated. The use of the glass ionomer to control caries should not be viewed, as a “material failure” but rather an attempt at “therapeutic success.”

 

Conclusion

Fluoride has been the single most effective chemical treatment in modern history of dentistry to reduce and prevent dental caries in populations. Fluoride-releasing glass ionomers can effectively seal and chemically treat areas of the tooth where it was damaged as the result of infection, and is finally rising to the forefront of contemporary dentistry. Material selection no longer should be based solely on physical and clinical properties. Rather, the clinician should also consider the biologic process of the disease itself, caries risk status of the patient, as well as the location, depth, and activity of the lesion.

*Associate Professor, Department of Diagnosis and Management, University of the Pacific, San Francisco, CA.

References

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  2. Yoshida Y, Van Meerbeek B, Nakayama Y, et al. Evidence of chemical bonding at biomaterial-hard tissue interfaces. J Dent Res 2000;79(2):709-714.
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  12. Mustafa NB, Chan DCN, Titus HW, Yang Z. Fluoride release from restorative materials after exposure to NaF. J Dent Res 1996;75(Special):382(Abstract No. 2917).
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  14. Sennou HE, Lebugle AA, Gregoire GL. X-ray photoelectron spectroscopy study of the dentin-glass ionomer cement interface. Dent Mater 1999;15(4):229-237.
  15. Ngo H, Mount GJ, Peters MC. A study of glass-ionomer cement and its interface with enamel and dentin using a low-temperature, high-resolution scanning electron microscopic technique. Quintessence Int 1997;28(1):63-69.
  16. Yiu CK, Tay FR, King NM, et al. Interaction of resin-modified glass-ionomer cements with moist dentine. J Dent 2004;32(7):521-530.
  17. Van Duinen RN, Davidson CL, De Gee AJ, Feilzer AJ. In situ transformation of glass-ionomer into an enamel-like material. Am J Dent 2004;17(4):223-227.
  18. Okada K, Tosaki S, Hirota K, Hume WR. Surface hardness change of restorative filling materials stored in saliva. Dent Mater 2001;17(1):34-39.
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