HISTORICAL DEVELOPMENT

The idea of physically occluding pits and fissures is hardly new, for as long ago as 1923 Hyatt suggested a technique he called “prophylactic odontotomy.”⁵² Developed in an age of severe and seemingly universal caries, Hyatt’s technique involved minimal operative preparation of sound fissures and restoration with amalgam. The idea was not fully accepted even before the days of modern preventive dentistry,⁵⁸ but it led to widespread use of the “preventive restoration,” meaning a full Black’s cavity restoration with “extension for prevention” in sound fissures, placed on the grounds that without intervention such areas would soon decay anyway. For many years, this type of restoration was considered good preventive practice, and perhaps it was when there were few other preventive options. Although we have no way of knowing just how much caries this method “prevented,” the extensive use of the preventive restoration served to artificially inflate DMF (decayed, missing, and filled) scores (see Chapter 15).⁵⁵

In the prefluoride era, various chemicals were painted onto the tooth surface in an effort to prevent caries, but none proved successful.^11,57 Even after fluoride entered dental practice, interest in a specific preventive agent for pit-and-fissure caries persisted, but it proved difficult to find a material that adhered successfully to enamel in the oral environment. The breakthrough came in 1955 with Buonocore’s development of the acid-etch technique.²¹ By the mid-1960s cyanoacrylates had been used as sealant materials with some success,³⁰ but their production was not continued.⁹¹ In the late 1960s the “bis-GMA” formulation (a sealant that is the reaction product of bisphenol A and glycidyl methacrylate with a methyl methacrylate monomer) was developed and proved successful in a feasibility trial.²² The bis-GMA formulation became the basis of a number of other products that soon came onto the market. The American Dental Association (ADA) issued provisional acceptance of the first bis-GMA material, Nuva-Seal, in 1972⁴ and full acceptance in 1976.⁵ The number and types of accepted materials have grown steadily since then and will likely continue to grow in the future. Currently, the most widely used sealant materials are either bis-GMA resin or urethane based.¹⁰⁹ There is also considerable interest in the potential use of glass ionomer– based materials and fluoride-containing varnishes as sealant materials, but the research literature to date shows their retention to be inferior to that of the conventional sealant materials.^9,16,17,56,^69–71, ^{96,97,105,116}

RATIONALE FOR SEALANTS

It has been recognized for years that fissured occlusal surfaces are the most vulnerable to caries. With the continuing caries decline among children, caries is more and more a disease of the fissured surfaces as the rate of interproximal caries development continues to decline faster than the overall rate of caries experience.^13,99 Occlusal surfaces are also those least protected by fluorides,¹² so the case for sealant application as a complementary procedure to fluoride use is even stronger. As of the early 1990s, at least 83% of all decayed or filled surfaces in the permanent teeth of 5- to 17-year-olds were in pit-and-fissure surfaces.¹⁹ In fact, the appropriate delivery of fluorides and sealants together, in theory at least, presents the prospect of controlling caries to low levels previously unimaginable.

SEALANT PRODUCTS AND PROCEDURES

The original bis-GMA materials, now referred to as first-generation sealants, polymerized under ultraviolet (UV) light, a procedure that required a bulky UV light source in the oral cavity. Second-generation sealants are chemically polymerized; that is, when they are mixed, the operator has a fixed time to apply the sealant before it hardens. A number of such sealants are currently available. Third-generation sealants are those cured by visible light, which gives the operator the advantage of curing the sealant only when satisfied that it is all correctly in place. That advantage also applied to first-generation UV-cured sealants, but the visible light sources are far more compact and less expensive than the original UV light sources. Some second- and third-generation sealants are colored or opaque to make them more visible at clinical examination.

It should also be noted that in 1996 a research report from Spain concluded that, shortly after placement of sealants, bisphenol A and bisphenol A dimethacrylate monomers could be detected in saliva and that these monomers showed estrogen-like activity when tested in in vitro cultures of human breast cell tumors.⁶⁸ This effect is of concern, because it theoretically could result in increased tumor cell growth. To date there is no evidence that the transient amounts of these chemicals in saliva represent an important exposure in humans. In addition, none of the sealants that currently carries the ADA seal of acceptance produces detectable levels of bisphenol A.¹ However, the Spanish finding does point out that any material used in dentistry must be thoroughly evaluated for potential risk and that, regardless of how safe it appears to be, practitioners must take care to use any procedure or material only when the patient is likely to benefit from it. The ADA provides continual updates on these and related matters on its website (http://www.ada.org/prof/resources/positions/statements/index.asp).

Sealant application is a simple though meticulous procedure that requires attention to all details of technique, especially moisture control. Even slight moisture contamination during sealant application and curing will result in failure. When applying a sealant, the operator begins by washing and drying the tooth surface, then etching with acid to demineralize the surface layers of enamel in and around the fissures. The etchant is supplied as either a liquid or a gel; 35%-37% orthophosphoric acid is the most commonly used agent.¹⁰⁹ Acid etching dissolves out some of the inorganic fraction of the enamel, which subsequently allows “tags” of sealant to penetrate and thus enhances retention. Some of these tags can extend up to 100 μm into enamel, although tags of 15-20 μm are more common.⁹² After etching, the tooth surface is again washed and dried thoroughly, and the liquid sealant is applied and worked into the fissures and pits. The sealant is then polymerized (by visible light or by self-curing) and trimmed if necessary. Detailed descriptions of the application process are available.¹⁰⁹

By the early twenty-first century, the application of sealants as a purely caries-preventive procedure was merging into the popularity of conservative restoration procedures, many of which also used the acid-etch technique. The trend was stimulated by the caries decline, which meant that practitioners increasingly had to manage small, slowly developing lesions rather than large cavities, and by the rapid developments in composite materials. Dentistry began moving away from placement of amalgams in traditionally prepared Black’s cavities with extension for prevention and toward minimum-preparation restorations, which were far less invasive, lasted longer, and were more esthetic.³⁵ The preventive resin and sealed composite restoration,^49,63,110 sealed glass ionomer restoration,⁴⁶ “tunnel” restoration for small proximal restorations,²⁸ and even sealed amalgams^63,64 are changing the face of restorative dentistry. The distinction between a purely preventive sealant placed on a sound tooth, sealant placed on an incipient lesion, and a minimum-preparation sealed restoration is becoming increasingly blurred. The challenge is to understand the indications for each and to use each appropriately.

SEALANT CLINICAL TRIAL DESIGN

The initial clinical trials for testing the efficacy of sealants necessarily differed from the classical model (see Chapter 13) in several respects:

Because of the overwhelming weight of the evidence for the efficacy of sealants, recent trials testing new products now most commonly apply an accepted sealant as a positive control on the control tooth or individual dentition. Because positive controls are used in comparative studies, the examiners should be blind as to which sealant is the test product and which is the positive control. Because differences in efficacy between the test product and the control are expected to be small, the numbers of subjects needed is fairly high.

SEALANT EFFICACY

A large number of well-conducted sealant studies have been carried out, which allows conclusions on their efficacy to be stated with some confidence. The panel at the National Institutes of Health Consensus Conference on dental sealants in 1983, one of the relatively few such conferences held on dental procedures, concluded that sealants were highly efficacious.⁶⁷ The panel also noted, however, that practitioners were slow to adopt their use and that insurance carriers were also hesitant about adding sealant application to their list of benefits. The Medicaid programs in all 50 states now cover sealant application, and although precise numbers are unavailable, the number of privately insured groups with sealant coverage continues to grow.

The first clinical sealant studies in the 1960s yielded spectacular results, with caries reductions of 99% reported.²² These initial studies, however, carefully selected both the patients and the teeth to be sealed. By the end of the 1970s, there was clear evidence from numerous clinical trials in different populations that sealants were highly efficacious when applied correctly.⁷⁶ Studies since then using second- and third-generation sealants have almost all yielded results highly favoring their use; reviews of what is now an extensive literature have all reached highly favorable conclusions regarding their efficacy.^{77,78,100,113} Well-controlled clinical trials have shown good results after 5 years,⁴⁸ 7 years,⁶⁵ and 10 years⁷⁸; and 10-year and 15-year retrospective reports also showed encouraging results.^94,95 The favorable evidence has led the ADA to strongly support the appropriate use of sealants in general practice.^2,3

Evidence for the efficacy of sealant application in private practice, although scanty, also appears favorable. In an observational study in Canada, sealed first permanent molars had a 75% lower incidence of new restorations than originally sound but unsealed molars.⁵³ The authors acknowledged that use of sealants was more common in caries-free children and in children whose parents had higher levels of education, which could account for some of the lower caries increment, but the differences in caries experience were so large that sealants had to have played a substantial role. It is nevertheless important to be cautious in interpreting outcomes from observational studies in which patients are not randomly assigned to receive or not to receive sealants. As has been pointed out in a study of the use of sealants in a Medicaid program, the children who actually received sealants tended to be at lower risk; that is, they were more likely to have been caries free initially and were more likely to have been classified by the study examiners as not needing sealants.⁸⁰ The authors pointed out that this pattern of nonrandom use of sealants in the least caries-prone children could lead to overestimates of sealant effectiveness. Nevertheless, there is ample reason to think that, with appropriate patient selection, sealant application is highly effective in private practice.

Findings from the earlier clinical studies of sealants that have been supported by later research include the following:

These results demonstrated unequivocally the considerable efficacy of sealants; they also gave hints of the more recent realization that sealants are more difficult to successfully apply and maintain on the very teeth that are most vulnerable, that is, the early-erupting molars in caries-prone children. On the other hand, sealants seem to be retained best on teeth that are least caries prone (e.g., bicuspids) and in children with low caries risk.¹⁸ This realization is part of what has lead to efforts to target sealant use to the most susceptible groups, individuals, and teeth, an issue discussed later in this chapter.

Later studies of sealant efficacy have led to four additional conclusions that have an important bearing on the way sealants are used in clinical practice. These conclusions are discussed in the following sections.