Caries Prevalence on Occlusal Surfaces

The occlusal surfaces constitute only around 12% of all the surfaces in the mouth, and yet they often account for 80%–90% of all the decay in younger populations.9,10 Although a net reduction of occlusal caries has occurred over the last decades, the proportion of occlusal decay has increased, mainly due to increased use of fluoride (Chapter 12). The pit and fissure system has been proportionally less affected by the caries preventive effect of fluoride than approximal or smooth surfaces. Figure 15.2 shows the considerable variation in shape found in the fissures. The pit and fissure system is usually the first site on the erupting tooth to be subject to caries, often because of lack of hygiene during the eruption period.11 Although the caries status of the erupting teeth can be affected, it takes a vigorous program of hygiene instruction, coupled with application of fluorides, to keep those teeth free of decay. Initial active lesions detected during the eruption period often revert to a status of inactivity once the teeth have reached full occlusion.12

While the occlusal surfaces are usually the first sites to show signs of caries, this may give the impression that 90% of caries could be successfully treated by sealant placement. This would be an overly optimistic assumption. The prevalence numbers are based on studies that use the methodology of visual examination, usually performed without prior cleaning of the teeth, and on a population of 12–15-year-old children. The occlusal lesions usually progress rapidly, and they lend themselves easily to direct visual examination, while progress of approximal lesions, for example, is more gradual, and their early development necessitates radiographic examination to be detected. The risk of caries formation is a stepwise process, whereby the sites of a group within the dentition with similar resistance for caries get affected as the patient′s individual defenses against the caries process become overwhelmed.13 Placing of sealants only assists with the risk of the pit and fissure system, and continued care is needed to protect other areas and sites of the dentition. There is no evidence so far of sealants reducing the risk of other surfaces, although hypotheses have been formed related to reduction in retention sites for oral bacteria. Even so, it is clear that sealants are an indispensable part of caries preventive strategies in clinical practice and in community programs.

The Sealant–Restoration Spectrum

There is a long tradition among dental professionals to view sealants as separate entities from fillings. The distinction was made on the basis of materials and procedures, since almost no tooth substance would be removed for the placement of a sealant. However, progress in materials and concepts of treatment has blurred this distinction. Really, it is best to regard sealants as almost noninvasive (microinvasive) therapy, as they are placed with no or very little mechanical modification of the surface. Nonetheless, the procedures and materials are similar to those used when a restoration is placed. Therefore, sealants require the same care and attention to detail as do fillings. Just as mishandling of filling procedures can compromise the adaptation and longevity of a restoration, the same applies to sealant placement: it is important to respect the advantages and limitations of the procedures and the materials that go toward making sealing into a successful caries-preventing procedure.

Historically, the reason for this separation is easy to understand. By the time sealants became available for use on occlusal surfaces of posterior teeth, the only other option was to place a class I amalgam restoration. The amalgam required technical skills, mechanical preparation with rotary instruments, understanding of engineering principles, good understanding of materials science, and an artistic touch in adapting and carving the material into a functional filling. The sealants, however, seemed simple and required little else but cleaning of the tooth surface, a little etching, and then the sealing material would flow into place. Indeed the procedure seemed so simple that the task could be referred to auxiliary members of the dental team. Dental hygienists and assistants could even offer sealants in school-based programs without the direct oversight of a dentist. Excellent staff and good training have proved the last statement to be true, but it is also recognized that the procedure requires attention to detail and adherence to certain principles to be successful.

Today the picture is a little more complicated. Amyriad of materials has been marketed for use as sealants. The different types of material make up an entire spectrum, from being unfilled resin materials, to resins with varying degrees of filler content, through various stages of compomers, resin-modified glass-ionomers, and glass-ionomers (see Chapters 14, 19). Fluoride is sometimes incorporated into some of these materials, and bonding agents may also be used. Even flowable composites are used by some dentists as sealant material. The procedure itself offers many options for how to place sealants, how to clean the surface, how to prepare the surface, and various other factors, as discussed below. The only clear procedural difference between placement of sealants and fillings lies in the preparation of the dental hard tissues.

A hybrid technique, so-called preventive resin restoration, was already advocated in the 1970s.14 Whether used in conjunction with amalgam or resin, this calls for removal of the caries-affected part of the lesion only and sealing of the rest of the fissure system, the sealing treatment becoming the predominant part of the operation. Sealants applied with amalgam fillings have also been shown to add to the longevity of the fillings.15 This sealant–restoration spectrum gives us many options to apply the correct treatment in various situations (Chapter 20).

Epidemiological considerations

A consequence of this blurred interface between sealants and fillings is how they are scored in clinical studies. In epidemiological studies there is now a problem in distinguishing sealants from small restorations underneath part of the sealant. The distinction is important in reporting of data, for example, national data measured with the DMFT index, as the sealed teeth would be regarded as sound, but filled teeth would contribute to the perceived disease burden of the participants (see Chapter 8). A few years ago, the sealants would have been either clear or white, but with the use of tooth-colored materials for the routine sealing, even that distinction has become blurred. More viscous materials do not flow as well into the fissures, and the proportion of overfilled fissures seems high.

The same problem is faced in clinical trials and other longitudinal studies. A surface with an overfilled tooth-colored sealant may resemble a filling at baseline, but with wear of the overfilled portion it may become clear that it was placed as a sealant. This would be counted as a reversal in the data set, and potentially obscure the effect of the treatment being tested.

Fissure Morphology

The fissures are developmental grooves, mainly in the occlusal surfaces of the teeth. They are considered to be faults which arose during development of the cuspal enamel, caused by the failure of the enamel lobes to coalesce perfectly during the formative stages, with their location based on the developmental lobes of the tooth formation ( Fig. 15.2 ). Although most common in molar teeth, the premolars have such grooves too ( Fig. 15.3 ). Also for palatal surfaces of upper anterior teeth, pits and fissures might be found. Since many of those pits and fissures become very narrow, they easily lead to plaque stagnation, which can then lead to lesion formation.

As part of risk assessment used to justify sealant placement, the phenomenon of “deep” fissures is commonly mentioned. In the North Carolina risk assessment study, one of the three risk predictors found to significantly correlate with subsequent caries formation was fissure morphology (deep fissures – high caries risk).16 Other studies have also linked deep fissures with prevalence of caries17–19 and dentists’ perception of the caries risk of deep fissures.20

Several methods have been used to examine fissure morphology. Serial sectioning,19,21,22 fissure splitting,23 resin replicas infused under vacuum24,25 and 3-D computer reconstruction of resin impregnated sections following dye infusion.26 Although there have been attempts to classify the different morphological types of fissure—into V-shaped, U-shaped, I-shaped, or combinations of these18,26—there are great variations in the shape, size, and width of fissures.21,22,27

The fissure system may not be an open canyon through the center of the teeth, but rather resembles a series of pits separated by cols that may change continuously as one travels along the system. It becomes obvious that those systems are vulnerable to plaque accumulation and caries formation ( Figs. 15.4 and 15.5 ).

The thickness of the enamel at the bottom of the fissures in the upper premolars is estimated to be around 0.2–0.35 mm.24 This varies to the extent that in rare cases some fissures can be found as having a very thin enamel covering at the bottom. The following dimensions on maxillary first premolars have been reported:27

• Fissure depth: 120–1050 µm

• Width in the middle part of the fissure: 40–156 µm

• Thickness of the enamel at the bottom of the fissure: 270–1000 µm

Thus, it can be stated that there is great variation in the shape and depth of the fissure system from one part to the next within the same surface, and it is likely that only some parts will need to be covered by sealants to reduce the caries risk.

A limitation of scientific studies concerning morphology of fissures is that they mostly refer to either third molars, which are known for their rich variation in anatomy and structure, or premolars extracted for orthodontic reasons. Extrapolating those results to the teeth most commonly recommended for sealing—that is, the first and second molars—needs to recognize this limitation, although the challenge is obvious: to be able to clean those narrow spaces, dry them, and get the sealant material to penetrate as far as possible downward into the fissure space. However, caries formation takes place at the upper part of the fissures, so penetration to the full depth of the fissure may not be needed.

The Fissure as Substrate for Resin Bonding

A fundamental question in planning placement of a sealant is to consider what the sealant is binding to. In this regard we must consider both the microscopic composition of the enamel, the morphological formation of the fissures, and the organic material found occupying the fissure space. At the time of eruption the fissures are filled with remnants of enamel protein, protein from tissue and blood fluids, and cellular remnants, forming a loose organic mesh inside the fissure ( Fig. 15.6 ).22 As soon as the tooth penetrates the mucosal lining, salivary proteins are added to the mix, forming acquired pellicle along with organic and inorganic ions from the saliva. At that time there is also an influx of bacteria, laying the foundation of the dental biofilm.19 Food debris gets compacted into the fissures, and calcifications can be seen within the fissures. After several weeks the biofilm has taken on a more distinct structure with successive layers of microorganisms and extracellular material.

In most cases sealants are placed on the teeth within a few months to a few years after their eruption. During this time there are changes taking place in the inorganic content of the enamel, and in the organic material that attaches to the enamel. When the enamel is formed it is immature, not all spaces are occupied by ions, there are intercrystalline spaces, remnants of enamel forming protein, and impurities. Also, a hypomineralized area has been described descending from the bottom of the fissure although its role in caries formation is not regarded as crucial.28 The enamel in the fissure does not all possess the regular keyhole structure found on smooth surfaces, but often a thin amorphous layer lines the fissure enamel that is different from the regular crystalline enamel rods.29 The importance of this must be understood in the context of using acid etching to create microscopic tags in the enamel for micromechanical retention of the sealant material.

With the fissure enamel constantly covered by biofilm and bathed in saliva, the enamel slowly transforms during the maturation phase, for up to two years, increasing the content of fluoride and the hardness of the enamel. The importance of the biofilm to sealant placement is that the phosphoric acid has poor capabilities for removing the biofilm, and therefore etching the enamel when it is covered by the biofilm may turn out to be incomplete. Studies also show that the organic content of the fissure is very hard to clean out,23 and that the methods of cleaning, mainly with pumice or prophylactic paste, can leave some of the cleaning agent in the fissure.29 Moreover, the fissures might be lined with remnant aprismatic enamel.30 In the last-cited study no full penetration of resins into the bottom of the fissures or gap-free interfaces could be observed in any of the specimens. Phosphoric acid did not penetrate well into the fissures and although hybridization of the etched aprismatic enamel was observed, etching was inconsistent and gaps were frequently observed. Entrapment of bacteria within the fissure walls was frequently seen.30

The goal is to make sealant penetration as complete as possible. The combination of three factors results in possible interference not only with enamel etching, but also with sealant penetration and therefore the proportion of the fissure that is filled with the sealant material:

• The microscopic structure of enamel

• The macroscopic fissure configuration

• The constant presence of a biofilm

Drying will be incomplete, and due to incompatibility of resin materials with hydrated environment, sealant penetration is suspect. This will result in fissures being at best partially filled,29 and in some cases with only limited retention available at the opening of the fissure.

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