Highlights
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Fluoride from dental materials works on caries similarly as fluoride from dentifrices.
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The effect of fluoride from dental materials is based on in vitro and in situ studies.
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The effect of fluoride from dental materials is still not based on clinical evidence.
Abstract
Objectives
(1) To describe caries lesions development and the role of fluoride in controlling disease progression; (2) to evaluate whether the use of fluoride-releasing pit and fissure sealants, bonding orthodontic agents and restorative materials, in comparison to a non-fluoride releasing material, reduces caries incidence in children or adults, and (3) to discuss how the anti-caries properties of these materials have been evaluated in vitro and in situ .
Methods
The search was performed on the Cochrane Database of Systematic Reviews and on Medline via Pubmed.
Results
Caries is a biofilm-sugar dependent disease and as such it provokes progressive destruction of mineral structure of any dental surface – intact, sealed or restored – where biofilm remains accumulated and is regularly exposed to sugar. The mechanism of action of fluoride released from dental materials on caries is similar to that of fluoride found in dentifrices or other vehicles of fluoride delivery. Fluoride-releasing materials are unable to interfere with the formation of biofilm on dental surfaces adjacent to them or to inhibit acid production by dental biofilms. However, the fluoride released slows down the progression of caries lesions in tooth surfaces adjacent to dental materials. This effect has been clearly shown by in vitro and in situ studies but not in randomized clinical trials.
Significance
The anti-caries effect of fluoride releasing materials is still not based on clinical evidence, and, in addition, it can be overwhelmed by fluoride delivered from dentifrices.
1
Introduction
Dental materials are used in Dentistry for many clinical purposes. If the material is used to rebuild the tooth and does not have properties that may help to control caries adjacent to the filling, the role of this material is only to make the tooth functional again and esthetically appealing to the patient. However, when dental materials have the ability to release fluoride, it is expected that, besides restoring function and esthetics, they may control the recurrence of caries on dental structure adjacent to the filling and/or even contribute to reduce caries incidence in the entire dentition.
In addition, fluoride-releasing materials are also used as sealants to prevent caries in pits and fissures and as materials for bonding and/or cementing orthodontic brackets and bands. In this case, it is also expected that the fluoride released by these materials will work reducing the progression of enamel caries around them.
The development of caries on dental surfaces adjacent to dental materials should not be considered different from that occurring on intact (natural) dental surfaces. Also, the mechanism of action of fluoride on caries control adjacent to these materials may be considered the same as that reported for other ways or vehicles of fluoride delivery, such as fluoride dentifrices (toothpastes). These concepts are discussed in Sections 2 and 3 of this paper.
2
Dental caries
Caries lesions develop on dental surfaces in which biofilms are formed, allowed to accumulate and retained for long periods of time ( e.g. , occlusal surfaces, interproximal areas, along gingival margins and on enamel-cementum junction) . Although necessary for caries lesion progression, biofilm accumulation alone is not enough. Sugars are the pivotal, negative factor, responsible for caries lesion progression . The acid pH produced from the fermentation of dietary sugars not only provokes dissolution of the underlying dental minerals, but also selects in the biofilm formed the most cariogenic bacteria .
Therefore, caries is a biofilm-sugar dependent disease and, among the dietary sugars, sucrose is the most cariogenic because besides being easily fermented into acids, it is the only carbohydrate that change the matrix of the biofilm formed, making the biofilm more cariogenic .
Thus, biofilm accumulation is the necessary factor and sugar exposure the negative determinant factor for caries progression in any dental surface, intact or restored ( Fig. 1 ). The only difference between caries progression on enamel or dentin adjacent to a filling, in comparison with a natural intact surface, is the possibility of biofilm accumulation in the gap between the wall of the cavity and the filling material . Every time sugar is ingested, biofilm bacteria produce acids and, consequently, the pH drops in the biofilm fluid. Thus, pH is the driving force governing the loss or gain of Ca and Pi from the mineral structure of the teeth. While pH remains below around 6.5 for dentin and 5.5 for enamel, the minerals of these tissues are dissolved (demineralization) ( Fig. 1 a). After around 20–30 min, the pH rises again and, above 5.5 for enamel and 6.5 for dentin, saliva tries to repair Ca and Pi loss (remineralization) ( Fig. 1 b). However, saliva alone is not 100% effective to repair all Ca and Pi minerals lost during the demineralizing process. The balance toward demineralization or remineralization will be dependent on the daily frequency of dietary sugars ingestion.
Thus, caries lesions progression on dental surfaces, restored or not, is provoked by the same factors, biofilm accumulation and sugar exposure. Also, the process (chemical events) of caries lesions development is the same in any dental surface ( Fig. 1 a and b).
Therefore, dental caries as a disease is, by nature, primary, and its control, either in intact dental surfaces or adjacent to dental materials (“secondary caries”), is achieved by biofilm mechanical disruption and sugar restriction.
Also, caries is not the result of fluoride deficiency but this ion is the only therapeutic agent known to effectively control caries progression, and fluoride-releasing materials may be considered a way or vehicle of fluoride delivery ( Table 1 ).
| Approaches for fluoride use | Vehicles (examples) |
|---|---|
| Community level | Water fluoridation |
| Individual level | Fluoride dentifrice, mouthrinse |
| Professionally applied | Fluoride-releasing dental materials, topical fluoride application (gel, varnish) |
| Combinations | Fluoride dentifrice + fluoride-releasing dental materials |
2
Dental caries
Caries lesions develop on dental surfaces in which biofilms are formed, allowed to accumulate and retained for long periods of time ( e.g. , occlusal surfaces, interproximal areas, along gingival margins and on enamel-cementum junction) . Although necessary for caries lesion progression, biofilm accumulation alone is not enough. Sugars are the pivotal, negative factor, responsible for caries lesion progression . The acid pH produced from the fermentation of dietary sugars not only provokes dissolution of the underlying dental minerals, but also selects in the biofilm formed the most cariogenic bacteria .
Therefore, caries is a biofilm-sugar dependent disease and, among the dietary sugars, sucrose is the most cariogenic because besides being easily fermented into acids, it is the only carbohydrate that change the matrix of the biofilm formed, making the biofilm more cariogenic .
Thus, biofilm accumulation is the necessary factor and sugar exposure the negative determinant factor for caries progression in any dental surface, intact or restored ( Fig. 1 ). The only difference between caries progression on enamel or dentin adjacent to a filling, in comparison with a natural intact surface, is the possibility of biofilm accumulation in the gap between the wall of the cavity and the filling material . Every time sugar is ingested, biofilm bacteria produce acids and, consequently, the pH drops in the biofilm fluid. Thus, pH is the driving force governing the loss or gain of Ca and Pi from the mineral structure of the teeth. While pH remains below around 6.5 for dentin and 5.5 for enamel, the minerals of these tissues are dissolved (demineralization) ( Fig. 1 a). After around 20–30 min, the pH rises again and, above 5.5 for enamel and 6.5 for dentin, saliva tries to repair Ca and Pi loss (remineralization) ( Fig. 1 b). However, saliva alone is not 100% effective to repair all Ca and Pi minerals lost during the demineralizing process. The balance toward demineralization or remineralization will be dependent on the daily frequency of dietary sugars ingestion.
Thus, caries lesions progression on dental surfaces, restored or not, is provoked by the same factors, biofilm accumulation and sugar exposure. Also, the process (chemical events) of caries lesions development is the same in any dental surface ( Fig. 1 a and b).
Therefore, dental caries as a disease is, by nature, primary, and its control, either in intact dental surfaces or adjacent to dental materials (“secondary caries”), is achieved by biofilm mechanical disruption and sugar restriction.
Also, caries is not the result of fluoride deficiency but this ion is the only therapeutic agent known to effectively control caries progression, and fluoride-releasing materials may be considered a way or vehicle of fluoride delivery ( Table 1 ).
| Approaches for fluoride use | Vehicles (examples) |
|---|---|
| Community level | Water fluoridation |
| Individual level | Fluoride dentifrice, mouthrinse |
| Professionally applied | Fluoride-releasing dental materials, topical fluoride application (gel, varnish) |
| Combinations | Fluoride dentifrice + fluoride-releasing dental materials |
3
Fluoride effect
The old concept that fluoride strengthens the teeth, making them more resistant to caries, is still prevalent. However, fluoride is not able to prevent caries lesion development because it does not avoid the formation of biofilm in any dental surface, either intact or adjacent to fluoride-releasing materials. Furthermore, the in vivo effect of fluoride inhibiting acid production from sugars in the biofim is negligible .
In fact, fluoride interferes with the caries process, reducing demineralization and enhancing remineralization of enamel and dentin . This physico-chemical mechanism occurs every time sugar is ingested and the pH falls in biofilm fluid; if fluoride is present, the amount of mineral dissolved is reduced because part of Ca and Pi lost as hydroxyapatite returns to the tooth as fluorapatite (reduction of demineralization) ( Fig. 2 a) . When the ingestion of sugar ceases and the pH rises again, fluoride present in the oral fluids enhances the natural phenomenon of remineralization ( Fig. 2 b). As a consequence, the progression of caries lesions is slowed down . Also, following the exchange of minerals between the biofilm fluid and enamel or dentin in the presence of fluoride, an enrichment of fluoride concentration in enamel or dentin surfaces occurs – a consequence of the effect of fluoride on the caries process, and not the cause of caries lesions reduction .
The anti-caries effect of fluoride can be obtained by the same mode of action irrespective of the ways or vehicles of fluoride use. Among them, toothbrushing with fluoride dentifrices is the only one whose results on caries reduction are strongly based on scientific evidence . Besides their effectiveness to control the incidence of caries in originally intact dental surfaces, fluoride dentifrices can also interfere with caries lesion progression adjacent to dental materials because during toothbrushing fluoride is spread throughout the mouth, and enriches remainings of biofilm not perfectly removed .
Fluoride-releasing materials should also be considered a way to maintain fluoride constantly in the mouth ( Table 1 ). For instance, glass ionomer cements, in addition to releasing fluoride for a long time, can also be recharged with the ion from other sources, such as fluoride dentifrice .
Furthermore, fluoride-releasing materials should be considered a unique way of fluoride use because the ion is maintained constant in the right place (the biofilm), is available at the right time (whenever sugar is ingested), and in enough concentration (low levels) to reduce de- and enhance remineralization ( Fig. 2 a and b).
Another advantage of fluoride-releasing dental materials in comparison with dentifrice is that the effect of fluoride does not depend on patient compliance. Fluoride from dental materials works passively.
However, considering that they all work by the same mode of action, a synergistic effect of the combination of ways of fluoride use would not be expected. In fact, there is no evidence of a synergistic or additive effect when fluoride dentifrices, rinses and gels or varnishes are used in combination . Thus, no significant additive effect would be expected if fluoride-releasing dental materials are used in a patient who brushes his/her teeth at least twice/day with dentifrice containing at least 1000 ppm F .
In fact, the anti-caries potential of fluoride-releasing materials has been extensively studied since the silicate cement era but most evaluations are based on in vitro and in situ studies . Therefore, there is a bulk of data recommending these materials to improve oral health in terms of lowering caries levels. However, these data accrue from studies with a low level of evidence to inform clinical practice. Currently, it is acknowledged that health interventions should be based on the best available evidence, and the only way to know if they work in real life is by conducting randomized clinical trials. The current available evidence from clinical studies on fluoride-releasing pit and fissure sealants, bonding orthodontic agents and restorative materials for caries control is presented below.
4
Clinical effects of fluoride-releasing dental materials on the development of caries lesions
The question we sought to answer in this section of the article was whether the use of fluoride-releasing pit and fissure sealants, bonding orthodontic agents or restorative materials, in comparison to a non-fluoride releasing material, reduces caries incidence in children or adults.
In order to gather the best, currently available evidence on the clinical effects of fluoride-releasing materials on caries control, we searched for published systematic reviews of randomized controlled trials (RCTs). The search was performed on the Cochrane Database of Systematic Reviews and on Medline via Pubmed, using the tool Clinical Queries and the option Systematic Reviews. Searches for published RCTs were also performed in order to seek more recent studies published after the date of the end of the search performed in the systematic reviews that we had identified and read. The search for RCTs was performed on Medline via Pubmed using the tool Clinical Queries and the clinical study category Therapy within the narrowed scope. We used different combinations of Mesh terms and free vocabulary: dental caries [mh], tooth demineralization [mh], orthodontics [mh], dental atraumatic restorative material [mh], fluorides [mh], dental materials [mh], pit and fissure sealants [mh], composite resins [mh], dental cements [mh], glass-ionomer cements [mh], compomers [mh], resin-modified glass-ionomer, giomer, nano-ionomer, and dental sealants. Commentaries on systematic reviews and RCTs were also searched for in the Database of Abstracts of Reviews of Effects (DARE) and in the secondary publications Evidence-based Dentistry and Journal of Evidence-based Dental Practice. All searches were performed in May 2015.
Cochrane systematic reviews are considered the highest standard in evidence-based health care and most of the Cochrane Oral Health Group reviews focus on the prevention of dental caries . Thus, higher priority was given to the results of Cochrane reviews whenever they could provide information on topics that were relevant to our research question.
4.1
Pit and fissure sealants
Sealing pit and fissures of occlusal surfaces with a thin layer of resin was introduced as a caries preventive method in the late 1960s. Resin dental materials used for this purpose evolved from ultraviolet to visible-light cured sealants, with the newest materials having fluoride incorporated into the resin . There is evidence to suggest that resin-based fissure sealants are effective in preventing occlusal caries in permanent molars of children and adolescents when compared to no sealants, but it is still not known whether fluoride-releasing resin-based sealants provide any additional benefit .
The potential of glass-ionomer cements (GIC) to form a chemical bond to enamel thus reducing the importance of maintaining a dry operating field during application, and their ability to release fluoride to provide a possible cariostatic effect, were strong motivations for their use as pit and fissure sealing materials, especially in very young children and in partially-erupted posterior permanent teeth . However, available evidence does not allow a definitive conclusion as to whether glass-ionomer or resin-modified glass-ionomer sealants are equally effective or superior to resin-based dental sealants in preventing occlusal dental caries in permanent or primary teeth .
A recent mapping of systematic reviews undertaken for topics considered clinically relevant for the practice of Pediatric Dentistry concluded that there is high to moderate-quality evidence of a caries preventive effect of fissure sealing with resin-based materials. This study also noted that there is still uncertainty concerning the effectiveness of fissure sealing with other materials . The Cochrane review on dental sealants for caries prevention that was updated and published in 2013 encourages conducting good quality primary studies that compare the caries protective effect of different types of sealants .
A Medline search via Pubmed using the Clinical Query option identified six clinical trials that evaluated the caries preventive effect of different dental sealing materials and were published after the date of the end of the search performed in the Cochrane review. These studies tested and compared the caries preventive effect of high-viscosity glass-ionomer cements, fluoride-releasing resin-based sealants, a flowable composite resin, glass carbomer sealants and an ormocer sealant . Most of them had a split-mouth design and only two found that one material performed better than the other; the results of one study favored a high-viscosity glass-ionomer sealant over an ormocer sealant and those of another study favored a high-viscosity glass-ionomer cement over a glass carbomer sealant. There is still a clear need for more clinical trials on this topic and studies aiming at investigating the possible benefits of local release of fluoride from sealants should not use a split-mouth design because it has been shown that the cariostatic effect of a fluoride-releasing dental sealant can be exerted beyond the pits and fissures of the sealed tooth; for example, on the distal surfaces of second primary molars adjacent to a tooth sealed with GIC .
4.2
Bonding orthodontic materials
The presence of orthodontic appliances increases the number of retentive sites in the dentition and also may cause pain and discomfort . This makes it more difficult for individuals to properly clean their teeth, which increases biofilm accumulation. Thus, orthodontic treatment carries the risk of caries lesion development. Initial enamel lesions may develop around orthodontic appliances as early as six months after bonding .
An alternative to compensate for the increased caries risk would be to enhance fluoride availability using fluoride-releasing orthodontic adhesives or cements. These materials would act as fluoride reservoirs and could potentially increase fluoride levels in saliva, biofilm and dental hard tissues.
One Cochrane systematic review we found aimed to evaluate the effects of fluoride in reducing the incidence of white spot lesions during orthodontic treatment . In the updated version of this review, three studies, among which only one had low risk of bias, met the inclusion criteria but none of them were RCTs testing materials containing fluoride that is released during treatment ( e.g. , fluoride-releasing composite resin-bonding materials, glass-ionomer cements, compomers and resin-modified glass-ionomers for bonding or banding). It should be noted that an earlier version of this review had less strict inclusion criteria and therefore included a higher number of studies. The updated review made substantial changes to the protocol regarding inclusion criteria, which resulted in the exclusion of all studies included previously. For example, quasi-randomized studies were excluded. Also, split-mouth trials were no longer accepted as they may lead to biased treatment efficacy estimates due to carry-across effects . The changes in the inclusion criteria resulted in changes in the conclusions; while the first version concluded that fluoride-releasing bonding materials during orthodontic treatment could reduce the occurrence and severity of white spot lesions, the updated version could not draw any conclusion regarding the effects of such materials due to the absence of studies meeting the inclusion criteria.
Other Cochrane reviews have evaluated the effectiveness of adhesives used to attach bands or bonded molar tubes during fixed appliance treatment on dental caries in the banded or bonded teeth. First permanent molars where bands had been cemented with glass-ionomer developed fewer white spot lesions than those where bands had been cemented with zinc phosphate. However, this evidence is considered weak as it comes from only two trials that did not perform any statistical analysis . Regarding the comparison between adhesives used to attach bonded molar tubes and adhesives used to attach bands, one trial found that participants whose first permanent molars had tubes bonded with light-cured composite developed more white spot lesions compared to patients whose first permanent molars had bands cemented with glass-ionomer, although enamel changes were considered minor as they could not be seen in wet teeth .
One non-Cochrane review concluded that fluoride-releasing bonding materials appear to have no significant effect on the prevention of white spot lesions development and another revealed weak evidence that glass-ionomer cements might be more effective than composite resins in preventing white spot lesions development in fixed orthodontic patients .
Finally, a search for recent RCTs in Medline via Pubmed failed to find any parallel RCT addressing the question of interest. Taken together, the evidence accrued from the systematic reviews described above makes it impossible to provide any evidence-based recommendation regarding the effectiveness of using fluoride-releasing material to bond or cement orthodontic appliances on white lesion spots development. There is a need to design high quality clinical trials on this subject. Researchers should prioritize RCTs with a parallel design and use standardized methods of caries diagnosis, including the assessment of white spot lesions as well as cavitated lesions at both enamel and dentin levels. Also, studies should cover longer follow-up periods in order to assess the impact of white spot lesions on patients’ esthetic perception and satisfaction.
4.3
Restorative materials
Fluoride-releasing restorative materials are seen as very attractive to the clinician because they could serve a double purpose; besides repairing carious or defective teeth, they could help prevent the development of new carious lesions. However, very few systematic reviews and clinical trials have considered the anti-caries effect of these dental materials as their outcome measure.
In 2006, Wiegand et al. reviewed existing evidence on the cariostatic effect of fluoride-releasing dental restoratives. They found few clinical longitudinal studies with an observation period of three years or more that evaluated the influence of fluoride-releasing dental materials on dental caries development and noted that these studies showed conflicting results as to whether or not these materials contribute to reduction of the risk of developing new dental caries lesions.
A Cochrane review compared survival and caries development for restorative materials used to treat dental caries lesions in children’s primary dentition. Of the three studies included, two compared a fluoride-releasing material to a non-fluoride releasing material. One split-mouth study compared a resin-modified glass-ionomer with an amalgam over a 36-month period, and another tested a compomer versus an amalgam. Only the 12-month data of the former and the 24-month data of the latter were useable. At 12 months no caries lesion was recorded and at 24 months there was no difference between compomer and amalgam restorations in terms of secondary caries.
A systematic review published in 2009 compared the absence of caries lesions at the margins of glass-ionomer cement (GIC) and amalgam restorations in primary and permanent posterior teeth. A meta-analysis combining the results of a parallel and a split-mouth study with a follow-up of 6 years showed an odds ratio (OR) of 2.64 (95% CI 1.39, 5.03) in favor of single-surface GIC restorations in permanent teeth. No other statistically significant differences were found. This systematic review has been updated; five new trials were included and one trial that was accepted in the original review was excluded . All of the included trials appear to be subjected to the risk of selection and detection/performance bias. Although new estimates of effect were produced, the original conclusions did not change and it was found that the margins of single-surface GIC restorations in permanent teeth had a 65% lower chance of developing carious lesions after 6 years than did similar teeth restored with amalgam (RR 0.35; 95% CI 0.19, 0.65).
Another systematic review sought to verify whether resin-modified glass-ionomer cements (RM-GIC) offer a more significant caries-preventive effect than composite resins (CR) . Four studies addressing dental restorations in children or in adult patients who had received radiation therapy to the head and neck were included. There was substantial heterogeneity among the studies and no meta-analysis was performed. Most of the comparisons made provided no statistically significant differences in terms of caries protection between the two materials. Moreover, all studies presented methodological problems that could compromise their internal validity. Therefore, no conclusion regarding the review question could be drawn.
The search for parallel RCTs in Medline via Pubmed identified one study that investigated the longevity of amalgam and compomer restorations in primary molars of 6–10 year old children . Placement of all restorations was preceded by the application of a bonding agent with a fluoride-release characteristic and children in the two treatment groups also received fluoride-releasing dental sealants. After a mean follow-up period of 1.4 years, an exploratory sub-analysis of the data showed that the frequency of restoration replacement due to caries in posterior primary teeth was significantly higher in teeth restored with compomer (3.0%) than in teeth restored with amalgam (0.5%). Additional analysis of data from the same trial assessed whether the occurrence of new caries on teeth other than the tooth where the restoration was placed varied by type of restorative material over a 5-year follow-up period. A very small, but statistically significant effect of dental material on new caries formed in different teeth was found in favor of amalgam restorations; caries lesions on other teeth appeared sooner after the placement of compomer than after the placement of amalgam. In contrast, among restorations with a full 5 years of follow-up, the mean number of new caries lesions on other teeth or on other surfaces of the same tooth was slightly lower in children who received compomer restorations than in children who received amalgam restorations. Overall, it was not possible to conclude whether one material was more successful than the other in preventing dental caries.
Glass-ionomer cements (GIC) have also been used in the atraumatic restorative treatment (ART). This technique, originally designed for non-dental settings, has now been included in dental curriculums and is used by private practitioners .
The outcome measure in all systematic reviews we identified comparing ART with conventional restorations was restoration survival, that is, tooth or restoration fracture or loss. One of these reviews also assessed the caries-preventive effect of ART sealants, but not of ART restorations . Survival rates of ART and conventional restorations appear to be similar . However, these similar survival rates do not answer the question whether the use of a fluoride-releasing material reduces the risk of new caries lesions. Most studies compared ART to amalgam restorations and these differ not only in the restorative material employed, but also in the amount of carious tissue removed, the instruments used to remove it and the preservation of remaining tooth structure.
A recent parallel RCT comparing amalgam restorations and ART with a high-viscosity GIC in primary molars showed that few restorations (5.4%) failed due to dentin caries alongside the restoration. After three years, both materials showed higher survival rates when used in single-surface restorations and no significant difference was found between the cumulative survival rates of amalgam and ART restorations. It should be noted that the high-viscosity GIC was used with a higher than usual powder-to-liquid ratio .
Besides the lack of assessment of caries as an outcome measure, the systematic reviews mentioned above have other limitations: the first studies of ART used low or medium-viscosity GIC, whereas more recent ones have used high-viscosity GIC, which has been the type of GIC required with ART since the mid-1990s; there are few long-term studies available; most studies focused on primary teeth, which have a shorter lifespan; there is a high risk of bias in the included studies, especially due to the inclusion of uncontrolled longitudinal clinical studies or RCTs with unclear randomized sequence allocation and/or allocation concealment; and publication bias cannot be ruled out due to language and database searching restrictions.
These shortcomings may be addressed in a planned Cochrane review that aims to assess the effects of ART for the treatment of decayed primary and permanent teeth in children and adults. In the published protocol, the authors stated that non-randomized studies will be excluded; other outcomes, in addition to restoration failure, such as caries, will be considered; all included trials will be evaluated regarding the potential of bias; and a comprehensive search will be performed .
In conclusion, it is still not known whether fluoride-releasing restoratives reduce caries risk in comparison to non fluoride-releasing restoratives and there is only weak evidence that GIC confers greater caries protection than amalgam in single-surface restorations in permanent teeth. It seems that there is an urgent need for well-designed, randomized controlled trials to investigate the anti-caries benefit of fluoride-releasing filling materials. In order to provide results that can be used to positively influence clinical practice, future trials should adhere to the CONSORT guidelines , adopt a parallel group design and report on outcomes that are important to both clinicians and patients.
In summary ( Table 2 ), if fluoride releasing materials are clinically effective on caries control is still a dogma but based on large numbers of in vitro and in situ studies made, they should work; this will be discussed in the next section of this paper.
