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Fluoride-releasing materials for orthodontic appliances

Glasspoole, E.A.

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Citation for published version (APA):Glasspoole, E. A. (2001). Fluoride-releasing materials for orthodontic appliances.

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Download date: 11 Dec 2020

CHAPTERR 1

Generall Introductio n

Problemss associated with orthodontic appliances

AA persistent concern associated with the widespread use of orthodontic appliances iss the accumulation of plaque and the subsequent problems related to this plaque accumulation.. The presence of orthodontic appliances, such as bands, brackets andd arch wires, often makes toothbrushing and cleaning difficult so plaque is moree likely to develop and accumulate around these appliances while they are in place.. Several overviews have discussed this issue [1-4]. It has been shown that similarr amounts of plaque accumulate around bands or brackets, however, with brackets,, the plaque accumulation tends to surround the brackets, while the bandedd teeth accumulate plaque more at the gingival margins [5]. Gwinnett and Ceenn [6,7] have demonstrated extensive patterns of new plaque accumulation associatedd with bonded orthodontic brackets. Plaque can result in detrimental effectss on periodontal health [3] and cause decalcification of tooth enamel [1,2,4]. Increasess in organisms associated with caries development have been found both inn plaque [8-10] and saliva [11,12] of patients after the placement of orthodontic appliances. .

Demineralizationn around orthodontic appliances can result in white spot lesions withinn one month after placing the appliances [13-15], and can persist for years followingg removal [16]. Factors such as a topical fluoride regime or patient oral hygienee may affect the incidence of white spot lesions during orthodontic treatmentt [2], In one study, where patients were not on a preventive fluoride programm other than routine use of fluoride toothpaste, 49.6% of patients and 10.8%% of teeth had white spot lesions after orthodontic bracket removal [17]. In a controll group of patients, examined prior to placement of orthodontic appliances, 24%% of patients and 3.6% of teeth had at least one white spot lesion in a location thatt might be similar to those that develop during orthodontic treatment. In anotherr study, where orthodontic patients were examined more than five years afterr their appliances were removed, a significant increase in the teeth with white spott lesions was found compared to the untreated control patients [16]. This

7 7

occurredd even though the patients had been instructed to use both a fluoride toothpastee and a 0.05% NaF rinse on a daily basis. The fluoride content in the drinkingg water was low, < 0.1 ppm. Of the maxillary teeth that had brackets, the overalll percentage with white spot lesions was 17.9%, while the same teeth in the controll group had only 4% with white spot lesions. In the mandibular teeth, the comparablee percentages were 20 and 4.1. This represents a 3 to 4-fold increase in whitee spot lesions for patients having had orthodontic brackets. For the teeth that weree most susceptible to development of white spot lesions, such as the maxillary laterall incisor or mandibular canine, there was closer to a 10-fold increase in whitee spot lesions. In the study by Gorelick, et al. [17], these same two teeth, the maxillaryy lateral incisor and the mandibular canine, had the greatest frequency of whitee spot formation at 23.0 and 18.0% respectively, while for the control group, thesee two teeth developed white spot lesions at the rate of 7.0 and 1.0% respectively.. These studies indicate that white spot lesions are a significant problemm associated with orthodontic treatment.

Preventivee measures

RemovalRemoval of plaque and prevention of plaque growth Sincee enamel demineralization that leads to white spot lesions is related to plaque accumulation,, the mechanical removal of plaque through toothbrushing and flossingflossing is of utmost importance. Meticulous oral hygiene has been shown to reducee the incidence of white spot lesions and has been discussed in several reviewss [1,2,4]. However, these authors point out that it is difficult to establish andd maintain such an oral hygiene regimen, due not only to the physical limitationss imposed by the orthodontic appliances, but also to patient compliance.

Thee use of antimicrobial agents to control plaque growth is another method for controllingg demineralization as well as periodontal related problems. The agent thatt has been most often investigated is chlorhexidine [2,3]. It has been shown effectivee against Streptococcus mutans, which is associated with caries, but not againstt lactobacilli which are also implicated in the caries process [12]; S. mutans wil ll however reestablish in a matter of weeks if not retreated. Compliance can alsoo be a problem given the objectionable taste and the brownish stain that developss upon use.

8 8

Sealants Sealants AA method for prevention of enamel demineralization that doesn't rely on patient compliancee is that of sealing the tooth enamel surrounding the orthodontic appliancess where plaque tends to accumulate. Several in vitro studies showed that chemicallyy cured unfilled resins were not able to provide complete sealing [18-20].. Failure was attributed to oxygen inhibition of polymerization of the resin film.. These studies were only interested in curing a film on the enamel and did nott investigate the effect on demineralization of enamel. However, in a clinical trial,, a viscous chemically cured sealant placed around the orthodontic brackets reducedd the number of teeth having decalcification by 38.7% compared to unsealedd control teeth [21]. Presumably this was due to the thick film being more resistantt to oxygen inhibition, since a non-viscous light-cured resin showed only a 4%% improvement over an unsealed control.

Otherr in vitro studies have shown that complete cured films can be formed on enamell surfaces with UV and visible light-cured systems, and demineralization wass eliminated except in instances where minor defects existed in the cured film [22-24].. However, a recent clinical study using a fluoride-releasing light-cured resinn sealant applied around brackets did not reduce the incidence of demineralizationn compared to an unsealed control [25]. The authors suggest that difficultyy in applying the viscous resin and the near proximity of the brackets to thee gingival tissue could have resulted in contamination and inadequate attachment off the sealant.

Twoo other in vitro studies using UV light-cured resins also showed some degree of protectionn from demineralization by subsurface resin tags that remained in the etchedd enamel after removal of the cured outer surface film [26,27]. In these studies,, the presence of a cured outer film, prior to its removal, assured good polymerizationn of the subsurface resin tags.

Fluoride Fluoride Thee caries preventive effect of fluoride in dentistry is well recognized and has beenn the subject of several recent reviews [28-31]. It has been pointed out that fluoridee inhibits demineralization of the enamel crystallites and enhances remineralizationn of the surface of the crystallites with a more stable fluoride-

9 9

substitutedd apatite layer. In vitro studies have suggested that relatively low levels off fluoride in saliva and plaque fluids are needed to provide this benefit [32-34]. Initiall salivary and plaque concentrations can be elevated substantially by the use off a fluoride-containing dentifrice or rinse, but salivary clearance reduces the concentrationn to near baseline levels after about one hour [35-37]. However, regularr use of a fluoride-containing dentifrice [37] or a fluoride-containing rinse [38]] results in an equilibrium level of salivary and plaque fluoride that is elevated comparedd to baseline by an amount directly related to the concentration of fluoridee in the dentifrice or rinse. In a 3-year clinical trial, plaque fluoride was increasedd in relation to the concentration of fluoride in the dentifrices used, and theree was an inverse relationship between these plaque fluoride levels and 3-year cariess increments [39]. In another study, initially caries-free children were examinedd over a period of 4 years and children with high salivary fluoride levels weree more frequently caries-free [40].

Despitee the ability of fluoride-containing dentifrices to reduce caries occurrence, theirr use has been found to be insufficient to prevent lesion formation surrounding orthodonticc brackets in a 1-month duration in vivo study [14,41]. In one of these studiess [14], the addition of a 0.05% NaF rinse eliminated lesion formation, but it shouldd be noted that compliance with the use of the NaF rinse was very high. Two clinicall studies also found a reduction in the incidence of white spot lesions associatedd with the use of a 0.05% NaF rinse, particularly if compliance was good [42,43].. In both studies, there was a clear relationship between white spot lesion formationn and compliance with the recommended daily use of the fluoride rinse. However,, compliance in these studies was generally poor with only about 13-27% off patients having excellent compliance.

Fluoride-releasingg bracket materials

ModelModel studies Bondingg of orthodontic brackets with fluoride releasing materials, that can provide fluoridee at the site, has appeal given the problems cited above in regard to compliancee with the use of topical fluoride regimens. The critical factor for such aa material is that it delivers adequate fluoride to the surrounding enamel to prevent demineralizationn throughout the course of treatment. The caries preventive effects

10 0

off fluoride releasing materials have been the subject of several reviews [44-46]. Twoo in vitro studies looked at the ability of fluoride releasing materials to prevent demineralizationn of enamel adjacent to brackets bonded with these materials in an acidicc challenge [47,48]. In both studies, the fluoride releasing materials protectedd the enamel from demineralization in a zone next to the bracket, the widthh being dependent on the level of fluoride release. A composite control had demineralizationn up to and undermining the bracket. Two other in vitro studies examinedd the ability of fluoride releasing materials to remineralize an initial lesionn adjacent to either an orthodontic band [49] or bracket [50]. These specimenss were cycled between an acidic medium and an artificial saliva containingg no fluoride. In each of the studies, remineralization was observed with thee fluoride releasing material, while the non-fluoride control material showed lesionn growth. These studies show evidence that this approach could be of value inn orthodontic practice.

Inn a rat model, miniature orthodontic brackets were bonded with either a fluoride releasingg resin or a non-fluoride releasing control resin, and were compared to rats wheree no brackets were bonded [51]. The rats were infected with S. sobrinus 67155 and fed a moderately cariogenic diet and non-fluoridated water. After 54 days,, the rats were sacrificed, the teeth examined and the mean percentage of linguall surfaces affected was determined. In the animals with no brackets, there weree no decalcified surfaces, while the group bonded with fluoride-releasing resin hadd 4.7% of surfaces with decalcification and the non-fluoride resin control group hadd 35.7% showing decalcification.

Ann in vivo model using orthodontic bands compared two glass-ionomer cements andd a cement that did not contain fluoride in a model that was meant to simulate conditionss that would exist under an orthodontic band in areas where cement was missingg [52]. Bands were cemented on premolars leaving a 6 mm wide gap where plaquee could accumulate during the 4-week study. After extraction, the teeth were sectionedd and examined by microradiography. A significant reduction in lesion depthh and mineral loss was found for the enamel surfaces of the two glass-ionomerr cemented groups compared to the group where the non-fluoride cement wass used. In another in vivo study [41], brackets were bonded to premolars with eitherr a resin-based composite or a fluoride releasing resin-based composite.

11 1

Afterr 4 weeks, these teeth were extracted and the enamel was examined by microradiography.. Lesion depth and mineral loss adjacent to the bracket were significantlyy less when the fluoride-releasing resin-based composite was used comparedd to the regular resin-based composite. These models also indicate that thiss approach may be feasible for preventing white spot lesions.

ClinicalClinical studies AA relatively small number of clinical studies have been published in which the effectt of a fluoride releasing bracket adhesive on demineralization during orthodonticc treatment have been studied [53-60]. Table 1 provides a summary of thee results found from these studies. Each of the studies compared a non-fluoride resin-basedd composite control to fluoride releasing materials that included fluoride releasingg resin-based composites (C), compomers (cmpr), conventional glass-ionomerss (GI) and resin-modified glass-ionomers (RMGI). Assessment of decalcificationn was made at debonding after the full course of orthodontic treatmentt and ranged from 13 to 22 months. Where data was available, the incidencee of white spot lesions as a percentage of teeth is also given, and for each off the studies, differences in white spot incidence between test and control materialss are identified as significant (S) or not significant (NS). None of the clinicall studies provided fluoride release data for the test material, however, the fluoridefluoride release for most of these materials has been examined in other studies and iss discussed in the next section. It is assumed that patients in all studies used a fluoridatedfluoridated toothpaste, but this was not specified in all studies and in only 2 of the studiess [55,57] was a fluoride rinse specifically recommended, but no measure of compliancee was provided.

Onlyy 3 studies showed a significant reduction in white spot lesion incidence for thee fluoride releasing materials.

Fluorid ee release

AA number of in vitro studies have examined fluoride release into water for materialss used for orthodontic bracket bonding [61-73]. These studies provided informationn in a number of different units. For purposes of comparison, these dataa were converted to a single unit (ug/cm2) using exact numbers where available,

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orr estimated values taken from release profiles presented in the publication. An arbitraryy time of 7-days was chosen to provide a relative comparison of the amountt of fluoride released by each material, and these data are given in Table 2. Bothh the cumulative amount of fluoride released in 7 days, and the amount releasedd in a 24-hour period at 7 days is provided.

Fromm this data and the previous discussion of clinical study results, it is interesting too note that, in general, for the fluoride releasing materials that release very low levelss of fluoride, there was no evidence of a protective effect from the material.

Thee above studies examined fluoride release into water, but in vivo, the release wouldd be into saliva or plaque fluid. In vitro studies have shown that the amount off fluoride released into an artificial saliva medium is 30 to 40%% of that released intoo water [74,75]. It has been suggested that this reduction is caused by precipitationn of CaF2 onto the surface of the material forming a physical barrier to fluoridee release. This could be a factor in vivo, however, for practical purposes, dataa for release into water is more readily available.

Bondingg brackets to enamel with fluoride-releasing materials

Inn the discussion above, the merits of a fluoride-releasing bracket material were reviewedd in the context of prevention of enamel demineralization. However, it is clearr that the material must provide adequate retention of the brackets to enamel too be a viable material.

Resin-basedd composite materials have been traditional for bonding brackets and therefore,, provide a benchmark forjudging other newer materials. Clinical failure ratess using such materials are typically found to be in the 4 to 10% range [76,77], particularlyy for the incisors through the first premolars, with somewhat higher failuress occurring in the more posterior regions. Laboratory bond strength measurementss are most frequently done by a shear-peel method, where the forces aree applied to the wings of the bracket. Typical bond strengths using this test rangee from 4 to 11 MPa [78,79]. Estimates of the minimum clinically acceptable bondd strength have ranged from about 1.4 to 8 MPa [80-82]. In another study, Mitchelll et al. [83], using a Weibull analysis, have speculated that the maximum

14 4

Tablee 2 FluorideFluoride release into water

Reference e

Foxx [61] (1990) )

Chadwick,, et al [62] (1995) )

Wiltshire,, et al [63] (1995) )

Young,, et al [64] (1996) )

Trimpeneers,, et al [65] (1998) )

Teminn [66] (1988) )

Gillgrass,, et al [67] (1999) )

Schmalz,, et al [68] (1997) )

Aboush,, et al [69] (1998) )

Verbeeck,, et al [70] (1998) )

Monteith,, et al [71] (1999) )

Karantakis,, et al [72] (2000) )

Evrenol,, et al [73] (1999) )

Testt material

Ketacc Cem Direct t

Ketacc Fil Vitrabond d FluorEver r Lightt Bond Rely-a-Bond d Rightt On

FluorEverr OBA

Ketacc Fil

Ketacc Cem Orthon n FluorEver r

FluorEver r

Ketacc Cem

Ketacc Fil Vitremer r Fujii II LC Dyract t

Vitremer r Fujii II LC

_Dy_ract__ _

Ketacc Fil Vitremer r

_J2Yract_ _

Vitremer r Dyractt Ortho

Vitremer r Fujii II LC Dyract t

Fujii Ortho LC Vitremerr Ortho

7-dayy cumulative (ug/cm2) )

125 5 12 2

127 7 163 3 52 2

11.5 5 6.4 4 2.4 4

55 5

64 4

80 0 16 6 80 0

70 0

51 1

NA A NA A NA A NA A

180 0 180 0 NA A

98 8 43 3 16 6

90 0 18 8

190 0 172 2 19 9

25 5 88 8

7-dayy daily rate (ug/cm2/day) )

10 0 0.8 8

NA A NA A NA A NA A NA A NA A

2 2

5 5

6 6 1.2 2 3.2 2

5 5

3.6 6

NA A 8.3 3 NA A 2 2

10 0 10 0 0.8 8

7 7 2.7 7 1.2 2

5.7 7 0.9 9

9 9 10 0 0.8 8

2.2 2 6 6

15 5

loadd that the brackets are subjected to is at the lower end of this range. In a study byy Willems et al [79], 22 commercially available resin-based composite bracket bondingg materials were tested with a single bracket using the shear-peel method, andd the range of bond strengths measured was 4.1 to 9.9 MPa, with a mean and standardd deviation of 6.75 MPa (1.44). The highest value was obtained with the Concisee orthodontic system which has frequently been used as the control materiall in laboratory testing. When this same system was tested in a shear-only test,, where the force was applied to the bracket at the point that it meets the enamell surface and then directed parallel to the surface, the bond strength has beenn shown to be about 29 MPa [84]. This latter method of applying a shear force iss more traditional in measuring bond strengths of materials to enamel. Not only iss the method of testing a factor in determining the bond strength, but for the case off the shear-peel test, the type of bracket used has a strong influence. When 28 differentt brackets were bonded to a metal bar with the Concise orthodontic bondingg system, the shear-peel bond strength ranged from 1.6 to 13.9 MPa [85]. Obviously,, a wide range of bond strengths may be obtained by combining various materialss and various brackets in a shear-peel test, however, one must assume that sincee all of these materials are commercially available and used for bracket bonding,, certain combinations can be successfully used clinically Because of this,, it is not possible to specify a bond strength number that will represent a clinicallyy acceptable material, but only a range of bond strengths that should representt a clinically acceptable material.

Severall studies have looked at fluoride-releasing resin-based composite materials ass bracket bonding materials, and as might be expected, the bracket bonding failuree rate was comparable to the resin-based composite control materials used in thee studies [55-57].

Brackett bond strengths of conventional glass-ionomers to enamel have been reviewedd by Millet and McCabe, who found them to be substantially lower than forr resin-based composite materials [86]. A recent study of 7 glass-ionomer cementss confirmed that result [87]. Millet and McCabe [86] also observed that withh conventional glass-ionomers, the debonding failures typically occur adhesivelyy between the bracket and cement. This is likely due to the relatively low fracturee strength of conventional glass-ionomers. Bond strengths to enamel cited

16 6

inn these two references were usually one-half or less of that found for resin-based composite. .

Severall conventional glass-ionomers have been tested in clinical studies with brackett failure rates being substantially greater than for a resin-based composite controll (Table 3). The lower bond strengths of conventional glass-ionomers comparedd to resin-based composites appear to be reflected in the clinical bracket bondingg failure rate, which is typically greater for the glass-ionomers.

Improvedd physical properties have been found for the newer resin-modified glass-ionomerss compared to conventional glass-ionomers [95,96]. The improved fracturee strength of RMGI's might be expected to result in a higher bond strength thann typically found for conventional glass- ionomers. Table 4 shows measured in vitroo bond strengths to enamel for the most frequently tested RMGI, Fuji Ortho LC,, under various conditions. Seven of the studies listed in the table compared thee bond strength of Fuji Ortho LC to one or more resin-based composite control materials.. There was littl e difference seen between the RMGI and the controls whenn the enamel surfaces were conditioned. It is interesting to note that while the bondd strengths between RMGI and the controls were similar, the enamel conditioningg for the Fuji Ortho LC was quite different, most often using 10% polyacrylicc acid (PAA) conditioner rather than the 37% phosphoric acid (PA) as typicallyy used with the control resin-based composites. Conditioning with PAA or PAA substantially increased the bond strength for the RMGI materials compared to noo enamel pretreatment. It can also be noted that ground enamel resulted in higherr bond strengths than enamel that was pumice cleaned only.

AA small number of clinical studies have examined retention rates of RMGI's comparedd to resin-based composite controls over periods of about one year. Both thee chemical and light-cured versions of the GC Fuji Ortho bonding system were tested,, and the results are shown in Table 5. From these studies it appears that conditioningg results in failure rates that are comparable to the failure rates of the resin-basedd composite controls. Although the results are mixed when the enamel surfacess are not conditioned, there is a tendency toward greater failure rates comparedd to the controls. The fact that laboratory bond strengths are substantially improvedd with conditioning would support this observation.

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Thee bond strength results and the limited clinical results may support the use of RMGII as a replacement for traditional bracket bonding materials. However, it is unfortunatee that the clinical studies for these materials, as well as those done on conventionall glass-ionomers, have not provided much information on their usefulnesss in reducing decalcification adjacent to brackets.

Anotherr category of fluoride-releasing materials that have been considered for orthodonticc bracket bonding are the compomers, which are poly-acid modified resin-composites.. There is littl e information in the literature regarding the use of thesee materials in orthodontics. Four studies have examined bond strength of compomerr to enamel [98,102,113,114], and found bond strengths from 50-80% of thatt found for the resin-based composite controls. Fluoride release of compomers intoo water is considerably lower that for conventional or resin-modified glass-ionomerr materials (Table 2). A clinical study of Dyract Ortho compomer found brackett failure rates comparable to the resin-based composite control [57]. In this studyy there appeared to be less decalcification when using the compomer comparedd to the control.

Aimm and outline of investigations

Thee purpose of this research was to examine aspects of enamel bonding and effectss on enamel demineralization of several types of fluoride-releasing materials, thatt are potential orthodontic bracket cements. Through this research project and aa critical analysis of the existing literature, much of which has been presented in thiss introduction, it is hoped that an understanding of the requirements for materialss used for orthodontic bracket bonding can be reached. These requirementss include adequate bracket retention and sufficient fluoride release to bee effective in reducing demineralization adjacent to orthodontic brackets.

Inn Chapters 2 and 3, the influence of surface pretreatments on bond strength to enamell was investigated for glass-ionomers and compomers. These studies also examinedd mechanism(s) of bonding.

Chapterr 4 details the development of a fluoride-releasing resin with favorable fluoridee release. This resin system was examined for its ability to inhibit

21 1

demineralizationn adjacent to simulated orthodontic brackets, when used as a surfacee sealant. A secondary aim was to address a potential defect in analysis of demineralizationn studies in the literature.

Inn Chapter 5, the basic resin system developed in Chapter 4 was used to make a fluoride-releasingfluoride-releasing resin-based composite for orthodontic bracket bonding. The specificationss for this composite were that it have physical properties comparable too other resin-based composite systems, but with fluoride release approaching glass-ionomers. .

Inn Chapter 6, the ability of the various restorative materials discussed in Chapters 2,, 3 and 5 to reduce demineralization adjacent to simulated brackets was examined.. This study was also designed to determine dose response relationships betweenn levels of fluoride release and effect on demineralization for these various materials. .

22 2

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23 3

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31 1