Effects of periodic fluoride treatment on fluoride ion release from fresh orthodontic adhesives

Effects of periodic fluoride treatment on fluoride ion release from fresh orthodontic adhesives

Journal of Dentistry, 2011-11-01, Volume 39, Issue 11, Pages 788-794, Copyright © 2011 Elsevier Ltd

Abstract

Objective

Periodic fluoride treatment may contribute to the ability of fresh orthodontic adhesives to provide long-term F release. The effects of periodic fluoride treatment on the amount of F release from fresh orthodontic adhesives was investigated.

Methods

F release was measured from a nonfluoride-releasing composite, a fluoride-releasing composite, a polyacid-modified composite (compomer), and two resin-modified glass-ionomer cements (RMGICs) at 1, 2, and 5 days after one of the following treatments: 225 ppm F solution, 900 ppm F solution, acidulated phosphate fluoride gel (APF), fluoridated dentifrice, and deionised water (control). F release was measured in a 5-day cycle, which was repeated 9 consecutive times. The amount of F release for each group was analysed using the repeated measures analysis of variance. Statistical significance was set at a level of α = 0.05.

Results

Periodic fluoride treatment temporarily increased F release in fresh fluoride-releasing orthodontic adhesives, but not in fresh nonfluoride-releasing composite. The order of effective fluoride-release was RMGICs > compomer > fluoride-releasing composite > nonfluoride-releasing composite. The application of APF or 900 ppm F solution was the most effective way to maintain F release from fresh orthodontic adhesives. However, the amount of F release gradually decreased with increasing specimen age.

Conclusion

Given the difficulty of routine use of APF at home, the results of this study show that a combination of RMGICs and high-dose fluoride mouth rinse is the most effective protocol to maintain F release from fresh orthodontic adhesives.

Clinical significance

Most studies have investigated fluoride-uptake abilities using aged materials in which fluoride had been lost for at least 1 month. This study has found that periodic fluoride treatment altered the conventional F release pattern of fresh fluoride-releasing materials and type of fluoride-containing medium plays a more critical role in fluoride recharging of the materials than fluoride concentration. This study will help clinicians to find the most effective fluoride treatment protocol of fresh materials.

Introduction

Enamel demineralisation around bonded orthodontic attachments is a common side-effect of fixed orthodontic treatments, because the presence of fixed orthodontic appliances predisposes to plaque accumulation. Enamel demineralisation has been demonstrated around orthodontic brackets after only 1 month. Furthermore, lesions may still present an aesthetic problem as many as 5 years after a fixed appliance treatment. Therefore, finding methods to reduce enamel demineralisation during and/or after orthodontic treatments is essential.

Fluoride is well-documented as an anticariogenic agent that prevents enamel demineralisation. A variety of mechanisms are involved in the anticariogenic effects of fluoride, including reduction of demineralisation, enhancement of remineralisation, interference in pellicle and plaque formation, and inhibition of microbial growth and metabolism. In this regard, the incorporation of fluoride into orthodontic adhesives has attracted considerable interest as a possible way to prevent enamel demineralisation during orthodontic treatment.

Several fluoride-releasing adhesives have been developed for orthodontic use including glass-ionomer cements, resin modified glass-ionomer cements (RMGICs), polyacid-modified composites (compomers), and composites. Many studies have shown that these materials have specific fluoride releasing abilities. The main advantage of fluoride-releasing orthodontic adhesives is the delivery of F to the bracket-enamel interface, the area at the greatest risk for enamel demineralisation, independent of patient co-operation. However, research has shown that all fluoride-releasing adhesives initially have high amounts of F release, but these amounts decline significantly within the first few days.

Several studies have demonstrated that fluoride-releasing materials can take up fluoride from exogenous sources of fluoride, such as dentifrice, mouth rinse, and fluoride gels in the oral environment. Periodic fluoride treatments may alter the usual fluoride-releasing patterns, which, as mentioned above, consisted of a significant decrease in F release after the initial burst. Such fluoride treatments may therefore contribute to the ability of these materials to inhibit enamel demineralisation throughout orthodontic treatment. In the past, most studies have investigated the fluoride-uptake ability of aged materials in which fluoride had been lost for at least 1 month. Little information is available regarding the re-release of F after periodic fluoride treatment in fresh materials. In addition, the uptake and re-release of F from orthodontic adhesives when used for bonding and banding of orthodontic materials, has received relatively limited attention. The purpose of this study is to assess the effects of periodic topical fluoride treatment on the amounts of F release from fresh orthodontic adhesives. The null hypothesis of this experiment was that periodic fluoride treatment would not affect the amount of F re-release from fresh orthodontic adhesives.

Materials and methods

Specimen preparation

Five light-cured orthodontic adhesives were used: a nonfluoride-releasing composite (Transbond XT, 3M, Monrovia, CA, USA), a fluoride-releasing composite (Light Bond, Reliance Orthodontics, Itasca, IL, USA), a compomer (Transbond Plus, 3M), and two RMGICs (Fuji Ortho LC; GC Corporation, Tokyo, Japan; and Multi-Cure, 3M).

Disc-shaped specimens were prepared using 1.1 mm-thick Teflon templates, each with a hole of 9.0 mm in diameter. The templates were positioned on top of glass slides, and each adhesive was placed into a hole until the material was flush with the top of the template. A second slide was placed on top, firmly pushed down to ensure a flat dorsal surface, and then gently removed. All materials were light-cured for 20 s from the top and 20 s from the bottom using an Ortholux LED (3M) according to the manufacturers’ instructions.

Fluoride treatment

Each disc was carefully rinsed with deionised water (DW) and treated with a different fluoride protocol; an aqueous solution of 225 ppm F , an aqueous solution of 900 ppm F , a fluoride gel, a fluoride-containing dentifrice, or DW as control. The reason for using aqueous solutions of both 225 ppm and 900 ppm F is that commercially available fluoride-containing mouth rinses contain 0.05% NaF (for daily rinse) or 0.2% NaF (weekly rinse), which is equal to the concentrations of 225 ppm and 900 ppm F , respectively. Discs in the 225 ppm and 900 ppm solution groups were immersed in 10 mL of NaF solutions containing 225 ppm and 900 ppm F , respectively, for 1 min. Discs in the fluoride gel group were immersed for 1 min in 10 mL of a foaming solution of acidulated phosphate fluoride gel (APF) (Oral-B, Belmont, CA) containing 1.23 wt.% NaF. Disks in the dentifrice group were brushed for 1 min with a slurry prepared from 1.0 g of a fluoridated dentifrice containing 1000 ppm F (Regular paste, Crest, Cincinnati, OH). Discs in the DW group were immersed in 10 mL DW for 1 min. After each treatment, the discs were carefully rinsed with DW, blotted dry, placed in a plastic vial with 10 mL DW, and stored in a shaking incubator at 37 °C.

Measurement of F

The discs were re-immersed in 10 mL of fresh DW at 1, 2 and 5 days after fluoride treatment. The collected 10 mL solutions were used for the following F measurements.

The ionic fluoride concentration of each solution was analysed using an Orion combination fluoride ion-selective electrode (Orion Research Electrode No 9069BN, Orion Research, Beverly, MA, USA) attached to an ion analyzer (Orion Research Expandable Ion Analyzer EA 940, Orion Research). One millilitre of the test solution was added to 1 mL of low-level total ionic strength adjustment buffer in a microsample dish, and the electrode and dish were then covered with cling film to minimise evaporation. The solution was stirred during the measurement procedure and the electrode was allowed to stabilise for 5 min before recording the reading in millivolts. The electrode membrane was gently rinsed with DW between measurements. A calibration curve was generated with the aid of a computer software program prior to each measuring session using standard fluoride solutions of various ionic concentrations (0.1–1000 ppm F ). Using this curve, fluoride measurements in mV were converted into corresponding fluoride concentrations in parts per million (ppm), which were further converted to ppm/cm 2 by dividing by the disc’s surface area.

Periodic fluoridation

Fluoride treatments were carried out in a 5-day cycle, which was repeated 9 consecutive times for 45 days. Measurement of F and changing of the solutions were performed at 1, 2, and 5 days after each fluoride treatment under the same procedure described above. Cumulative fluoride levels during each 5-day cycle were calculated to provide values in ppm/cm 2 /day ( Table 1 ).

Table 1
Cumulative amounts of fluoride ion release (ppm/cm 2 /day) from fresh orthodontic adhesives after various topical fluoride treatment.
1st fluoridation Mean ± SD 2nd fluoridation Mean ± SD 3rd fluoridation Mean ± SD 4th fluoridation Mean ± SD 5th fluoridation Mean ± SD 6th fluoridation Mean ± SD 7th fluoridation Mean ± SD 8th fluoridation Mean ± SD 9th fluoridation Mean ± SD
Deionised water
Transbond XT 0.01 ± 0.00 0.01 ± 0.00 0.02 ± 0.00 0.01 ± 0.00 0.01 ± 0.00 0.01 ± 0.00 0.01 ± 0.00 0.01 ± 0.00 0.01 ± 0.00
Light Bond 1.01 ± 0.10 0.15 ± 0.01 0.09 ± 0.02 0.07 ± 0.02 0.06 ± 0.01 0.03 ± 0.01 0.02 ± 0.00 0.02 ± 0.01 0.02 ± 0.00
Transbond Plus 1.18 ± 0.08 0.28 ± 0.04 0.21 ± 0.03 0.17 ± 0.03 0.13 ± 0.01 0.05 ± 0.01 0.01 ± 0.00 0.04 ± 0.03 0.04 ± 0.00
Fuji Ortho LC 4.21 ± 0.68 1.35 ± 0.20 0.74 ± 0.10 0.55 ± 0.10 0.42 ± 0.07 0.12 ± 0.01 0.04 ± 0.01 0.06 ± 0.01 0.05 ± 0.01
Multi-Cure 4.30 ± 0.68 1.27 ± 0.13 0.81 ± 0.11 0.57 ± 0.10 0.42 ± 0.03 0.16 ± 0.01 0.09 ± 0.02 0.06 ± 0.01 0.03 ± 0.00
Fluoridated dentifrice
Transbond XT 0.01 ± 0.00 0.02 ± 0.00 0.01 ± 0.00 0.01 ± 0.00 0.01 ± 0.00 0.01 ± 0.00 0.01 ± 0.00 0.02 ± 0.00 0.02 ± 0.00
Light Bond 0.57 ± 0.24 0.15 ± 0.05 0.09 ± 0.03 0.06 ± 0.01 0.08 ± 0.02 0.08 ± 0.02 0.02 ± 0.00 0.03 ± 0.01 0.02 ± 0.00
Transbond Plus 1.25 ± 0.24 0.53 ± 0.10 0.42 ± 0.08 0.28 ± 0.03 0.28 ± 0.03 0.17 ± 0.01 0.12 ± 0.02 0.19 ± 0.04 0.16 ± 0.02
Fuji Ortho LC 3.00 ± 0.17 1.29 ± 0.11 1.09 ± 0.05 0.86 ± 0.18 0.80 ± 0.11 0.54 ± 0.07 0.32 ± 0.03 0.51 ± 0.03 0.44 ± 0.01
Multi-Cure 4.40 ± 0.78 1.52 ± 0.28 1.11 ± 0.11 0.98 ± 0.14 0.83 ± 0.07 0.56 ± 0.08 0.39 ± 0.04 0.53 ± 0.06 0.35 ± 0.03
225 ppm F solution
Transbond XT 0.02 ± 0.00 0.02 ± 0.00 0.03 ± 0.00 0.02 ± 0.00 0.01 ± 0.00 0.01 ± 0.00 0.01 ± 0.00 0.01 ± 0.00 0.01 ± 0.00
Light Bond 0.60 ± 0.29 0.14 ± 0.04 0.13 ± 0.04 0.07 ± 0.03 0.07 ± 0.01 0.04 ± 0.01 0.01 ± 0.00 0.04 ± 0.01 0.02 ± 0.00
Transbond Plus 1.20 ± 0.29 0.56 ± 0.14 0.55 ± 0.14 0.33 ± 0.09 0.30 ± 0.05 0.19 ± 0.02 0.12 ± 0.03 0.18 ± 0.03 0.13 ± 0.02
Fuji Ortho LC 3.41 ± 0.51 1.34 ± 0.23 1.39 ± 0.16 1.09 ± 0.16 0.99 ± 0.07 0.53 ± 0.05 0.46 ± 0.05 0.53 ± 0.16 0.40 ± 0.07
Multi-Cure 4.35 ± 1.32 1.47 ± 0.34 1.50 ± 0.30 1.13 ± 0.28 0.86 ± 0.14 0.60 ± 0.06 0.42 ± 0.04 0.47 ± 0.06 0.33 ± 0.04
900 ppm F solution
Transbond XT 0.01 ± 0.00 0.01 ± 0.00 0.02 ± 0.00 0.02 ± 0.00 0.01 ± 0.00 0.01 ± 0.00 0.01 ± 0.00 0.01 ± 0.00 0.01 ± 0.00
Light Bond 0.57 ± 0.39 0.14 ± 0.04 0.11 ± 0.03 0.07 ± 0.02 0.06 ± 0.02 0.05 ± 0.01 0.03 ± 0.00 0.03 ± 0.01 0.03 ± 0.01
Transbond Plus 1.00 ± 0.12 0.75 ± 0.10 0.75 ± 0.12 0.55 ± 0.08 0.44 ± 0.07 0.33 ± 0.03 0.05 ± 0.01 0.21 ± 0.03 0.22 ± 0.01
Fuji Ortho LC 3.13 ± 0.30 2.28 ± 0.27 2.37 ± 0.24 1.99 ± 0.16 1.66 ± 0.21 0.94 ± 0.25 0.75 ± 0.06 0.66 ± 0.15 0.59 ± 0.13
Multi-Cure 3.22 ± 0.17 2.22 ± 0.38 2.39 ± 0.27 2.07 ± 0.19 1.50 ± 0.14 1.35 ± 0.33 0.99 ± 0.12 0.77 ± 0.14 0.65 ± 0.14
Acid phosphate fluoride gel
Transbond XT 0.02 ± 0.00 0.02 ± 0.00 0.01 ± 0.00 0.02 ± 0.00 0.02 ± 0.00 0.01 ± 0.00 0.01 ± 0.00 0.02 ± 0.00 0.02 ± 0.00
Light Bond 0.79 ± 0.24 0.17 ± 0.05 0.13 ± 0.03 0.07 ± 0.01 0.07 ± 0.01 0.06 ± 0.01 0.03 ± 0.00 0.05 ± 0.01 0.03 ± 0.00
Transbond Plus 1.12 ± 0.19 0.83 ± 0.17 0.79 ± 0.11 0.74 ± 0.11 0.85 ± 0.11 0.53 ± 0.12 0.47 ± 0.08 0.57 ± 0.06 0.57 ± 0.05
Fuji Ortho LC 3.35 ± 0.16 1.94 ± 0.16 2.02 ± 0.21 1.91 ± 0.21 1.83 ± 0.17 1.30 ± 0.17 0.99 ± 0.10 0.82 ± 0.06 0.89 ± 0.05
Multi-Cure 4.17 ± 0.36 1.83 ± 0.18 1.72 ± 0.25 1.67 ± 0.21 1.73 ± 0.29 1.40 ± 0.29 1.18 ± 0.11 0.88 ± 0.16 0.98 ± 0.09
SD, standard deviation.

Statistical analysis

F release experiments were performed on 5 independent specimen groups. The data were evaluated using the language R. The cumulative amount of F release for each group during the observation were analysed using the repeated measures analysis of variance and multiple comparisons were performed by t tests using the Bonferroni correction at a significance level of α = 0.05.

Results

Figs. 1–5 show the F release pattern from fresh orthodontic adhesives after a different fluoride treatment. Without fluoridation, all fluoride-releasing adhesives initially showed higher amounts of F release, which subsequently declined to lower, but stable levels ( Fig. 1 ). However, periodic fluoride treatment temporarily increased F release from fresh fluoride-releasing orthodontic adhesives after the second treatment ( Figs. 2–5 ). The increase in F release lasted for 2 days after exposure, and the amounts of F release were decreased after 2 days.

Fluoride ion release from various orthodontic adhesives after treatment with deionised water.
Fig. 1
Fluoride ion release from various orthodontic adhesives after treatment with deionised water.
Fluoride ion release from various orthodontic adhesives after treatment with fluoridated dentifrice.
Fig. 2
Fluoride ion release from various orthodontic adhesives after treatment with fluoridated dentifrice.
Fluoride ion release from various orthodontic adhesives after treatment with 225 ppm F − solution.
Fig. 3
Fluoride ion release from various orthodontic adhesives after treatment with 225 ppm F solution.
Fluoride ion release from various orthodontic adhesives after treatment with 900 ppm F − solution.
Fig. 4
Fluoride ion release from various orthodontic adhesives after treatment with 900 ppm F solution.
Fluoride ion release from various orthodontic adhesives after treatment with acidulated phosphate fluoride gel.
Fig. 5
Fluoride ion release from various orthodontic adhesives after treatment with acidulated phosphate fluoride gel.

The temporal increase in F release after fluoride treatment varied amongst adhesives. Periodic fluoride treatment significantly increased F release in the RMGICs and compomer, but not in the nonfluoride-releasing composite ( Figs. 2–5 ). Although fluoride-releasing composite showed slightly higher F release than nonfluoride-releasing composite after fluoride treatment, F release from fluoride-releasing composite was significantly lower than those from RMGICs and compomer ( P < .05).

The temporal increase in F release after fluoride treatment influenced the cumulative amounts of F release of fresh materials ( Table 1 ). The RMGICs showed the largest cumulative amounts of F release (RMGICs > compomer > fluoride-releasing composite > nonfluoride-releasing adhesive, P < .05). There was no significant difference in the cumulative amounts of F release between the two RMGICs. The nonfluoride-releasing composite released very little amount of F during the entire experimental period, even with periodic fluoridation ( Figs. 2–5 and Table 1 ).

The cumulative amounts of F release significantly differed according to fluoride treatment method. The application of APF or 900 ppm F solution maintained the highest amount of F release from the RMGICs and compomer during the experimental period ( Table 1 ). Both fluoridated dentifrice and 225 ppm F solution also increased F release from the RMGICs and compomer, but at lower levels (900 ppm F solution and APF > 225 ppm F solution and dentifrice > DW).

After periodic fluoride treatments, F release from the fluoride-releasing adhesives gradually decreased with increasing specimen age ( Figs. 2–5 ). The most significant decrease in F release occurred after the first of 9 fluoride treatments. Furthermore, the decrease in F release over time differed between fluoridation methods. The decreases in F release from the RMGICs and compomer were significantly delayed in the APF treatment group ( Fig. 3 ) compared to the other fluoride treatment groups ( Figs. 3 and 4 ). The changes in F release according to specimen age and fluoridation methods is consistent with the cumulative changes in F release ( Table 1 ).

Discussion

The hypothesis of the present study was rejected because periodic fluoride treatment can alter the conventional F release pattern of fresh fluoride-releasing orthodontic adhesives. Without periodic fluoridation, all fluoride-releasing adhesives exhibited an initial short-term increase that stabilised after approximately 30 days ( Fig. 1 ), which is consistent with the classic profile of F release demonstrated by previous studies.

Generally, F release from fluoride-releasing orthodontic adhesives was increased after periodic exposure to various topical fluorides, which rapidly returned to near pre-exposure levels within 3 days ( Figs. 2–5 ). However, the F level even after the last fluoride treatment did not subsequently return to the fluoride level present after immersion in DW, particularly in the case of RMGICs. The temporary increased F release after topical fluoride treatment has been reported to be associated with washout of fluoride ions that are retained on the surface or in the pores of the materials during refluoridation. These findings indicate that fresh fluoride-releasing orthodontic adhesives have an ability to be recharged with fluorides introduced from exogenous fluoride source.

However, there was no significant increase in F release immediately after the first exposure to topical fluoride treatments compared to F release after the first immersion in DW ( Figs. 1–5 ). This is confirmed by the cumulative amount of F release from the adhesives during the first fluoride treatment phase ( Table 1 ). This may be due to the fact that in the initial stage, the amount of inherent fluoride in the fluoride-releasing adhesives is higher than the amounts of fluoride supplied from exogenous fluoride source. The inherent fluoride may be rapidly exhausted after the initial profound F release, which may allow fluoride-releasing adhesives to uptake fluoride from the exogenous fluoride source after the next fluoride treatment.

After the second fluoride treatment, periodic exposure to various topical fluorides increased cumulative F release from fresh fluoride-releasing adhesives, specifically RMGICs and compomer ( Table 1 ). However, periodic fluoride treatments did not significantly promote additional F release from nonfluoride-releasing composites. In addition, exposure to exogenous fluoride temporarily increased by a tiny amount of F release from fluoride-releasing composites ( Table 1 ). This may be due to relative impermeability of the composites compared to the compomer or RMGICs, which may prevent the absorption of F deep into their substructures in composites.

These results indicate that fluoride uptake capacity varies widely between different classes of fresh fluoride-releasing adhesives. In this study, we found that RMGICs are a significantly more effective fluoride reservoir than composite-based materials ( Table 1 ). The compomer’s intermediate fluoride uptake capacity can be explained by its composition, which can be considered intermediate between RMGICs and composites. Our data suggest that the material with higher F release has a higher fluoride uptake capacity. It seems that a material’s fluoride uptake may be limited by its inherent fluoride-releasing capacity, as the sites occupied by inherent fluoride are fixed and limitative within the materials.

When comparing the amount of F release according to fluoridation method, fresh RMGICs and compomer are more easily recharged by APF or 900 ppm F solution than by a 225 ppm F solution or fluoridated dentifrice ( Figs. 2–5 ). It seems that solutions may easily infiltrate the adhesive’s substructures compared to gels and slurries, given that APF, 900 ppm F solution, fluoridated dentifrice, and 225 ppm F solution contain 1.23%, 0.2%, 0.21%, and 0.05% NaF, respectively. These results suggested that type of fluoride-containing medium plays a more critical role in fluoride recharging than F concentration.

However, even APF and 900 ppm F solution did not significantly increase fluoride uptake of nonfluoride-releasing or fluoride-releasing composites. This indicates that topical fluoride treatments are only effective in maintaining F release from fresh glass-ionomer based adhesives. As stated above, this may be associated with relative impermeability of the composites.

Our data showed a trend towards decreasing F release of RMGIs and compomer following fluoride treatment with increasing specimen age ( Table 1 and Figs. 2–5 ). This trend suggests that the specimens’ uptake potential diminished over time with increased exposure to supplemental fluoride, which has several possible explanations. First, this reduction may reflect reduced uptake capacity due to age-related material degradation. Second, it may be due to concomitant binding of F to the adhesive matrix, which then could be subsequently released. Third, it may be due to erosion and disintegration of adhesive components occurring during the fluoride treatment itself. Nevertheless, APF treatment delayed the decrease in F release from RMGICs and compomer, compared to other fluoride treatments. This may be associated with the fact that fluoride-releasing adhesives can incur significant damage in an acidic environment caused by APF, resulting in relatively continuous F release from the adhesive matrix.

It is important to maintain high levels of F release in orthodontic adhesives in order to prevent enamel demineralisation during orthodontic treatment. A previous study showed that demineralisation is inhibited in enamel adjacent to an adhesive releasing as little as 0.65–1.3 ppm F /cm 2 /day. In our study, although F release gradually decreased over time, we found that fresh RMGICs released a clinically meaningful amount of F for over 40 days when periodically recharged with APF or 900 ppm F solution ( Table 1 ). In addition, it has been reported that aged RMGICs can also release F at an amount over the critical level when recharged with a high-dose fluoride solution or APF. However, care must be exercised with frequent APF application because it can cause erosive damage of both RMGICs and compomers, which may weaken mechanical bond strength of those materials.

Enamel demineralisation has been demonstrated around orthodontic brackets only 1 month after the placement of fixed appliances. Thus whilst 900 ppm F solution contains a high fluoride concentration and should be used under supervision, our results show that the regular use of high-fluoride mouth rinse with RMGICs maintains the highest amount of F release during the initial critical stage of orthodontic treatment. Our findings can apply most strongly to patients with a high risk of caries, where all preventive measures must be considered.

Although this study provides valuable information on the effective fluoride treatment protocol, further studies are required to apply the results of the present study to the clinical situation, because fluoride uptake and re-release process can be affected by several experimental factors, such as solubility of the materials and the presence of saliva. In addition, composite adhesive should be used with primer to enhance bond strength. Furthermore, only small area of orthodontic adhesives may expose to F around orthodontic brackets in the oral cavity. Further study simulating in vivo clinical condition will help to develop a promising approach for long-term F release from fluoride-releasing dental materials.

Conclusions

This study showed that the periodic application of topical fluorides is not a good option to recharge fresh composite adhesives with fluoride. Instead, the combination of RMGICs with periodic application of mouth rinse containing high-dose fluoride needs to be considered to maintain F release at the bracket-enamel interface from the beginning of the treatment.

Acknowledgements

This study was supported by a grant of the Korea Healthcare Technology R&D Project, Ministry for Health, Welfare and Family Affairs, Republic of Korea (A091074).

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Effects of periodic fluoride treatment on fluoride ion release from fresh orthodontic adhesives Bum-Soon Lim , Shin-Jae Lee , Young-Jun Lim and Sug-Joon Ahn Journal of Dentistry, 2011-11-01, Volume 39, Issue 11, Pages 788-794, Copyright © 2011 Elsevier Ltd Abstract Objective Periodic fluoride treatment may contribute to the ability of fresh orthodontic adhesives to provide long-term F − release. The effects of periodic fluoride treatment on the amount of F − release from fresh orthodontic adhesives was investigated. Methods F − release was measured from a nonfluoride-releasing composite, a fluoride-releasing composite, a polyacid-modified composite (compomer), and two resin-modified glass-ionomer cements (RMGICs) at 1, 2, and 5 days after one of the following treatments: 225 ppm F − solution, 900 ppm F − solution, acidulated phosphate fluoride gel (APF), fluoridated dentifrice, and deionised water (control). F − release was measured in a 5-day cycle, which was repeated 9 consecutive times. The amount of F − release for each group was analysed using the repeated measures analysis of variance. Statistical significance was set at a level of α = 0.05. Results Periodic fluoride treatment temporarily increased F − release in fresh fluoride-releasing orthodontic adhesives, but not in fresh nonfluoride-releasing composite. The order of effective fluoride-release was RMGICs > compomer > fluoride-releasing composite > nonfluoride-releasing composite. The application of APF or 900 ppm F − solution was the most effective way to maintain F − release from fresh orthodontic adhesives. However, the amount of F − release gradually decreased with increasing specimen age. Conclusion Given the difficulty of routine use of APF at home, the results of this study show that a combination of RMGICs and high-dose fluoride mouth rinse is the most effective protocol to maintain F − release from fresh orthodontic adhesives. Clinical significance Most studies have investigated fluoride-uptake abilities using aged materials in which fluoride had been lost for at least 1 month. This study has found that periodic fluoride treatment altered the conventional F − release pattern of fresh fluoride-releasing materials and type of fluoride-containing medium plays a more critical role in fluoride recharging of the materials than fluoride concentration. This study will help clinicians to find the most effective fluoride treatment protocol of fresh materials. 1 Introduction Enamel demineralisation around bonded orthodontic attachments is a common side-effect of fixed orthodontic treatments, because the presence of fixed orthodontic appliances predisposes to plaque accumulation. Enamel demineralisation has been demonstrated around orthodontic brackets after only 1 month. Furthermore, lesions may still present an aesthetic problem as many as 5 years after a fixed appliance treatment. Therefore, finding methods to reduce enamel demineralisation during and/or after orthodontic treatments is essential. Fluoride is well-documented as an anticariogenic agent that prevents enamel demineralisation. A variety of mechanisms are involved in the anticariogenic effects of fluoride, including reduction of demineralisation, enhancement of remineralisation, interference in pellicle and plaque formation, and inhibition of microbial growth and metabolism. In this regard, the incorporation of fluoride into orthodontic adhesives has attracted considerable interest as a possible way to prevent enamel demineralisation during orthodontic treatment. Several fluoride-releasing adhesives have been developed for orthodontic use including glass-ionomer cements, resin modified glass-ionomer cements (RMGICs), polyacid-modified composites (compomers), and composites. Many studies have shown that these materials have specific fluoride releasing abilities. The main advantage of fluoride-releasing orthodontic adhesives is the delivery of F − to the bracket-enamel interface, the area at the greatest risk for enamel demineralisation, independent of patient co-operation. However, research has shown that all fluoride-releasing adhesives initially have high amounts of F − release, but these amounts decline significantly within the first few days. Several studies have demonstrated that fluoride-releasing materials can take up fluoride from exogenous sources of fluoride, such as dentifrice, mouth rinse, and fluoride gels in the oral environment. Periodic fluoride treatments may alter the usual fluoride-releasing patterns, which, as mentioned above, consisted of a significant decrease in F − release after the initial burst. Such fluoride treatments may therefore contribute to the ability of these materials to inhibit enamel demineralisation throughout orthodontic treatment. In the past, most studies have investigated the fluoride-uptake ability of aged materials in which fluoride had been lost for at least 1 month. Little information is available regarding the re-release of F − after periodic fluoride treatment in fresh materials. In addition, the uptake and re-release of F − from orthodontic adhesives when used for bonding and banding of orthodontic materials, has received relatively limited attention. The purpose of this study is to assess the effects of periodic topical fluoride treatment on the amounts of F − release from fresh orthodontic adhesives. The null hypothesis of this experiment was that periodic fluoride treatment would not affect the amount of F − re-release from fresh orthodontic adhesives. 2 Materials and methods 2.1 Specimen preparation Five light-cured orthodontic adhesives were used: a nonfluoride-releasing composite (Transbond XT, 3M, Monrovia, CA, USA), a fluoride-releasing composite (Light Bond, Reliance Orthodontics, Itasca, IL, USA), a compomer (Transbond Plus, 3M), and two RMGICs (Fuji Ortho LC; GC Corporation, Tokyo, Japan; and Multi-Cure, 3M). Disc-shaped specimens were prepared using 1.1 mm-thick Teflon templates, each with a hole of 9.0 mm in diameter. The templates were positioned on top of glass slides, and each adhesive was placed into a hole until the material was flush with the top of the template. A second slide was placed on top, firmly pushed down to ensure a flat dorsal surface, and then gently removed. All materials were light-cured for 20 s from the top and 20 s from the bottom using an Ortholux LED (3M) according to the manufacturers’ instructions. 2.2 Fluoride treatment Each disc was carefully rinsed with deionised water (DW) and treated with a different fluoride protocol; an aqueous solution of 225 ppm F − , an aqueous solution of 900 ppm F − , a fluoride gel, a fluoride-containing dentifrice, or DW as control. The reason for using aqueous solutions of both 225 ppm and 900 ppm F − is that commercially available fluoride-containing mouth rinses contain 0.05% NaF (for daily rinse) or 0.2% NaF (weekly rinse), which is equal to the concentrations of 225 ppm and 900 ppm F − , respectively. Discs in the 225 ppm and 900 ppm solution groups were immersed in 10 mL of NaF solutions containing 225 ppm and 900 ppm F − , respectively, for 1 min. Discs in the fluoride gel group were immersed for 1 min in 10 mL of a foaming solution of acidulated phosphate fluoride gel (APF) (Oral-B, Belmont, CA) containing 1.23 wt.% NaF. Disks in the dentifrice group were brushed for 1 min with a slurry prepared from 1.0 g of a fluoridated dentifrice containing 1000 ppm F − (Regular paste, Crest, Cincinnati, OH). Discs in the DW group were immersed in 10 mL DW for 1 min. After each treatment, the discs were carefully rinsed with DW, blotted dry, placed in a plastic vial with 10 mL DW, and stored in a shaking incubator at 37 °C. 2.3 Measurement of F − The discs were re-immersed in 10 mL of fresh DW at 1, 2 and 5 days after fluoride treatment. The collected 10 mL solutions were used for the following F − measurements. The ionic fluoride concentration of each solution was analysed using an Orion combination fluoride ion-selective electrode (Orion Research Electrode No 9069BN, Orion Research, Beverly, MA, USA) attached to an ion analyzer (Orion Research Expandable Ion Analyzer EA 940, Orion Research). One millilitre of the test solution was added to 1 mL of low-level total ionic strength adjustment buffer in a microsample dish, and the electrode and dish were then covered with cling film to minimise evaporation. The solution was stirred during the measurement procedure and the electrode was allowed to stabilise for 5 min before recording the reading in millivolts. The electrode membrane was gently rinsed with DW between measurements. A calibration curve was generated with the aid of a computer software program prior to each measuring session using standard fluoride solutions of various ionic concentrations (0.1–1000 ppm F − ). Using this curve, fluoride measurements in mV were converted into corresponding fluoride concentrations in parts per million (ppm), which were further converted to ppm/cm 2 by dividing by the disc's surface area. 2.4 Periodic fluoridation Fluoride treatments were carried out in a 5-day cycle, which was repeated 9 consecutive times for 45 days. Measurement of F − and changing of the solutions were performed at 1, 2, and 5 days after each fluoride treatment under the same procedure described above. Cumulative fluoride levels during each 5-day cycle were calculated to provide values in ppm/cm 2 /day ( Table 1 ). Table 1 Cumulative amounts of fluoride ion release (ppm/cm 2 /day) from fresh orthodontic adhesives after various topical fluoride treatment. 1st fluoridation Mean ± SD 2nd fluoridation Mean ± SD 3rd fluoridation Mean ± SD 4th fluoridation Mean ± SD 5th fluoridation Mean ± SD 6th fluoridation Mean ± SD 7th fluoridation Mean ± SD 8th fluoridation Mean ± SD 9th fluoridation Mean ± SD Deionised water Transbond XT 0.01 ± 0.00 0.01 ± 0.00 0.02 ± 0.00 0.01 ± 0.00 0.01 ± 0.00 0.01 ± 0.00 0.01 ± 0.00 0.01 ± 0.00 0.01 ± 0.00 Light Bond 1.01 ± 0.10 0.15 ± 0.01 0.09 ± 0.02 0.07 ± 0.02 0.06 ± 0.01 0.03 ± 0.01 0.02 ± 0.00 0.02 ± 0.01 0.02 ± 0.00 Transbond Plus 1.18 ± 0.08 0.28 ± 0.04 0.21 ± 0.03 0.17 ± 0.03 0.13 ± 0.01 0.05 ± 0.01 0.01 ± 0.00 0.04 ± 0.03 0.04 ± 0.00 Fuji Ortho LC 4.21 ± 0.68 1.35 ± 0.20 0.74 ± 0.10 0.55 ± 0.10 0.42 ± 0.07 0.12 ± 0.01 0.04 ± 0.01 0.06 ± 0.01 0.05 ± 0.01 Multi-Cure 4.30 ± 0.68 1.27 ± 0.13 0.81 ± 0.11 0.57 ± 0.10 0.42 ± 0.03 0.16 ± 0.01 0.09 ± 0.02 0.06 ± 0.01 0.03 ± 0.00 Fluoridated dentifrice Transbond XT 0.01 ± 0.00 0.02 ± 0.00 0.01 ± 0.00 0.01 ± 0.00 0.01 ± 0.00 0.01 ± 0.00 0.01 ± 0.00 0.02 ± 0.00 0.02 ± 0.00 Light Bond 0.57 ± 0.24 0.15 ± 0.05 0.09 ± 0.03 0.06 ± 0.01 0.08 ± 0.02 0.08 ± 0.02 0.02 ± 0.00 0.03 ± 0.01 0.02 ± 0.00 Transbond Plus 1.25 ± 0.24 0.53 ± 0.10 0.42 ± 0.08 0.28 ± 0.03 0.28 ± 0.03 0.17 ± 0.01 0.12 ± 0.02 0.19 ± 0.04 0.16 ± 0.02 Fuji Ortho LC 3.00 ± 0.17 1.29 ± 0.11 1.09 ± 0.05 0.86 ± 0.18 0.80 ± 0.11 0.54 ± 0.07 0.32 ± 0.03 0.51 ± 0.03 0.44 ± 0.01 Multi-Cure 4.40 ± 0.78 1.52 ± 0.28 1.11 ± 0.11 0.98 ± 0.14 0.83 ± 0.07 0.56 ± 0.08 0.39 ± 0.04 0.53 ± 0.06 0.35 ± 0.03 225 ppm F − solution Transbond XT 0.02 ± 0.00 0.02 ± 0.00 0.03 ± 0.00 0.02 ± 0.00 0.01 ± 0.00 0.01 ± 0.00 0.01 ± 0.00 0.01 ± 0.00 0.01 ± 0.00 Light Bond 0.60 ± 0.29 0.14 ± 0.04 0.13 ± 0.04 0.07 ± 0.03 0.07 ± 0.01 0.04 ± 0.01 0.01 ± 0.00 0.04 ± 0.01 0.02 ± 0.00 Transbond Plus 1.20 ± 0.29 0.56 ± 0.14 0.55 ± 0.14 0.33 ± 0.09 0.30 ± 0.05 0.19 ± 0.02 0.12 ± 0.03 0.18 ± 0.03 0.13 ± 0.02 Fuji Ortho LC 3.41 ± 0.51 1.34 ± 0.23 1.39 ± 0.16 1.09 ± 0.16 0.99 ± 0.07 0.53 ± 0.05 0.46 ± 0.05 0.53 ± 0.16 0.40 ± 0.07 Multi-Cure 4.35 ± 1.32 1.47 ± 0.34 1.50 ± 0.30 1.13 ± 0.28 0.86 ± 0.14 0.60 ± 0.06 0.42 ± 0.04 0.47 ± 0.06 0.33 ± 0.04 900 ppm F − solution Transbond XT 0.01 ± 0.00 0.01 ± 0.00 0.02 ± 0.00 0.02 ± 0.00 0.01 ± 0.00 0.01 ± 0.00 0.01 ± 0.00 0.01 ± 0.00 0.01 ± 0.00 Light Bond 0.57 ± 0.39 0.14 ± 0.04 0.11 ± 0.03 0.07 ± 0.02 0.06 ± 0.02 0.05 ± 0.01 0.03 ± 0.00 0.03 ± 0.01 0.03 ± 0.01 Transbond Plus 1.00 ± 0.12 0.75 ± 0.10 0.75 ± 0.12 0.55 ± 0.08 0.44 ± 0.07 0.33 ± 0.03 0.05 ± 0.01 0.21 ± 0.03 0.22 ± 0.01 Fuji Ortho LC 3.13 ± 0.30 2.28 ± 0.27 2.37 ± 0.24 1.99 ± 0.16 1.66 ± 0.21 0.94 ± 0.25 0.75 ± 0.06 0.66 ± 0.15 0.59 ± 0.13 Multi-Cure 3.22 ± 0.17 2.22 ± 0.38 2.39 ± 0.27 2.07 ± 0.19 1.50 ± 0.14 1.35 ± 0.33 0.99 ± 0.12 0.77 ± 0.14 0.65 ± 0.14 Acid phosphate fluoride gel Transbond XT 0.02 ± 0.00 0.02 ± 0.00 0.01 ± 0.00 0.02 ± 0.00 0.02 ± 0.00 0.01 ± 0.00 0.01 ± 0.00 0.02 ± 0.00 0.02 ± 0.00 Light Bond 0.79 ± 0.24 0.17 ± 0.05 0.13 ± 0.03 0.07 ± 0.01 0.07 ± 0.01 0.06 ± 0.01 0.03 ± 0.00 0.05 ± 0.01 0.03 ± 0.00 Transbond Plus 1.12 ± 0.19 0.83 ± 0.17 0.79 ± 0.11 0.74 ± 0.11 0.85 ± 0.11 0.53 ± 0.12 0.47 ± 0.08 0.57 ± 0.06 0.57 ± 0.05 Fuji Ortho LC 3.35 ± 0.16 1.94 ± 0.16 2.02 ± 0.21 1.91 ± 0.21 1.83 ± 0.17 1.30 ± 0.17 0.99 ± 0.10 0.82 ± 0.06 0.89 ± 0.05 Multi-Cure 4.17 ± 0.36 1.83 ± 0.18 1.72 ± 0.25 1.67 ± 0.21 1.73 ± 0.29 1.40 ± 0.29 1.18 ± 0.11 0.88 ± 0.16 0.98 ± 0.09 SD, standard deviation. 2.5 Statistical analysis F − release experiments were performed on 5 independent specimen groups. The data were evaluated using the language R. The cumulative amount of F − release for each group during the observation were analysed using the repeated measures analysis of variance and multiple comparisons were performed by t tests using the Bonferroni correction at a significance level of α = 0.05. 3 Results Figs. 1–5 show the F − release pattern from fresh orthodontic adhesives after a different fluoride treatment. Without fluoridation, all fluoride-releasing adhesives initially showed higher amounts of F − release, which subsequently declined to lower, but stable levels ( Fig. 1 ). However, periodic fluoride treatment temporarily increased F − release from fresh fluoride-releasing orthodontic adhesives after the second treatment ( Figs. 2–5 ). The increase in F − release lasted for 2 days after exposure, and the amounts of F − release were decreased after 2 days. Fig. 1 Fluoride ion release from various orthodontic adhesives after treatment with deionised water. Fig. 2 Fluoride ion release from various orthodontic adhesives after treatment with fluoridated dentifrice. Fig. 3 Fluoride ion release from various orthodontic adhesives after treatment with 225 ppm F − solution. Fig. 4 Fluoride ion release from various orthodontic adhesives after treatment with 900 ppm F − solution. Fig. 5 Fluoride ion release from various orthodontic adhesives after treatment with acidulated phosphate fluoride gel. The temporal increase in F − release after fluoride treatment varied amongst adhesives. Periodic fluoride treatment significantly increased F − release in the RMGICs and compomer, but not in the nonfluoride-releasing composite ( Figs. 2–5 ). Although fluoride-releasing composite showed slightly higher F − release than nonfluoride-releasing composite after fluoride treatment, F − release from fluoride-releasing composite was significantly lower than those from RMGICs and compomer ( P < .05). The temporal increase in F − release after fluoride treatment influenced the cumulative amounts of F − release of fresh materials ( Table 1 ). The RMGICs showed the largest cumulative amounts of F − release (RMGICs > compomer > fluoride-releasing composite > nonfluoride-releasing adhesive, P < .05). There was no significant difference in the cumulative amounts of F − release between the two RMGICs. The nonfluoride-releasing composite released very little amount of F − during the entire experimental period, even with periodic fluoridation ( Figs. 2–5 and Table 1 ). The cumulative amounts of F − release significantly differed according to fluoride treatment method. The application of APF or 900 ppm F − solution maintained the highest amount of F − release from the RMGICs and compomer during the experimental period ( Table 1 ). Both fluoridated dentifrice and 225 ppm F − solution also increased F − release from the RMGICs and compomer, but at lower levels (900 ppm F − solution and APF > 225 ppm F − solution and dentifrice > DW). After periodic fluoride treatments, F − release from the fluoride-releasing adhesives gradually decreased with increasing specimen age ( Figs. 2–5 ). The most significant decrease in F − release occurred after the first of 9 fluoride treatments. Furthermore, the decrease in F − release over time differed between fluoridation methods. The decreases in F − release from the RMGICs and compomer were significantly delayed in the APF treatment group ( Fig. 3 ) compared to the other fluoride treatment groups ( Figs. 3 and 4 ). The changes in F − release according to specimen age and fluoridation methods is consistent with the cumulative changes in F − release ( Table 1 ). 4 Discussion The hypothesis of the present study was rejected because periodic fluoride treatment can alter the conventional F − release pattern of fresh fluoride-releasing orthodontic adhesives. Without periodic fluoridation, all fluoride-releasing adhesives exhibited an initial short-term increase that stabilised after approximately 30 days ( Fig. 1 ), which is consistent with the classic profile of F − release demonstrated by previous studies. Generally, F − release from fluoride-releasing orthodontic adhesives was increased after periodic exposure to various topical fluorides, which rapidly returned to near pre-exposure levels within 3 days ( Figs. 2–5 ). However, the F − level even after the last fluoride treatment did not subsequently return to the fluoride level present after immersion in DW, particularly in the case of RMGICs. The temporary increased F − release after topical fluoride treatment has been reported to be associated with washout of fluoride ions that are retained on the surface or in the pores of the materials during refluoridation. These findings indicate that fresh fluoride-releasing orthodontic adhesives have an ability to be recharged with fluorides introduced from exogenous fluoride source. However, there was no significant increase in F − release immediately after the first exposure to topical fluoride treatments compared to F − release after the first immersion in DW ( Figs. 1–5 ). This is confirmed by the cumulative amount of F − release from the adhesives during the first fluoride treatment phase ( Table 1 ). This may be due to the fact that in the initial stage, the amount of inherent fluoride in the fluoride-releasing adhesives is higher than the amounts of fluoride supplied from exogenous fluoride source. The inherent fluoride may be rapidly exhausted after the initial profound F − release, which may allow fluoride-releasing adhesives to uptake fluoride from the exogenous fluoride source after the next fluoride treatment. After the second fluoride treatment, periodic exposure to various topical fluorides increased cumulative F − release from fresh fluoride-releasing adhesives, specifically RMGICs and compomer ( Table 1 ). However, periodic fluoride treatments did not significantly promote additional F − release from nonfluoride-releasing composites. In addition, exposure to exogenous fluoride temporarily increased by a tiny amount of F − release from fluoride-releasing composites ( Table 1 ). This may be due to relative impermeability of the composites compared to the compomer or RMGICs, which may prevent the absorption of F − deep into their substructures in composites. These results indicate that fluoride uptake capacity varies widely between different classes of fresh fluoride-releasing adhesives. In this study, we found that RMGICs are a significantly more effective fluoride reservoir than composite-based materials ( Table 1 ). The compomer's intermediate fluoride uptake capacity can be explained by its composition, which can be considered intermediate between RMGICs and composites. Our data suggest that the material with higher F − release has a higher fluoride uptake capacity. It seems that a material's fluoride uptake may be limited by its inherent fluoride-releasing capacity, as the sites occupied by inherent fluoride are fixed and limitative within the materials. When comparing the amount of F − release according to fluoridation method, fresh RMGICs and compomer are more easily recharged by APF or 900 ppm F − solution than by a 225 ppm F − solution or fluoridated dentifrice ( Figs. 2–5 ). It seems that solutions may easily infiltrate the adhesive's substructures compared to gels and slurries, given that APF, 900 ppm F − solution, fluoridated dentifrice, and 225 ppm F − solution contain 1.23%, 0.2%, 0.21%, and 0.05% NaF, respectively. These results suggested that type of fluoride-containing medium plays a more critical role in fluoride recharging than F − concentration. However, even APF and 900 ppm F − solution did not significantly increase fluoride uptake of nonfluoride-releasing or fluoride-releasing composites. This indicates that topical fluoride treatments are only effective in maintaining F − release from fresh glass-ionomer based adhesives. As stated above, this may be associated with relative impermeability of the composites. Our data showed a trend towards decreasing F − release of RMGIs and compomer following fluoride treatment with increasing specimen age ( Table 1 and Figs. 2–5 ). This trend suggests that the specimens’ uptake potential diminished over time with increased exposure to supplemental fluoride, which has several possible explanations. First, this reduction may reflect reduced uptake capacity due to age-related material degradation. Second, it may be due to concomitant binding of F − to the adhesive matrix, which then could be subsequently released. Third, it may be due to erosion and disintegration of adhesive components occurring during the fluoride treatment itself. Nevertheless, APF treatment delayed the decrease in F − release from RMGICs and compomer, compared to other fluoride treatments. This may be associated with the fact that fluoride-releasing adhesives can incur significant damage in an acidic environment caused by APF, resulting in relatively continuous F − release from the adhesive matrix. It is important to maintain high levels of F − release in orthodontic adhesives in order to prevent enamel demineralisation during orthodontic treatment. A previous study showed that demineralisation is inhibited in enamel adjacent to an adhesive releasing as little as 0.65–1.3 ppm F − /cm 2 /day. In our study, although F − release gradually decreased over time, we found that fresh RMGICs released a clinically meaningful amount of F − for over 40 days when periodically recharged with APF or 900 ppm F − solution ( Table 1 ). In addition, it has been reported that aged RMGICs can also release F − at an amount over the critical level when recharged with a high-dose fluoride solution or APF. However, care must be exercised with frequent APF application because it can cause erosive damage of both RMGICs and compomers, which may weaken mechanical bond strength of those materials. Enamel demineralisation has been demonstrated around orthodontic brackets only 1 month after the placement of fixed appliances. Thus whilst 900 ppm F − solution contains a high fluoride concentration and should be used under supervision, our results show that the regular use of high-fluoride mouth rinse with RMGICs maintains the highest amount of F − release during the initial critical stage of orthodontic treatment. Our findings can apply most strongly to patients with a high risk of caries, where all preventive measures must be considered. Although this study provides valuable information on the effective fluoride treatment protocol, further studies are required to apply the results of the present study to the clinical situation, because fluoride uptake and re-release process can be affected by several experimental factors, such as solubility of the materials and the presence of saliva. In addition, composite adhesive should be used with primer to enhance bond strength. 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