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Journal of Orthodontics, Vol. 30, No. 4, 317-322, December 2003
© 2003 British Orthodontic Society

Scientific Section

Fluoridated elastomers: in vivo versus in vitro fluoride release

D. Tinsley1, J. J. O’Dwyer1 and Philip E. Benson2

1 Charles Clifford Dental Hospital, Sheffield, and Chesterfield Royal Hospital, UK
2 Department of Oral Health and Development, University of Sheffield, UK

David Tinsley, Orthodontic Department, Charles Clifford Dental Hospital, Wellesley Road, Sheffield S10 2SZ, UK. E-mail: dtinsley.harrogate{at}virgin.net


   Abstract
Top
 Abstract
Introduction
 Materials and methods
 Results
 Discussion
 Conclusions
 References
 
Objectives: To compare (i) the in vivo release of fluoride from fluoridated elastomers to the in vitro release, and (ii) the residual fluoride content of the elastomers after 1 week in the mouth with and without fluoride toothpaste and mouthrinse.

Design: A prospective, longitudinal, cross-over study.

Subjects and method: Six subjects were recruited by poster to take part in the study. Each subject had one premolar in each quadrant to which a bracket could be fixed and exemplary oral hygiene. Elastomers were then placed on these brackets.

Intervention: The study was divided into two parts: (i) subjects used oral hygiene products with fluoride and (ii) oral hygiene products with fluoride were excluded. Both groups of elastomers were left in the mouth for 1 week. After collection the elastomers were stored in distilled water.

Main outcome measures: The amount of residual fluoride in the ligatures after they have been placed in the mouth for 1 week was compared with the cumulative fluoride release in vitro over 1 week and 6 months.

Results: Only 13 per cent of the total amount of fluoride in fluoridated elastomers was released during the first week in vitro, compared with 90 per cent in vivo. There was a significantly greater amount (P = 0.001) of residual fluoride when the elastomers were in the mouth for 1 week in the presence of fluoride toothpaste and mouthrinse, than when fluoride supplements were excluded.

Conclusions: (1) Higher levels of fluoride are lost from the fluoride elastomers in vivo than in vitro during the first week. (2) A significantly greater amount of residual fluoride was released from the elastomers placed in the mouth when fluoride toothpaste and mouthrinse were used.

Key words: Demineralization, fluoride, elastomer, orthodontic


   Introduction
Top
 Abstract
 Introduction
Materials and methods
 Results
 Discussion
 Conclusions
 References
 
Fluoride supplements have been shown to reduce the incidence of demineralization during orthodontic treatment.1,Go2Go One problem with these methods of administrating fluoride is that they are dependent on patient compliance.

Recently, elastomers that release stannous fluoride have been made available for orthodontic patients. The elastomers leach fluoride close to the site of vulnerability next to the bracket. Because they are changed every 4–6 weeks, they may be the perfect vehicle to provide a low concentration of fluoride over a period of orthodontic treatment. Clinical studies using these elastomers have shown promising results with a reduction in the prevalence3Go and severity4Go of demineralization during orthodontic treatment. In vitro studies, however, have shown that although the fluoride release is initially high it soon becomes low and not sustainable over clinically relevant time periods.5Go

To-date few studies have examined the in vivo fluoride release from fluoridated ligatures. Wiltshire6Go has suggested that the fluoridated elastomers absorb fluoride in the mouth, as well as release it. This has also been demonstrated with some of the dental cements.7Go It would obviously be difficult to directly measure the amount of fluoride released or absorbed from the elastomers in vivo. However, by measuring the amount of fluoride left in the elastomers after they have been in the mouth for a period of time and comparing this with the total amount of fluoride released from the elastomers in vitro, it should be possible to provide an indirect estimate of the amount of fluoride released. Further to this, the in vitro part of the study will also allow comparisons to be made between the fluoride release into a test tube to that in the mouth.

The aims of this in vivo study were:


   Materials and methods
Top
 Abstract
 Introduction
 Materials and methods
Results
 Discussion
 Conclusions
 References
 
Ethical approval was obtained from the South Sheffield Research Ethics Committee. They made the proviso that a poster should be used to recruit volunteers. A sample size calculation showed that five individuals were needed to show a difference of 0.56 µgF/ml/elastomer, to a power of 0.95 and significance of 0.01.

The study group
Six subjects (three males, three females) were recruited to take part in the study. Subjects were eligible for inclusion if they had at least one premolar in each quadrant, to which a bracket could be fixed and their oral hygiene was of an exemplary standard. For all the subjects that were eligible for inclusion, the trial was explained and each received a patient information sheet. At the next appointment written consent was obtained.

The intervention
Each subject had an orthodontic bracket (GAC international. Inc, 185 Oval Drive, Islanda, NY 11749) placed onto a premolar in each quadrant (Figure 1Go). Standard methods and materials were used to place the bracket, including the use of 30 per cent phosphoric etchant gel to treat the surface of the premolar. They were attached with a non-fluoride leaching composite bonding cement (Rely-a –Bond®, Reliance Ortho Prod. Inc., Itasca, IL, USA). Following bracket placement the subjects were given oral hygiene instruction. All subjects were asked to maintain good oral hygiene for the duration of the study.



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  		FIG. 1 Brackets positioned on the premolars, with fluoridated elastomers in place.

 
The in vivo study with fluoride supplements. One fluoridated elastomer (Fluor-I-Ties®, Ortho Arch Company Inc) was tied onto each of the four brackets and left in the mouth for 7 days. During this period, the subjects were asked to use a standardized fluoridated mouthwash (Colgate FluoriGard®, Sodium fluoride 0.05 per cent, Colgate Palmolive (UK) Manchester M5 3FS) and toothpaste (Colgate Fluoride toothpaste, Sodium monofluorophosphate 1000 ppm, sodium fluoride 450 ppm). The subjects were asked to use these products at set times of the day, namely 7.30 a.m. and 7.30 p.m.

The elastomers were removed after 7 days, then rinsed with distilled water to remove debris and protein accumulations. The four ties from each volunteer were placed collectively into an air-tight, polyethelene beaker containing 10 ml of distilled water and stored at room temperature.

The in vivo study without fluoride supplements. The method outlined above was repeated, except for the 7-days that the elastomers were left in the mouth the subjects were prescribed a non-fluoridated mouthwash and toothpaste (Sainsbury’s Confident Naturally Fresh, Stamford Street, London SE11 9LL). Subjects were also asked to avoid other fluoride supplements for the duration of the study.

Fluoride analysis
In order to calculate the fluoride concentrations of the distilled water a fluoride/fluoride combination electrode (Thermo Orion) was used. The electrode was connected to a pH/mV with readability to 0.1 mV. Before analysing the samples the electrode was calibrated, using two standard solutions of fluoride (1 and 10 ppm). To maintain a standard pH all solutions were mixed with equal amounts of total ionic solution adjustment buffer (TISAB II). Calibration of the electrode was repeated every 2 hours to ensure the readings were accurate and consistent.

The in vivo elastomers. Fluoride analysis was carried out incrementally (approximately every 6 weeks) over 6 months until most of the fluoride had been released. Six weeks was considered to be a sufficiently long time period to allow fluoride to leach out of the elastomers without allowing super saturation of the solutions. At each of the analysing sessions the four elastomers were removed from each sample and transferred to a fresh beaker containing 10 ml of distilled water. The new sample was then stored for a further 6 weeks in the laboratory before analysis. A 1-ml pipette was used to draw equal quantities of the solution under test (taken from the original beaker) and TISAB II, which were mixed together in a disposable container. This solution was then analysed with the fluoride electrode and once the voltmeter had stabilized, the reading was recorded. Between readings the electrode was carefully cleaned with distilled water and dried with a tissue. At the completion of each session the electrode was cleaned and stored according to the manufacturers instructions.

The in vitro elastomers. Four elastomers were placed onto orthodontic brackets, with 1 ml of distilled water in a polyethelene beaker and stored in an incubator at 37°C. The fluoride/fluoride combination electrode was used to analyse the fluoride content of the distilled water after 24 hours. The same four elastomers were placed in a fresh 1-ml sample of distilled water and again stored for 24 hours. The fluoride concentration of the distilled water was again measured. Daily measurements of fluoride release were carried out during the first week. This was followed by incremental measurements over the next 6 months until it was considered that most of the fluoride had been released from the elastomers.

Throughout the testing period of the study a distilled water control was used. This was obtained from the same source as the water used as the storage medium for the elastomers. The same conditions were applied to the control as to the solutions under test.

Statistical testing
The following statistics were employed in this study:

  1. The total amount of fluoride released in vitro was calculated by adding the areas under the curve (AUC) between each pair of observations as follows. If we have fluoride concentrations y1 and y2 at times t1 and t2, the AUC between those two times is the product of the time difference and the average of the two measurements. Thus, we get:


    This is known as the trapezium rule because of the shape of each segment of the area of the curve. The total amount of fluoride left in the in vivo elastomers was calculated by adding the incremental fluoride measurements taken over the 6 months.

  2. To test the null hypothesis that there was no difference between the fluoride released from the elastomers worn during the fluoride supplement period compared with the non-fluoride supplement period, a paired t-test was used. Prior to testing, the data was examined for normality using the Shapiro–Wilks test.


   Results
Top
 Abstract
 Introduction
 Materials and methods
 Results
Discussion
 Conclusions
 References
 
The mean total fluoride concentration per module in vitro over the 6-months for each of the ten groups is shown in Table 1Go. The mean area under the curve for the 10 samples was 138.65 µgF/day/ml/elastomer (SD 3.75). The mean total amount of fluoride available for release from each ligature was therefore 0.139 mg. The in vitro mean cumulative 7-day fluoride concentration per module was 18.07 µgF/ml/elastomer. The mean total amount of fluoride released from each ligature during the first 7 days in vitro was therefore 0.018 mgF. Hence, 13 per cent of the available fluoride was released during the first week in vitro.


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  		TABLE 1 The cumulative fluoride concentration per elastomer (µgF/day/ml/elastomer) from 10 in vitro samples of fluoridated elastomers over 6 months calculated using the area under the curve (AUC) method
 
The cumulative residual fluoride concentration from the elastomers that had been placed in the mouth is shown in Table 2Go. The mean cumulative residual fluoride concentration was 13.30 µgF/ml/elastomer (SD 0.74) for the fluoride elastomers worn without fluoride supplements and 15.85 µgF/ml/elastomer (SD 0.72) for those worn with fluoride supplements. The mean total amount of residual fluoride in each elastomer was therefore 0.013 mg for the fluoride elastomers worn without fluoride supplements and 0.016 mg for those worn with fluoride supplements.


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  		TABLE 2 Cumulative residual fluoride concentration per module (µgF/ml/elastomer) measured over 6 months for fluoridated elastomers worn by six individuals for 1 week, with and without fluoride mouthwash and toothpaste
 
To calculate the amount of fluoride lost in the mouth during the first week a subtraction method was used. The mean total amount of residual fluoride from the in vivo part of the study was subtracted from the mean total amount of fluoride from the in vitro part of the study. The mean total amount of fluoride in each elastomer calculated in vitro was 0.139 mg. The mean residual amount of fluoride per elastomer was 0.013 mg for the fluoride elastomers worn without fluoride supplements. The difference in these amounts is very large, and suggests that over 0.12 mg of fluoride or 90 per cent of the available fluoride was lost per module, in the mouth of each volunteer during the first week.

Throughout the study a control was used. This was distilled water obtained from the same source as that used for the fluoride analysis. The fluoride levels remained constant and extremely low (<0.02 µgF/ml) for the whole study indicating that the increase in fluoride was coming solely from the elastomers and not from any other source (i.e. the beaker).

At the end of the 6-month experimental period both in vivo and in vitro samples were still releasing a low level of fluoride. To ensure that the amount of residual fluoride was the same for both the in vivo and in vitro elastomers the samples were placed in fresh distilled water and the fluoride concentration was tested after 24 hours. The fluoride release was found to be the same for both the in vivo (0.41 µgF/ml/elastomer) and in vitro groups (0.41 µgF/ml/elastomer). Therefore, as the amount was small and similar in both groups it was considered possible to compare the results from the two groups.

The effect of fluoride supplementation on the in vivo modules
To test the hypothesis that there was a difference in the residual fluoride concentration between the in vivo elastomers collected with fluoride supplements compared with the elastomers collected without fluoride supplements the data were first checked for normality using the Shapiro–Wilks test. This test was non-significant thus allowing the paired t-test to be utilized. The mean difference, standard deviation and 95 per cent confidence limits of the differences between the two groups are shown in Table 3Go. The significance level (P = 0.001) indicates very strong evidence that there is a difference between the residual fluoride levels in the two groups.


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  		TABLE 3 Mean difference, standard deviation, 95 per cent CI and results of paired t-test for the cumulative residual fluoride concentration per elastomer (µgF/ml/elastomer) measured over 6 months from the fluoridated elastomers worn by six individuals for 1 week, with and without fluoride toothpaste and mouthrinse
 

   Discussion
Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
Conclusions
 References
 
The results of this study suggest that fluoride is released from fluoridated elastomers more rapidly when placed in the mouth compared with in the test tube. The amount of fluoride released from each elastomer in vivo was approximately 0.12 mg. If the average orthodontic patient has 20 modules placed at each appointment, 2.4 mg of fluoride is released during the first week in the mouth. It has been suggested that the daily intake of fluoride in children should not exceed 0.10 mg/kg of body weight.8Go If the release of fluoride quickly drops off after the first few days, the fluoride release may be extremely high immediately after the elastomers are placed. Caution is warranted when using these modules, particularly for younger patients, those with an already high fluoride intake from other sources, such as fluoride supplements or in areas where the water is fluoridated. One solution to reduce this initial high dose of fluoride is to place them only on the teeth at risk of demineralization or alternate them with regular modules. Further work is required to investigate fluoride loss from these elastomers in the first 24 hours after placement.

Another finding of this investigation was that there is a significant difference in the amount of residual fluoride in the modules when fluoride supplements were used compared with when they were avoided by the subjects. This confirms work carried out previously,6Go which suggested that the modules not only release fluoride, but also absorb it from their environment. One criticism of the fluoridated elastomers is that their ability to release fluoride is not sustainable over time.5Go If the modules are able to recharge with fluoride introduced into the environment from toothpastes or mouthwashes, then sufficient amounts of fluoride might be released over longer time periods to prevent demineralization. Several factors are likely to be involved in this recharge,7Go including the permeability of the material, and the form and concentration of the fluoride used; however, this study appears to suggest that the elastomers do have a recharge potential. The clinical significance of this recharge potential is speculative. The patients who are susceptible to demineralization are likely to be non-compliant having an irregular intake of fluoride from toothpastes or mouthwashes. Ideally, there should be sufficient amounts of fluoride released from the elastomers over clinically relevant time periods to prevent demineralization without relying on fluoride recharge.

The pattern of fluoride release in the in vitro part of this study was similar to that reported by Wiltshire.5Go However, the amount of fluoride released at each time interval in this study was much higher.

One explanation for this finding is that the elastomers may not be the same in both studies. The appearance of the elastomers certainly has changed; instead of being injection moulded they are now cut from a tube. This is illustrated in Figure 2Go.



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  		FIG. 2 Recent change in physical appearance of the elastomers.

 
It is likely that not only has the appearance of the modules changed since Wiltshire’s work, but so have their physical and chemical properties. Several clinicians have found that the original fluoridated elastomers performed poorly in the mouth.3,Go9Go They reported high breakages and unacceptable swelling of the modules. Wiltshire6Go found that they swelled and doubled in weight after 4 weeks in the mouth, whereas little change was measured with conventional elastomers. It is possible that the manufacturer has addressed these problems and changed the elastomers. It would appear that elastomers produced by a die punching technique are 50–80 per cent stronger than those produced by injection moulding.10Go From their appearance the elastomers have changed from injection moulded to die punched.

It was found that, even after 6 months, the elastomers were still releasing fluoride; therefore, our calculation of the total amount of fluoride available for release from each elastomer might be an underestimation. This is possibly because there is an outer, loosely bound layer of fluoride that is released initially and an inner, more tightly bound core of fluoride that is released more slowly. However, because the 24-hour release from the in vivo and in vitro elastomers was the same after 196 days, there is likely to be a similar amount of fluoride left in both groups of elastomers; therefore, the proportion of fluoride released after 7 days in the mouth will not be altered.

Although 90 per cent of the fluoride is released during the first week in the mouth, the remaining 10 per cent might be released more slowly and could be sufficient to reduce the prevalence and severity of demineralization.3,Go4Go Margolis et al.11Go found that concentrations of fluoride as low as 0.024 ppm offered ‘remarkable protection’ of the enamel surface in vitro. If this is true in vivo, then it is likely that fluoridated elastomers, particularly combined with fluoride toothpaste and mouthrinse, will raise the fluoride level in plaque sufficiently to enhance remineralization. This would be a useful area of further investigation.

This study had a small sample size, but one surprising finding was that the variability in the amount of residual fluoride left in the elastomers after 1 week was low between individuals. This suggests that the fluoride release from these modules is consistent regardless of differences in diet, flow of saliva and fluoride clearance patterns. Because this variability was low, the power of the study was sufficient to show a positive difference in the residual fluoride when fluoride supplements were used.

This study explored fluoride release from fluoridated elastomers after 1 week in the mouth. Further investigations leaving them in place in the mouth for 2, 3, 4, and 6 weeks are warranted. This would enable the rate of fluoride release to be more accurately profiled and help ascertain the optimal period for replacing fluoridated elastomers in the clinical situation. It would also be interesting to determine if non-fluoridated elastomers have the same recharge potential.


   Conclusions
Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 Conclusions
References
 

  1. In the first week after placement, fluoridated elastomers release high levels of fluoride.
  2. The in vitro and in vivo fluoride release does not appear to be similar.
  3. There was a significantly greater amount (P = 0.001) of residual fluoride when the elastomers were in the mouth for 1 week in the presence of fluoride containing hygiene products than when they were excluded.
  4. Fluoridated elastomers may imbibe fluoride from their environment.


   References
Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 Conclusions
 References
 
1 O’Reilly MM, Featherstone JD. Demineralization and remineralization around orthodontic appliances: an in vivo study. Am J Orthod Dentofac Orthop 1987; 92: 33–40.[CrossRef][Medline]

2 Geiger AM, Gorelick L, Gwinnett AJ, Benson BJ. Reducing white spot lesions in orthodontic populations with fluoride rinsing. Am J Orthod Dentofac Orthop 1992; 101: 403–7.[Medline]

3 Banks PA, Chadwick SM, Asher-McDade C, Wright JL. Fluoride-releasing elastomerics—a prospective controlled clinical trial. Eur J Orthod 2000; 22: 401–7.[Abstract/Free Full Text]

4 Mattick CR, Mitchell L, Chadwick SM, Wright J. Fluoride-releasing elastomeric modules reduce decalcification: a randomized controlled trial. J Orthod 2001; 28: 217–19.[Abstract/Free Full Text]

5 Wiltshire WA. Determination of fluoride from fluoride-releasing elastomeric ligature ties. Am J Orthod Dentofac Orthop 1996; 110: 383–7.[CrossRef][Medline]

6 Wiltshire WA. In vitro and in vivo fluoride release from orthodontic elastomeric ligature ties. Am J Orthod Dentofac Orthop 1999; 115: 288–292.[CrossRef][Medline]

7 Preston AJ, Higham SM, Agalamanyi EA, Mair LH. Fluoride recharge of aesthetic dental materials. J Oral Rehabil 1999; 26: 936–40.[CrossRef][Medline]

8 Levy SM, Guha-Chowdhury N. Total fluoride intake and implications for dietary fluoride supplementation. J Publ Hlth Dent, 1999; 59: 211–23.

9 Miethke RR, Comment on determination of fluoride from ligature ties, Am J Orthod Dentofac Orthop 1997; III 33A.

10 Dowling PA, Jones WB, Lagerstrom L, Sandham JA. An investigation into the behavioural characteristics of orthodontic elastomeric modules. Br J Orthod 1998; 25: 197–202.[Abstract]

11 Margolis HC, Moreno EC, Murphy BJ. Effect of low levels of fluoride in solution on enamel demineralization in vitro. J Dent Res 1986; 65: 23–9.[Abstract/Free Full Text]

Received December 20, 2002; accepted March 19, 2003





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