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In vitro and in vivo fluoride release from orthodontic elastomeric ligature ties
ORIGINAL ARTICLE In vitro and in vivo fluoride release from orthodontic elastomeric ligature ties William A. Wiltshire, BChD, BChD(Hons), MDent, MChD, DSc Winnipeg, Manitoba, Canada Clinically, demineralization of enamel around orthodontic attachments can occur after only 1 month. Fluoride incorporation into elastomeric ligature ties may provide additional protection against decalcification through fluoride release. This study compared the fluoride release of fluoride-impregnated and nonfluoride elastomeric ligature ties (Ortho Arch Company) both in vitro and in vivo. A total of 260 fluoride-impregnated and 260 nonfluoride elastomerics were evaluated in this study, 400 in vitro and 120 in vivo. For the in vivo part of the study, six patients had fluoride and nonfluoride elastomerics placed in cross-quadrant fashion in their mouths; these were removed and tested for residual fluoride release after 1 month. With the use of the potentiometric analytical method, the fluoride release of the elastomerics was determined in distilled water as the 24-hour residual release, to compare the in vitro and in vivo fluoride leached into solution. The data was analyzed with the Wilcoxon matched-pairs signed ranks test. The distilled water control yielded an F− reading of 0.03 ± 0.01 µgF/mL. In the in vitro part of the study, an average of 0.38µgF/mL/elastomeric was released over the 1 month period by the fluoride-impregnated elastomerics; this decreased significantly (P < .05) to a 24-hour residual value at 1 month of 0.02 µgF/mL/elastomeric ligature, which is in the same order of magnitude as the distilled water control solution. The nonfluoride ties produced a calculated 24 hour residual fluoride release of 0.003 µg/F/mL/elastomeric after 1 month; this is much less than the distilled water control and would not be possible to measure accurately. After 1 month in vivo, significantly greater (P > .05) amounts of 24-hour residual fluoride were apparent: F− elastomerics = 1.43µgF/mL/elastomeric and nonfluoride elastomerics = 0.44µgF/mL/elastomeric. Fluoride ties gained weight intra-orally. Residual, leachable fluoride was present in fluoride-impregnated and nonfluoride elastomeric ligature ties after 1 month of intraoral use, due to imbibition. The clinical efficacy of fluoride-impregnated elastomeric ligature ties to prevent decalcification in the presence of plaque needs to be investigated. (Am J Orthod Dentofacial Orthop 1999;115:288-92)
F
ixed orthodontic appliance treatment significantly increases the risk of white spot lesions and enamel decalcification.1-7 Enamel decalcification is caused by an imbalance between demineralizing and remineralizing of enamel, and the resultant white spot lesion is considered to be a precursor of enamel caries.7 In orthodontics, white spots and decalcification are attributed to prolonged accumulation and retention of bacterial plaque on the enamel surface adjacent to the attachments7,8 Of concern in clinical orthodontics, is that demineralization of enamel has been reported to occur around orthodontic brackets after only 1 month.8 The addition of fluoride into the hydroxyapatite structure of enamel causes the formation of fluorapatite, which is known to aid in the remineralization of aProfessor and Chairman, Section of Orthodontics, Faculty of Dentistry, University of Manitoba. Reprint requests to: William A. Wiltshire, BChD, BChD(Hons), MDent, MChD, DSc, Professor and Chairman, Section of Orthodontics, Faculty of Dentistry, University of Manitoba,790 Bannatyne Avenue, Winnipeg, Manitoba, R3E OW2, Canada; e-mail,[email protected] Copyright © 1999 by the American Association of Orthodontists. 0889-5406/99/$8.00 + 0 8/1/92383
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decalcified enamel lesions as well as reducing decalcifications.9-11 Reduction of carious lesions are reported with fluoride concentrations of less than 0.05 ppm12,13 and even a small concentration of only 0.2 µgF−/cm2 released from a material in vivo, induced an uptake of 5400 ppm in the outer 10µm of enamel after only 24 hours of contact with the teeth.14 Fluoride-releasing elastomeric ligature ties containing tin fluoride (SnF2) are currently available to orthodontists. The manufacturers claim a low concentration of fluoride release from these elastomerics over a sustained period of time; this could decrease plaque formation and aid in remineralization of enamel around the bracket bases that are difficult areas to keep clean.15 In a recent in vitro study,15 it was shown that fluoridecontaining elastomeric ligature ties released significant amounts of fluoride; this was characterized by an initial burst of fluoride during the first 2 days and was followed by a logarithmic decrease over the remainder of the 6 month test period. It was estimated that adequate amounts of fluoride were released over the test period in order to theoretically aid in the prevention of de-
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Table I. In
µgF/mL
vitro and in vivo fluoride release from fluoride-impregnated and nonfluoride elastomeric ligature ties in In vitro
In vivo
1 Month
Fluoride elastomerics Nonfluoride elastomerics Distilled water control
24 Hours
Measured total mean for group of 20 elastomerics
Calculated total mean per individual elastomeric
7.64 ± 1.31 0.14 ± 0.02 0.03 ± 0.01
0.38 0.007*
Measured mean for group of 20 elastomerics 0.37 ± 0.12 0.06 ± 0.01*
24 Hours
Calculated mean per individual elastomeric
Measured mean per individual elastomeric
0.02* 0.003*
1.43 ± 0.37 0.44 ± 0.05
*Quantity of F− insufficient to accurately determine with the potentiometric analytical method.
mineralization and enhancement of remineralization of enamel.15 The amount of fluoride released from elastomerics in vivo may be influenced by mouth temperature fluctuations, saliva, plaque, diet and tooth-brushing procedures. It was thus the purpose of this study to compare the in vitro levels of fluoride release from SnF2 impregnated elastomeric ligatures with their fluoride release after 1 month of intraoral use. One month intraorally was selected as the test period, because it was proposed that, for optimum clinical benefit, the fluoride-releasing ligatures should be replaced monthly,15 and in addition, many orthodontists do follow-up appointments of full fixed banded cases on a monthly basis. MATERIAL AND METHODS
A total of 260 fluoride-impregnated elastomeric ligatures (Fluor-I-Ties, Ortho Arch Company, Inc., Hoffman Estates, Ill) and 260 nonfluoride elastomerics (Ortho Arch Company) were evaluated in this study. The fluoride release of the elastomeric ligatures were evaluated both in vitro and in vivo. For the in vitro part of the study, 200 fluorideimpregnated and 200 nonfluoride elastomerics were placed in groups of 20 in 1 mL of distilled water in separate polyethylene bottles. For the in vivo part of the study, each of six full-fixed banded orthodontic patients had 60 fluoride-impregnated and 60 nonfluoride elastomeric ligature ties placed in a cross-quadrant fashion on their orthodontic attachments from second premolar to second premolar, in both maxillary and mandibular arches. Accordingly, 20 elastomerics were placed per patient. No dietary or specific additional toothbrushing instructions were given to the patients. The patients were not told what type of toothpaste to use nor were they given any specific mouthrinsing instructions. The patients were equally matched by sex and were all in their permanent dentition. The elastomerics were removed after 1 month in the mouth and then thorough-
ly rinsed with distilled water to remove any debris and protein accumulations that could have affected the electrode sensitivity during the fluoride analyses. After having been removed from the patients mouths and rinsed, the elastomerics were regrouped in two groups, either the “fluoride impregnated” or the “nonfluoride” group and then randomly and individually placed in 1 mL of distilled water. No attempt was made to distinguish patient-specific fluoride-release patterns. The total in vitro released fluoride was determined after 1 month. In addition, the 24-hour released fluoride content of each solution was determined after 1 month for both in vitro and in vivo samples. For the 24hour time period before testing, the polyethylene bottles with the in vivo elastomeric samples were maintained in an incubator at 37°C. The in vitro samples were stored at 37°C for the entire 1 month test period. A blank with only distilled water, served as the test control. The fluoride content ([F]) of the test solutions was determined by means of the potentiometric analytic method.16 A fluoride-ion selective electrode (F1052F, Radiometer, Copenhagen, Denmark) and a reference electrode (K4040, Radiometer, Copenhagen, Denmark) coupled to an analyzer (ION 85 Ionanalyzer, Radiometer, Copenhagen, Denmark) were used. Electrode calibration was accomplished with a commercial fluoride standard (Ionplus, ATI orion, Boston, Mass) containing 100 ± 0.5 ppm sodium fluoride diluted with double distilled and deionized water. Total ionic strength adjustment buffer (TISAB) was used for pH adjustment, ion strength adjustment, and for masking metals that complex fluoride. By taking the dilution factor into account, the F– release was calculated and expressed as the total F release in µgF/mL. The elastomeric ligatures were weighed before placement in the mouth and after removal and rinsing with distilled water, on a balanced scale (Prescisca 205A, Switzerland). The resultant data were analyzed with the Wilcoxon matched-pairs signed ranks test.17
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RESULTS
The results are presented in Table I. The in vitro control group, which contained only distilled water, yielded a constant F− reading of 0.03 ± 0.01 µgF/mL. After 1 month of immersion in distilled water, the fluoride-impregnated elastomerics had released an accumulated total mean fluoride of 7.64 ± 1.31 µgF/mL, which is calculated to be to 0.38 µgF/mL per individual elastomeric. In contrast, the nonfluoride elastomerics produced an accumulated total mean fluoride of 0.14 ± 0.02 µgF/mL after 1 month of immersion, which is calculated to be 0.007 µgF/mL per individual non-F elastomeric and is significantly less (P < .05) fluoride than the F− elastomerics and distilled water control. The mean 24-hour in vitro released F− was 0.37 ± 0.12 µgF/mL for the fluoride-impregnated elastomerics; this is calculated to be 0.02 µgF/mL per individual elastomeric and is in the same order of magnitude as the distilled water control solution. For the nonfluoride elastomerics the in vitro 24 hour F− release was calculated at 0.003 µgF/mL/elastomeric, which is significantly less than the distilled water control (P < .05) and experimentally, this magnitude would have been too small to have been accurately determined with the potentiometric method. Accordingly, the F− release had to be determined from groups of 20 elastomerics, and then calculated for estimated individual F− release per elastomeric ligature tie. Obviously, total monthly F− release from the elastomerics was not possible to determine intraorally because of the salivary flow, F− ingestion in vivo and the impossibility of measuring it intraorally. Accordingly, following 1 month intraorally, the 24-hour released F− was determined in the laboratory with the potentiometric analytical method, in order to facilitate in vitro and in vivo comparisons. After 1 month in vivo, the 24-hour residual released fluoride for fluoride-impregnated elastomerics was 1.43 ± 0.37 µgF/mL/elastomeric which is significantly greater (P < .05) than the in vitro release (0.02 µgF/mL/elastomeric). After 1 month intraorally, the nonfluoride elastomerics also released significantly (P < .05) greater quantities of F−, on average 0.44 ± 0.05 µgF/mL/elastomeric, than the group of nonfluoride elastomerics (0.003 µgF/mL/elastomeric). The 24-hour residual leachable fluoride, tested after 1 month of intraoral use, increased significantly (P < .05) for both nonfluoride and fluoride impregnated elastomeric ligature ties, compared with those elastomerics that were not tested intraorally. Fluoride elastomerics doubled their weight after serving for 1 month intraorally (0.011 g vs 0.005 g),
American Journal of Orthodontics and Dentofacial Orthopedics March 1999
while the weight of the nonfluoride elastomerics remained virtually unchanged, on average (0.006 g vs 0.005 g). The significant weight increase observed with fluoride-impregnated elastomerics, may be due to the significant uptake in fluoride content from the oral milieu and water/saliva imbibition during the same time period intraorally. DISCUSSION
The simulated oral environment may grossly underestimate the loss of physical properties of biomaterials as a result of dynamic and continual intraoral variations in temperature, pH, salivary enzymes, and bacterial activity among others.15,18 The patterns of in vitro F− release reported in several studies15,18,19 are characterized by an initial high burst of fluoride release during the first 48 hours, followed by a diminishing release over time. Clinically, however, a different pattern is apparent judging from the results of the present study. After having served for 1 month intraorally, the fluoride-impregnated elastomerics, tested for residual 24-hour leachable fluoride, provided a residual and leachable fluoride that is over 70 times more than was measured in vitro (1.43 ± 0.37 vs 0.02 µgF/mL). Even the nonfluoride elastomerics, on average, leached significantly more fluoride after serving for a month in the mouth (0.44 µgF/mL/elastomeric vs 0.003 µgF/mL/ elastomeric). Fluoride elastomerics are known to expel large amounts of F− within the first month, and the majority of fluoride, judging from in vitro estimations,15,19 is probably released within the first few days of intraoral insertion. Comparing the in vitro and in vivo F− release patterns, however, shows that clinically, both fluorideimpregnated and nonfluoride elastomerics tend to imbibe fluoride. The fluoride imbibition could be a result of fluoride introduction into the oral milieu from toothpastes, drinking water, mouthrinses, foods, and beverages. Furthermore, the present study has shown that the imbibed F− is available for further release into solution, when tested after 1 month, intraorally. It should be kept in mind that, in the oral environment, the leaching of fluoride may be convection controlled, whereas in the in vitro situation, the leaching is diffusion limited. It is also possible that the nonfluoride elastomerics may have imbibed relatively large amounts of fluoride set free during the initial burst of fluoride release from the fluoride-elastomerics, especially because they were placed in the same mouth during the clinical experiment. This study also indicates that clinically there is a reservoir of fluoride remaining in both fluorideimpregnated and even nonfluoride elastomeric ligature
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ties after 1 month of intraoral service. Cognizant of previous research12,13 that demonstrated that concentrations of fluoride of less than 0.05 ppm are beneficial for caries reduction, this study shows the potential that sufficient leachable fluoride may be available in elastomeric ligature ties after 1 month of clinical use to be effective in preventing decalcification as well as enhancing remineralization.12,13 The continual availability of low doses of fluoride that are released during the third to fourth weeks in vivo are potentially important because the fluoride is released exactly where it is needed, around the bracket bases15 that may assist in the prevention of decalcification through the formation of fluorapatite.20,21 This is in contrast to calcium fluoride formation that is formed when the initial high doses of fluoride are released during the first 48 hours.15,22-24 Some of the fluoride from the elastomeric ligatures will be ingested,18 but the remainder, depending on salivary flow rate,19 will be available to act locally on enamel at the orthodontic bracket site to the benefit of both calcium fluoride and fluorapatite formation.15 It has been advised that for optimal clinical benefit, elastomeric ligature ties should be replaced monthly.15 One report, however, indicates that the elastomerics would need to be replaced even sooner.25 Replenishment of fluoride-releasing elastomeric ties would reintroduce a monthly high dose of fluoride release that would once again benefit calcium fluoride formation in the enamel surrounding the brackets. Studies have shown that stannous fluoride, in addition to preventing caries through enamel remineralization, also exhibits antibacterial properties at low concentrations.7,26-28 The association between low quantities of released fluoride and enamel demineralization is a matter of speculation in the literature.29 Moreover, it has been reported that the frequency of fluoride application and not merely the level of concentration, is important in anticariogenicity,22,23,29 highlighting the importance of slow-release properties of F− from materials. Of importance is that inhibition of demineralization of enamel seems to be related to the initial amounts of fluoride released, and the material that shows high initial bursts of fluoride release within the first few days shows more encouraging results.29 The fluoride-impregnated elastomerics that show high bursts of fluoride release initially are the materials of choice, and their monthly replacement would provide an advantageous cumulative effect.15 However, it may be possible that, in time, there is a reduction in the constant release of fluoride from fluoride-releasing materials or, alternatively, that the surface of the materials becomes saturated with fluoride,30 and thus diminishes the beneficial effects.
The amount of fluoride necessary to provide significant reductions in enamel demineralization and the conditions of its optimal functioning are unclear.30 In a recent study,31 fluoride release in the order of 200 to 300 mg/cm2 of material per month was calculated to completely inhibit enamel demineralization in the presence of plaque. On the basis of the present study, it is doubtful if fluoride-impregnated elastomeric ligature ties would be able to significantly inhibit enamel demineralization during orthodontics with plaque present around the brackets. Because the clinical part of this study attempted to mimic normal oral conditions and accordingly did not prescribe a nonfluoride diet or specific oral care regimen, it is not possible to comment on the relationship between fluoride introduction intraorally, the amount of imbibed fluoride clinically, and the amount of fluoride in reserve in the elastomeric materials. However, it is speculated that dietary and oral-care fluoride introduced into the mouth, would play a significant role in the eventual fluoride uptake by elastomeric ligatures and affect the residual fluoride that may be rereleased into the oral cavity. Although strictly not part of this study, it was found that clinically, the fluoride elastomerics swelled up significantly intraorally and were discolored, swollen, and unaesthetic after 1 month of service and had to be replaced. This is in agreement with similar reports in the literature.15,25 From a biomaterial improvement point of view, it would be worthwhile for manufacturers to investigate the solving of these clinical problems. Nevertheless, fluoride-impregnated orthodontic elastomeric ligature ties remain a promising, continual-release, low-dose, fluoride delivery system. Future research should focus on the anticariogenic clinical efficacy15 and antibacterial properties7 of fluoride-releasing elastomeric ligature ties. CONCLUSIONS 1.
2. 3. 4. 5.
Residual, leachable fluoride was present in fluorideimpregnated and nonfluoride elastomeric ligature ties after 1 month intraorally. Fluoride-impregnated elastomeric ligature ties released significantly more fluoride than nonfluoride ties. In vitro testing significantly underestimates in vivo residual fluoride release. Fluoride is imbibed by elastomerics in vivo. Future research should focus on the anticariogenic clinical efficacy and antibacterial properties of fluorideimpregnated elastomeric ligature ties.
I am grateful to Ms Sonia Janse van Rensburg, Ms Rene Sekenal and Dr Francien Botha of the University of Pretoria for assistance with the fluoride analyses,
292 Wiltshire
and Mrs Yvonne Skinner of the University of Pretoria and Judy Pitsanuk of the University of Manitoba for the manuscript preparation. REFERENCES 1. Balensiefen JW, Madonia JV. Study of dental plaque in orthodontic patients. J Dent Res 1970;49:320-4. 2. Gwinnet AJ, Ceen RF. Plaque distribution on bonded brackets; a scanning microscope study. Am J Orthod 1979;75:667-77. 3. Ceen RF, Gwinnett AJ. White spot formation associated with sealants used in orthodontics. Pediatric Dent 1961;3:174-8. 4. Boyd RL. Comparison of three self-applied topical fluoride preparations for control of decalcification. Angle Orthod 1993;63:25-30. 5. Artun J, Brobakken BO. Prevalence of carious white spots after treatment with multibanded appliances. Eur J Orthod 1986;8:229-34. 6. Wiltshire WA, Janse van Rensburg SD. Fluoride release from four visible light-cured orthodontic adhesive resins. Am J Orthod Dentofacial Orthop 1995;108:278-83. 7. Wilson TG, Gregory RL. Clinical effectiveness of fluoride-releasing elastomers. I: Salivary Streptococcus mutans numbers. Am J Orthod Dentofacial Orthop 1995;107: 293-7. 8. O’Reilly M, Featherstone JD. De and remineralization around orthodontic appliances: an in vitro study. J Dent Res 1985;64:301. 9. Dean HT, Arnold FA, Elvove E. Domestic water and dental caries. Publ Health Res 1938;53:1443-52. 10. Arnold FA, Likins RC, Russel AL, Scott DB. Fifteenth year of the Grand Rapids fluoridation study. J Am Dent Assoc 1962;65:780-5. 11. Mellberg JR, Mallon DE. Acceleration of remineralization in vitro by sodium mono fluori phosphate and sodium fluoride. J Dent Res 1984;63:1130-5. 12. Margolis JC, Mareno EC, Murphy BJ. Effect of low levels of fluoride in solution on enamel demineralization in vitro. J Dent Res 1986;65:23-9. 13. Ten Cate JM, Arend J. Remineralization of artificial enamel lesions in vitro. Caries Res 1977;11:277-86. 14. Tenbirojh D, Retief DH, Russel CM. Enamel, cementum and dentine fluoride uptake from a fluoride releasing resin composite. Am J Dent 1992;5:226-32. 15. Wiltshire WA. Determination of fluoride from fluoride-releasing elastomeric ligature ties. Am J Orthod Dentofacial Orthop 1996;110:383-7.
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16. Nicholson K, Duff EJ. Fluoride determination in water: an optimum buffer system for use with the fluoride selective electrode. Anal Let 1981;14:493-517. 17. Siegel S. Nonparametric statistics for the behavioral sciences. New York: McGraw Hill; 1956. p. 68-83. 18. Boyd R, Kapila S. Commentary: fluoride-releasing elastomeric chains. Angle Orthod 1994;64:210. 19. Joseph VP, Grobler ST, Rossouw PE. Fluoride release from orthodontic elastic chain. J Clin Orthod 1993;27:101-4. 20. Fox NA. Fluoride release from orthodontic bonding materials. Br Dent J 1990;17: 293-8. 21. Grobler SR, Ogaard BR, Rolla G. The uptake and retention of fluoride in sound enamel in vivo after a single application of neutral 2% sodium fluoride. In: Rolla G, Sonju T, Embery G, editors. Tooth surface interactions and preventive dentistry. London: IRL Press Ltd; 1981. p. 17-26. 22. Geiger AM, Gorelick L, Gwinnett AJ, Griswold PG. The effect of a fluoride program on white spot formation during orthodontic treatment. Am J Orthod Dentofacial Orthop 1988;97:29-37. 23. Barkvoll P, Rolla G, Lagerlof F. Effect of sodium lauryl sulfate on the deposition of alkali-soluble fluoride on enamel in vitro. Caries Res 1988;22:139-44. 24. Rolla G, Saxegaard E. Critical evaluation of the composition and use of topical fluorides with emphasis on the role of calcium fluoride in caries inhibition. J Dent Res 1990;69:780-5. 25. Miethke RR. Comment on determination of fluoride from ligature ties. Am J Orthod Dentofacial Orthop 1997;111:33A. 26. Whitford GM, Schuster GS, Paschley DH, Venkateswarlu P. Fluoride uptake by Streptococcus mutans 6715. Infect Immun 1977;18:660-87. 27. Ota K, Kikuchi S, Beierle JW. Stannous fluoride and its effects on oral microbial adhesive properties in vitro. Ped Dent 1989;11:21-4. 28. Camosci DA, Tinanoff N. Anti-bacterial determinants of stannous fluoride. J Dent Res 1984;63:1121-5. 29. Basdra EK, Huber H, Komposch G. Fluoride released from orthodontic bonding agents alters the enamel surface and inhibits enamel demineralization in vitro. Am J Orthod Dentofacial Orthop 1996;109:466-72. 30. Young A, Von der Fehr FR. Sonju T, Nordbo H. Fluoride release and uptake in vitro from a composite resin and two orthodontic adhesives. Acta Odontol Scand 1996;54:223-8. 31. Dijkman GEHM, de Vries J, Lodding A, Arends J. Long-term fluoride release of visible light-activated composites in vitro: a correlation with in-situ demineralisation data. Caries Res 1993;27:117-23.
F
ixed orthodontic appliance treatment significantly increases the risk of white spot lesions and enamel decalcification.1-7 Enamel decalcification is caused by an imbalance between demineralizing and remineralizing of enamel, and the resultant white spot lesion is considered to be a precursor of enamel caries.7 In orthodontics, white spots and decalcification are attributed to prolonged accumulation and retention of bacterial plaque on the enamel surface adjacent to the attachments7,8 Of concern in clinical orthodontics, is that demineralization of enamel has been reported to occur around orthodontic brackets after only 1 month.8 The addition of fluoride into the hydroxyapatite structure of enamel causes the formation of fluorapatite, which is known to aid in the remineralization of aProfessor and Chairman, Section of Orthodontics, Faculty of Dentistry, University of Manitoba. Reprint requests to: William A. Wiltshire, BChD, BChD(Hons), MDent, MChD, DSc, Professor and Chairman, Section of Orthodontics, Faculty of Dentistry, University of Manitoba,790 Bannatyne Avenue, Winnipeg, Manitoba, R3E OW2, Canada; e-mail,[email protected] Copyright © 1999 by the American Association of Orthodontists. 0889-5406/99/$8.00 + 0 8/1/92383
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decalcified enamel lesions as well as reducing decalcifications.9-11 Reduction of carious lesions are reported with fluoride concentrations of less than 0.05 ppm12,13 and even a small concentration of only 0.2 µgF−/cm2 released from a material in vivo, induced an uptake of 5400 ppm in the outer 10µm of enamel after only 24 hours of contact with the teeth.14 Fluoride-releasing elastomeric ligature ties containing tin fluoride (SnF2) are currently available to orthodontists. The manufacturers claim a low concentration of fluoride release from these elastomerics over a sustained period of time; this could decrease plaque formation and aid in remineralization of enamel around the bracket bases that are difficult areas to keep clean.15 In a recent in vitro study,15 it was shown that fluoridecontaining elastomeric ligature ties released significant amounts of fluoride; this was characterized by an initial burst of fluoride during the first 2 days and was followed by a logarithmic decrease over the remainder of the 6 month test period. It was estimated that adequate amounts of fluoride were released over the test period in order to theoretically aid in the prevention of de-
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Table I. In
µgF/mL
vitro and in vivo fluoride release from fluoride-impregnated and nonfluoride elastomeric ligature ties in In vitro
In vivo
1 Month
Fluoride elastomerics Nonfluoride elastomerics Distilled water control
24 Hours
Measured total mean for group of 20 elastomerics
Calculated total mean per individual elastomeric
7.64 ± 1.31 0.14 ± 0.02 0.03 ± 0.01
0.38 0.007*
Measured mean for group of 20 elastomerics 0.37 ± 0.12 0.06 ± 0.01*
24 Hours
Calculated mean per individual elastomeric
Measured mean per individual elastomeric
0.02* 0.003*
1.43 ± 0.37 0.44 ± 0.05
*Quantity of F− insufficient to accurately determine with the potentiometric analytical method.
mineralization and enhancement of remineralization of enamel.15 The amount of fluoride released from elastomerics in vivo may be influenced by mouth temperature fluctuations, saliva, plaque, diet and tooth-brushing procedures. It was thus the purpose of this study to compare the in vitro levels of fluoride release from SnF2 impregnated elastomeric ligatures with their fluoride release after 1 month of intraoral use. One month intraorally was selected as the test period, because it was proposed that, for optimum clinical benefit, the fluoride-releasing ligatures should be replaced monthly,15 and in addition, many orthodontists do follow-up appointments of full fixed banded cases on a monthly basis. MATERIAL AND METHODS
A total of 260 fluoride-impregnated elastomeric ligatures (Fluor-I-Ties, Ortho Arch Company, Inc., Hoffman Estates, Ill) and 260 nonfluoride elastomerics (Ortho Arch Company) were evaluated in this study. The fluoride release of the elastomeric ligatures were evaluated both in vitro and in vivo. For the in vitro part of the study, 200 fluorideimpregnated and 200 nonfluoride elastomerics were placed in groups of 20 in 1 mL of distilled water in separate polyethylene bottles. For the in vivo part of the study, each of six full-fixed banded orthodontic patients had 60 fluoride-impregnated and 60 nonfluoride elastomeric ligature ties placed in a cross-quadrant fashion on their orthodontic attachments from second premolar to second premolar, in both maxillary and mandibular arches. Accordingly, 20 elastomerics were placed per patient. No dietary or specific additional toothbrushing instructions were given to the patients. The patients were not told what type of toothpaste to use nor were they given any specific mouthrinsing instructions. The patients were equally matched by sex and were all in their permanent dentition. The elastomerics were removed after 1 month in the mouth and then thorough-
ly rinsed with distilled water to remove any debris and protein accumulations that could have affected the electrode sensitivity during the fluoride analyses. After having been removed from the patients mouths and rinsed, the elastomerics were regrouped in two groups, either the “fluoride impregnated” or the “nonfluoride” group and then randomly and individually placed in 1 mL of distilled water. No attempt was made to distinguish patient-specific fluoride-release patterns. The total in vitro released fluoride was determined after 1 month. In addition, the 24-hour released fluoride content of each solution was determined after 1 month for both in vitro and in vivo samples. For the 24hour time period before testing, the polyethylene bottles with the in vivo elastomeric samples were maintained in an incubator at 37°C. The in vitro samples were stored at 37°C for the entire 1 month test period. A blank with only distilled water, served as the test control. The fluoride content ([F]) of the test solutions was determined by means of the potentiometric analytic method.16 A fluoride-ion selective electrode (F1052F, Radiometer, Copenhagen, Denmark) and a reference electrode (K4040, Radiometer, Copenhagen, Denmark) coupled to an analyzer (ION 85 Ionanalyzer, Radiometer, Copenhagen, Denmark) were used. Electrode calibration was accomplished with a commercial fluoride standard (Ionplus, ATI orion, Boston, Mass) containing 100 ± 0.5 ppm sodium fluoride diluted with double distilled and deionized water. Total ionic strength adjustment buffer (TISAB) was used for pH adjustment, ion strength adjustment, and for masking metals that complex fluoride. By taking the dilution factor into account, the F– release was calculated and expressed as the total F release in µgF/mL. The elastomeric ligatures were weighed before placement in the mouth and after removal and rinsing with distilled water, on a balanced scale (Prescisca 205A, Switzerland). The resultant data were analyzed with the Wilcoxon matched-pairs signed ranks test.17
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RESULTS
The results are presented in Table I. The in vitro control group, which contained only distilled water, yielded a constant F− reading of 0.03 ± 0.01 µgF/mL. After 1 month of immersion in distilled water, the fluoride-impregnated elastomerics had released an accumulated total mean fluoride of 7.64 ± 1.31 µgF/mL, which is calculated to be to 0.38 µgF/mL per individual elastomeric. In contrast, the nonfluoride elastomerics produced an accumulated total mean fluoride of 0.14 ± 0.02 µgF/mL after 1 month of immersion, which is calculated to be 0.007 µgF/mL per individual non-F elastomeric and is significantly less (P < .05) fluoride than the F− elastomerics and distilled water control. The mean 24-hour in vitro released F− was 0.37 ± 0.12 µgF/mL for the fluoride-impregnated elastomerics; this is calculated to be 0.02 µgF/mL per individual elastomeric and is in the same order of magnitude as the distilled water control solution. For the nonfluoride elastomerics the in vitro 24 hour F− release was calculated at 0.003 µgF/mL/elastomeric, which is significantly less than the distilled water control (P < .05) and experimentally, this magnitude would have been too small to have been accurately determined with the potentiometric method. Accordingly, the F− release had to be determined from groups of 20 elastomerics, and then calculated for estimated individual F− release per elastomeric ligature tie. Obviously, total monthly F− release from the elastomerics was not possible to determine intraorally because of the salivary flow, F− ingestion in vivo and the impossibility of measuring it intraorally. Accordingly, following 1 month intraorally, the 24-hour released F− was determined in the laboratory with the potentiometric analytical method, in order to facilitate in vitro and in vivo comparisons. After 1 month in vivo, the 24-hour residual released fluoride for fluoride-impregnated elastomerics was 1.43 ± 0.37 µgF/mL/elastomeric which is significantly greater (P < .05) than the in vitro release (0.02 µgF/mL/elastomeric). After 1 month intraorally, the nonfluoride elastomerics also released significantly (P < .05) greater quantities of F−, on average 0.44 ± 0.05 µgF/mL/elastomeric, than the group of nonfluoride elastomerics (0.003 µgF/mL/elastomeric). The 24-hour residual leachable fluoride, tested after 1 month of intraoral use, increased significantly (P < .05) for both nonfluoride and fluoride impregnated elastomeric ligature ties, compared with those elastomerics that were not tested intraorally. Fluoride elastomerics doubled their weight after serving for 1 month intraorally (0.011 g vs 0.005 g),
American Journal of Orthodontics and Dentofacial Orthopedics March 1999
while the weight of the nonfluoride elastomerics remained virtually unchanged, on average (0.006 g vs 0.005 g). The significant weight increase observed with fluoride-impregnated elastomerics, may be due to the significant uptake in fluoride content from the oral milieu and water/saliva imbibition during the same time period intraorally. DISCUSSION
The simulated oral environment may grossly underestimate the loss of physical properties of biomaterials as a result of dynamic and continual intraoral variations in temperature, pH, salivary enzymes, and bacterial activity among others.15,18 The patterns of in vitro F− release reported in several studies15,18,19 are characterized by an initial high burst of fluoride release during the first 48 hours, followed by a diminishing release over time. Clinically, however, a different pattern is apparent judging from the results of the present study. After having served for 1 month intraorally, the fluoride-impregnated elastomerics, tested for residual 24-hour leachable fluoride, provided a residual and leachable fluoride that is over 70 times more than was measured in vitro (1.43 ± 0.37 vs 0.02 µgF/mL). Even the nonfluoride elastomerics, on average, leached significantly more fluoride after serving for a month in the mouth (0.44 µgF/mL/elastomeric vs 0.003 µgF/mL/ elastomeric). Fluoride elastomerics are known to expel large amounts of F− within the first month, and the majority of fluoride, judging from in vitro estimations,15,19 is probably released within the first few days of intraoral insertion. Comparing the in vitro and in vivo F− release patterns, however, shows that clinically, both fluorideimpregnated and nonfluoride elastomerics tend to imbibe fluoride. The fluoride imbibition could be a result of fluoride introduction into the oral milieu from toothpastes, drinking water, mouthrinses, foods, and beverages. Furthermore, the present study has shown that the imbibed F− is available for further release into solution, when tested after 1 month, intraorally. It should be kept in mind that, in the oral environment, the leaching of fluoride may be convection controlled, whereas in the in vitro situation, the leaching is diffusion limited. It is also possible that the nonfluoride elastomerics may have imbibed relatively large amounts of fluoride set free during the initial burst of fluoride release from the fluoride-elastomerics, especially because they were placed in the same mouth during the clinical experiment. This study also indicates that clinically there is a reservoir of fluoride remaining in both fluorideimpregnated and even nonfluoride elastomeric ligature
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ties after 1 month of intraoral service. Cognizant of previous research12,13 that demonstrated that concentrations of fluoride of less than 0.05 ppm are beneficial for caries reduction, this study shows the potential that sufficient leachable fluoride may be available in elastomeric ligature ties after 1 month of clinical use to be effective in preventing decalcification as well as enhancing remineralization.12,13 The continual availability of low doses of fluoride that are released during the third to fourth weeks in vivo are potentially important because the fluoride is released exactly where it is needed, around the bracket bases15 that may assist in the prevention of decalcification through the formation of fluorapatite.20,21 This is in contrast to calcium fluoride formation that is formed when the initial high doses of fluoride are released during the first 48 hours.15,22-24 Some of the fluoride from the elastomeric ligatures will be ingested,18 but the remainder, depending on salivary flow rate,19 will be available to act locally on enamel at the orthodontic bracket site to the benefit of both calcium fluoride and fluorapatite formation.15 It has been advised that for optimal clinical benefit, elastomeric ligature ties should be replaced monthly.15 One report, however, indicates that the elastomerics would need to be replaced even sooner.25 Replenishment of fluoride-releasing elastomeric ties would reintroduce a monthly high dose of fluoride release that would once again benefit calcium fluoride formation in the enamel surrounding the brackets. Studies have shown that stannous fluoride, in addition to preventing caries through enamel remineralization, also exhibits antibacterial properties at low concentrations.7,26-28 The association between low quantities of released fluoride and enamel demineralization is a matter of speculation in the literature.29 Moreover, it has been reported that the frequency of fluoride application and not merely the level of concentration, is important in anticariogenicity,22,23,29 highlighting the importance of slow-release properties of F− from materials. Of importance is that inhibition of demineralization of enamel seems to be related to the initial amounts of fluoride released, and the material that shows high initial bursts of fluoride release within the first few days shows more encouraging results.29 The fluoride-impregnated elastomerics that show high bursts of fluoride release initially are the materials of choice, and their monthly replacement would provide an advantageous cumulative effect.15 However, it may be possible that, in time, there is a reduction in the constant release of fluoride from fluoride-releasing materials or, alternatively, that the surface of the materials becomes saturated with fluoride,30 and thus diminishes the beneficial effects.
The amount of fluoride necessary to provide significant reductions in enamel demineralization and the conditions of its optimal functioning are unclear.30 In a recent study,31 fluoride release in the order of 200 to 300 mg/cm2 of material per month was calculated to completely inhibit enamel demineralization in the presence of plaque. On the basis of the present study, it is doubtful if fluoride-impregnated elastomeric ligature ties would be able to significantly inhibit enamel demineralization during orthodontics with plaque present around the brackets. Because the clinical part of this study attempted to mimic normal oral conditions and accordingly did not prescribe a nonfluoride diet or specific oral care regimen, it is not possible to comment on the relationship between fluoride introduction intraorally, the amount of imbibed fluoride clinically, and the amount of fluoride in reserve in the elastomeric materials. However, it is speculated that dietary and oral-care fluoride introduced into the mouth, would play a significant role in the eventual fluoride uptake by elastomeric ligatures and affect the residual fluoride that may be rereleased into the oral cavity. Although strictly not part of this study, it was found that clinically, the fluoride elastomerics swelled up significantly intraorally and were discolored, swollen, and unaesthetic after 1 month of service and had to be replaced. This is in agreement with similar reports in the literature.15,25 From a biomaterial improvement point of view, it would be worthwhile for manufacturers to investigate the solving of these clinical problems. Nevertheless, fluoride-impregnated orthodontic elastomeric ligature ties remain a promising, continual-release, low-dose, fluoride delivery system. Future research should focus on the anticariogenic clinical efficacy15 and antibacterial properties7 of fluoride-releasing elastomeric ligature ties. CONCLUSIONS 1.
2. 3. 4. 5.
Residual, leachable fluoride was present in fluorideimpregnated and nonfluoride elastomeric ligature ties after 1 month intraorally. Fluoride-impregnated elastomeric ligature ties released significantly more fluoride than nonfluoride ties. In vitro testing significantly underestimates in vivo residual fluoride release. Fluoride is imbibed by elastomerics in vivo. Future research should focus on the anticariogenic clinical efficacy and antibacterial properties of fluorideimpregnated elastomeric ligature ties.
I am grateful to Ms Sonia Janse van Rensburg, Ms Rene Sekenal and Dr Francien Botha of the University of Pretoria for assistance with the fluoride analyses,
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and Mrs Yvonne Skinner of the University of Pretoria and Judy Pitsanuk of the University of Manitoba for the manuscript preparation. REFERENCES 1. Balensiefen JW, Madonia JV. Study of dental plaque in orthodontic patients. J Dent Res 1970;49:320-4. 2. Gwinnet AJ, Ceen RF. Plaque distribution on bonded brackets; a scanning microscope study. Am J Orthod 1979;75:667-77. 3. Ceen RF, Gwinnett AJ. White spot formation associated with sealants used in orthodontics. Pediatric Dent 1961;3:174-8. 4. Boyd RL. Comparison of three self-applied topical fluoride preparations for control of decalcification. Angle Orthod 1993;63:25-30. 5. Artun J, Brobakken BO. Prevalence of carious white spots after treatment with multibanded appliances. Eur J Orthod 1986;8:229-34. 6. Wiltshire WA, Janse van Rensburg SD. Fluoride release from four visible light-cured orthodontic adhesive resins. Am J Orthod Dentofacial Orthop 1995;108:278-83. 7. Wilson TG, Gregory RL. Clinical effectiveness of fluoride-releasing elastomers. I: Salivary Streptococcus mutans numbers. Am J Orthod Dentofacial Orthop 1995;107: 293-7. 8. O’Reilly M, Featherstone JD. De and remineralization around orthodontic appliances: an in vitro study. J Dent Res 1985;64:301. 9. Dean HT, Arnold FA, Elvove E. Domestic water and dental caries. Publ Health Res 1938;53:1443-52. 10. Arnold FA, Likins RC, Russel AL, Scott DB. Fifteenth year of the Grand Rapids fluoridation study. J Am Dent Assoc 1962;65:780-5. 11. Mellberg JR, Mallon DE. Acceleration of remineralization in vitro by sodium mono fluori phosphate and sodium fluoride. J Dent Res 1984;63:1130-5. 12. Margolis JC, Mareno EC, Murphy BJ. Effect of low levels of fluoride in solution on enamel demineralization in vitro. J Dent Res 1986;65:23-9. 13. Ten Cate JM, Arend J. Remineralization of artificial enamel lesions in vitro. Caries Res 1977;11:277-86. 14. Tenbirojh D, Retief DH, Russel CM. Enamel, cementum and dentine fluoride uptake from a fluoride releasing resin composite. Am J Dent 1992;5:226-32. 15. Wiltshire WA. Determination of fluoride from fluoride-releasing elastomeric ligature ties. Am J Orthod Dentofacial Orthop 1996;110:383-7.
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16. Nicholson K, Duff EJ. Fluoride determination in water: an optimum buffer system for use with the fluoride selective electrode. Anal Let 1981;14:493-517. 17. Siegel S. Nonparametric statistics for the behavioral sciences. New York: McGraw Hill; 1956. p. 68-83. 18. Boyd R, Kapila S. Commentary: fluoride-releasing elastomeric chains. Angle Orthod 1994;64:210. 19. Joseph VP, Grobler ST, Rossouw PE. Fluoride release from orthodontic elastic chain. J Clin Orthod 1993;27:101-4. 20. Fox NA. Fluoride release from orthodontic bonding materials. Br Dent J 1990;17: 293-8. 21. Grobler SR, Ogaard BR, Rolla G. The uptake and retention of fluoride in sound enamel in vivo after a single application of neutral 2% sodium fluoride. In: Rolla G, Sonju T, Embery G, editors. Tooth surface interactions and preventive dentistry. London: IRL Press Ltd; 1981. p. 17-26. 22. Geiger AM, Gorelick L, Gwinnett AJ, Griswold PG. The effect of a fluoride program on white spot formation during orthodontic treatment. Am J Orthod Dentofacial Orthop 1988;97:29-37. 23. Barkvoll P, Rolla G, Lagerlof F. Effect of sodium lauryl sulfate on the deposition of alkali-soluble fluoride on enamel in vitro. Caries Res 1988;22:139-44. 24. Rolla G, Saxegaard E. Critical evaluation of the composition and use of topical fluorides with emphasis on the role of calcium fluoride in caries inhibition. J Dent Res 1990;69:780-5. 25. Miethke RR. Comment on determination of fluoride from ligature ties. Am J Orthod Dentofacial Orthop 1997;111:33A. 26. Whitford GM, Schuster GS, Paschley DH, Venkateswarlu P. Fluoride uptake by Streptococcus mutans 6715. Infect Immun 1977;18:660-87. 27. Ota K, Kikuchi S, Beierle JW. Stannous fluoride and its effects on oral microbial adhesive properties in vitro. Ped Dent 1989;11:21-4. 28. Camosci DA, Tinanoff N. Anti-bacterial determinants of stannous fluoride. J Dent Res 1984;63:1121-5. 29. Basdra EK, Huber H, Komposch G. Fluoride released from orthodontic bonding agents alters the enamel surface and inhibits enamel demineralization in vitro. Am J Orthod Dentofacial Orthop 1996;109:466-72. 30. Young A, Von der Fehr FR. Sonju T, Nordbo H. Fluoride release and uptake in vitro from a composite resin and two orthodontic adhesives. Acta Odontol Scand 1996;54:223-8. 31. Dijkman GEHM, de Vries J, Lodding A, Arends J. Long-term fluoride release of visible light-activated composites in vitro: a correlation with in-situ demineralisation data. Caries Res 1993;27:117-23.