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Fluoride release from orthodontic band cements—a comparison of two in vitro models
Journal of Dentistry 31 (2003) 19–24
www.elsevier.com/locate/jdent
Fluoride release from orthodontic band cements—a comparison of two in vitro models G.R. Cavesa, D.T. Milletta,*, S.L. Creanorb, R.H. Foyeb, W.H. Gilmourc a
Unit of Orthodontics, University of Glasgow Dental Hospital and School, 378 Sauchiehall Street, Glasgow G2 3JZ, UK b Oral Biology and New Technology Research Group, University of Glasgow Dental School, Glasgow, UK c Departments of Statistics and Public Health, University of Glasgow, Glasgow, UK Received 20 September 2001; accepted 5 November 2002
KEYWORDS Fluoride release; Orthodontic band cements; In vitro model
Summary Objectives. To compare, in vitro, the fluoride release from a conventional glass ionomer cement (Ketac-Cem), a resin-modified glass ionomer cement (3MMulticure) and a polyacid modified composite (Ultra Band-Lok) using a banded tooth model and a disc model with the same mean cement weight. Methods. Forty pairs of caries-free third molars were collected and divided into two groups, each of 20 teeth. One tooth from each pair was banded with Ketac-Cem and the other with Ultra Band-Lok or 3M-Multicure; the average band size for each cement group was the same. Two coats of nail varnish were painted on each tooth to within 1 mm of the band margin. Five discs (4.5 mm diameter and 2 mm depth) were prepared for each cement, these dimensions having been calculated so that the mean cement weight of the banded tooth model matched that of the disc model for each cement. The fluoride released into 2 ml of deionised water, from each banded tooth or disc, was measured at regular intervals over 30 days using an Orion ion-selective electrode connected to an ion analyser. Results. At 30 days, for both banded tooth and disc models, the mean cumulative fluoride release was greatest from 3M-Multicure followed by Ketac-Cem, which in turn released more fluoride than Ultra Band-Lok. These differences were all significant ( p , 0.05). Despite having the same mean cement weight, the banded tooth model for Ketac-Cem and 3M-Multicure released approximately 3–4 times more cumulative fluoride than the disc model after 30 days ( p , 001). For Ultra Band-Lok, both models released comparable levels of fluoride ( p . 0.05). Conclusions. Cement type, specimen geometry and surface area appear to influence significantly fluoride release characteristics. Q 2003 Elsevier Science Ltd. All rights reserved.
Introduction Historically the use of zinc phosphate cement for orthodontic band cementation has been associated *Corresponding author. Tel.: þ 44-141-211-9666; fax: þ 44141-331-2798. E-mail address: [email protected]
with enamel decalcification. 1 Bands are now cemented routinely with glass ionomer cements, which release and uptake fluoride, thereby affording greater cariostatic potential.2 Studies which have assessed fluoride release from orthodontic cements have been conducted primarily using a disc model, but wide variation
0300-5712/03/$ - see front matter Q 2003 Elsevier Science Ltd. All rights reserved. doi: 1 0 . 1 0 1 6 / S 0 3 0 0 - 5 7 1 2 ( 0 2 ) 0 0 0 8 7 - 8
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exists in the methodology used. The sample size has ranged from one specimen3 to 254 and the medium into which fluoride is released has varied in both type and volume. Although distilled or deionised water has been employed mostly, saliva has been used also as the specimen immersant.5 – 8 Fluoride release, however, is significantly less into artificial or human saliva,5,9,10 though this medium does not replicate entirely the clinical situation.8 The volume of the immersant has ranged from 16,11 to 50 ml3 and the observation time has varied from 1 h9 to 2 years.12 Specimen shape and size have also varied considerably. Discs as small as 3 mm £ 1.5 mm 13,14 ranging up to 20 mm £ 1.5 mm15 have been used. Most studies have demonstrated a similar pattern of fluoride release over the first 2 days. Then there followed an exponential decay with a plateau reached within 1 – 2 weeks.12,16 The amount of fluoride released varies substantially and is dependent on the type of cement being tested. Though most studies demonstrate greatest fluoride release from conventional glass ionomers when compared with resin-modified glass ionomers or polyacidmodified composites, some have shown comparable fluoride release between these products16 and others the reverse.17 A number of factors effect fluoride release from glass ionomer cements. When the surface area is kept constant, an almost linear relationship between cement volume and fluoride release has been found18 A non-linear relationship between surface area and fluoride release, however, has been demonstrated when the volume is kept constant.14 A 50% reduction in surface area reduced fluoride release by almost 25%. Williams et al.19 also showed that fluoride release is strongly dependent on surface area rather than volume and recommended that units of fluoride release should be quoted in terms of specimen surface area, particularly relevant in an orthodontic model. When the volume to surface area ratio is lowered, fluoride release rises.18,20 To date, no standard clinically appropriate specimen size has been recommended for use in orthodontically related fluoride release studies. It seems pertinent, therefore, to question the relevance of using non-standardised discs that bear no relationship to the mass or geometry of cement found clinically beneath a band or bracket. Furthermore, no attempt has been made to calculate the amount of cement beneath an orthodontic band that would enable the construction of a disc model that emulates more the clinical situation. It appears that no study has yet developed a banded tooth model akin to
G.R. Caves et al.
the bracket/tooth model used by other workers in assessing fluoride release from orthodontic cements.14,21 This study aims to address both of these aspects. The aims of this study were to compare the fluoride release characteristics of a conventional glass ionomer cement to that of a resin-modified glass ionomer and a polyacid-modified composite using both a disc model and a banded tooth model where the mean weight of the cement discs is the same as that found beneath the bands. The null hypotheses were that firstly, there was no difference between the cements with regard to their daily and cumulative fluoride release; secondly, there was no difference between the banded tooth model and the disc model with regard to the daily and cumulative fluoride release from each cement.
Materials and methods Banded tooth model preparation Forty pairs of caries-free intact human third molars were obtained from subjects resident in nonfluoridated areas. Each pair of molars originated from the same subject to minimise any potential differences in enamel fluoride levels. 22 After debridement, teeth were stored in a 0.12% thymol solution and were cleaned thoroughly with a water and pumice mixture. An orthodontic first molar band (3M Unitek, Bradford, England) was sized for each tooth using the smallest one possible in each case and contouring the edges to ensure close adaptation to the enamel surface. Each tooth was dried using compressed air and weighed with its band on a Sartorius MCI Balance (Sartorius, Goettingen, Germany). A conventional glass ionomer cement, KetacCem (ESPE, Seefeld, Oberbay Gmbh, Germany), a modified composite, Ultra Band-Lok (Reliance Orthodontic Products Inc. Itasca, ILL, USA) and a resin-modified glass ionomer cement, 3M-Multicure (3M Unitek, Monrovia, CA, USA) were chosen as the test materials. The teeth were divided into two groups of 20 pairs of teeth. One tooth from each pair was banded with Ketac-Cem and the other with Ultra-Bandlok or 3M-Multicure. The assignment of teeth took into account the band size for each tooth so that the mean band size for each group was the same. This ensured that the mean crown size of each group and, therefore, the mean cement volume was as similar as possible.
Fluoride release from orthodontic band cements—a comparison of two in vitro models
21
Each cement was prepared according to the manufacturer’s instructions from the same batch to minimise variation in fluoride release profiles due to differences between batches.23 The operator wore latex examination gloves (Microtouch, Johnson and Johnson Medical Inc. Arlington, Texas. USA) throughout all of the experimental procedures to avoid contamination of the test materials and gloves were changed between cements. Prior to loading with cement, each band had a strip of masking tape placed over the occlusal surface. This encouraged an even distribution of material beneath the band during cementation by forcing it to move gingivally, as recommended for clinical practise.24 Following band cementation, the Ultra BandLok and 3M-Multicure samples were light-cured for 40 s in accordance with the manufacturer’s instructions using a dental curing light (Visilux 2, 3M Unitek, Monrovia, CA, USA) while Ketac-Cem specimens were allowed to bench cure for 5 min. Each banded tooth was then re-weighed to allow calculation of the cement weight. The teeth were covered with an acid-resistant nail varnish (Max Factor Diamond Hard, Procter and Gamble, Surrey, UK) leaving only 1 mm of exposed enamel around the band periphery, to ensure that the full cement surface was exposed. Once dry, each specimen was immersed in a sealed plastic vial with 2 ml of deionised water. After 24 h, each tooth was removed from its vial, blotted dry to prevent crossover contamination of fluoride and immersed in 2 ml of fresh deionised water. The solutions were changed daily for 10 days and thereafter on days 15, 20 and 30. One millilitre samples were taken at each changeover and stored at 2 20 8C until fluoride analysis could be carried out.
where the disc volume was calculated from the formula
Disc construction
Measurement of fluoride was undertaken using a fluoride ion selective electrode (Orion Combination Fluoride Specific Electrode No. 9609BN), attached to an ion-analyser (Orion Research Expandable Ion Analyser EA940, Boston, Massachusetts, USA). The fluoride electrode was calibrated using a series of standards of known concentration on the morning of each measuring session. 0.5 ml of total ionic strength adjustment buffer was added to 0.5 ml of each sample and pipetted into a microsample dish before being placed over a non-heating magnetic stirrer (H1304N. Jencons Scientific Limited, Leighton Buzzard, England). Cling film was wrapped around the electrode and microsample dish to minimise evaporation during the measuring procedure. Readings were taken after a 5 min immersion period. Between measurements, the electrode membrane was gently rinsed with deionised water, then blotted dry.
Using data for all banded specimens, the mean weight of cement for band cementation was calculated. To allow the density of the cement to be evaluated, five discs of known dimensions (3 mm diameter £ 2 mm height) were prepared. These were weighed immediately following setting of the cement and their mean weight was then estimated. The density of the cement was assessed using the formula Density ¼ Weightðdisc meanÞ=Volume The volume of cement needed to match the mean volume used to band the teeth was calculated using the formula Volume ¼ Weight ðcement mean per banded toothÞ= Density
Volume ¼ p £ radius2 £ height As the required volume for each disc was then known, the dimensions required to create that volume were generated from the formula Radius2 £ Height ¼ Volume=p This resulted in discs with dimensions of 4.5 mm diameter £ 2 mm height.
Preparation of discs Precision milled stainless steel cylindrical discs were manufactured to the dimensions calculated above (4.5 mm diameter and 2.0 mm height). A custom-made silicone putty mould was prepared using the stainless steel discs as a template. Five discs of each cement were made using this mould, the cements having been prepared and set as per manufacturers’ instructions. The discs were then immersed individually in a sealed plastic vial with 2 ml of deionised water. After 24 h each disc was removed from its vial, blotted dry to prevent crossover contamination and immersed in 2 ml of fresh deionised water. The solutions were changed on a daily basis for 10 days and thereafter on days 15, 20 and 30. One millilitre samples were taken at each changeover and stored at 2 20 8C until fluoride analysis could be carried out.
Assessment of fluoride release
22
G.R. Caves et al.
Table 1 Banded tooth model-daily and cumulative fluoride release, days 1, 5, 15, and 30 (units are ppm with SD in brackets). Day 1 Daily Ketac-Cem (Group 1) Ultra Band-Lok Ketac-Cem (group 2) 3M-Multicure
Day 5 Cumulative
Day 15
Day 30
Daily
Cumulative
Daily
Cumulative
Daily
Cumulative
4.46 (0.93)
4.46 (0.93)
1.87 (0.33)
11.62 (2.14)
0.80 (0.17)
25.43 (4.49)
0.91 (0.23)
37.56 (6.87)
1.23 (0.24) 3.36 (0.80)
1.23 (0.24) 3.36 (0.80)
0.22 (0.03) 2.21 (0.52)
2.43 (0.37) 12.04 (2.49)
0.04 (0.01) 0.93 (0.24)
3.55 (0.49) 28.60 (5.75)
0.15 (0.05) 1.14 (0.30)
4.80 (0.73) 43.95 (9.13)
10.53 (2.90)
10.53 (2.90)
3.75 (1.06)
26.46 (7.49)
1.24 (0.34)
51.91 (12.99)
1.64 (0.66)
72.68 (19.01)
Statistical analyses Mean daily and cumulative fluoride release were calculated for each cement and model. For an intramodel comparison for both discs and banded teeth, 2-tailed paired t-tests were used, whilst a 2-tailed 2sample t-test was used to compare the test materials with regard to both daily and cumulative fluoride release from each material. Inter-model comparisons were made using 2-tailed 2-sample t-tests.
both daily ( p ¼ 0.9) and cumulative ( p ¼ 0.66) fluoride release. This equated to 3 –4 times more fluoride release from the banded tooth model than from the disc model for Ketac-Cem and 3M-Multicure at 30 days. For Ultra Band-Lok, both models released comparable levels of fluoride at the same time point.
Discussion Pattern of fluoride release
Results Daily and cumulative fluoride release from the banded tooth and disc models are displayed in,Tables 1 and 2, respectively. Whereas 3M-Multicure demonstrated a consistently greater fluoride release than Ketac-Cem for both models, Ultra Band-Lok exhibited a lower level of fluoride release at all time points compared to Ketac-Cem. An inter-material comparison for each model indicated significant differences in both daily and cumulative fluoride release at all time intervals ( p , 0.05), except for comparisons between KetacCem and Ultra Band-Lok for the disc models at days 5 ( p ¼ 0.06), 15 ( p ¼ 0.05) and 30 ( p ¼ 0.13). In addition, no significant difference was found for day 30 comparisons between the disc models of KetacCem and 3M-Multicure ( p ¼ 0.08) and Ultra BandLok and 3M-Multicure ( p ¼ 0.06). An inter-model comparison for each cement indicated significant differences in daily ( p , 0.01) and cumulative ( p , 0.01) fluoride release except for Ultra Band-Lok at day 30 for
The pattern of fluoride release from the disc model for Ketac-Cem and Ultra Band-Lok was similar, and is comparable with other studies using KetacCem.13,23,25 No previous studies have assessed fluoride release from 3M-Multicure, though the pattern produced is similar to Ketac-Cem, Ultra Band-Lok and other resin-modified glass ionomers.3,14,26 The pattern of fluoride release from the banded tooth model matched the disc model for each cement.
Daily and cumulative fluoride release For both models, the daily mean and cumulative mean fluoride release was greatest from 3MMulticure, followed by Ketac-Cem and then Ultra Band-Lok. Greater fluoride release has been demonstrated from conventional glass ionomer cements than from resin-modified glass ionomers or polyacid-modified composites,27 – 29 while other studies have reported conflicting findings.16,17 The differences in study outcomes may be the result of
Table 2 Disc model—daily and cumulative fluoride release days 1, 5, 15, and 30 (units are ppm with SD in brackets). Day 1
Ketac-Cem Ultra Band-Lok 3M-Multicure
Day 5
Day 15
Day 30
Daily
Cumulative
Daily
Cumulative
Daily
Cumulative
Daily
Cumulative
1.77 (0.26) 0.66 (0.07) 5.88 (0.69)
1.77 (0.26) 0.66 (0.07) 5.88 (0.69)
0.24 (0.05) 0.13 (0.02) 0.70 (0.09)
4.00 (0.70) 1.51 (0.15) 13.51 (1.96)
0.24 (0.05) 0.10 (0.01) 0.53 (0.09)
6.71 (1.21) 2.54 (0.20) 20.48 (3.01)
0.19 (0.04) 0.15 (0.02) 0.46 (0.13)
9.22 (1.63) 4.61 (0.84) 27.08 (4.04)
Fluoride release from orthodontic band cements—a comparison of two in vitro models
either intrinsic (cement-related) and extrinsic (study-related) variables.30 The amount of fluoride release from discs of Ketac-Cem in the study reported here is similar to that observed in similar in vitro studies. Only two of these studies, however, are directly comparable. Gillgrass et al.13 measured fluoride release from discs of Ketac-Cem measuring 3 mm £ 1.5 mm. The pattern of fluoride release seen is very similar to that obtained in this study when the units of fluoride measurement are adjusted to ppm/cm2 to take account of differences in surface area.19 For example, on day 1 a fluoride release of 2.21 ppm/cm2 was reported,13 while the present study found a level of 2.83 ppm/ cm2. On day 15, the cumulative fluoride release 13 was 9.68 ppm/cm2 while a value of 10.74 ppm/ 2 cm was recorded in the present study. Both studies used 2 ml of deionised water and similar experimental protocols. Fox25 used slabs of Ketac-Cem and found different results to the present study, even when the units of fluoride release are adjusted for surface area. On day 2, he reported 9.61 ppm/ cm2 whilst Gillgrass et al.13 found 3.89 ppm/cm2 and the present study 4.23 ppm/cm2. Specimens, however, were immersed in 10 ml of distilled water by Fox25 compared with 2 ml in both this study and that of Gillgrass et al.13 Direct comparison of the findings cannot be made. Only one study has investigated the fluoride release profile of Ultra Band-Lok, demonstrating a similar pattern of both daily and cumulative fluoride release when the values are adjusted to account for surface area.13 Two studies have assessed fluoride release from Band-Lok31,32 a two-paste system of similar composition to Ultra Band-Lok. Interestingly, one study found Band-Lok to release more fluoride than resinmodified glass ionomers,31 whereas the other found less fluoride release from this product than from a resin-modified (Vitremer, 3M Dental Products Division, St Paul, MN, USA) or conventional (Fuji I, GC International Corp., Tokyo, Japan) glass ionomer.32 No studies have assessed fluoride release from 3MMulticure.
Inter-model comparison Despite having the same mean cement weight, the banded tooth model for Ketac-Cem and 3M-Multicure demonstrated approximately 2 – 3 times more daily and 3 – 4 times more cumulative fluoride release than the disc model. This result may be explained by considering the variables that influence fluoride release.
23
Though the cements for each model were matched with regard to weight/volume,18 powder – liquid ratio18,20 and mixing time,33 they could not be matched with regard to cement surface area. The surface area of the discs used in the study reported here was 0.6 cm2. Despite careful banding technique, the band cement margins varied slightly in height, width and bulk for each tooth, due mainly to variation in crown morphology and resultant band fit. It was not possible, therefore, to calculate exactly the band cement marginal surface area that was exposed. An estimation, however, of this is likely to fall well below the surface area of the disc model value of 0.6 cm2. Hence, the banded tooth model would have been expected to release less fluoride than the disc model. Differences in cement geometry for each model, in particular the compact nature of the cement in the disc model versus the thin lute of cement used in the banded tooth model, may explain partly the different results. Specimen geometry may alter cement fluoride release kinetics, though the exact mechanism by which this process occurs is still not entirely clear.30 In addition, the possibility of microleakage resulting in an increased available surface area for fluoride release cannot be ruled out, though the banded teeth were not subject to any thermocycling or other physical insult that might have compromised the cement/band or cement/enamel interface.13 For Ultra Band-Lok, the banded tooth model released significantly more fluoride than the disc model for days 1, 5 and 15. These inter-model differences, however, were of a much lesser magnitude than those seen for Ketac-Cem and 3MMulticure. This is likely to be due to Ultra Band-Lok releasing relatively low amounts of fluoride throughout the study and exhausting its fluoride reserves early on for both models, regardless of any inter-model differences in surface area. The standard deviations of mean fluoride release measurement were higher for the banded tooth model than for the disc model. This was to be expected, as the cement volume for each banded tooth varied considerably due to differences in crown morphology and band size. From the results of this study, it would seem that the disc model used here is not directly comparable to the banded tooth model with regard to fluoride release.
Conclusions 1. At 30 days, for both banded tooth and disc models, the mean cumulative fluoride release was greatest from 3M-Multicure followed by
24
Ketac-Cem, which in turn released more fluoride than Ultra Band-Lok. These differences were all significant ( p , 0.05). 2. Despite having the same mean cement weight, the banded tooth model for Ketac-Cem and 3MMulticure released approximately 3 – 4 times more cumulative fluoride than the disc model after 30 days ( p , 001). For Ultra Band-Lok, both models released comparable levels of fluoride ( p . 0.05). 3. Cement type, specimen geometry and surface area appear to influence significantly fluoride release characteristics.
G.R. Caves et al.
14.
15. 16.
17.
18.
19.
References 20. 1. Kvam E, Broch J, Nissen-Meyer IH. Comparison between a zinc phosphate cement and a glass ionomer cement for cementation of orthodontic bands. European Journal of Orthodontics 1983;5:307—13. 2. Maijer R, Smith DC. A comparison between zinc phosphate and glass ionomer cement in orthodontics. American Journal of Orthodontics and Dentofacial Orthopedics 1988;93: 273—9. 3. Bertacchini SM, Abate PF, Blank A, et al. Solubility and fluoride release in ionomers and compomers. Quintessence International 1999;30:193—7. 4. Momoi Y, McCabe JF. Fluoride release from light-activated glass ionomer restorative cements. Dental Materials 1993;9: 151—4. 5. El Mallakh BF, Sarkar NK. Fluoride release from glass ionomer cements in de-ionised water and artificial saliva. Dental Materials 1990;6:118—22. 6. DeSchepper EJ, Berr EA, Cailleteau JG, et al. A comparative study of fluoride release from glass—ionomer cements. Quintessence International 1991;22:215—20. 7. Rezk-Lega F, Øgaard B, Arends J. An in vivo study on the merits of two glass ionomers for the cementation of orthodontic bands. American Journal of Orthodontics and Dentofacial Orthopedics 1991;99:162—7. 8. Øgaard B, Rezk-Lega F, Rubens J, et al. Cariostatic effect and fluoride release from a visible light-curing adhesive for bonding of orthodontic brackets. American Journal of Orthodontic and Dentofacial Orthopedics 1992;101:303—7. 9. Rezk-Lega F, Øgaard B, Rølla G. Availability of fluoride from glass—ionomer cements in human saliva. Scandinavian Journal of Dental Research 1991;99:60—3. 10. MacSweeney F, St George G, McCabe JF. Fluoride release and uptake of glass ionomers using various storage media. Journal of Dental Research 1999;78:1073. 11. Wiltshire WA, Janse van Rensburg SD. Fluoride release from four visible light-cured orthodontic adhesive resins. American Journal of Orthodontics and Dentofacial Orthopedics 1995;108:278—83. 12. Forsten L. Short- and long-term fluoride release from glass ionomers and other fluoride-containing filling materials in vitro. Scandinavian Journal of Dental Research 1990;98: 179—85. 13. Gillgrass TJ, Millett DT, Creanor SL, et al. Fluoride release, microbial inhibition and microleakage pattern of two
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orthodontic band cements. Journal of Dentistry 1999;27: 455—61. Monteith VL, Millett DT, Creanor SL, et al. Fluoride release from orthodontic bonding agents: a comparison of three in vitro models. Journal of Dentistry 1999;27:53—61. Newman SM, Rudolf SL. Fluoride release from composite resin. Journal of Dental Research 1994;93:180. Creanor SL, Carruthers LM, Saunders WP, et al. Fluoride uptake and release characteristics of glass ionomer cements. Caries Research 1994;28:322—8. Chadwick SM, Gordon PHH. An investigation into the fluoride release of a variety of orthodontic bonding agents. British Journal of Orthodontics 1995;22:29—33. Cranfield M, Kuhn AT, Winter GB. Factors relating to the rate of fluoride-ion release from glass—ionomer. Journal of Dentistry 1982;10:333—41. Williams JA, Billington RW, Pearson GJ. The influence of sample dimensions on fluoride ion release from a glass ionomer restorative cement. Biomaterials 1999;20: 1327—37. Muzynski BL, Greener E, Jameson L, et al. Fluoride release from glass ionomers used as luting agents. Journal of Prosthetic Dentistry 1988;60:41—4. Chan DCN, Swift EJ, Bishara SE. In vitro evaluation of a fluoride-releasing orthodontic resin. Journal of Dental Research 1990;69:1576—9. Weatherell JA, Robinson C, Hallsworth AS. Variations in the chemical composition of human enamel. Journal of Dental Research 1974;53:180—92. Trimpeneers M, Verbeeck RMH, Dermaut LR. Long-term fluoride release of some orthodontic bonding resins: a laboratory study. Dental Materials 1998;14:142—9. Proffit WR. Contemporary orthodontic appliances in contemporary orthodontics, 3rd ed. St Louis: Mosby; 2000. p. 385—416. Fox NA. Fluoride release from orthodontic bonding materials. An in vitro study. British Journal of Orthodontics 1990;17:293—8. Galvan RI, Robertello FJ, Lynde TA. In vitro comparison of fluoride release from core materials. Journal of Dental Research 1999;78:115. Wyness DM, Sherriff M. Fluoride release from a variety of commercially available orthodontic bonding systems. Journal of Dental Research 1996;75:1177. Young A, von der Fehr FR, Sonju T, et al. Fluoride release and uptake in vitro from a composite resin and two orthodontic adhesives. Acta Odontologica Scandinavica 1996;54:223—8. Shaw AJ, Carrrick T, McCabe JF. Fluoride release from glass—ionomer and compomer restorative materials: 6month data. Journal of Dentistry 1998;26:355—9. Verbeeck RMH, De Maeyer EA, Marks LA, et al. Fluoride release process of (resin-modified) glass—ionomer cements versus (polyacid-modified) composite resins. Biomaterials 1998;19:509—19. Ashcraft DB, Staley RN, Jakobsen JR. Fluoride release and shear bond strengths of three light-cured glass ionomer cements. American Journal of Orthodontics and Dentofacial Orthopedics 1997;111:260—5. Wiltshire WA. In vitro and in vivo fluoride release from orthodontic elastomeric ligature ties. American Journal of Orthodontic and Dentofacial Orthopedics 1999;115: 288—92. Swift EJ. The effect of mixing time on fluoride release from a glass ionomer cement. American Journal of Dentistry 1988; 1:132—4.
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Fluoride release from orthodontic band cements—a comparison of two in vitro models G.R. Cavesa, D.T. Milletta,*, S.L. Creanorb, R.H. Foyeb, W.H. Gilmourc a
Unit of Orthodontics, University of Glasgow Dental Hospital and School, 378 Sauchiehall Street, Glasgow G2 3JZ, UK b Oral Biology and New Technology Research Group, University of Glasgow Dental School, Glasgow, UK c Departments of Statistics and Public Health, University of Glasgow, Glasgow, UK Received 20 September 2001; accepted 5 November 2002
KEYWORDS Fluoride release; Orthodontic band cements; In vitro model
Summary Objectives. To compare, in vitro, the fluoride release from a conventional glass ionomer cement (Ketac-Cem), a resin-modified glass ionomer cement (3MMulticure) and a polyacid modified composite (Ultra Band-Lok) using a banded tooth model and a disc model with the same mean cement weight. Methods. Forty pairs of caries-free third molars were collected and divided into two groups, each of 20 teeth. One tooth from each pair was banded with Ketac-Cem and the other with Ultra Band-Lok or 3M-Multicure; the average band size for each cement group was the same. Two coats of nail varnish were painted on each tooth to within 1 mm of the band margin. Five discs (4.5 mm diameter and 2 mm depth) were prepared for each cement, these dimensions having been calculated so that the mean cement weight of the banded tooth model matched that of the disc model for each cement. The fluoride released into 2 ml of deionised water, from each banded tooth or disc, was measured at regular intervals over 30 days using an Orion ion-selective electrode connected to an ion analyser. Results. At 30 days, for both banded tooth and disc models, the mean cumulative fluoride release was greatest from 3M-Multicure followed by Ketac-Cem, which in turn released more fluoride than Ultra Band-Lok. These differences were all significant ( p , 0.05). Despite having the same mean cement weight, the banded tooth model for Ketac-Cem and 3M-Multicure released approximately 3–4 times more cumulative fluoride than the disc model after 30 days ( p , 001). For Ultra Band-Lok, both models released comparable levels of fluoride ( p . 0.05). Conclusions. Cement type, specimen geometry and surface area appear to influence significantly fluoride release characteristics. Q 2003 Elsevier Science Ltd. All rights reserved.
Introduction Historically the use of zinc phosphate cement for orthodontic band cementation has been associated *Corresponding author. Tel.: þ 44-141-211-9666; fax: þ 44141-331-2798. E-mail address: [email protected]
with enamel decalcification. 1 Bands are now cemented routinely with glass ionomer cements, which release and uptake fluoride, thereby affording greater cariostatic potential.2 Studies which have assessed fluoride release from orthodontic cements have been conducted primarily using a disc model, but wide variation
0300-5712/03/$ - see front matter Q 2003 Elsevier Science Ltd. All rights reserved. doi: 1 0 . 1 0 1 6 / S 0 3 0 0 - 5 7 1 2 ( 0 2 ) 0 0 0 8 7 - 8
20
exists in the methodology used. The sample size has ranged from one specimen3 to 254 and the medium into which fluoride is released has varied in both type and volume. Although distilled or deionised water has been employed mostly, saliva has been used also as the specimen immersant.5 – 8 Fluoride release, however, is significantly less into artificial or human saliva,5,9,10 though this medium does not replicate entirely the clinical situation.8 The volume of the immersant has ranged from 16,11 to 50 ml3 and the observation time has varied from 1 h9 to 2 years.12 Specimen shape and size have also varied considerably. Discs as small as 3 mm £ 1.5 mm 13,14 ranging up to 20 mm £ 1.5 mm15 have been used. Most studies have demonstrated a similar pattern of fluoride release over the first 2 days. Then there followed an exponential decay with a plateau reached within 1 – 2 weeks.12,16 The amount of fluoride released varies substantially and is dependent on the type of cement being tested. Though most studies demonstrate greatest fluoride release from conventional glass ionomers when compared with resin-modified glass ionomers or polyacidmodified composites, some have shown comparable fluoride release between these products16 and others the reverse.17 A number of factors effect fluoride release from glass ionomer cements. When the surface area is kept constant, an almost linear relationship between cement volume and fluoride release has been found18 A non-linear relationship between surface area and fluoride release, however, has been demonstrated when the volume is kept constant.14 A 50% reduction in surface area reduced fluoride release by almost 25%. Williams et al.19 also showed that fluoride release is strongly dependent on surface area rather than volume and recommended that units of fluoride release should be quoted in terms of specimen surface area, particularly relevant in an orthodontic model. When the volume to surface area ratio is lowered, fluoride release rises.18,20 To date, no standard clinically appropriate specimen size has been recommended for use in orthodontically related fluoride release studies. It seems pertinent, therefore, to question the relevance of using non-standardised discs that bear no relationship to the mass or geometry of cement found clinically beneath a band or bracket. Furthermore, no attempt has been made to calculate the amount of cement beneath an orthodontic band that would enable the construction of a disc model that emulates more the clinical situation. It appears that no study has yet developed a banded tooth model akin to
G.R. Caves et al.
the bracket/tooth model used by other workers in assessing fluoride release from orthodontic cements.14,21 This study aims to address both of these aspects. The aims of this study were to compare the fluoride release characteristics of a conventional glass ionomer cement to that of a resin-modified glass ionomer and a polyacid-modified composite using both a disc model and a banded tooth model where the mean weight of the cement discs is the same as that found beneath the bands. The null hypotheses were that firstly, there was no difference between the cements with regard to their daily and cumulative fluoride release; secondly, there was no difference between the banded tooth model and the disc model with regard to the daily and cumulative fluoride release from each cement.
Materials and methods Banded tooth model preparation Forty pairs of caries-free intact human third molars were obtained from subjects resident in nonfluoridated areas. Each pair of molars originated from the same subject to minimise any potential differences in enamel fluoride levels. 22 After debridement, teeth were stored in a 0.12% thymol solution and were cleaned thoroughly with a water and pumice mixture. An orthodontic first molar band (3M Unitek, Bradford, England) was sized for each tooth using the smallest one possible in each case and contouring the edges to ensure close adaptation to the enamel surface. Each tooth was dried using compressed air and weighed with its band on a Sartorius MCI Balance (Sartorius, Goettingen, Germany). A conventional glass ionomer cement, KetacCem (ESPE, Seefeld, Oberbay Gmbh, Germany), a modified composite, Ultra Band-Lok (Reliance Orthodontic Products Inc. Itasca, ILL, USA) and a resin-modified glass ionomer cement, 3M-Multicure (3M Unitek, Monrovia, CA, USA) were chosen as the test materials. The teeth were divided into two groups of 20 pairs of teeth. One tooth from each pair was banded with Ketac-Cem and the other with Ultra-Bandlok or 3M-Multicure. The assignment of teeth took into account the band size for each tooth so that the mean band size for each group was the same. This ensured that the mean crown size of each group and, therefore, the mean cement volume was as similar as possible.
Fluoride release from orthodontic band cements—a comparison of two in vitro models
21
Each cement was prepared according to the manufacturer’s instructions from the same batch to minimise variation in fluoride release profiles due to differences between batches.23 The operator wore latex examination gloves (Microtouch, Johnson and Johnson Medical Inc. Arlington, Texas. USA) throughout all of the experimental procedures to avoid contamination of the test materials and gloves were changed between cements. Prior to loading with cement, each band had a strip of masking tape placed over the occlusal surface. This encouraged an even distribution of material beneath the band during cementation by forcing it to move gingivally, as recommended for clinical practise.24 Following band cementation, the Ultra BandLok and 3M-Multicure samples were light-cured for 40 s in accordance with the manufacturer’s instructions using a dental curing light (Visilux 2, 3M Unitek, Monrovia, CA, USA) while Ketac-Cem specimens were allowed to bench cure for 5 min. Each banded tooth was then re-weighed to allow calculation of the cement weight. The teeth were covered with an acid-resistant nail varnish (Max Factor Diamond Hard, Procter and Gamble, Surrey, UK) leaving only 1 mm of exposed enamel around the band periphery, to ensure that the full cement surface was exposed. Once dry, each specimen was immersed in a sealed plastic vial with 2 ml of deionised water. After 24 h, each tooth was removed from its vial, blotted dry to prevent crossover contamination of fluoride and immersed in 2 ml of fresh deionised water. The solutions were changed daily for 10 days and thereafter on days 15, 20 and 30. One millilitre samples were taken at each changeover and stored at 2 20 8C until fluoride analysis could be carried out.
where the disc volume was calculated from the formula
Disc construction
Measurement of fluoride was undertaken using a fluoride ion selective electrode (Orion Combination Fluoride Specific Electrode No. 9609BN), attached to an ion-analyser (Orion Research Expandable Ion Analyser EA940, Boston, Massachusetts, USA). The fluoride electrode was calibrated using a series of standards of known concentration on the morning of each measuring session. 0.5 ml of total ionic strength adjustment buffer was added to 0.5 ml of each sample and pipetted into a microsample dish before being placed over a non-heating magnetic stirrer (H1304N. Jencons Scientific Limited, Leighton Buzzard, England). Cling film was wrapped around the electrode and microsample dish to minimise evaporation during the measuring procedure. Readings were taken after a 5 min immersion period. Between measurements, the electrode membrane was gently rinsed with deionised water, then blotted dry.
Using data for all banded specimens, the mean weight of cement for band cementation was calculated. To allow the density of the cement to be evaluated, five discs of known dimensions (3 mm diameter £ 2 mm height) were prepared. These were weighed immediately following setting of the cement and their mean weight was then estimated. The density of the cement was assessed using the formula Density ¼ Weightðdisc meanÞ=Volume The volume of cement needed to match the mean volume used to band the teeth was calculated using the formula Volume ¼ Weight ðcement mean per banded toothÞ= Density
Volume ¼ p £ radius2 £ height As the required volume for each disc was then known, the dimensions required to create that volume were generated from the formula Radius2 £ Height ¼ Volume=p This resulted in discs with dimensions of 4.5 mm diameter £ 2 mm height.
Preparation of discs Precision milled stainless steel cylindrical discs were manufactured to the dimensions calculated above (4.5 mm diameter and 2.0 mm height). A custom-made silicone putty mould was prepared using the stainless steel discs as a template. Five discs of each cement were made using this mould, the cements having been prepared and set as per manufacturers’ instructions. The discs were then immersed individually in a sealed plastic vial with 2 ml of deionised water. After 24 h each disc was removed from its vial, blotted dry to prevent crossover contamination and immersed in 2 ml of fresh deionised water. The solutions were changed on a daily basis for 10 days and thereafter on days 15, 20 and 30. One millilitre samples were taken at each changeover and stored at 2 20 8C until fluoride analysis could be carried out.
Assessment of fluoride release
22
G.R. Caves et al.
Table 1 Banded tooth model-daily and cumulative fluoride release, days 1, 5, 15, and 30 (units are ppm with SD in brackets). Day 1 Daily Ketac-Cem (Group 1) Ultra Band-Lok Ketac-Cem (group 2) 3M-Multicure
Day 5 Cumulative
Day 15
Day 30
Daily
Cumulative
Daily
Cumulative
Daily
Cumulative
4.46 (0.93)
4.46 (0.93)
1.87 (0.33)
11.62 (2.14)
0.80 (0.17)
25.43 (4.49)
0.91 (0.23)
37.56 (6.87)
1.23 (0.24) 3.36 (0.80)
1.23 (0.24) 3.36 (0.80)
0.22 (0.03) 2.21 (0.52)
2.43 (0.37) 12.04 (2.49)
0.04 (0.01) 0.93 (0.24)
3.55 (0.49) 28.60 (5.75)
0.15 (0.05) 1.14 (0.30)
4.80 (0.73) 43.95 (9.13)
10.53 (2.90)
10.53 (2.90)
3.75 (1.06)
26.46 (7.49)
1.24 (0.34)
51.91 (12.99)
1.64 (0.66)
72.68 (19.01)
Statistical analyses Mean daily and cumulative fluoride release were calculated for each cement and model. For an intramodel comparison for both discs and banded teeth, 2-tailed paired t-tests were used, whilst a 2-tailed 2sample t-test was used to compare the test materials with regard to both daily and cumulative fluoride release from each material. Inter-model comparisons were made using 2-tailed 2-sample t-tests.
both daily ( p ¼ 0.9) and cumulative ( p ¼ 0.66) fluoride release. This equated to 3 –4 times more fluoride release from the banded tooth model than from the disc model for Ketac-Cem and 3M-Multicure at 30 days. For Ultra Band-Lok, both models released comparable levels of fluoride at the same time point.
Discussion Pattern of fluoride release
Results Daily and cumulative fluoride release from the banded tooth and disc models are displayed in,Tables 1 and 2, respectively. Whereas 3M-Multicure demonstrated a consistently greater fluoride release than Ketac-Cem for both models, Ultra Band-Lok exhibited a lower level of fluoride release at all time points compared to Ketac-Cem. An inter-material comparison for each model indicated significant differences in both daily and cumulative fluoride release at all time intervals ( p , 0.05), except for comparisons between KetacCem and Ultra Band-Lok for the disc models at days 5 ( p ¼ 0.06), 15 ( p ¼ 0.05) and 30 ( p ¼ 0.13). In addition, no significant difference was found for day 30 comparisons between the disc models of KetacCem and 3M-Multicure ( p ¼ 0.08) and Ultra BandLok and 3M-Multicure ( p ¼ 0.06). An inter-model comparison for each cement indicated significant differences in daily ( p , 0.01) and cumulative ( p , 0.01) fluoride release except for Ultra Band-Lok at day 30 for
The pattern of fluoride release from the disc model for Ketac-Cem and Ultra Band-Lok was similar, and is comparable with other studies using KetacCem.13,23,25 No previous studies have assessed fluoride release from 3M-Multicure, though the pattern produced is similar to Ketac-Cem, Ultra Band-Lok and other resin-modified glass ionomers.3,14,26 The pattern of fluoride release from the banded tooth model matched the disc model for each cement.
Daily and cumulative fluoride release For both models, the daily mean and cumulative mean fluoride release was greatest from 3MMulticure, followed by Ketac-Cem and then Ultra Band-Lok. Greater fluoride release has been demonstrated from conventional glass ionomer cements than from resin-modified glass ionomers or polyacid-modified composites,27 – 29 while other studies have reported conflicting findings.16,17 The differences in study outcomes may be the result of
Table 2 Disc model—daily and cumulative fluoride release days 1, 5, 15, and 30 (units are ppm with SD in brackets). Day 1
Ketac-Cem Ultra Band-Lok 3M-Multicure
Day 5
Day 15
Day 30
Daily
Cumulative
Daily
Cumulative
Daily
Cumulative
Daily
Cumulative
1.77 (0.26) 0.66 (0.07) 5.88 (0.69)
1.77 (0.26) 0.66 (0.07) 5.88 (0.69)
0.24 (0.05) 0.13 (0.02) 0.70 (0.09)
4.00 (0.70) 1.51 (0.15) 13.51 (1.96)
0.24 (0.05) 0.10 (0.01) 0.53 (0.09)
6.71 (1.21) 2.54 (0.20) 20.48 (3.01)
0.19 (0.04) 0.15 (0.02) 0.46 (0.13)
9.22 (1.63) 4.61 (0.84) 27.08 (4.04)
Fluoride release from orthodontic band cements—a comparison of two in vitro models
either intrinsic (cement-related) and extrinsic (study-related) variables.30 The amount of fluoride release from discs of Ketac-Cem in the study reported here is similar to that observed in similar in vitro studies. Only two of these studies, however, are directly comparable. Gillgrass et al.13 measured fluoride release from discs of Ketac-Cem measuring 3 mm £ 1.5 mm. The pattern of fluoride release seen is very similar to that obtained in this study when the units of fluoride measurement are adjusted to ppm/cm2 to take account of differences in surface area.19 For example, on day 1 a fluoride release of 2.21 ppm/cm2 was reported,13 while the present study found a level of 2.83 ppm/ cm2. On day 15, the cumulative fluoride release 13 was 9.68 ppm/cm2 while a value of 10.74 ppm/ 2 cm was recorded in the present study. Both studies used 2 ml of deionised water and similar experimental protocols. Fox25 used slabs of Ketac-Cem and found different results to the present study, even when the units of fluoride release are adjusted for surface area. On day 2, he reported 9.61 ppm/ cm2 whilst Gillgrass et al.13 found 3.89 ppm/cm2 and the present study 4.23 ppm/cm2. Specimens, however, were immersed in 10 ml of distilled water by Fox25 compared with 2 ml in both this study and that of Gillgrass et al.13 Direct comparison of the findings cannot be made. Only one study has investigated the fluoride release profile of Ultra Band-Lok, demonstrating a similar pattern of both daily and cumulative fluoride release when the values are adjusted to account for surface area.13 Two studies have assessed fluoride release from Band-Lok31,32 a two-paste system of similar composition to Ultra Band-Lok. Interestingly, one study found Band-Lok to release more fluoride than resinmodified glass ionomers,31 whereas the other found less fluoride release from this product than from a resin-modified (Vitremer, 3M Dental Products Division, St Paul, MN, USA) or conventional (Fuji I, GC International Corp., Tokyo, Japan) glass ionomer.32 No studies have assessed fluoride release from 3MMulticure.
Inter-model comparison Despite having the same mean cement weight, the banded tooth model for Ketac-Cem and 3M-Multicure demonstrated approximately 2 – 3 times more daily and 3 – 4 times more cumulative fluoride release than the disc model. This result may be explained by considering the variables that influence fluoride release.
23
Though the cements for each model were matched with regard to weight/volume,18 powder – liquid ratio18,20 and mixing time,33 they could not be matched with regard to cement surface area. The surface area of the discs used in the study reported here was 0.6 cm2. Despite careful banding technique, the band cement margins varied slightly in height, width and bulk for each tooth, due mainly to variation in crown morphology and resultant band fit. It was not possible, therefore, to calculate exactly the band cement marginal surface area that was exposed. An estimation, however, of this is likely to fall well below the surface area of the disc model value of 0.6 cm2. Hence, the banded tooth model would have been expected to release less fluoride than the disc model. Differences in cement geometry for each model, in particular the compact nature of the cement in the disc model versus the thin lute of cement used in the banded tooth model, may explain partly the different results. Specimen geometry may alter cement fluoride release kinetics, though the exact mechanism by which this process occurs is still not entirely clear.30 In addition, the possibility of microleakage resulting in an increased available surface area for fluoride release cannot be ruled out, though the banded teeth were not subject to any thermocycling or other physical insult that might have compromised the cement/band or cement/enamel interface.13 For Ultra Band-Lok, the banded tooth model released significantly more fluoride than the disc model for days 1, 5 and 15. These inter-model differences, however, were of a much lesser magnitude than those seen for Ketac-Cem and 3MMulticure. This is likely to be due to Ultra Band-Lok releasing relatively low amounts of fluoride throughout the study and exhausting its fluoride reserves early on for both models, regardless of any inter-model differences in surface area. The standard deviations of mean fluoride release measurement were higher for the banded tooth model than for the disc model. This was to be expected, as the cement volume for each banded tooth varied considerably due to differences in crown morphology and band size. From the results of this study, it would seem that the disc model used here is not directly comparable to the banded tooth model with regard to fluoride release.
Conclusions 1. At 30 days, for both banded tooth and disc models, the mean cumulative fluoride release was greatest from 3M-Multicure followed by
24
Ketac-Cem, which in turn released more fluoride than Ultra Band-Lok. These differences were all significant ( p , 0.05). 2. Despite having the same mean cement weight, the banded tooth model for Ketac-Cem and 3MMulticure released approximately 3 – 4 times more cumulative fluoride than the disc model after 30 days ( p , 001). For Ultra Band-Lok, both models released comparable levels of fluoride ( p . 0.05). 3. Cement type, specimen geometry and surface area appear to influence significantly fluoride release characteristics.
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14.
15. 16.
17.
18.
19.
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