Australian Dental Journal
The official journal of the Australian Dental Association
Australian Dental Journal 2014; 59: 208–214 doi: 10.1111/adj.12168
Fluoride release, recharge and flexural properties of polymethylmethacrylate containing fluoridated glass fillers IA Al-Bakri,* MV Swain,* SJ Naoum,* WM Al-Omari,† E Martin,‡ A Ellakwa§ *Department of Biomaterials, Faculty of Dentistry, The University of Sydney, New South Wales, Australia. †Faculty of Dentistry, The University of Sydney, New South Wales, Australia and Department of Prosthodontics, Faculty of Dentistry, Jordan University of Science and Technology, Irbid, Jordan. ‡Department of Conservative Dentistry, Faculty of Dentistry, The University of Sydney, New South Wales, Australia. §Department of Biomaterials, Faculty of Dentistry, The University of Sydney, New South Wales and Faculty of Dentistry, Tanta University, Tanta, Egypt.
ABSTRACT Background: The purpose of this study was to investigate the effect of fluoridated glass fillers on fluoride release, recharge and the flexural properties of modified polymethylmethacrylate (PMMA). Methods: Specimens of PMMA denture base material with various loading of fluoridated glass fillers (0%, 1%, 2.5%, 5% and 10% by weight) were prepared. Flexural properties were evaluated on rectangular specimens (n = 10) aged in deionized water after 24 hours, 1 and 3 months. Disc specimens (n = 10) were aged for 43 days in deionized water and lactic acid (pH 4.0) and fluoride release was measured at numerous intervals. After ageing, specimens were recharged and fluoride re-release was recorded at 1, 3 and 7 days after recharge. Results: Samples containing 2.5%, 5% and 10% glass fillers showed significantly (p < 0.05) greater levels of fluoride release compared with the control and 1% glass fillers specimens. All experimental specimens exhibited fluoride release in both media. The flexural strength of specimens decreased in proportion to the percentage filler inclusion with the modulus of elasticity values remaining within ISO Standard 1567. Conclusions: The modified PMMA with fluoridated glass fillers has the ability to release and re-release fluoride ion. Flexural strength decreased as glass filler uploading increased. Keywords: Flexural strength, fluoride release, glass fillers, modulus of elasticity, polymethylmethacrylate. Abbreviations and acronyms: ANOVA = analysis of variance; PMMA = polymethylmethacrylate; S-PRG = surface pre-reacted giomer. (Accepted for publication 10 September 2013.)
INTRODUCTION With the increasing lifespan of the general population, the need for the prosthodontic replacement of missing natural teeth will increase.1 Despite the wide use of implant prostheses to replace lost teeth, the removable partial denture remains one of the most widely accepted means of tooth replacement. As the population ages the proportion of patients seeking this type of prosthesis is likely to increase, especially as the cost of multiple tooth replacement using implants and the lack of sufficient bone in areas of long-term tooth loss limit implant use.2,3 Notably however, the risk of root caries is reported to be higher in older patients as a result of gingival recession, and in those patients wearing removable partial dentures.3 Abutment teeth that are used to secure and support a partial denture are at particular risk of caries development as denture clasps and rests 208
function as niches for the collection of food debris and plaque.4,5 Hence, preventing caries of abutment teeth associated with removable partial dentures is crucial, and delivery of fluoride into the oral cavity using a variety of vehicles is recommended as a means of preventing the formation of caries, and the development of recurrent caries.6,7 One possible mode of fluoride delivery to tooth surfaces could be through the use of fluoride releasing materials contained within the denture base. It has been reported in longitudinal clinical studies that sound approximal surfaces in contact with restorations capable of fluoride ion release exhibit a lower rate of caries incidence compared to sound tooth surfaces contacting non-fluoride releasing materials.8 Although the quantities of fluoride released from a restorative material may be substantially smaller than that provided by the application of a dentifrice, a sustained level of minimal fluoride ion released from a © 2014 Australian Dental Association
Fluoride release and properties of modified polymethylmethacrylate restorative material adjacent to a tooth structure might result in a caries inhibitive effect.9 A recent study reported that an experimental heatpolymerizing polymethylmethacrylate (PMMA) resin, containing surface pre-reacted giomer (S-PRG) filler particles was able to demonstrate both fluoride release and fluoride recharge capability.10 Mukai et al. found that fluoride release from the S-PRG particles contained in a denture base resin resulted in the percentage mineral loss of contacting dentine subjected to a demineralizing solution to be decreased by up to 60%.11 However, despite the potential benefit that the incorporation of fluoride releasing glass fillers within denture bases could provide to abutment tooth longevity, few studies have assessed the potential of fluoride releasing denture acrylic materials.10 The aim of this study was to evaluate the flexural strength, elastic modulus and fluoride release levels after ageing and following recharge with sodium fluoride of a PMMA denture base material modified by the addition of different percentages of fluoridated glass fillers. MATERIALS AND METHODS Specimen preparation To fabricate the experimental denture base materials, white silane-coated glass fillers (ultrafine GM35429) (Table 1) with an average particle size of approximately 1.5 lm were added to PMMA powder. The mixture was dispersed vigorously for 5 minutes using a vacuum mixer. Powder mixtures of four different experimental denture bases with various filler content (1.0%, 2.5%, 5% and 10% by weight) and a control group with 0% glass fillers content were prepared. A metal mould was employed to prepare 150 rectangular specimens (65 9 10 9 3.5 mm, n = 10/5 groups/3 ageing interval) to measure flexural strength and modulus of elasticity in accordance with ISO 1567 specifications.12 To measure fluoride release, another metal mould was used to prepare 50 disc specimens (10 9 1.5 mm, n = 10/group). The metal moulds were placed in conventional dental metal
Table 1. The manufacturers and composition of the fluoridated glass fillers and their silane coating Material
Manufacturer
Composition
Glass fillers (ultrafine GM35429)
Urban, clear pink, C2, Shofu Inc., Kyoto, Japan Schott, Landshut, Germany
SiO2 (30% wt)CaO (10% wt)Al2O3 (30% wt)F (15% wt)P2O5 (2.5% filler >1% filler >0% filler). All resin base samples continued to release fluoride for the 43-day ageing period except the control group (0% filler). At the completion of 43 days ageing, the level of fluoride release had decreased in all testing groups to 0.4 lg/cm2 or less. Each of the experimental denture base samples, with the exception of those containing 2.5% filler, demonstrated significantly greater fluoride release into lactic acid compared to the release into water. Significant differences in fluoride release were found between groups (p < 0.05) within the same testing interval and among the same experimental groups over the 9 testing intervals. Figure 5 shows the weekly cumulative fluoride ion release for the experimental resin samples aged in deionized water and lactic acid, following a weekly 5-minute fluoride recharge (5000 ppm) for three consecutive weeks following 43 days of ageing. All experimental resin bases demonstrated fluoride recharge, the level of fluoride ion release from each resin base 8 7 6 5 4 week 3 week 2 week 1
3 2 1 LA
LA 2.5 % LA 5% LA 10 % LA
1%
0%
%
DW
0
10
Figure 4 shows the cumulative fluoride ion release from the denture base samples aged in deionized water and lactic acid over 43 days. During the 43-day analysis period the cumulative fluoride release demonstrated by each resin base sample was positively correlated to the level of fluoridated glass fillers contained
1% fluoride DW
0
DW
Fluoride release and recharge
1% fluoride LA
DW
deterioration between the control group (0% fillers) and the denture base samples containing 10% glass filler content was recorded as 4.6% for the (24 hours – 1 month period), which dropped to 1.9% for the (1 month – 3 month period). For the samples containing 2.5%, 5% and 10% filler particle loading the differences compared with the control group (0% fillers) at (24 hours – 1 month period) were 2.5, 4.5 and 4.6 respectively. These differences dropped to 0, 0.2 and 1.9 in the (1 month – 3 months period) test. There were statistically significant differences in the modulus of elasticity at 24 hours ageing between the resin samples containing 5% filler (recording the highest reading of 2.87 Gpa) and the samples containing 2.5% and 10% filler (Fig. 2), otherwise no significant differences (p > 0.05) were noted between the samples at 1 month and 3 months containing varied filler loadings.
2.5% fluoride DW
2.5% fluoride LA
5%
Fig. 3 The percentage change in flexural strength for the experimental resins over the testing intervals.
0.6 0.4 0.2
5% fluoride DW
DW
Fig. 2 The means of modulus of elasticity (GPa) recorded for all groups at different ageing intervals.
5% fluoride LA
10% fluoride DW
%
0 1 2.5 5 10 Experimental groups with different loading percentages (%)
1.2 1 0.8
1%
3 months
10% fluoride LA
2.5
1 month
1.6 1.4
DW
24 hours
Fluoride ion concentraion (µg/cm2/day)
1.8
Fluoride ion concentration (µg/cm2)
2.9 2.8 2.7 2.6 2.5 2.4 2.3 2.2 2.1 2
0%
Modulus of elasticity (GPa)
Fluoride release and properties of modified polymethylmethacrylate
Testing groups in storage medium
Fig. 5 Cumulative fluoride ion release from each group aged in deionized water (DW) and lactic acid (LA) for 43 days ageing. Followed by a weekly 5-minute fluoride recharge (5000 ppm), for three consecutive weeks. 211
IA Al-Bakri et al. into both solutions increased markedly after fluoride recharge compared to the release following 43 days ageing. The level of fluoride ion release following recharge was positively correlated with the level of glass filler within each experimental resin base. The denture base samples containing 10% filler demonstrated the greatest cumulative release among the groups, in both media, for each of the three weeks. The unfilled PMMA samples demonstrated fluoride release after recharge with sodium fluoride particularly in the first day post recharge which then declined to a negligible amount with the passage of time in contrast with the denture base specimens of 2.5%, 5% and 10% filler loading, which showed continuous re-release capability. DISCUSSION It is essential that any chemical or mechanical modifications made to materials used for removable partial dentures should either sustain or improve the mechanical properties of the material. In the present study, the fluoridated filler particles were treated using a silane coupling agent before incorporation within the resin material. This was undertaken in an attempt to improve bonding between the filler particles and denture base resin matrix, potentially enhancing the mechanical properties of the denture base material. All the experimental resins tested post 24 hours exhibited flexural strength values greater than 65 MPa and modulus of elasticity values greater than 2 GPa and therefore complied with the requirement of ISO 1567. However, despite the silane treatment, it was observed that the flexural strength recordings of the experimental base samples decreased as the percentage addition of glass fillers increased. Further, with the exception of the denture base samples containing 1% filler, all the experimental denture base samples demonstrated significantly (p < 0.05) lower strength values compared to the control group (0% filler) at 24 hours, 1 month and 3 months. Similar results were also reported in a previous study in which non-silane treated S-PRG fluoride containing fillers were incorporated within a denture base resin.10 In order to further investigate the flexural property changes through the fluoride release procedure, the 1 month and 3 month results were statistically analysed. The results clearly showed that the difference in deterioration between the control group and the other experimental resins decreased over time. As observed from the flexural strength test results, the control group with 0% glass fillers decreased and at 1–3 months was similar to the bases containing glass fillers. 212
Additionally, the results of this study are suggestive of a threshold of filler incorporation. In order to prevent further detrimental effects on the mechanical properties of the denture base resin, 10% wt filler addition was considered an appropriate upper limit. This is supported by Kamijo et al. who recommended a filler content of less than 20 wt%.10 This study demonstrated that denture base materials incorporating 2.5% or more fluoridated filler were able to exhibit sustained fluoride release during ageing but declined with the passage of time, in addition to fluoride release following recharge. This finding is of potential clinical relevance as the development of denture base resins capable of fluoride release and recharge has the potential to deliver fluoride to abutment teeth susceptible to the development of dental caries. By comparison, the denture base samples containing 1% wt glass fillers demonstrated undetectable fluoride release. These observations indicate that the level of fluoride release correlates with the amount of fluoride filler contained in the denture base resin samples. This finding is in agreement with a previous investigation that reported the proportion of S-PRG fillers added to PMMA correlated with the amount of fluoride ion released during ageing and following recharge.10,11 The mechanism of denture base fluoride release observed in this study resulted from dissolution of the inorganic fluorides contained within the fluoroalumino-silicate glass fillers.13 This pattern of fluoride release has been described in previous studies where leachable glass fillers dispersed in a polymer matrix are water soluble and capable of releasing fluoride ions into the storage medium.14,15 However, the exact mechanism of fluoride release from the denture base resin materials has not been investigated. It is acknowledged that incorporation of inorganic fluoride containing particles into restorative materials results in fluoride release, but this also has the potential to create voids within the matrix as the fluoride leaches out of the material.16,17 A similar effect might have occurred with the denture base materials modified by the addition of fluoridated fillers. The final porosity of the material may have an adverse effect on the mechanical properties, but might also allow deeper diffusion of the recharge solution, thus resulting in greater potential for fluoride storage and release.14 Further, the hydrophilic nature of PMMA denture base material may play a key role in the absorption of solutions which may enhance the dissolution of fluoridated glass fillers with a resultant increase in fluoride uptake and release. Further studies are warranted to understand the underlying mechanism of fluoride release from denture base material containing fluoridated fillers. © 2014 Australian Dental Association
Fluoride release and properties of modified polymethylmethacrylate The experimental denture base containing 10% fluoridated glass filler showed 27.2% greater cumulative fluoride release in lactic acid than in deionized water after 43 days ageing. Attin et al.18 and Karantakis et al.19 have explained the increased fluoride release in the acidic medium as a result of the low pH increasing the dissolution of the material. However, the potential effect of fluoride ion release on the surface topography and subsequently the possible plaque adhesion to surface irregularities would be an area for further investigation. The results of this study demonstrated the ability of fluoridated glass fillers within a denture base resin to facilitate fluoride release following recharge involving immersion of the samples within a 5000 ppm NaF gel for 5 minutes. Although previous studies employed different recharge regimens,10,14 the current protocol can be recommended for use by patients as a routine daily toothbrushing regimen, as it has been reported that many denture wearers do not remove their dentures for daily cleaning.9,20 Seemingly after the recharge regimen, only the adsorbed fluoride to the surface was released and thus depleted shortly for the 0% filler group, whilst the fluoridated glass fillers in the other experimental groups served as a reservoir for storage of fluoride ions and a vehicle for continuous release. For all groups tested in both media the amount of fluoride released in the 24 hours post recharge was significantly (p < 0.05) greater than the amount of fluoride released over the first 24 hours of ageing; for the experimental denture base samples containing 10% fluoridated glass particles, the difference was 106.5% and 83.3% in deionized water and lactic acid respectively. Although a similar pattern of fluoride release was observed by Kamijo et al. after recharge, the cumulative amount of fluoride release was higher in the current study.10 However, differences exist in the methods between the two studies which render a direct comparison of the results difficult. Some differences included dry curing of the denture base material, the use of S-PRG fillers, the use of different particle size fillers, and the use of uncoated particles. The fluoride released from restorative materials occurs as a result of many factors including size of the fluoridated fillers, type of particles, and type of resin and silane treatment of the filler particles.21 The level of fluoride released from the denture base samples in this study was considerably lower than the amount reportedly released from glass ionomer and polyacid-modified resin composite (compomer) materials investigated in past studies.9,22 However, this does not negate the potential clinical benefit of fluoride release from denture base resins as this would contribute a much larger surface area for potential fluoride release than that available from intracoronal © 2014 Australian Dental Association
restorations, thus enabling a greater cumulative amount of fluoride to be released into the oral environment. Importantly, it is not only the ability to release fluoride but sustained release through fluoride recharge that might facilitate caries inhibition. The anticariogenic action of fluoride occurs as a result of three different mechanisms; reduction of demineralization, enhancement of remineralization, and the inhibition of microbial growth and metabolism. According to ten Cate et al. the concentrations of fluoride ion release with ageing and following recharge from both the 5% and 10% filled samples would be adequate to inhibit demineralization of enamel lesions, in addition to inhibiting approximal dentine demineralization by up to 40% in a pH 4.5 medium.23 The findings of this study show that fluoride was released from the denture base resins in concentrations that may have the potential to enhance remineralization and inhibit demineralization around the abutment teeth of partial denture wearers, especially following a simple regular fluoride recharge regimen. This could provide benefit to those at high risk of developing new carious lesions such as the elderly and special needs patients. However, the efficacy of the fluoride containing denture base resin in the remineralization of demineralized abutment teeth requires further investigation. Within the limitations of the present laboratory study, the experimental heat polymerized PMMA denture base material containing fluoridated glass fillers with 2.5%, 5% and 10% wt demonstrated fluoride release and recharge in deionized water and lactic acid. The modulus of elasticity did not diminish with ageing over the 43-day period. However, flexural strength decreased in proportion with the incorporated percentage of glass fillers, although in the longer term the ageing factor appeared to have a greater influence on the flexural properties than the proportion of fillers incorporated within the polymer matrix. REFERENCES 1. Reddy NS, Reddy NA, Navendra R, Reddy SD. Epidemiological survey on edentulousness. J Contemp Dent Pract 2012;13:562–570. 2. Zitzmann NU, Staehelin K, Walls AW, Menghini G, Weiger R, Zemp Stutz E. Changes in oral health over a 10-yr period in Switzerland. Eur J Oral Sc 2008;116:52–59. 3. Preshaw PM, Walls AW, Jakubovics NS, Moynihan PJ, Jepson NJ, Loewy Z. Association of removable partial denture use with oral and systemic health. J Dent 2011;39:711–719. 4. Drake CW, Beck JD. The oral status of elderly removable partial denture wearers. J Oral Rehabil 1993;20:53–60. 5. Jensen ME, Kohout F. The effect of a fluoridated dentifrice on root and coronal caries in an older adult population. J Am Dent Assoc 1988;117:829–832. 213
IA Al-Bakri et al. 6. Han L, Cv E, Li M, et al. Effect of fluoride mouth rinse on fluoride releasing and recharging from aesthetic dental materials. Dent Mater J 2002;21:285–295.
17. Attar N, Onen A. Fluoride release and uptake characteristics of aesthetic restorative materials. J Oral Rehabil 2002;29:791– 798.
7. Delbem AC, Carvalho LP, Morihisa RK, Cury JA. Effect of rinsing with water immediately after APF gel application on enamel demineralization in situ. Caries Res 2005;39:258–260.
18. Attin T, Buchalla W, Siwert C, Hellwig E. Fluoride release/ uptake of polyacid-modified resin composites (compomers) in neutral acidic buffer solutions. J Oral Rehabil 1999;26:388– 393.
8. Bergman B, Hugoson A, Olsson CO. Caries and periodontal status in patients fitted with removable partial dentures. J Clin Periodontol 1977;4:134–146. 9. Naoum S, Ellakwa A, Martin F, Swain M. Fluoride release, recharge and mechanical property stability of various fluoridecontaining resin composites. Oper Dent 2011;36:422–432. 10. Kamijo K, Mukai Y, Tominaga T, Iwaya I, Fujino F, Hirata Y. Fluoride release and recharge characteristics of denture base resins containing surface pre-reacted glass-ionomer filler. Dent Mater J 2009;28:227–233. 11. Mukai Y, Kamijo K, Fujino F, Hirata Y, Teranaka T, ten Cate JM. Effect of denture base-resin with prereacted glass-ionomer filler on dentin demineralization. Eur J Oral Sci 2009;117:750–754. 12. International Organization for Standardization. ISO 1567:1999. Dentistry – Denture base polymers. Geneva: ISO, 1999:12. 13. Itota T, Al-Naimi OT, Carrick TE, Yoshiyama M, McCabe JF. Fluoride release from aged resin composites containing fluoridated glass filler. Dent Mater 2005;21:1033–1038. 14. Xu X, Burgess JO. Compressive strength, fluoride release and recharge of fluoride-releasing materials. Biomaterials 2003;24:2451–2461. 15. Dijkman GE, de Vries J, Lodding A, Arends J. Long-term fluoride release of visible light-activated composites in vitro a correlation with in situ demineralization data. Caries Res 1993;27:117–123. 16. Sales D, Sae-Lee D, Matsuya S, Ana ID. Short-term fluoride and cations release from polyacid-modified composites in a distilled water, and an acidic lactate buffer. Biomaterials 2003;24:1687–1696.
214
19. Karantakis P, Helvatjoglou-Antoniades M, Theodoridou-Pahini S, Papadogiannis Y. Fluoride release from three glass ionomers, a compomer, and a composite resin in water, artificial saliva, and lactic acid. Oper Dent 2000;25:20–25. 20. Zissis A, Yannikakis S, Harrison A. Comparison of denture stomatitis prevalence in 2 population groups. Int J Prosthodont 2006;19:621–625. 21. Wiegand A, Buchalla W, Attin T. Review on fluoride-releasing restorative materials– fluoride release and uptake characteristics, antibacterial activity and influnce on caries formation. Dent Mater 2007;23:343–362. 22. Shaw AJ, Carrick T, McCabe JF. Fluoride release from glassionomer and compomer restorative materials: 6-month data. J Dent 1998;26:355–359. 23. ten Cate JM, Damen JJ, Buijs MJ. Inhibition of dentin demineralization by fluoride in vitro. Caries Res 1998;32:141–147.
Address for correspondence: Dr Ayman Ellakwa Associate Professor Department of Biomaterials Faculty of Dentistry The University of Sydney Sydney NSW 2006 Email: [email protected]
© 2014 Australian Dental Association
The official journal of the Australian Dental Association
Australian Dental Journal 2014; 59: 208–214 doi: 10.1111/adj.12168
Fluoride release, recharge and flexural properties of polymethylmethacrylate containing fluoridated glass fillers IA Al-Bakri,* MV Swain,* SJ Naoum,* WM Al-Omari,† E Martin,‡ A Ellakwa§ *Department of Biomaterials, Faculty of Dentistry, The University of Sydney, New South Wales, Australia. †Faculty of Dentistry, The University of Sydney, New South Wales, Australia and Department of Prosthodontics, Faculty of Dentistry, Jordan University of Science and Technology, Irbid, Jordan. ‡Department of Conservative Dentistry, Faculty of Dentistry, The University of Sydney, New South Wales, Australia. §Department of Biomaterials, Faculty of Dentistry, The University of Sydney, New South Wales and Faculty of Dentistry, Tanta University, Tanta, Egypt.
ABSTRACT Background: The purpose of this study was to investigate the effect of fluoridated glass fillers on fluoride release, recharge and the flexural properties of modified polymethylmethacrylate (PMMA). Methods: Specimens of PMMA denture base material with various loading of fluoridated glass fillers (0%, 1%, 2.5%, 5% and 10% by weight) were prepared. Flexural properties were evaluated on rectangular specimens (n = 10) aged in deionized water after 24 hours, 1 and 3 months. Disc specimens (n = 10) were aged for 43 days in deionized water and lactic acid (pH 4.0) and fluoride release was measured at numerous intervals. After ageing, specimens were recharged and fluoride re-release was recorded at 1, 3 and 7 days after recharge. Results: Samples containing 2.5%, 5% and 10% glass fillers showed significantly (p < 0.05) greater levels of fluoride release compared with the control and 1% glass fillers specimens. All experimental specimens exhibited fluoride release in both media. The flexural strength of specimens decreased in proportion to the percentage filler inclusion with the modulus of elasticity values remaining within ISO Standard 1567. Conclusions: The modified PMMA with fluoridated glass fillers has the ability to release and re-release fluoride ion. Flexural strength decreased as glass filler uploading increased. Keywords: Flexural strength, fluoride release, glass fillers, modulus of elasticity, polymethylmethacrylate. Abbreviations and acronyms: ANOVA = analysis of variance; PMMA = polymethylmethacrylate; S-PRG = surface pre-reacted giomer. (Accepted for publication 10 September 2013.)
INTRODUCTION With the increasing lifespan of the general population, the need for the prosthodontic replacement of missing natural teeth will increase.1 Despite the wide use of implant prostheses to replace lost teeth, the removable partial denture remains one of the most widely accepted means of tooth replacement. As the population ages the proportion of patients seeking this type of prosthesis is likely to increase, especially as the cost of multiple tooth replacement using implants and the lack of sufficient bone in areas of long-term tooth loss limit implant use.2,3 Notably however, the risk of root caries is reported to be higher in older patients as a result of gingival recession, and in those patients wearing removable partial dentures.3 Abutment teeth that are used to secure and support a partial denture are at particular risk of caries development as denture clasps and rests 208
function as niches for the collection of food debris and plaque.4,5 Hence, preventing caries of abutment teeth associated with removable partial dentures is crucial, and delivery of fluoride into the oral cavity using a variety of vehicles is recommended as a means of preventing the formation of caries, and the development of recurrent caries.6,7 One possible mode of fluoride delivery to tooth surfaces could be through the use of fluoride releasing materials contained within the denture base. It has been reported in longitudinal clinical studies that sound approximal surfaces in contact with restorations capable of fluoride ion release exhibit a lower rate of caries incidence compared to sound tooth surfaces contacting non-fluoride releasing materials.8 Although the quantities of fluoride released from a restorative material may be substantially smaller than that provided by the application of a dentifrice, a sustained level of minimal fluoride ion released from a © 2014 Australian Dental Association
Fluoride release and properties of modified polymethylmethacrylate restorative material adjacent to a tooth structure might result in a caries inhibitive effect.9 A recent study reported that an experimental heatpolymerizing polymethylmethacrylate (PMMA) resin, containing surface pre-reacted giomer (S-PRG) filler particles was able to demonstrate both fluoride release and fluoride recharge capability.10 Mukai et al. found that fluoride release from the S-PRG particles contained in a denture base resin resulted in the percentage mineral loss of contacting dentine subjected to a demineralizing solution to be decreased by up to 60%.11 However, despite the potential benefit that the incorporation of fluoride releasing glass fillers within denture bases could provide to abutment tooth longevity, few studies have assessed the potential of fluoride releasing denture acrylic materials.10 The aim of this study was to evaluate the flexural strength, elastic modulus and fluoride release levels after ageing and following recharge with sodium fluoride of a PMMA denture base material modified by the addition of different percentages of fluoridated glass fillers. MATERIALS AND METHODS Specimen preparation To fabricate the experimental denture base materials, white silane-coated glass fillers (ultrafine GM35429) (Table 1) with an average particle size of approximately 1.5 lm were added to PMMA powder. The mixture was dispersed vigorously for 5 minutes using a vacuum mixer. Powder mixtures of four different experimental denture bases with various filler content (1.0%, 2.5%, 5% and 10% by weight) and a control group with 0% glass fillers content were prepared. A metal mould was employed to prepare 150 rectangular specimens (65 9 10 9 3.5 mm, n = 10/5 groups/3 ageing interval) to measure flexural strength and modulus of elasticity in accordance with ISO 1567 specifications.12 To measure fluoride release, another metal mould was used to prepare 50 disc specimens (10 9 1.5 mm, n = 10/group). The metal moulds were placed in conventional dental metal
Table 1. The manufacturers and composition of the fluoridated glass fillers and their silane coating Material
Manufacturer
Composition
Glass fillers (ultrafine GM35429)
Urban, clear pink, C2, Shofu Inc., Kyoto, Japan Schott, Landshut, Germany
SiO2 (30% wt)CaO (10% wt)Al2O3 (30% wt)F (15% wt)P2O5 (2.5% filler >1% filler >0% filler). All resin base samples continued to release fluoride for the 43-day ageing period except the control group (0% filler). At the completion of 43 days ageing, the level of fluoride release had decreased in all testing groups to 0.4 lg/cm2 or less. Each of the experimental denture base samples, with the exception of those containing 2.5% filler, demonstrated significantly greater fluoride release into lactic acid compared to the release into water. Significant differences in fluoride release were found between groups (p < 0.05) within the same testing interval and among the same experimental groups over the 9 testing intervals. Figure 5 shows the weekly cumulative fluoride ion release for the experimental resin samples aged in deionized water and lactic acid, following a weekly 5-minute fluoride recharge (5000 ppm) for three consecutive weeks following 43 days of ageing. All experimental resin bases demonstrated fluoride recharge, the level of fluoride ion release from each resin base 8 7 6 5 4 week 3 week 2 week 1
3 2 1 LA
LA 2.5 % LA 5% LA 10 % LA
1%
0%
%
DW
0
10
Figure 4 shows the cumulative fluoride ion release from the denture base samples aged in deionized water and lactic acid over 43 days. During the 43-day analysis period the cumulative fluoride release demonstrated by each resin base sample was positively correlated to the level of fluoridated glass fillers contained
1% fluoride DW
0
DW
Fluoride release and recharge
1% fluoride LA
DW
deterioration between the control group (0% fillers) and the denture base samples containing 10% glass filler content was recorded as 4.6% for the (24 hours – 1 month period), which dropped to 1.9% for the (1 month – 3 month period). For the samples containing 2.5%, 5% and 10% filler particle loading the differences compared with the control group (0% fillers) at (24 hours – 1 month period) were 2.5, 4.5 and 4.6 respectively. These differences dropped to 0, 0.2 and 1.9 in the (1 month – 3 months period) test. There were statistically significant differences in the modulus of elasticity at 24 hours ageing between the resin samples containing 5% filler (recording the highest reading of 2.87 Gpa) and the samples containing 2.5% and 10% filler (Fig. 2), otherwise no significant differences (p > 0.05) were noted between the samples at 1 month and 3 months containing varied filler loadings.
2.5% fluoride DW
2.5% fluoride LA
5%
Fig. 3 The percentage change in flexural strength for the experimental resins over the testing intervals.
0.6 0.4 0.2
5% fluoride DW
DW
Fig. 2 The means of modulus of elasticity (GPa) recorded for all groups at different ageing intervals.
5% fluoride LA
10% fluoride DW
%
0 1 2.5 5 10 Experimental groups with different loading percentages (%)
1.2 1 0.8
1%
3 months
10% fluoride LA
2.5
1 month
1.6 1.4
DW
24 hours
Fluoride ion concentraion (µg/cm2/day)
1.8
Fluoride ion concentration (µg/cm2)
2.9 2.8 2.7 2.6 2.5 2.4 2.3 2.2 2.1 2
0%
Modulus of elasticity (GPa)
Fluoride release and properties of modified polymethylmethacrylate
Testing groups in storage medium
Fig. 5 Cumulative fluoride ion release from each group aged in deionized water (DW) and lactic acid (LA) for 43 days ageing. Followed by a weekly 5-minute fluoride recharge (5000 ppm), for three consecutive weeks. 211
IA Al-Bakri et al. into both solutions increased markedly after fluoride recharge compared to the release following 43 days ageing. The level of fluoride ion release following recharge was positively correlated with the level of glass filler within each experimental resin base. The denture base samples containing 10% filler demonstrated the greatest cumulative release among the groups, in both media, for each of the three weeks. The unfilled PMMA samples demonstrated fluoride release after recharge with sodium fluoride particularly in the first day post recharge which then declined to a negligible amount with the passage of time in contrast with the denture base specimens of 2.5%, 5% and 10% filler loading, which showed continuous re-release capability. DISCUSSION It is essential that any chemical or mechanical modifications made to materials used for removable partial dentures should either sustain or improve the mechanical properties of the material. In the present study, the fluoridated filler particles were treated using a silane coupling agent before incorporation within the resin material. This was undertaken in an attempt to improve bonding between the filler particles and denture base resin matrix, potentially enhancing the mechanical properties of the denture base material. All the experimental resins tested post 24 hours exhibited flexural strength values greater than 65 MPa and modulus of elasticity values greater than 2 GPa and therefore complied with the requirement of ISO 1567. However, despite the silane treatment, it was observed that the flexural strength recordings of the experimental base samples decreased as the percentage addition of glass fillers increased. Further, with the exception of the denture base samples containing 1% filler, all the experimental denture base samples demonstrated significantly (p < 0.05) lower strength values compared to the control group (0% filler) at 24 hours, 1 month and 3 months. Similar results were also reported in a previous study in which non-silane treated S-PRG fluoride containing fillers were incorporated within a denture base resin.10 In order to further investigate the flexural property changes through the fluoride release procedure, the 1 month and 3 month results were statistically analysed. The results clearly showed that the difference in deterioration between the control group and the other experimental resins decreased over time. As observed from the flexural strength test results, the control group with 0% glass fillers decreased and at 1–3 months was similar to the bases containing glass fillers. 212
Additionally, the results of this study are suggestive of a threshold of filler incorporation. In order to prevent further detrimental effects on the mechanical properties of the denture base resin, 10% wt filler addition was considered an appropriate upper limit. This is supported by Kamijo et al. who recommended a filler content of less than 20 wt%.10 This study demonstrated that denture base materials incorporating 2.5% or more fluoridated filler were able to exhibit sustained fluoride release during ageing but declined with the passage of time, in addition to fluoride release following recharge. This finding is of potential clinical relevance as the development of denture base resins capable of fluoride release and recharge has the potential to deliver fluoride to abutment teeth susceptible to the development of dental caries. By comparison, the denture base samples containing 1% wt glass fillers demonstrated undetectable fluoride release. These observations indicate that the level of fluoride release correlates with the amount of fluoride filler contained in the denture base resin samples. This finding is in agreement with a previous investigation that reported the proportion of S-PRG fillers added to PMMA correlated with the amount of fluoride ion released during ageing and following recharge.10,11 The mechanism of denture base fluoride release observed in this study resulted from dissolution of the inorganic fluorides contained within the fluoroalumino-silicate glass fillers.13 This pattern of fluoride release has been described in previous studies where leachable glass fillers dispersed in a polymer matrix are water soluble and capable of releasing fluoride ions into the storage medium.14,15 However, the exact mechanism of fluoride release from the denture base resin materials has not been investigated. It is acknowledged that incorporation of inorganic fluoride containing particles into restorative materials results in fluoride release, but this also has the potential to create voids within the matrix as the fluoride leaches out of the material.16,17 A similar effect might have occurred with the denture base materials modified by the addition of fluoridated fillers. The final porosity of the material may have an adverse effect on the mechanical properties, but might also allow deeper diffusion of the recharge solution, thus resulting in greater potential for fluoride storage and release.14 Further, the hydrophilic nature of PMMA denture base material may play a key role in the absorption of solutions which may enhance the dissolution of fluoridated glass fillers with a resultant increase in fluoride uptake and release. Further studies are warranted to understand the underlying mechanism of fluoride release from denture base material containing fluoridated fillers. © 2014 Australian Dental Association
Fluoride release and properties of modified polymethylmethacrylate The experimental denture base containing 10% fluoridated glass filler showed 27.2% greater cumulative fluoride release in lactic acid than in deionized water after 43 days ageing. Attin et al.18 and Karantakis et al.19 have explained the increased fluoride release in the acidic medium as a result of the low pH increasing the dissolution of the material. However, the potential effect of fluoride ion release on the surface topography and subsequently the possible plaque adhesion to surface irregularities would be an area for further investigation. The results of this study demonstrated the ability of fluoridated glass fillers within a denture base resin to facilitate fluoride release following recharge involving immersion of the samples within a 5000 ppm NaF gel for 5 minutes. Although previous studies employed different recharge regimens,10,14 the current protocol can be recommended for use by patients as a routine daily toothbrushing regimen, as it has been reported that many denture wearers do not remove their dentures for daily cleaning.9,20 Seemingly after the recharge regimen, only the adsorbed fluoride to the surface was released and thus depleted shortly for the 0% filler group, whilst the fluoridated glass fillers in the other experimental groups served as a reservoir for storage of fluoride ions and a vehicle for continuous release. For all groups tested in both media the amount of fluoride released in the 24 hours post recharge was significantly (p < 0.05) greater than the amount of fluoride released over the first 24 hours of ageing; for the experimental denture base samples containing 10% fluoridated glass particles, the difference was 106.5% and 83.3% in deionized water and lactic acid respectively. Although a similar pattern of fluoride release was observed by Kamijo et al. after recharge, the cumulative amount of fluoride release was higher in the current study.10 However, differences exist in the methods between the two studies which render a direct comparison of the results difficult. Some differences included dry curing of the denture base material, the use of S-PRG fillers, the use of different particle size fillers, and the use of uncoated particles. The fluoride released from restorative materials occurs as a result of many factors including size of the fluoridated fillers, type of particles, and type of resin and silane treatment of the filler particles.21 The level of fluoride released from the denture base samples in this study was considerably lower than the amount reportedly released from glass ionomer and polyacid-modified resin composite (compomer) materials investigated in past studies.9,22 However, this does not negate the potential clinical benefit of fluoride release from denture base resins as this would contribute a much larger surface area for potential fluoride release than that available from intracoronal © 2014 Australian Dental Association
restorations, thus enabling a greater cumulative amount of fluoride to be released into the oral environment. Importantly, it is not only the ability to release fluoride but sustained release through fluoride recharge that might facilitate caries inhibition. The anticariogenic action of fluoride occurs as a result of three different mechanisms; reduction of demineralization, enhancement of remineralization, and the inhibition of microbial growth and metabolism. According to ten Cate et al. the concentrations of fluoride ion release with ageing and following recharge from both the 5% and 10% filled samples would be adequate to inhibit demineralization of enamel lesions, in addition to inhibiting approximal dentine demineralization by up to 40% in a pH 4.5 medium.23 The findings of this study show that fluoride was released from the denture base resins in concentrations that may have the potential to enhance remineralization and inhibit demineralization around the abutment teeth of partial denture wearers, especially following a simple regular fluoride recharge regimen. This could provide benefit to those at high risk of developing new carious lesions such as the elderly and special needs patients. However, the efficacy of the fluoride containing denture base resin in the remineralization of demineralized abutment teeth requires further investigation. Within the limitations of the present laboratory study, the experimental heat polymerized PMMA denture base material containing fluoridated glass fillers with 2.5%, 5% and 10% wt demonstrated fluoride release and recharge in deionized water and lactic acid. The modulus of elasticity did not diminish with ageing over the 43-day period. However, flexural strength decreased in proportion with the incorporated percentage of glass fillers, although in the longer term the ageing factor appeared to have a greater influence on the flexural properties than the proportion of fillers incorporated within the polymer matrix. REFERENCES 1. Reddy NS, Reddy NA, Navendra R, Reddy SD. Epidemiological survey on edentulousness. J Contemp Dent Pract 2012;13:562–570. 2. Zitzmann NU, Staehelin K, Walls AW, Menghini G, Weiger R, Zemp Stutz E. Changes in oral health over a 10-yr period in Switzerland. Eur J Oral Sc 2008;116:52–59. 3. Preshaw PM, Walls AW, Jakubovics NS, Moynihan PJ, Jepson NJ, Loewy Z. Association of removable partial denture use with oral and systemic health. J Dent 2011;39:711–719. 4. Drake CW, Beck JD. The oral status of elderly removable partial denture wearers. J Oral Rehabil 1993;20:53–60. 5. Jensen ME, Kohout F. The effect of a fluoridated dentifrice on root and coronal caries in an older adult population. J Am Dent Assoc 1988;117:829–832. 213
IA Al-Bakri et al. 6. Han L, Cv E, Li M, et al. Effect of fluoride mouth rinse on fluoride releasing and recharging from aesthetic dental materials. Dent Mater J 2002;21:285–295.
17. Attar N, Onen A. Fluoride release and uptake characteristics of aesthetic restorative materials. J Oral Rehabil 2002;29:791– 798.
7. Delbem AC, Carvalho LP, Morihisa RK, Cury JA. Effect of rinsing with water immediately after APF gel application on enamel demineralization in situ. Caries Res 2005;39:258–260.
18. Attin T, Buchalla W, Siwert C, Hellwig E. Fluoride release/ uptake of polyacid-modified resin composites (compomers) in neutral acidic buffer solutions. J Oral Rehabil 1999;26:388– 393.
8. Bergman B, Hugoson A, Olsson CO. Caries and periodontal status in patients fitted with removable partial dentures. J Clin Periodontol 1977;4:134–146. 9. Naoum S, Ellakwa A, Martin F, Swain M. Fluoride release, recharge and mechanical property stability of various fluoridecontaining resin composites. Oper Dent 2011;36:422–432. 10. Kamijo K, Mukai Y, Tominaga T, Iwaya I, Fujino F, Hirata Y. Fluoride release and recharge characteristics of denture base resins containing surface pre-reacted glass-ionomer filler. Dent Mater J 2009;28:227–233. 11. Mukai Y, Kamijo K, Fujino F, Hirata Y, Teranaka T, ten Cate JM. Effect of denture base-resin with prereacted glass-ionomer filler on dentin demineralization. Eur J Oral Sci 2009;117:750–754. 12. International Organization for Standardization. ISO 1567:1999. Dentistry – Denture base polymers. Geneva: ISO, 1999:12. 13. Itota T, Al-Naimi OT, Carrick TE, Yoshiyama M, McCabe JF. Fluoride release from aged resin composites containing fluoridated glass filler. Dent Mater 2005;21:1033–1038. 14. Xu X, Burgess JO. Compressive strength, fluoride release and recharge of fluoride-releasing materials. Biomaterials 2003;24:2451–2461. 15. Dijkman GE, de Vries J, Lodding A, Arends J. Long-term fluoride release of visible light-activated composites in vitro a correlation with in situ demineralization data. Caries Res 1993;27:117–123. 16. Sales D, Sae-Lee D, Matsuya S, Ana ID. Short-term fluoride and cations release from polyacid-modified composites in a distilled water, and an acidic lactate buffer. Biomaterials 2003;24:1687–1696.
214
19. Karantakis P, Helvatjoglou-Antoniades M, Theodoridou-Pahini S, Papadogiannis Y. Fluoride release from three glass ionomers, a compomer, and a composite resin in water, artificial saliva, and lactic acid. Oper Dent 2000;25:20–25. 20. Zissis A, Yannikakis S, Harrison A. Comparison of denture stomatitis prevalence in 2 population groups. Int J Prosthodont 2006;19:621–625. 21. Wiegand A, Buchalla W, Attin T. Review on fluoride-releasing restorative materials– fluoride release and uptake characteristics, antibacterial activity and influnce on caries formation. Dent Mater 2007;23:343–362. 22. Shaw AJ, Carrick T, McCabe JF. Fluoride release from glassionomer and compomer restorative materials: 6-month data. J Dent 1998;26:355–359. 23. ten Cate JM, Damen JJ, Buijs MJ. Inhibition of dentin demineralization by fluoride in vitro. Caries Res 1998;32:141–147.
Address for correspondence: Dr Ayman Ellakwa Associate Professor Department of Biomaterials Faculty of Dentistry The University of Sydney Sydney NSW 2006 Email: [email protected]
© 2014 Australian Dental Association
