This is "Advance Publication Article". J-STAGE Advance Published Date:Jun 21,2010 Dental Materials Journal Dental Materials Journal 2010; 29(4): ▲–▲
Detection of ions released from S-PRG fillers and their modulation effect Yoshihiro FUJIMOTO1, Mika IWASA1, Ryosuke MURAYAMA1, Masashi MIYAZAKI1, Akihiro NAGAFUJI2 and Toshiyuki NAKATSUKA2
1Department of Operative Dentistry, Nihon University School of Dentistry, 1-8-13 Kanda-Surugadai, Chiyoda-ku, Tokyo 101-8310, Japan 2Department of Research and Development, Shofu Inc., 11 Kamitakamatsu-cho, Fukuine, Higashiyama-ku, Kyoto 605-0983, Japan Corresponding author, Masashi MIYAZAKI; E-mail: [email protected]
The purpose of this study was to analyze the ions released from a surface pre-reacted glass ionomer (S-PRG) filler in distilled water or lactic acid solution. S-PRG filler was mixed with either solution at 1000:1, 100:1, 10:1, and 1:1 ratios by weight. By means of inductively coupled plasma atomic emission spectroscopy and a fluoride electrode, elements released from S-PRG filler were identified to be Al, B, Na, Si, Sr, and F. To investigate the effect of solution pH on ion release, the pH values of the solutions before mixing and after 24 hours’ mixing with S-PRG were measured. After 24 hours’ mixing, the pH values of solutions at all ratios became more neutral or weakly alkaline regardless of their initial pH levels before mixing. In conclusion, results showed that S-PRG filler released several types of ions, and that ion release was influenced by the mixing ratio of the solution rather than the initial pHofthe solution.
Keywords: Giomer, Ion release, pH
cements makes them capable of releasing many types INTRODUCTION of ions. Nonetheless, the release rate of fluoride and Previous studies on fluoride release from restorative other ions is controlled by a diffusion mechanism of the materials have mostly focused on fluoride-releasing ions through the matrix and is influenced by myriad polymers1,2). With methacryloyl fluoride-methyl factors —such as the duration of fluoride release, pH of methacrylate copolymers, fluoride release was achieved the extraction medium, and the surface area and via hydrolysis of the monomer followed by an anhydride degree of erosive wear of the glass ionomer cement9). formation3). With restorative materials, fluoride is On the effect of medium pH on ion release behavior, incorporated either into the filler or the polymer acidic conditions are known to enhance the release of matrix4). For fluoride-containing dental restorative all ions. Nonetheless, it must be pointed out that this materials, NaF employed as fluoride-releasing fillers enhanced release in acids is not uniform, but occurs to can exhibit extremely high levels of fluoride release. different extents for each ion10). Interestingly, despite However, the drawback is that elution of Na and F these differences, ion release tends to lead to fairly would result in a loss of the mechanical integrity of the uniform shifts in the pH of the extraction medium dental adhesive and/or restorative materials. towards neutral11). To gain a more detailed insight into In the formulations of glass ionomer cements, the latter phenomenon, cement-solution samples of fluoride release is achieved through an acid-base varied ratios were prepared in this study to investigate reaction between an ion-leachable fluoroaluminosilicate the effect of cement concentration on medium pH glass and a polyalkenoic acid. Therefore in changes. contemporary resin-based restorative materials, Pre-reacted glass-ionomer (PRG) fillers are fluoride is released from ion-leachable glass fillers — prepared via an acid-base reaction (of the traditional but with varying fluoride release rates5). Ion exchange glass ionomer) between fluoroaluminosilicate glass takes place between the glass ionomer cement and the (base) and a polyacrylic acid in the presence of water, oral fluids as well as between the glass ionomer cement whereby the preliminary product was a stable glass and the tooth tissue6), leading to formation of ionic ionomer phase within the glass particles12-14). Upon bonds between the synthesized polyalkenoic acid and freeze-drying, the desiccated xerogel is further milled the surface calcium ions of exposed hydroxyapatite7,8). and silane-treated to form PRG fillers of a specific size On this premise, the unique properties of self- range. Two types of PRG fillers are thereby prepared adhesiveness between glass ionomer cements and —full reaction type (F-PRG) and surface reaction type human hard tissues such as bones and teeth can be (S-PRG), and they are included in the formulations of attributed to the presence of ionic bonding7,8). “Giomer” products14). The use of both types of PRG In general, the water-mediated ion exchange in fillers promotes rapid fluoride release through a ligand glass ionomer cements begins during initial hardening exchange within the pre-reacted hydrogel15). and continues thereafter at a slower rate. The With polymerized composites, release of fluoride availability of a variety of ions in glass ionomer ions from a polymerized resin matrix can occur only in
Received Feb 3, 2010: Accepted Mar 8, 2010 doi:10.4012/dmj.2010-015 JOI JST.JSTAGE/dmj/2010-015 This is "Advance Publication Article". J-STAGE Advance Published Date:Jun 21,2010 Dental Materials Journal Dent Mater J 2010; 29(4): ▲–▲ the presence of water. According to several reports, the The mixture was stirred for 24 hours and centrifuged rate of ion release depended on the rate of water to precipitate the S-PRG filler. The supernatant sorption and the segmental mobility of polymer chains solution was then filtered using a Chromato Disk (25A within the resin phase of the polymerized resin Hydrophilic Type, diameter: 25 mm, pore size: 0.2 µm; composites16-18). Although the types of ions released GL Sciences Inc., Tokyo, Japan) to obtain the test from glass ionomer cements have been identified19), this solution. was not so for the types of ions released from S-PRG fillers. In view of the scarcity of information in relation Elemental analysis to the S-PRG fillers, the purpose of this study was to Elemental analysis of ions (Al, B, Na, Si, and Sr) analyze the ions released from S-PRG fillers mixed released from S-PRG filler was performed using with different ratios of distilled water and lactic acid inductively coupled plasma atomic emission solution. Further, changes to the pH values of the spectroscopy (ICP-AES; ICPS-8000, Shimadzu Co., solutions —associated with and arising from the release Kyoto, Japan). Analysis was conducted after preparing of ions from S-PRG fillers— were also examined. calibration curves corresponding to each element (standard solution concentration; Si: 0, 0.5, 1, and 5 ppm; Sr: 0, 5, 20, and 50 ppm; B: 0, 10, 50, and 100 MATERIALS AND METHODS ppm; Al: 0, 0.5, 5, and 10 ppm; Na: 0, 0.5, 20, and 50 Specimen preparation ppm). Fluoroboroaluminosilicate glass was prepared by fusing Concentration of F was also determined using a
14.0 wt% of silica (SiO2), 27.0 wt% of mullite fluoride ion electrode method after preparing its
(3Al2O3•2SiO2), 19.0 wt% of boric oxide (B2O3), 5.0 wt% calibration curves (standard solution concentration: 0.1, of cryolite (Na3AlF6), 29.0 wt% of strontium fluoride 1, 5, and 10 ppm). A fluoride electrode (Model 9609BN,
(SrF2), and 6.0 wt% of strontium carbonate (SrCO3). Orion Research Inc., MA, USA) connected to a pH/ion The mixture was heated and melted in an arc furnace meter (720A, Orion Research Inc.) was used to measure at 1,400°C for 2 hours. The melted mixture in liquid the F concentration of each solution. Each test solution form was removed from the furnace and quenched in was diluted to obtain a fluoride ion level of less than 5 running water, then dried for 12 hours at 150°C in an ppm, and 0.5 mL of an ionic strength adjuster (TISAB air oven to obtain glass frit. Analysis of the glass frit III, Orion Research Inc.) was added to 5 mL of each with an X-ray fluorescence spectrometer (ZSX100e, diluted solution to measure F ion concentration. Rigaku Corp., Tokyo, Japan) and an X-ray For each ion under investigation, the amount diffractometer (Multiflex 2kW, Rigaku Corp.) revealed released into the solution was expressed in ppm, as that the glass structure was amorphous and consisted well as a cumulative amount in per gram of S-PRG of 21.6 wt% of SiO2, 21.6 wt% of Al2O3, 16.6 wt% of filler weight (mg/g).
B2O3, 27.2 wt% of SrO, 2.6 wt% of Na2O, and 10.4 wt% of F. pH measurement The glass frit was coarsely ground with a ball mill The pH value of each test solution was measured by (BM-10, Seiwa Giken Co., Hiroshima, Japan) and then means of a combination glass electrode connected to a wet-ground with an agitator bead mill in the presence pH meter (Twin pH B-212, Horiba, Kyoto, Japan). of water to obtain irregular-shaped filler particles. The Before the pH measurement commenced, the pH resultant glass slurry was agitated with the addition of electrodes were calibrated with standard solutions at a polysiloxane solution (SiO2 content: 16 wt%, degree of pH 7 and pH 4 (Horiba). Three specimens were condensation: 2−6; Mitsubishi Chemical Co., Tokyo, prepared and the mean value thereof was calculated. Japan), then aged at 50°C for 40 hours followed by heat treatment at 120°C for 6 hours in a heat dryer. The Statistical analysis heat-treated, solidified material was then disintegrated The data were subjected to one-way analysis of by a high-speed mixer (FS-GC-20JE, Fukae Powtec Co., variance (ANOVA) followed by Tukey’s Honestly Osaka, Japan) to obtain surface-treated glass filler. Significant Difference (HSD) test at a significance level While being stirred in the cutter-mixer, the glass filler of 0.05. Statistical analysis was carried out with a was subjected to spray treatment with a polyacrylic statistical software (SigmaStat Ver. 3.1, SPSS Inc., acid aqueous solution (polymer content: 13.0 wt%), Chicago, IL, USA). followed by heat treatment in the heat dryer at 150°C for 3 hours to obtain S-PRG filler14). Mean particle size RESULTS of S-PRG filler was 3.0 µm, as measured using a laser diffraction particle size analyzer (Microtrac HRA 9320- Effect of mixing ratios of solutions on ion release X100, Nikkiso Co., Tokyo, Japan). On the release of Al, B, Na, Si, and Sr ions, their To obtain solution samples of varied ratios, leached concentrations in distilled water and lactic acid distilled water (pH=5.9) or lactic acid aqueous solution aqueous solution are shown in Tables 1 and 2 (pH=3.8; Nacalai Tesque Inc., Kyoto, Japan) was mixed respectively. In Tables 3 and 4, these selfsame values with S-PRG filler in ratios of 1000:1 (1 L:1 g), 100:1 (1 in ppm were recalculated into mg/g of S-PRG filler L:10 g), 10:1 (1 L:100 g), or 1:1 (1 L:1,000 g) by weight. weight. This is "Advance Publication Article". J-STAGE Advance Published Date:Jun 21,2010 Dental Materials Journal Dent Mater J 2010; 29(4): ▲–▲
Despite their different initial pH values, similar increased with the highest amount of ion release from ion release behaviors were detected in both distilled Sr, followed by F and Si. As for B and Na, they water and lactic acid aqueous solution after they were exhibited a gradual increase in the amount of ion mixed with S-PRG filler. The release of Sr, F, and Si release as the ratio of solution increased. The release ions tended to be suppressed when the mixing ratio of of B ion, which was less affected by the solution ratio, solution to S-PRG filler was lower (1:1 and 10:1). was consistently higher than that of Na at all mixing However, when the ratio of solution was higher ratios. For Al, its ion release amount remained low (1000:1), their release amounts became significantly regardless of the solution ratio. On the overall, the
Table 1 Amounts of ions released (ppm) from S-PRG filler into distilled water (DW) DW:S-PRG Al B F Na Si Sr 1000:1 0.23 3.73 8.1 2.36 1.53 21.28 100:1 1.67 29.69 30.5 18.12 4.46 57.96 10:1 4.42 238.44 69.5 117.30 7.53 56.46 1:1 44.25 2067.18 90.1 512.29 31.11 252.81
Table 2 Amounts of ions released (ppm) from S-PRG filler into lactic acid solution (LA) DW:S-PRG Al B F Na Si Sr 1000:1 0.53 4.71 8.6 2.85 2.48 28.91 100:1 1.78 31.96 27.7 19.27 4.35 60.35 10:1 2.78 254.10 66.0 120.87 5.53 56.17 1:1 21.26 1771.28 94.3 490.88 15.78 169.07
Table 3 Amounts of ions released (mg/g) from S-PRG filler into distilled water (DW) DW:S-PRG Al B F Na Si Sr 1000:1 0.23 3.73 8.12 2.36 1.53 21.28 100:1 0.17 2.97 3.05 1.18 0.45 5.80 10:1 0.04 2.38 0.70 1.12 0.08 0.56 1:1 0.04 2.07 0.09 0.51 0.03 0.25
Table 4 Amounts of ions released (mg/g) from S-PRG filler into lactic acid solution (LA) DW:S-PRG Al B F Na Si Sr 1000:1 0.53 4.71 8.60 2.85 2.48 28.91 100:1 0.18 3.20 2.77 1.93 0.44 6.04 10:1 0.03 2.54 0.66 1.21 0.06 0.56 1:1 0.02 1.77 0.09 0.49 0.02 0.17
Table 5 pH changes of distilled water (DW) and lactic acid solution (LA) mixed with S-PRG filler Solution:S-PRG DW LA start after 24 h start after 24 h 1000:1 5.9 (0.1)a 6.7 (0.1)b 3.8 (0.1)d 6.7 (0.1)b 100:1 5.9 (0.1)a 7.1 (0.1)b 3.8 (0.1)d 7.1 (0.1)b 10:1 5.9 (0.1)a 7.7 (0.1)c 3.8 (0.1)d 7.8 (0.1)c 1:1 5.9 (0.2)a 7.8 (0.1)c 3.8 (0.1)d 7.8 (0.1)c n=3. Values in parentheses indicate standard deviations. Values with same superscript letters indicate that they are not significantly different (p>0.05). This is "Advance Publication Article". J-STAGE Advance Published Date:Jun 21,2010 Dental Materials Journal Dent Mater J 2010; 29(4): ▲–▲ higher the solution ratio, the more ions were released during the attachment of acid groups to the glass from S-PRG filler. powder surface. Together, the liberated F and Na ions formed soluble fluoride salt23), whereby the latter would Effect of pH changes on ion release contribute to a rapid release of F into the surrounding After 24 hours of mixing with S-PRG filler, the pH medium. values of both types of solutions at all mixing ratios On the contribution of F towards forming acid- shifted to the neutral and weak alkaline regions, resistant tooth substrates, it has been well regardless of their initial pH levels before mixing documented24). However, the influence of other ions is (Table 5). When the solution:S-PRG filler ratio was the not so well defined. Among the ions detected in this same (i.e., 1:1), both types of solutions arrived at a study, Sr was thought to play a role in tooth similar pH level of 7.8 after 24 hours of mixing. mineralization6,25). In a recently published study, it was reported that Sr had the capacity to enhance enamel remineralization when used in conjunction with DISCUSSION F26). Owing to a variety of ions present in the formulation of In another study, it was reported that Si seemed to glass ionomer cements, the latter are capable of promote hydroxyapatite formation as silica gel induced releasing these ions into their surrounding solutions19). apatite nucleation on its surface27). The surface silanol For the recently developed S-PRG fillers, they are made groups of hydrated silica gel interacted with the through an acid-base reaction (of the traditional glass calcium and phosphorous ions from the surrounding ionomer) between acid-reactive fluoride-containing environment, thereby generating biologically active glass and polyacids in the presence of water, such that apatite on the silica gel surface. Alternatively, it was ions might also be released from these fillers. reported that Si ions released from bioactive glass In this study, considerable amounts of Al, B, F, Na, particles were adsorbed on the substrate surface, Si, and Sr ions were released from the S-PRG filler into thereby providing sites for heterogeneous apatite distilled water as well as lactic acid solution. Moreover, nucleation. Once nucleated, it will spontaneously grow similar ion releasing behavior was observed for each to form a bone-like apatite layer28). For bonelike mixing ratio in both types of solutions. Although the apatite coating on materials with complex shapes, it mechanism of ion release from S-PRG filler is not was reported that an apatite layer was formed when completely understood, it was believed to have occurred sodium silicate was used as a catalyst for apatite because of the presence of a glass ionomer phase nucleation —whereby particular silicate oligomers with around the glass core of the filler. Since soluble ions structures such as dimer, linear trimer, and cyclic were produced based on the reaction between polyacid tetramer contributed most to apatite nucleation29). On and glass powder, it was thus suggested that the gel its contribution to dentin mineralization, it was phase of the glass core was the source of released ions. reported that Si promoted dentin mineralization by a The release of Sr, F, and Si ions was suppressed mechanism based on the condensation of silicic acid to when the mixing ratio of solution to S-PRG filler was oligomers30). When a sufficient quantity of Si was low (1:1 and 10:1), but became significantly increased adsorbed on dentin with anionic groups (SiO−), it acted when the ratio was high (1000:1). The highest amount as a nucleation centre for subsequent, increased CaP of ions released was observed for Sr, followed by F and formation30). Si. Similarly, B and Na showed a gradual increase in In the present study, pH measurements indicated the amount of ions released as the ratio of solution that S-PRG filler altered the pH values of both distilled increased. It is also noteworthy that the release of B water and lactic acid solution closer to neutral. These ion, which was less affected by the solution ratio, was results thus showed that S-PRG filler, like the consistently higher than that of Na at all mixing ratios. conventional glass ionomer cements, had a modulation As for Al, its ion release amount remained low effect on acidic solutions9,10). When the solution: S-PRG regardless of the solution ratio. filler ratio was the same, the pH values of both types of In glasses, F is likely to be combined with Al and solutions became similar at pH 7.8, regardless of their alkaline earth metal ions20). In S-PRG fillers, Na is different pH levels before mixing. Moreover, the higher present in the composition and has been shown to be the solution ratio, the more ions were released from S- released with Al21). The release of Al is associated with PRG filler. enhanced F release, leading to increases in the The acidic attack on glass ionomer cements has formation of aluminofluoro complexes10). Although only been shown to occur mainly on the gel phase rather a minor amount of Na was present in the S-PRG filler than the unreacted glass cores, indicating a release of when compared with the major amounts of F and Si, ions from the matrix phase of set cement11). the amount of Na released was much higher than the Nonetheless, acidity of the surrounding solution has other ions. This was probably because Na was the only been shown to degrade the glass cores of glass ionomer cation eluted to preserve electroneutrality, which cements, thereby increasing the number of ions meant that an equivalent amount of Na ions was released9). In contrast, S-PRG filler —unlike glass expected to be released to match the eluting cations22). ionomer cements— was relatively stable in acidic As for F, it was liberated into the matrix with Na conditions; but its interactions at different pH This is "Advance Publication Article". J-STAGE Advance Published Date:Jun 21,2010 Dental Materials Journal Dent Mater J 2010; 29(4): ▲–▲ conditions are complex, involving ion release and water 7) Yoshida Y, Van Meerbeek B, Nakayama Y, Snauwaert J, uptake. Hellemans L, Lambrechts P, Vanherle G, Wakasa K. On the clinical implications of S-PRG fillers, Evidence of chemical bonding at biomaterial-hard tissue interfaces. J Dent Res 2000; 79: 709-714. results of this study suggested that they could wield a 8) Fukuda R, Yoshida Y, Nakayama Y, Okazaki M, Inoue S, two-fold impact: able to release ions that contribute to Sano H, Suzuki K, Shintani H, Van Meerbeek B. Bonding tooth mineralization as well as have a modulation efficacy of polyalkenoic acids to hydroxyapatite, enamel and effect on the acidic conditions produced by oral dentin. Biomaterials 2003; 24: 1861-1867. cariogenic microorganisms. Therefore, the favorable 9) Gandolfi MG, Chersoni S, Acquaviva GL, Piana G, Prati C, effects and encouraging results observed in the present Mongiorgi R. Fluoride release and absorption at different pH from glass-ionomer cements. Dent Mater 2006; 22: 441- study augured well for S-PRG fillers to provide 449. sustained clinical benefit after long-term clinical service 10) Czarnecka B, Nicholson JW. Ion release by resin-modified in the oral environment. glass-ionomer cements into water and lactic acid solutions. J Dent 2006; 34: 539-543. 11) Nicholson JW, Aggarwal A, Czarnecka B, Limanowska- CONCLUSIONS Shaw H. The rate of change of pH of lactic acid exposed to glass-ionomer dental cements. Biomaterials 2000; 21: 1989- Within the limits of the present study, the following 1993. conclusions were drawn: 12) Tay FR, Sano H, Tagami J, Hashimoto M, Moulding KM, 1. S-PRG filler was capable of releasing several Yiu C, Pashley DH. Ultrastructural study of a glass types of ions. ionomer-based, all-in-one adhesive. J Dent 2001; 29: 489- 2. Amount of ion release was influenced by the 498. mixing ratio of solution to S-PRG filler rather 13) Ikemura K, Tay FR, Kouro Y, Endo T, Yoshiyama M, Miyai than the initial pH of the solution. K, Pashley DH. Optimizing filler content in an adhesive system containing pre-reacted glass-ionomer fillers. Dent 3. S-PRG filler had a modulation effect on acidic Mater 2003; 19: 137-146. conditions, causing the pH of the surrounding 14) Ikemura K, Tay FR, Endo T, Pashley DH. A review of environment to become weakly alkaline upon chemical-approach and ultramorphological studies on the contact with water or acidic solutions. Such development of fluoride-releasing dental adhesives modulation effect might be brought about by the comprising new pre-reacted glass ionomer (PRG) fillers. release of Sr, B, Na, and F ions. Dent Mater J 2008; 27: 315-339. 15) Han L, Okamoto A, Fukushima M, Okiji T. Evaluation of a new fluoride-releasing one-step adhesive. Dent Mater J ACKNOWLEDGMENTS 2006; 25: 509-515. 16) Itota T, Carrick TE, Yoshiyama M, McCabe JF. Fluoride This work was supported in part by a Grant-in-aid for release and recharge in giomer, compomer and resin Scientific Research (C) (No. 20592237) from the Japan composite. Dent Mater 2004; 20: 789-795. Society for the Promotion of Science (JSPS), and by 17) Itota T, Carrick TE, Rusby S, Al-Naimi OT, Yoshiyama M, McCabe JF. Determination of fluoride ions released from Special Research Grants for the Development of resin-based dental materials using ion-selective electrode Distinctive Education from the Promotion and Mutual and ion chromatograph. J Dent 2004; 32: 117-122. Aid Corporation for Private Schools of Japan. In 18) Itota T, Al-Naimi OT, Carrick TE, Yoshiyama M, McCabe addition, this project was supported in part by the Sato JF. Fluoride release from aged resin composites containing Fund and by a grant from the Dental Research Center fluoridated glass filler. Dent Mater 2005; 21: 1033-1038. of Nihon University School of Dentistry, Japan. 19) Billington RW, Williams JA, Pearson GJ. Ion processes in glass ionomer cements. J Dent 2004; 34: 544-555.
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