- Email: [email protected]
Fluoride release from light-activated glass ionomer restorative cements
Dent Mater 9:151-154, May, 1993
Fluoride release from light-activated glass ionomer restorative cements Y. Momoi L2J.F. McCabe I
Dental School, Newcastle upon Tyne, England, UK Wow at: Department of Operative Dentistry, School of Dental Medicine, Tsurumi University, Yokohama, JAPAN
Abstract. Fluoride release from four light-activated glass ionomer cements, including newly developed restorative cements, was evaluated and compared with four conventional acid-base glass ionomer cements. There was no significant difference between the group of light-activated cements and the group of conventional cements, and light-activated cements were found to have a potential for releasing fluoride equivalent to that of conventional cements. The amount and rate of release varied among cements. It may be affected not only by the formation of complex fluoride compounds and their interaction with polyacrylic acid but also by the type and amount of resin used for the photochemical polymerization reaction.
INTRODUCTION Conventional glass ionomer cements which undergo setting by an acid-base reaction should be protected from both moisture contamination and dehydration during the early stages of setting to avoid a considerable reduction of physical properties. Normally these cements require at least 24 h to achieve a reasonably stable state. To overcome this shortcoming, Mathis and Ferracane (1989) produced a resin-modified glass ionomer cement and reported its lower moisture sensitivity. They modifed a conventional glass ionomer cement by simply mixing the liquid from a commercial cement restorative with a resin used in light-activated composites. Recently developed commercial resin-modified dual-setting glass ionomer cements contain the essential components of both an aqueous glass ionomer cement and a photocurable resin. Wilson (1990) reported that these novel cements combine the advantages of a long working time and a rapid setting and show a higher early strength. Mathis and Ferracane (1989) mentioned in their work on experimental resinmodified glass ionomer cements that although a reduction in solubility was noted, compared with conventional cements, its value was still great enough to suggest that fluoride release would be significant. Mitra (1991) investigated the fluoride release from a light-activated glass ionomer liner/base cement and revealed that it was not hindered by the presence of lightactivated resin. The objective of this study was to evaluate fluoride release from newly developed light-activated glass ionomer restorative cements and to compare the release rate with that achieved using conventional glass ionomer cements.
MATERIALS AND METHODS Four light-activated glass ionomer cements and four conventional acid-base glass ionomer cements were tested. A posterior composite which contains 5% ytterbium trifluoride (YbF3)was also tested. Details of the materials used are given in Table 1. Twenty-five cylindrical specimens, each having a diameter of 3.0 mm and a height of 2.7 mm, were prepared for each material using disposable injection molded polypropylene molds. Materials were handled according to the manufacturers' instructions. Light-activated glass ionomer cements were activated with a halogen light source (Luxor, Model 4000, ICI, Macclesfield, Cheshire, England, UK) using 40 s exposure to each end of the cylindrical specimen. Specimens were removed from the molds after setting and placed in 2 mL of distilled water in individual plastic containers. They were maintained at 37°C until the time of the measurement, at 24 h, 3 d, 7 d, 1 mon and 3 mon. Five specimens of each material at each test time were prepared. Fluoride release was measured using a specific ion electrode (Russell, Type ISE96-6099/11, Auchtermuchty, Fife, Scotland, UK). At each testing time, standard solutions were prepared from a sodium fluoride solution of concentration 10-1 mol/L F (Fluoride Standard, Type 949060, Orion Research Incorporated, 529 Main St., Boston, MA, USA), to which TISAB (total ionic strength adjuster)(Application solution, Russell, Type 949110, Auchtermuchty, Scotland, UK) was added to obtain a constant background ionic strength. Using the standard fluoride solutions, a calibration curve was obtained plotting the measured voltage (mV) against F" concentration (tool/L) on a semi-logarithmic scale using a computer. One milliliter of sample solution was buffered with TISAB and its fluoride ion concentration was determined by reference to the calibration curve. From the value ofF concentration ofeach sample solution, the total mass (l~g) of fluoride ions released into the storage solution (2 mL ofwater) was calculated. Fluoride release from the materials was expressed as the mass of released fluoride ions from the sample (!~g) per unit volume of the specimen (mm3). Results were analyzed by ANOVA and the Student-NewmanKeuls range test (p<0.05). RESULTS The mean values and standard deviations of fluoride release at
Dental Materials~May 1993
151
TABLE 1: MATERIALS USED IN THIS STUDY Brand
Code
Manufacturer
Type
Batch No.
Photac-Fil Aplicap
PF
ESPE, Seefeld, Germany
L
I
Fuji II LC
FLC
GC, Tokyo, Japan
L
P:A2061111 L:071011
Vitrebond
VB
3M, St. Paul, MN, USA
L
P:8W2
Photo-bond Aplicap
PB
ESPE
L
008
Ketac-Fil
KF
ESPE
C
T191
Ketac-Bond Aplicap
KB
ESPE
C
T331
Fuji l
F
GC
C
P:LY 290394 L:120491
Miracle Mix
MM
GC
C
P:16039 P:061489 L:130491
Heliomolar Radiopaque (composite)
HM
Vivadent Schaan, Liechtenstein
L
413701
L: light-activated type C: conventional acid-base type
7 d and 3 mon are shown in Table 2 as representatives for the early and late stage, respectively. All glass ionomer cements showed significant levels of fluoride release compared with the posterior composite. Overall, there was no significant difference in fluoride release between the group of light-activated cements and the group of conventional cements. Accumulative values after 3 mon of water storage were greatest for VB in the lightactivated cement group and for MM in the conventional cement group.
DISCUSSION An extensive amount of work has been done to evaluate fluoride release from dental materials in the past, and the documented values of released fluoride vary considerably from one study to another (Cranfield et al., 1982; E1 Mallakh and Sarkar, 1990). This may be caused by a lack of uniformity in specimen shape, experimental regime, nature of the aqueous environment used and even the units used to express fluoride release. Therefore, despite a large number of reports, it is difficult to compare values directly. The aim ofthis study was simply to evaluate the fluoride release rate of light-activated glass ionomer cements recently developed as filling materials and to compare these values with those for light-activated liner/base cements and conventional glass ionomer cements. In some studies, the fluoride release has been measured using thin disk specimens and the values quoted as fluoride ion concentration in the storage medium (ppm) or as mass of released fluoride per unit surface area of the specimen. Cranfield et al. (1982) reported on factors affecting fluoride ion release. This previous study showed that fluoride ion release is affected by the sample geometry and that ions are derived not only from the near-surface regions but also from the bulk ofthe samples. In this study, in order to observe the fluoride release from a specimen with a bulky configuration closer to that encountered in practice,
152 Momoi & McCabe/Fluoride release of glass ionomer cements
specimens were prepared in a squat cylindrical shape. Values were expressed as a mass of Application released fluoride ion per unit volume of the speciRestoration men (~g/mm3). This implied that fluoride release can occur from the whole body of the specimen and not just from the surface, as implied in Restoration calculations using mass per unit area. It could be argued that the early fluoride release rate is a Liner/Base function of specimen area, and the long-term value is more likely to be a function of volume. For this study, the latter approach was chosen Liner/Base since there is some evidence that the fluoride release values had stabilized by the completion of Restoration the experiment (3 mon). Although fluoride forms a major constituent Liner/Base of many glass ionomer cements, it appears to have no role in the cement-forming process (Crisp Luting and Wilson, 1974), as opposed to other components such as calcium and aluminium, which are involved in cement matrix formation and play a Core build up basic role in hardening and strengthening the ionomer salt hydrogel (Crisp et al., 1976). After powder and liquid are mixed together, the ionRestoration leachable glass is decomposed by proton attack at the surface, and subsequently fluoride ions are liberated from the glass particles. Swift and Dogan (1990) reported that fluoride is dispersed homogeneously throughout the matrix regions of a set cement. Fluoride in the matrix is available for elution for a long period after setting (Crisp et al., 1976) and it is of concern that in light-activated glass ionomer cements, fluoride may not be available to the same extent as for conventional acid-base reacted cements. The setting reaction of light-activated glass ionomer cements is different from that of conventional glass ionomer cements. Wilson (1990) described the setting reaction of lightactivated glass ionomer cements as dual-setting, in which both acid-base and polymerization reactions take place. This setting mechanism was experimentally confirmed using Fourier Transform Infrared Spectroscopy (FTIR) and Differential Photocalorimetry (DPC) by Mitra et al. (1992) and Differential Thermal Analysis (DTA) by Bourke et al. (1992). The acid-base reaction is a normal glass ionomer cement reaction which is commenced by mixing powder and liquid. The second reaction is a photochemical polymerization process similar to that of a lightactivated composite, which is commenced by irradiation with visible light of the appropriate wavelength and intensity. The extent to which each reaction develops depends upon the age of the cement and the time after mixing at which irradiation takes place. There are two matrices in this type of cement, consisting of the ionomer salt hydrogel and polymer, and to prevent phase separation, the cement has been formulated such that the photoinitiated polymer is chemically linked to the polyacrylate (Wilson, 1990). Resin reinforcement was found to produce cements with better physical properties than conventional cements where only the acid-base reaction is responsible for setting (Mathis and Ferracane, 1989). From these findings, it was assumed that, in the set material, fluoride ions might be firmly encapsulated by the resin matrix and that consequently its fluoride release rate into an aqueous environment might be smaller and slower than that of conventional glass ionomer cements. This hypothesis was further supported by a pilot study where fluoride release was measured for experimental specimens in which a fluoride-con-
fABLE 2: FLUORIDE RELEASE FROM MATERIALS TESTED (pg/mm 3) Mean and (S.D.) Material
Code
N
7d
3 mon
Heliomolar
HM
5
0.0 (0.0)
0.1 (0.0)
Ketac-Bond Aplicap
KB
5
0.7 (0.4) a
1.2 (0.5) A
Fuji II LC
LC
5
1.0 (0.2) a
1.7 (0.1) A
Photac-Bond Aplicap
PB
5
1.3 (0.2) ab
2.1 (0.2) B
Photac-Fil Aplicap
PF
5
1.5 (0.1) b
3.3 (0.1) c
Fuji l
F
5
1.5 (0.1) b
1.4 (0.1) A
Miracle Mix
MM
5
1.7 (0.4) =
3.2 (0.7) c
Ketac-Fil Aplicap
KF
5
1.8 (0.1) 0
2.5 (0.1) B
Vitrebond
VB
5
2.0 (0.2) °
3.9 (0.3) D
Means with same letters are not significantly different (p < 0.05); comparisons at 7 d are indicated by lower-case letters and those at 3 mon are indicated by upper-case letters.
taining material was encapsulated in resin. A high fluoridecontaining material was made by mixing sodium fluoride and Bis-GMA/TEGDMAwhere fluoride loading was varied from 10 to 70 wt%. This experimental material showed a high level of fluoride release (4-30 wt% of fluoride release after 2 wk of water storage), and the amount released was in proportion to the fluoride content of the material. However, once this material was encased in a commercial dental resin (one unfilled and two microfilled resins), fluoride release was hindered even by a thin layer (about 100 pm) of surrounding resin, resulting in a very small (less than 0.6 wt%) or almost zero value after as long as 6 mon of water storage. Wilson (1990) suggested that the fluoride release rate may be adversely affected by replacement of part of the water in the cement by resin and further, the fluoride content of restorative resins is limited by the need for translucency, particularly for the restorative cements. In contrast to the assumption of this investigation, the results demonstrated that light-activated glass ionomer cements have a potential for releasing fluoride equivalent to that of conventional acid-base glass ionomer cements. The most rapid release of fluoride occurred during the first 7 d for both conventional and light-activated glass ionomer cements. Regarding the two restorative products, PF released more fluoride than FLC and their fluoride-releasing time profiles were different. FLC reached an equilibrium value of released fluoride after 1 mon, while for PF, fluoride release continued to increase over the full period of the experiment (3 mon). In conventional glass ionomer cements, the fluoride release rate depends on the formation of complex fluorides and their interaction with polyacrylic acid (Crisp and Wilson, 1974). For light-activated glass ionomer cements, in addition to this factor, the type and amount of resin used for the photchemical polymerization reaction may affect the rate of fluoride release. In VB, a copolymer of methacrylate groups which are pendent to a polycarboxylate chain, and 2-hydroxyethyl
methacrylate (HEMA) dissolved in water are employed in the liquid (Mitra et al., 1992). In FLC, only HEMA added to water is responsible for the radical reaction (Tozaki and Hirota, 1992). The composition of resin components used in PF and PB is unknown; however, HEMA is presumably a primary component. When HEMA is polymerized, it forms poly-HEMA which is an excellent biocompatible polymer and has strong affinity to water with its hydrophilic hydroxyl group. Pedley et al. (1980) reported that the equilibrium water content of poly-HEMA is typically 40 wt%. Although by copolymerization with hydrophobic monomers such as methyl methacrylate, the equilibrium water content of HEMA becomes lower, the value reported is still as much as 12.5%. One possible explanation for the fluoride release shown in light-activated glass ionomer cements is that poly-HEMA absorbs sufficient water to enable diffusion of fluoride ions which may otherwise be firmly encapsulated within the polyacrylate matrix. Ideally, fluoride should be released slowly by a diffusion process and not by dissolution of the material, that is, fluoride release should not result in a deterioration of other physical properties of the materials. From this point of view, the cement showing a large and continuous increase of fluoride release may need to be evaluated for other physical properties such as longterm flexural strength and water solubility. The clinical significance of the results reported here is unknown, as the mechanism of the fluoride enrichment of enamel through contact with restorative materials is not fully understood. It can be concluded, however, that the light-activated dualsetting glass ionomer cements appear to have fluoride-leaching characteristics at least as good as those of the conventional acidbase setting glass ionomer cements. ReceivedJanuary 14, 1993/AcceptedApril 18, 1993
Addresscorrespondenceand reprint requeststo: J.F. McCabe Dental Materials ScienceUnit, DentalSchool UniversityofNewcastleuponTyne Framlington Place NewcastleuponTyne,NE2 4BW England, U.K.
REFERENCES Bourke AM, Walls AWG, McCabe JF (1992). Light-activated glass polyalkenoate (ionomer) cements: the setting reaction. J Dent 20:115-120. Cranfield M, Kuhn AT, Winter GB (1982). Factors relating to the rate of fluoride release from glass-ionomer cement. J Dent 10:333-341. Crisp S, Wilson AD (1974). Reactions in glass ionomer cements: 1. Decomposition of the powder. J Dent Res 53:1408-1413. Crisp S, Lewis BG, Wilson AD (1976). Glass ionomer cements: Chemistry of erosion. J Dent Res 55:1032-1041. E1 Mallakh BF, Sarkar NK (1990). Fluoride release from glassionomer cements in de-ionized water and artificial saliva. Dent Mater 6:118-122. Mathis RS, Ferracane JL (1989). Properties of a glass ionomer/ resin-composite hybrid material. Dent Mater 5:355-358.
Dental Materials~May 1993 153
Mitra SB (1991). In vitro fluoride release from a light-cured glass ionomer liner/base. JDent Res 70:75-78. Mitra SB, Li MY, Culler SR (1992). Setting reaction of Vitrebond light-cure glass ionomer liner/base. Proceedings of a joint symposium of the Academy of Dental Materials and the Dental Materials Group/IADR on Setting Mechanisms of Dental Materials: Jun 30, Loch Lomond, Scotland, UK. Pedley DG, Skelly PJ, Tighe BJ (1980). Hydrogels in biomedical
154 Momoi & McCabe/Fluoride release of glass ionomer cements
applications. Br Polym J 12:99-109. Swift EJ, Dogan AU (1990). Analysis of glass ionomer cement with use of scanning electron microscopy. J Prosthet Dent 64:167-174. Tozaki S, Hirota K (1992). New restorative glass ionomer cements. J Dent Engin (July):130-133. Wilson AD (1990). Resin-modified glass-ionomer cements. Int J Prosthodont 3:425-429.
Fluoride release from light-activated glass ionomer restorative cements Y. Momoi L2J.F. McCabe I
Dental School, Newcastle upon Tyne, England, UK Wow at: Department of Operative Dentistry, School of Dental Medicine, Tsurumi University, Yokohama, JAPAN
Abstract. Fluoride release from four light-activated glass ionomer cements, including newly developed restorative cements, was evaluated and compared with four conventional acid-base glass ionomer cements. There was no significant difference between the group of light-activated cements and the group of conventional cements, and light-activated cements were found to have a potential for releasing fluoride equivalent to that of conventional cements. The amount and rate of release varied among cements. It may be affected not only by the formation of complex fluoride compounds and their interaction with polyacrylic acid but also by the type and amount of resin used for the photochemical polymerization reaction.
INTRODUCTION Conventional glass ionomer cements which undergo setting by an acid-base reaction should be protected from both moisture contamination and dehydration during the early stages of setting to avoid a considerable reduction of physical properties. Normally these cements require at least 24 h to achieve a reasonably stable state. To overcome this shortcoming, Mathis and Ferracane (1989) produced a resin-modified glass ionomer cement and reported its lower moisture sensitivity. They modifed a conventional glass ionomer cement by simply mixing the liquid from a commercial cement restorative with a resin used in light-activated composites. Recently developed commercial resin-modified dual-setting glass ionomer cements contain the essential components of both an aqueous glass ionomer cement and a photocurable resin. Wilson (1990) reported that these novel cements combine the advantages of a long working time and a rapid setting and show a higher early strength. Mathis and Ferracane (1989) mentioned in their work on experimental resinmodified glass ionomer cements that although a reduction in solubility was noted, compared with conventional cements, its value was still great enough to suggest that fluoride release would be significant. Mitra (1991) investigated the fluoride release from a light-activated glass ionomer liner/base cement and revealed that it was not hindered by the presence of lightactivated resin. The objective of this study was to evaluate fluoride release from newly developed light-activated glass ionomer restorative cements and to compare the release rate with that achieved using conventional glass ionomer cements.
MATERIALS AND METHODS Four light-activated glass ionomer cements and four conventional acid-base glass ionomer cements were tested. A posterior composite which contains 5% ytterbium trifluoride (YbF3)was also tested. Details of the materials used are given in Table 1. Twenty-five cylindrical specimens, each having a diameter of 3.0 mm and a height of 2.7 mm, were prepared for each material using disposable injection molded polypropylene molds. Materials were handled according to the manufacturers' instructions. Light-activated glass ionomer cements were activated with a halogen light source (Luxor, Model 4000, ICI, Macclesfield, Cheshire, England, UK) using 40 s exposure to each end of the cylindrical specimen. Specimens were removed from the molds after setting and placed in 2 mL of distilled water in individual plastic containers. They were maintained at 37°C until the time of the measurement, at 24 h, 3 d, 7 d, 1 mon and 3 mon. Five specimens of each material at each test time were prepared. Fluoride release was measured using a specific ion electrode (Russell, Type ISE96-6099/11, Auchtermuchty, Fife, Scotland, UK). At each testing time, standard solutions were prepared from a sodium fluoride solution of concentration 10-1 mol/L F (Fluoride Standard, Type 949060, Orion Research Incorporated, 529 Main St., Boston, MA, USA), to which TISAB (total ionic strength adjuster)(Application solution, Russell, Type 949110, Auchtermuchty, Scotland, UK) was added to obtain a constant background ionic strength. Using the standard fluoride solutions, a calibration curve was obtained plotting the measured voltage (mV) against F" concentration (tool/L) on a semi-logarithmic scale using a computer. One milliliter of sample solution was buffered with TISAB and its fluoride ion concentration was determined by reference to the calibration curve. From the value ofF concentration ofeach sample solution, the total mass (l~g) of fluoride ions released into the storage solution (2 mL ofwater) was calculated. Fluoride release from the materials was expressed as the mass of released fluoride ions from the sample (!~g) per unit volume of the specimen (mm3). Results were analyzed by ANOVA and the Student-NewmanKeuls range test (p<0.05). RESULTS The mean values and standard deviations of fluoride release at
Dental Materials~May 1993
151
TABLE 1: MATERIALS USED IN THIS STUDY Brand
Code
Manufacturer
Type
Batch No.
Photac-Fil Aplicap
PF
ESPE, Seefeld, Germany
L
I
Fuji II LC
FLC
GC, Tokyo, Japan
L
P:A2061111 L:071011
Vitrebond
VB
3M, St. Paul, MN, USA
L
P:8W2
Photo-bond Aplicap
PB
ESPE
L
008
Ketac-Fil
KF
ESPE
C
T191
Ketac-Bond Aplicap
KB
ESPE
C
T331
Fuji l
F
GC
C
P:LY 290394 L:120491
Miracle Mix
MM
GC
C
P:16039 P:061489 L:130491
Heliomolar Radiopaque (composite)
HM
Vivadent Schaan, Liechtenstein
L
413701
L: light-activated type C: conventional acid-base type
7 d and 3 mon are shown in Table 2 as representatives for the early and late stage, respectively. All glass ionomer cements showed significant levels of fluoride release compared with the posterior composite. Overall, there was no significant difference in fluoride release between the group of light-activated cements and the group of conventional cements. Accumulative values after 3 mon of water storage were greatest for VB in the lightactivated cement group and for MM in the conventional cement group.
DISCUSSION An extensive amount of work has been done to evaluate fluoride release from dental materials in the past, and the documented values of released fluoride vary considerably from one study to another (Cranfield et al., 1982; E1 Mallakh and Sarkar, 1990). This may be caused by a lack of uniformity in specimen shape, experimental regime, nature of the aqueous environment used and even the units used to express fluoride release. Therefore, despite a large number of reports, it is difficult to compare values directly. The aim ofthis study was simply to evaluate the fluoride release rate of light-activated glass ionomer cements recently developed as filling materials and to compare these values with those for light-activated liner/base cements and conventional glass ionomer cements. In some studies, the fluoride release has been measured using thin disk specimens and the values quoted as fluoride ion concentration in the storage medium (ppm) or as mass of released fluoride per unit surface area of the specimen. Cranfield et al. (1982) reported on factors affecting fluoride ion release. This previous study showed that fluoride ion release is affected by the sample geometry and that ions are derived not only from the near-surface regions but also from the bulk ofthe samples. In this study, in order to observe the fluoride release from a specimen with a bulky configuration closer to that encountered in practice,
152 Momoi & McCabe/Fluoride release of glass ionomer cements
specimens were prepared in a squat cylindrical shape. Values were expressed as a mass of Application released fluoride ion per unit volume of the speciRestoration men (~g/mm3). This implied that fluoride release can occur from the whole body of the specimen and not just from the surface, as implied in Restoration calculations using mass per unit area. It could be argued that the early fluoride release rate is a Liner/Base function of specimen area, and the long-term value is more likely to be a function of volume. For this study, the latter approach was chosen Liner/Base since there is some evidence that the fluoride release values had stabilized by the completion of Restoration the experiment (3 mon). Although fluoride forms a major constituent Liner/Base of many glass ionomer cements, it appears to have no role in the cement-forming process (Crisp Luting and Wilson, 1974), as opposed to other components such as calcium and aluminium, which are involved in cement matrix formation and play a Core build up basic role in hardening and strengthening the ionomer salt hydrogel (Crisp et al., 1976). After powder and liquid are mixed together, the ionRestoration leachable glass is decomposed by proton attack at the surface, and subsequently fluoride ions are liberated from the glass particles. Swift and Dogan (1990) reported that fluoride is dispersed homogeneously throughout the matrix regions of a set cement. Fluoride in the matrix is available for elution for a long period after setting (Crisp et al., 1976) and it is of concern that in light-activated glass ionomer cements, fluoride may not be available to the same extent as for conventional acid-base reacted cements. The setting reaction of light-activated glass ionomer cements is different from that of conventional glass ionomer cements. Wilson (1990) described the setting reaction of lightactivated glass ionomer cements as dual-setting, in which both acid-base and polymerization reactions take place. This setting mechanism was experimentally confirmed using Fourier Transform Infrared Spectroscopy (FTIR) and Differential Photocalorimetry (DPC) by Mitra et al. (1992) and Differential Thermal Analysis (DTA) by Bourke et al. (1992). The acid-base reaction is a normal glass ionomer cement reaction which is commenced by mixing powder and liquid. The second reaction is a photochemical polymerization process similar to that of a lightactivated composite, which is commenced by irradiation with visible light of the appropriate wavelength and intensity. The extent to which each reaction develops depends upon the age of the cement and the time after mixing at which irradiation takes place. There are two matrices in this type of cement, consisting of the ionomer salt hydrogel and polymer, and to prevent phase separation, the cement has been formulated such that the photoinitiated polymer is chemically linked to the polyacrylate (Wilson, 1990). Resin reinforcement was found to produce cements with better physical properties than conventional cements where only the acid-base reaction is responsible for setting (Mathis and Ferracane, 1989). From these findings, it was assumed that, in the set material, fluoride ions might be firmly encapsulated by the resin matrix and that consequently its fluoride release rate into an aqueous environment might be smaller and slower than that of conventional glass ionomer cements. This hypothesis was further supported by a pilot study where fluoride release was measured for experimental specimens in which a fluoride-con-
fABLE 2: FLUORIDE RELEASE FROM MATERIALS TESTED (pg/mm 3) Mean and (S.D.) Material
Code
N
7d
3 mon
Heliomolar
HM
5
0.0 (0.0)
0.1 (0.0)
Ketac-Bond Aplicap
KB
5
0.7 (0.4) a
1.2 (0.5) A
Fuji II LC
LC
5
1.0 (0.2) a
1.7 (0.1) A
Photac-Bond Aplicap
PB
5
1.3 (0.2) ab
2.1 (0.2) B
Photac-Fil Aplicap
PF
5
1.5 (0.1) b
3.3 (0.1) c
Fuji l
F
5
1.5 (0.1) b
1.4 (0.1) A
Miracle Mix
MM
5
1.7 (0.4) =
3.2 (0.7) c
Ketac-Fil Aplicap
KF
5
1.8 (0.1) 0
2.5 (0.1) B
Vitrebond
VB
5
2.0 (0.2) °
3.9 (0.3) D
Means with same letters are not significantly different (p < 0.05); comparisons at 7 d are indicated by lower-case letters and those at 3 mon are indicated by upper-case letters.
taining material was encapsulated in resin. A high fluoridecontaining material was made by mixing sodium fluoride and Bis-GMA/TEGDMAwhere fluoride loading was varied from 10 to 70 wt%. This experimental material showed a high level of fluoride release (4-30 wt% of fluoride release after 2 wk of water storage), and the amount released was in proportion to the fluoride content of the material. However, once this material was encased in a commercial dental resin (one unfilled and two microfilled resins), fluoride release was hindered even by a thin layer (about 100 pm) of surrounding resin, resulting in a very small (less than 0.6 wt%) or almost zero value after as long as 6 mon of water storage. Wilson (1990) suggested that the fluoride release rate may be adversely affected by replacement of part of the water in the cement by resin and further, the fluoride content of restorative resins is limited by the need for translucency, particularly for the restorative cements. In contrast to the assumption of this investigation, the results demonstrated that light-activated glass ionomer cements have a potential for releasing fluoride equivalent to that of conventional acid-base glass ionomer cements. The most rapid release of fluoride occurred during the first 7 d for both conventional and light-activated glass ionomer cements. Regarding the two restorative products, PF released more fluoride than FLC and their fluoride-releasing time profiles were different. FLC reached an equilibrium value of released fluoride after 1 mon, while for PF, fluoride release continued to increase over the full period of the experiment (3 mon). In conventional glass ionomer cements, the fluoride release rate depends on the formation of complex fluorides and their interaction with polyacrylic acid (Crisp and Wilson, 1974). For light-activated glass ionomer cements, in addition to this factor, the type and amount of resin used for the photchemical polymerization reaction may affect the rate of fluoride release. In VB, a copolymer of methacrylate groups which are pendent to a polycarboxylate chain, and 2-hydroxyethyl
methacrylate (HEMA) dissolved in water are employed in the liquid (Mitra et al., 1992). In FLC, only HEMA added to water is responsible for the radical reaction (Tozaki and Hirota, 1992). The composition of resin components used in PF and PB is unknown; however, HEMA is presumably a primary component. When HEMA is polymerized, it forms poly-HEMA which is an excellent biocompatible polymer and has strong affinity to water with its hydrophilic hydroxyl group. Pedley et al. (1980) reported that the equilibrium water content of poly-HEMA is typically 40 wt%. Although by copolymerization with hydrophobic monomers such as methyl methacrylate, the equilibrium water content of HEMA becomes lower, the value reported is still as much as 12.5%. One possible explanation for the fluoride release shown in light-activated glass ionomer cements is that poly-HEMA absorbs sufficient water to enable diffusion of fluoride ions which may otherwise be firmly encapsulated within the polyacrylate matrix. Ideally, fluoride should be released slowly by a diffusion process and not by dissolution of the material, that is, fluoride release should not result in a deterioration of other physical properties of the materials. From this point of view, the cement showing a large and continuous increase of fluoride release may need to be evaluated for other physical properties such as longterm flexural strength and water solubility. The clinical significance of the results reported here is unknown, as the mechanism of the fluoride enrichment of enamel through contact with restorative materials is not fully understood. It can be concluded, however, that the light-activated dualsetting glass ionomer cements appear to have fluoride-leaching characteristics at least as good as those of the conventional acidbase setting glass ionomer cements. ReceivedJanuary 14, 1993/AcceptedApril 18, 1993
Addresscorrespondenceand reprint requeststo: J.F. McCabe Dental Materials ScienceUnit, DentalSchool UniversityofNewcastleuponTyne Framlington Place NewcastleuponTyne,NE2 4BW England, U.K.
REFERENCES Bourke AM, Walls AWG, McCabe JF (1992). Light-activated glass polyalkenoate (ionomer) cements: the setting reaction. J Dent 20:115-120. Cranfield M, Kuhn AT, Winter GB (1982). Factors relating to the rate of fluoride release from glass-ionomer cement. J Dent 10:333-341. Crisp S, Wilson AD (1974). Reactions in glass ionomer cements: 1. Decomposition of the powder. J Dent Res 53:1408-1413. Crisp S, Lewis BG, Wilson AD (1976). Glass ionomer cements: Chemistry of erosion. J Dent Res 55:1032-1041. E1 Mallakh BF, Sarkar NK (1990). Fluoride release from glassionomer cements in de-ionized water and artificial saliva. Dent Mater 6:118-122. Mathis RS, Ferracane JL (1989). Properties of a glass ionomer/ resin-composite hybrid material. Dent Mater 5:355-358.
Dental Materials~May 1993 153
Mitra SB (1991). In vitro fluoride release from a light-cured glass ionomer liner/base. JDent Res 70:75-78. Mitra SB, Li MY, Culler SR (1992). Setting reaction of Vitrebond light-cure glass ionomer liner/base. Proceedings of a joint symposium of the Academy of Dental Materials and the Dental Materials Group/IADR on Setting Mechanisms of Dental Materials: Jun 30, Loch Lomond, Scotland, UK. Pedley DG, Skelly PJ, Tighe BJ (1980). Hydrogels in biomedical
154 Momoi & McCabe/Fluoride release of glass ionomer cements
applications. Br Polym J 12:99-109. Swift EJ, Dogan AU (1990). Analysis of glass ionomer cement with use of scanning electron microscopy. J Prosthet Dent 64:167-174. Tozaki S, Hirota K (1992). New restorative glass ionomer cements. J Dent Engin (July):130-133. Wilson AD (1990). Resin-modified glass-ionomer cements. Int J Prosthodont 3:425-429.