Short-Term Fluoride Release from Ion- Releasing Dental Materials

Abstract

Objective

To compare short-term release of fluoride ions from ion-releasing dental restorative materials.

Material and methods

Seven experimental groups were prepared using the following six different materials: alkasite (Cention Forte), resin-modified glass ionomer cement (Fuji II LC), bioactive composite (ACTIVA BioACTIVE-RESTORATIVE), fluoride-containing nano-hybrid composite (Luminos UN), coat-free glass hybrid (EQUIA Forte HT), coat-applied glass hybrid (EQUIA Forte HT), and glass ionomer cement (Fuji IX). A total of 40 samples for each group (n=40) were prepared in Teflon molds (8 mm x 2 mm) and placed in polyethylene vials with 5 ml of deionized water. Fluoride release was measured after 6, 24, 48 hours, and for 5 weeks using an ion-selective electrode. The results were expressed in mg/l and the data were statistically analyzed using ANOVA.

Results

Significant differences in fluoride release were observed within the first 6 hours (ANOVA p<0.001). EQUIA Forte HT had the highest release, while the other materials showed no significant differences. After 24 hours, EQUIA Forte HT (p<0.001) and Luminos UN (p<0.05) exhibited significantly higher releases, compared to other tested materials. EQUIA Forte HT maintained the highest release at 48 hours (p<0.001), followed by Cention Forte (p<0.05) and Luminos UN (p<0.05). All material pairs showed significant differences in fluoride release at 5 weeks (p<0.001).

Conclusion

Coat-free EQUIA Forte HT had the overall highest fluoride release, while Cention Forte demonstrated the greatest increase over time. ACTIVA BioACTIVE-RESTORATIVE exhibited the lowest fluoride release in this study.

Introduction

Fluoride is one of the best known anti-cariogenic remedies (1). It acts through several mechanisms, most notably by replacing hydroxyl ions in the hydroxyapatite crystal and forming more acid-resistant fluorapatite (2), influencing bacterial metabolism (3), and affecting the formation of biofilm near the restorations (4). Such desirable properties prompted the use of fluoride as an active ingredient in various dental products, in order to prevent caries formation (5).

Glass ionomer cements (GICs) are most widely used and critically acclaimed fluoride-containing restorative materials (6). They are characterized by their excellent potential for fluoride release, which helps in preventing enamel demineralization, promotes remineralization and reduces plaque growth (6, 7). As cements, GICs set through acid-base reaction, during which fluoride ions that are not a part of matrix formation, are released, but can also be absorbed into the cement. Thus, GICs can serve as fluoride reservoirs, keeping the fluoride amount around the tooth relatively stable (8 - 11). Despite their beneficial anti-cariogenic effect, GICs failed to become long-term restorative materials, due to their poor mechanical properties, such as low compressive strength, wear resistance and elastic modulus (12, 13). To overcome those issues, different materials combining preferable features of fluoride-releasing GICs, and mechanically resilient resin-based composites were introduced. Resin-modified glass ionomer cements (RM-GICs) consist of GIC base, with the addition of water-soluble methacrylate monomers, with boosting their mechanical features in mind (14). However, such modifications did not offer the expected results. Therefore, a major improvement of GICs was recently introduced; a glass hybrid (GH). This material is improved by adding more reactive fluoroaluminosilicate (FAS) glass fillers, which are smaller than main glass fillers, together with higher-molecular weight polyacrylic acid molecules. FAS fillers release more metal ions, improving the crosslinking of polyacrylic acid, which enchases the physical properties (15) (16). Additionally, GHs should be used in combination with self-adhesive resin-based coating material, which further brushes up their properties. An alkasite, often listed as a subgroup of composites, is a recently introduced restorative material. It is capable of releasing fluoride, calcium, and hydroxyl ions, resulting in anti-cariogenic effect (17). Resin-based composite materials, which already possess favorable mechanical and esthetic properties, have been modified by the addition of fluorides and different hybrid versions are nowadays available on the market (18).

In order for fluoride-containing products to be efficient in combating caries development, fluoride ions need to be released (17). Therefore, fluoride release (FR), which is triggered by hydrophilic or ionic environment causing the transport of fluoride ions in and out of the material, is a crucial feature of such products (19). It is influenced by different factors, including the composition and permeability of the material, storage conditions, surface characteristics and curing technique (20 – 22). Although there are numerous commercially available products which are considered to have beneficial effects of fluoride actions, a large number of them are not sufficiently investigated. Considering the importance of FR for achieving the advertised anti-cariogenic effect, the aim of this study was to determine and compare the amount of released fluoride ions from different types of restorative dental materials. The null hypothesis is that there are no differences in FR among the tested materials.

Material and methods

This study was performed using protocols approved by the Ethics Committee of the University of Zagreb, School of Dental Medicine, approval number 05-PA-4-7-X-I-1/2020.

Six commercially available fluoride-containing restorative dental materials were used in this study. They are listed in Table 1 along with their composition provided by the manufacturers.

Table 1. Type and composition of the materials used in the study.

Materials	Type	Composition	Manufacturer
Luminos UN	Fluoride-containing nano hybrid composite	Bis-GMA/TEGDMA resin, multifunctional filler (including micronized fluoroboroaluminosilicate glass)	UnoDent Ltd, Witham, United Kingdom
Cention Forte	Alkasite (Resin composite with reactive glass fillers)	UDMA, DCP, Aromatic aliphatic-UDMA, PEG-400 DMA Barium aluminium silicate glass, Ytterbium trifluoride, Isofiller, Calcium barium aluminium fluorosilicate glass, calciumfluoro silicate glass	Ivoclar Vivadent, Schaan, Liechtenstein
Fuji IX GP Extra (Fuji IX)	Conventional glass ionomer material	Liquid Distilled water, Polyacrylic acid, Powder: Fluoroaluminosilicate glass.	GC Corporation, Tokyo, Japan
ACTIVA BioACTIVE-RESTORATIVE (Activa)	Bioactive composite	Blend of diurethane and other methacrylates with modified polyacrylic acid, silica, amorphous, sodium fluoride	Pulpdent Corporation, Watertown, MA, USA
Fuji II LC	Resin modified glass ionomer material	Fluroaluminisilicate glass/ Liquid destiled water, polyacrylic acid, HEMA, UDMA, Comphorquinone	GC Corporation, Tokyo, Japan
EQUIA Forte HT, without coat	Glass hybrid	Powder: fluoroaluminosilicate glass, polyacrylic acid, iron oxide Liquid: polybasic carboxylic acid, water	GC Corporation, Tokyo, Japan
EQUIA Forte HT, with coat	Glass hybrid, covered with resin-based coat	Coat: silica fillers, multifunctional monomers. Powder and liquid same as EQUIA Forte HT.	GC Corporation, Tokyo, Japan

Specimen preparation

Disc-shaped specimens of each material were prepared using Teflon molds (8 mm wide, 2 mm thick). For each tested material, 40 samples were made (n=40). All tested materials, except for Luminos UN and Activa, were in encapsulated form and were mixed according to the manufacturer’s instructions using Silver Mix capsule mixer (GC Corporation Tokyo, Japan). To avoid any air trapping, polyester strips were placed on both sides of the molds, with material being gently compressed using glass plates. Light curable materials (Luminos UN, Activa, Fuji II LC, Cention Forte) were cured for 40 seconds using the light cure unit Woodpecker LED-C (Guilin Tucano Medical Apparatus and Instruments Limited Company, Guilin, China), curing light output: 850 W/cm² wavelength: 420 nm-480 nm. The rest of the samples made from self-curing materials were left to set for 1 hour before taking them out of the moulds. In addition, the samples for coated EQUIA Forte HT were covered with coat after material setting. The coat was applied for 10 seconds using a brush, air-dried for 5 seconds and light-cured for 10 seconds.

Fluoride release measurement

The samples were individually placed in polyethylene vials (12 mm x 38 mm) (Laboroprema, Zagreb, Croatia) and 5 ml of deionized water was added. They were then left to rest at 37°C in a cooled incubator ES 120 (NÜVE, Ankara, Turkey) for 6 hours. The concentrations of fluoride released into the water were measured after 6 h, 24 h, 48 h and 5 weeks. Deionized water was replaced after every testing procedure. For the measurements, each disk was removed from the water, dried on filter paper and immediately immersed in 5 mL fresh deionized water for further measurements. The fluoride concentrations in the water samples were measured using an ionoselective electrode (F800 DIN, Xylem Analytics Germany, Weilheim, Germany) connected to an ion analyzer (inoLab Multi 9630 DS; Xylem Analytics Germany, Weilheim, Germany). The ion selective electrode was prepared for analysis using WTW outer chamber filling solution ELY/BR/503. The ISE was then calibrated using standards of 50, 100 and 200 mg/l. Once the standard checks had passed quality control, the samples were analyzed by removing the sample and adding 5 ml of TISAB II solution (Total Ionic Strength Adjustment Buffer; Merck KGaA, Darmstadt, Germany). FR from each sample was measured three times and expressed in mg/L (ppm F−).

Statistical analysis

Cumulative values for individual time points were compared using analysis of variance with Scheffe post-hoc test, and differences in release rate were analyzed using one-way analysis of variance with one dependent measurement (release rate per time point) and one independent (material). The significance level was set at 0.05, and the analysis was conducted using SPSS for Windows 22.0 (IBM, Armonk, NY, USA).

Results

The results of fluoride release measurement are shown in Table 2.

Table 2. Fluoride release (mg/l) and SD values after 6 hours, 24 hours, 48 hours and 5 weeks (p<0.001).

	6 hours	24 hours	48 hours	5 weeks
Activa	0.0027±0.0019	0.0042±0.0019	0.0055±0.0022	0.8706±0.0969
Cention Forte	0.0234±0.0203	0.4412±0.1929	9.2491±2.5479	12.6301±2.6305
EQUIA Forte HT	8.0410±3.4183	11.4452±4.0049	13.0840±4.1424	19.5236±4.1696
EQUIA Forte HT with coating	0.7451±0.7158	0.7699±0.7267	0.7860±0.7331	4.9574±1.1639
Fuji II LC	0.1977±0.0901	0.2154±0.0904	0.23340±0.0917	1.8351±0.5204
Fuji IX	0.7657±0.0842	1.1433±0.1265	1.2887±0.1458	9.6002±0.8396
Luminos UN	0.5653±0.1704	1.8870±0.4938	3.0663±1.5647	3.1369±1.5658

The values expressed in Table 2 represent the arithmetic mean of three measurements, specifically for each material and time point. The data for the time points were given cumulatively. For methodological reasons, samples the values of which ​​deviated by more than 2.5 standard deviations from the distribution average were excluded from the analysis. A total of 17 samples were eliminated in this way: Activa (2 samples), Cention Forte (2samples), EQUIA Forte HT without coat (2 samples), EQUIA Forte HT with coat (4 samples), Luminos UN (3 samples), Fuji II LC (3 samples) and Fuji IX (1 sample).

In the first 6 hours, there was a statistically significant difference in the amount of fluoride release between the tested materials (ANOVA p<0.001). EQUIA Forte HT scored significantly higher than the other materials, all of which do not differ significantly, compared to each other. After 24 hours, there was again a statistically significant difference in fluoride among the materials (p<0.001). This time, both EQUIA Forte HT (p<0.001) and Luminos UN (p<0.05) had statistically significantly higher score than other tested materials. In the first 48 hours, EQUIA Forte HT again scored statistically significantly higher than other materials (p<0.001), followed by Cention Forte (p<0.05) and Luminos UN (p<0.05), with other materials having statistically significantly lower values. In 5 weeks, there were statistically significant differences in the amount of fluoride released between all pairs of materials (p<0.001).

The rate of change of release was different among materials (repeated measures ANOVA for concentration variables per hour, p<0.001). The release rate of EQUIA Forte HT decreased over time, while for Cention Forte it increased between 24h and 48h, and again decreased in 48h and 5 weeks. The rates for coated EQUIA Forte HT, Fuji IX and Luminos UN differed from the other materials, but not from each other; in all three cases there was a slight continuous decrease of fluoride release rates. Hourly release rates are shown in Figure 1.

Figure 1.

Hourly fluoride release rates (mg/l).

Discussion

The results of this study showed that uncoated EQUIA Forte HT released the highest amount of fluorides among the tested materials in all testing periods, thus, the null-hypothesis was rejected. In addition, it was shown that FR rates of the materials varied greatly compared to each other, owing to differences in their composition.

The reason why uncoated EQUIA Forte HT, a glass hybrid material, outperformed other materials lies in its composition, as glass hybrids developed from glass ionomer cements, with several significant modifications (23). The initial ‘burst effect’, which is characterized by releasing high amounts of fluorides in the first 24 hours, is a well-known property of glass ionomers (24), also observed in glass hybrids. It is one of their biggest advantages, as it helps neutralizing the bacteria and promotes dentine remineralization (25). One of glass hybrid improvements is the replacement of Ca²⁺ with Sr²⁺ ions; this enhanced the fluoride release since strontium fluoride complex dissolves faster than calcium fluoride complex (26). However, it is interesting to notice that the same material, this time coated with the recommended EQUIA Coat, released much lower amounts of fluorides. This can be attributed to the fact that this type of coat is resin-based and contains nanofillers, which obstructs the gaps in the material and protects it, thus effectively reducing the ion release (27). These findings are also supported by the research of Tiwari et al. (28) and McKnight-Hanes et al. (29) who also concluded that coat application significantly decreases the fluoride release. However, the introduction of the coating brings about a notable enhancement in the mechanical properties of glass hybrids. It is important to emphasize that the improvement in mechanical properties was, in fact, the primary reason behind the application of the coating in the first place. Yet, certain volume of fluoride release is still retained. The amalgamation of the aforementioned effects highlights the multifaceted benefits that arise from the utilization of the coating. Not only does it sustain the desirable anti-cariogenic properties associated with fluoride, but it also serves as a catalyst for reinforcing the mechanical characteristics of glass hybrids. This symbiotic relationship between the coating and the material not only enhances their overall performance, but also extends their potential applications in diverse fields (30).

In the current study, during the early testing periods, some of the light cured materials, namely Activa, Fuji II LC, Cention Forte and coated EQUIA Forte HT, released somewhat lower amount of fluorides than self-cured glass hybrid and glass ionomer cement. The main contributing factor to this result is the effect of the curing method; previous research has shown that the initiation of polymerization through light exposure enhances the formation of chemical bonds, leading to an increased density of cross-linking. As a result, the permeability of the resin matrix towards fluoride ions is ultimately decreased (31, 32). While composites continued to release relatively low amounts of fluorides, Cention Forte, an alkasite material, outperformed all other tested material after 5 weeks. Singbal et al. reported similar findings (33). Initial low rates of fluoride release observed in Cention Forte could be attributed to the fact that the fillers present in this material undergo surface modification, resulting in increased resistance to degradation and potentially leading to a release of a reduced quantity of fluoride ions (7). This suggests that there is a certain period of time needed for the material matrix to be maturated enough to release fluorides.

In the current study, Activa, a composite material, released the lowest amounts of fluorides in all testing periods. Similar findings were reported by Rifai et al. (34), who compared Activa to EQUIA Forte HT, concluding that Glass hybrid material outperforms composite materials. In addition, Hokii et al. (35) compared the fluoride release from six different restorative materials. They also concluded that Activa releases lower amounts of fluorides compared to glass hybrids and glass ionomers. In contrast to our study, Activa released a significant amount of fluoride ions in the study conducted by Vicente et al. (36). The explanation for this is the exposure of the material to acidic pH of 3, 5 environments in their study, leading to a significant increase in ion release rate in certain measuring periods, meaning that Activa expresses anti-cariogenic properties in caries challenging conditions.

Luminos UN composite performed better than Activa. Yet it was inferior to other materials. The explanation for this could lie in the fact that the water uptake/dissolution process in composite resins is less efficient in delivering fluoride compared to mechanisms observed in glass-ionomer-based materials (37). Alternatively, it could be indicative of lower fluoride concentrations in the composite formulation.

To simulate the oral environment, other studies tested the materials in various storage mediums, for instance artificial saliva or different types of acids, with different pH cycling solutions (38). However, for the purposes of the current study, deionized water was chosen as the storage medium. This selection was made because deionized water is free from fluoride traces or minerals, thus allowing for an accurate measurement of fluoride ion release. This was further considered sufficient, as it has been demonstrated that fully replicating oral conditions are exceedingly challenging due to various factors, such as the rate of saliva flow and individual habit variations (39).

While these studies offer valuable preliminary data, it is essential to recognize that the outcomes may be influenced by numerous factors such as the specific protocols employed, the concentrations of the bioactive materials tested, and the environmental conditions in which the experiments took place, therefore, experimental conditions under which they were conducted should be considered. In addition, this study investigated solely the fluoride release, while other ions, most notably calcium ions, also play a significant role in the remineralization process. By conducting additional studies, researchers can delve deeper into the mechanisms underlying fluoride ion release from these dental materials, exploring their long-term release patterns and their ability to provide sustained anti-cariogenic benefits. Moreover, investigating the potential of different coating techniques or modifications to enhance the fluoride ion release of EQUIA Forte HT and other materials could lead to the development of more effective preventive strategies in the field of dentistry. Furthermore, it would be valuable to explore the correlation between the released fluoride ion concentrations and the actual clinical outcomes in terms of caries prevention. Determining the effectiveness of these materials in real-world scenarios and evaluating their long-term clinical performance could provide a more comprehensive understanding of their practical utility and help guide clinicians in selecting the most appropriate materials for their patients.

Conclusions

Based on the findings of this study, we can draw several significant conclusions regarding the fluoride ion release characteristics of various dental materials. Specifically, the uncoated EQUIA Forte HT, a glass hybrid material, demonstrated a notable advantage in terms of releasing a higher concentration of fluoride ions within a short-time period compared to other materials examined in this investigation. This observation highlights the potential of EQUIA Forte HT to effectively contribute to the prevention of dental caries by delivering a substantial amount of fluorides. All other tested materials exhibited some degree of fluoride ion release, which could possibly aid in preventing the formation and progression of dental cavities. However, further research is needed to comprehensively validate anti-cariogenic effects of these materials, especially in clinical conditions.