In this study two different composite materials were used to measure the effect of storage media volume and refreshing of it on monomer elution.
Bulkfill composites like FBF are in high demand these days because of their increased depth of cure and reduced clinical time of operation [8]. BG can provide esthetics in conservative restorations with gingival recession [9]. Despite the manufacturer’s information, elution of TEGDMA from FBF- as seen in Pongprueksa P et al’s [10] study, and also UDMA from BG was observed. According to Cokic SM et al [6] manufacturers do not release the exact component of composites because of trade reasons.
Ethanol was used as storage media to intensify monomer elution. In aqueous media such as artificial saliva, monomers are released in smaller amounts [11, 12]. Also, FDA declared that ethanol can simulate exposure to nutrition materials such as light drinks and chocolate [13, 14]. According to VanLanduyt et al [1] UDMA wasn’t detectable during first day of elution.
According to Cokic SM et al [6] storage media volume had major effect on monomer elution. The larger the volume of the storage media, the higher the amounts of monomer elution that happens. Also, incubation time between 7 and 30 days had no significant effect in monomer release. These two observations lead to considering the limitation of monomer solubility in storage media, as a major factor in monomer elution. Polydorou O et al [14] measured monomer elution in 1, 7 and 28 days and one year and storage media were refreshed in 28 days. Monomer elution was equal in 28 days and one year which indicates saturation of storage media. In fact, saturation of storage media prevents further elution of monomers. Higher amount of storage media reaches saturation point later, so monomer elution is higher. According to Van Landuyt KL et al’s [1] meta-analysis, release of monomers is a chemical equilibrium reaction and in smaller amounts of storage media equilibrium was achieved faster and prevented more monomer release. Constant refreshing of saliva and pulpal fluid in in vivo situation prevent saturation status. High volume of storage media and refreshing of it could be used to overcome this condition.
In this study 2 volumes of storage media were used to evaluate effect of storage media volume on monomer elution. In order to evaluate the effect of storage media refreshing on monomer elution, in half of the samples, storage media was refreshed daily. Thus, daily pattern of monomer elution was obtained. Alshali et al [15] measured monomer elution after one day, one month and three months. In their study, 70% of total three month-elution of TEGDMA and 50% of the total BisGMA elution occurred on the first day. Nalcaci et al [16] declared that TEGDMA eluted faster than BisGMA in methanol as in one sample group, 92% of TEGDMA eluted in first 6 hours while at the same time BisGMA reached 57% elution. Sideridou ID et al [17] measured monomer elution in 3,6 and 24 hours and also 3,6 and 30 days. Similar to the current study, the highest amount of release of TEGDMA and UDMA was observed in first day, unlike BisGMA.
The first mechanism of monomer elution, is elution from the composite surface that occurs in the first 24 hours. Subsequently monomer elution continues with a slower rate, since increasing the volume of polymeric chains and release of unreacted monomers from composite, takes substantial time [11].
In the current study TEGDMA and UDMA from FBF and UDMA from BG eluted more in 3 ml storage media (when not refreshing the solvent) because of the faster saturation of 1ml storage media. In cases of refreshing storage media, two different volumes had no significant difference in reaching saturation status and thus monomer elution.
According to Cokic SM et al there are two barriers for monomer release; decreasing the concentration gradient between the sample and the storage media [6] and saturation of storage media with monomers. Thus, rather than the storage media volume and concentration of monomer in the storage media, the concentration of unreacted monomers in the composite plays an important role in reaching equilibrium. Elution of TEGDMA from BG (in cases of not refreshing the solvent) had no significant difference between two volumes of storage media. It can be observed that high concentrations of unreacted TEGDMA in BG led to the continued elution of TEGDMA in 1ml storage media. Daily pattern of TEGDMA elution also confirms this result. Since elution of this monomer from BG, despite of its high rate in elution, continued till day 7. While FBF released TEGDMA only on the first day. Elution of BisGMA from FBF and BG was in small amounts, which justifies not reaching saturation in 1ml volume and not having a significant difference in elution of this monomer in two storage media volumes.
Refreshing of storage media had significant effect on BisGMA elution. Considering that BisGMA couldn’t reach saturation during 7 days because of its small amounts, refreshing of storage media should cause another phenomenon rather than preventing saturation. Refreshing storage media will reduce concentration of monomers in storage media and increase the concentration gradient. BisGMA has a heavy aromatic core, a higher molecular weight [17] and a lower elution rate than TEGDMA and UDMA. BisGMA is the only monomer which eluted lower than 50% of its total elution in day one. Storage media refreshing and increasing the concentration gradient, affect more than 50% of BisGMA ‘s total elution while for TEGDMA and UDMA this effect is not significant.
TEGDMA elution from FBF is significantly higher in case of refreshing storage media despite its daily elution pattern and rate of elution. Gonzalez-Bonet A et al [18] described that TEGDMA has two hydrolysable ester groups and hydrolyzing these groups creates methacrylate acid (MA), 2-(2-(2-(2-hydroxyethoxy)ethoxy)ethoxy)ethyl methacrylate (TEGMA) and TEG. Finer Y et al [19] detected TEGDMA products such as TEGMA, triethylene glycol and methacrylicacid. They suggested that total TEGDMA elution is equal to amount of the main TEGDMA plus its products. It can be obtained that FBF releasing products such as additives and filler components could have hydrolyzed TEGDMA to its products while refreshing the storage media released TEGDMA and prevented hydrolyzing of this monomer.
3ml storage media of BG composites at the first day, eluted monomers in small amounts; Which caused more monomer elution in 1ml storage media cases with refreshing and also more elution of TEGDMA in 3ml storage media in cases without refreshing.