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Effects of Curing Modes on the Microhardness of Resin-modified Glass Ionomer Cements

Objective This study evaluated the effects of curing modes on surface microhardness of visible light-cured resin-modified glass ionomer cements (VLC RMGIC) and a giomer after different storage periods in comparison to auto-cured resin-modified glass ionomer cements (AC RMGIC). Materials and Methods The following materials were used: VLC RMIC: Fuji II LC Improved, Photac Fil Quick Aplicap, AC RMGIC: Fuji Plus, Fuji VIII and Giomer: Beautifil II. The measurements of microhardness were performed using a Vickers test (100 g loads were applied for 10 s) in the following time intervals: immediately after the recommended cure and after 1, 7 and 14 days of immersion in distilled water. Five samples (d=4 mm, h=2 mm) were prepared for each combination of curing mode and tested material. Results After 14 days, an improvement of microhardness was evident in all tested materials. The full factorial ANOVA identified a highly significant (p<0.001) effect of the factors “material”, “time” and “curing mode (“low”, “soft“, „high”) for the light-cured materials Beautifil II, Fuji II LC and Photac Fil Quick. There was a statistically significant difference in the microhardness between different material types (Beautifil II˃Fuji II LC˃Photac Fil Quick˃Fuji Plus˃Fuji VIII) and curing modes (low ˂soft ˂high). Conclusions Material type had the greatest impact on microhardness, followed by the factor of time, while curing modes showed a considerably smaller influence on microhardness of the light-cured materials.


Introduction
After almost 45 years of clinical use glass ionomer cements (GICs) have undergone some changes aiming to improve their properties, handling characteristics, efficacy and longevity of restorations. One of important changes is hydrophilic resin monomer incorporation to GIC, resulting in resin-modified GIC (RMGIC) (1).
This modification was an attempt to overcome moisture sensitivity and low physical properties of conventional GIC while still maintaining unique properties of GIC such as chemical adhesion to tooth structure and fluoride release (2).
Resin monomer incorporation modifies the chemistry of the GIC setting reaction (8). The fundamental acid-base reaction begins as soon as the powder (base) and liquid (acid) are combined, forming the network of polysalts. This reaction lasts approximately several minutes, although further maturation continues over extended times (9). Acid-base reaction is supplemented by free-radical mediated polymerization of methacrylate monomers, whereas both reactions occur concurrently, if not disturbed. Monomer polymerization can be induced chemically or photo-chemically depending on the initiator system used. Thus, RMGICs available on the market can be designated as dual cure (acid-base reaction + monomer light-cure or acid-base reaction + monomer autocure) or triple cure (acid-base reaction + monomer light-cure + monomer auto-cure) materials.
The set of RMGIC has a complicated structure of interpenetrating networks of poly (HEMA) and polyacrylate salts in which the unreacted (10), remaining parts of glass particles are embedded.
At an early stage of the setting process, there is a competitive nature of network-forming reactions (11,12) and a sensitive balance exists between these two processes (12).
The setting reaction of light-cured RMGIC (VLC RMGIC) involves a rapid polymerization reaction of the resinous component and a slower acid-base reaction, both of which mutually interact (13). Visible light command set of RMGIC has an advantage in clinical procedure, because polymerized monomer immediately protects the acid-base reaction from the problems of water balance and stabilizes the setting cement thus allowing the continuation of clinical procedure.
It is important to distinguish RMGICs from the materials that resemble GICs, namely polyacid modified resin composite (3) and giomers that are compositionally more similar to light cured resin composites. Giomers contain surface reaction type of pre-reacted glass-ionomer (S-PRG) fillers (14) obtained by reacting acid-reactive fluoride-containing glass with polyacids in the presence of water (15) and are capable of fluoride release/recharge in a wet environment.
RMGICs represent an established class of restorative materials with good clinical performance. Nevertheless, potential cytotoxicity may occur from the presence of the residual monomer (16). Palmer et al. (17) investigated HEMA release from Fuji Lining LC, Vitremer, Vitrebond and Fuji II LC and found different material behavior with regards to the effect of curing time.
Apart from the potential cytotoxicity, it is very important to achieve an adequate polymerization of the resinous component in order to ensure basic mechanical properties and longevity of restorations (18). Polymerization effectiveness is influenced by the type of light curing unit (LCU), its irradiance and exposure time (18).
Multiple studies have compared the effectiveness of the halogen and LED LCU with similar (600 mW/cm 2 ) (21), or different light intensities (halogen 350 mW/cm 2 , LED 1400 mW/cm 2 , LED 1100 mW/cm 2 ) (22), and with different light intensity and exposure time (halogen 700 mW/cm 2 , LED 500 mW/cm 2 ; 20 s and 40 s) on the microhardness (MH) of RMGIC (23). Microhardness (MH) value was dependent on the light used to cure them (21,22). In relation to the curing time, similar MH results were obtained with halogen and LED 40 s time exposure, while a significant difference was found between halogen 40 s and LED 20 s (23). Parisay et al. (24) also compared halogen and LED curing units with different light intensity (halogen 600 mW/cm 2 , LED 700 mW/ cm 2 ), and exposure time (20 s, 30 s, 40 s) on top and bottom MH. The top surface MH was higher in all experimental groups. An increase in both the top and the bottom MH was observed when exposure time was increased. In the halogen group, there were no significant differences between 20, 30 and 40 s, while in the LED group, 40 s curing yielded significantly better results than 20 and 30 s.
Considering the depth of cure, VLC RMGIC should be placed in layers which by manufactures recommendations should not exceed 2 mm. Contrary, auto-cured RMGIC (AC RMGIC) may be placed in bulk and are clinically more convenient.
The aim of this study was to assess the MH of AC RMGICs, as well as VLC RMGICs and giomer light-cured with three different curing modes of LED unit after artificial aging for up to 14 days.
Research hypotheses tested were: 1) MH values will be affected by material type; giomer is expected to show higher MH than GICs, whereas AC and VLC RMGICs are expected to show similar MH values 2) MH values of VLC RMGICs will be influenced by three different curing modes 3) MH values will improve with artificial aging.
Vickers MH was evaluated at four time intervals: immediately after specimen preparation and manufacturer recommended setting time 0 day, 1 day, 7 days and 14 days. The surfaces of aged specimens were gently blotted to remove water before the measurements. A diamond pyramid was indented into irradiated specimen surfaces using the Leitz Miniload 2 microhardness tester (Leitz, Oberkochen, Germany) with a load of 100 g, dwell time of 10 s and 5 indentations per specimen. MH was calculated by the following equation: HV=1.8544xF/d 2 , where d is the diagonal of the indentation and F=m×g (g=9.81 N/kg, m=load).

Statistical analysis
Normality of distribution and homogeneity of variances was confirmed using the Shapiro-Wilk's and the increase of both the top and the bottom MH was observed the Levene's test, respectively. The assumption of sphericity for repeated measurements was confirmed using the Mauchly's test. Mean values of MH for each experimental group were compared using a mixed model ANOVA, which accounted for the clustering effect of the repeated measurements performed on individual specimens (31). Post-hoc multiple comparisons were made using a Tukey's HSD correction. Partial eta-squared statistics were used to evaluate relative influences of the factors "time", "material" and "curing mode". Statistical software SPSS 20 (IBM, Armonk, NY, USA) was used, with the level of significance set at 0.05.

Results
Mean MH values as a function of material, aging time and curing mode are presented in Figure 1. The full factorial ANOVA which considered the light-cured materials B2, F2LC and PFQ identified a highly significant (p<0.001) effect of the factors "material", "time" and "curing mode", with the relative influences of individual factors represented by partial eta squared values of 0.991, 0.778 and 0.171, respectively.
The highest partial eta squared for the factor "material" indicates that the highest amount of variation of the dependent variable MH is attributable to the material type. The factor "time" showed a significant interaction (p<0.001) with the factor "material", reflecting a more pronounced MH improvement with time in the group of glassionomer-based materials compared to the giomer B2. After 14 days, the MH values for B2 improved for 2.9-5.7% compared to the immediate (0-day) values, whereas a much more extensive improvement was identified in the glassionomer-based materials: 20.9-22.4% in F2LC, 47.7-51.0% in PFQ, 30.9% in FP and 38.7% in FVIII.
Although statistically significant, the influence of the factor "curing mode" on MH was the weakest among all tested factors. This modest effect of different curing modes is visu-simulirano je pohranom u deioniziranoj vodi u inkubatoru (Cultura, Ivoclar-Vivadent, Schaan, Lihtenštajn) pri 37 ± 1 °C u razdoblju do 14 dana.
Iako je statistički značajan, učinak čimbenika polimerizacijski program na MT bio je najslabiji od svih ispitanih čimbenika. Taj skromni učinak različitih polimerizacijskih ally observable in Figure 1 within all combinations of factors "material" and "time" as a trend of increase in the order of low < soft < high. However, due to the small effect size, the differences among curing modes were statistically significant only in B2 after 1 and 7 days and in F2LC after 14 days. In these cases, the MH appeared significantly higher for the "high" curing mode compared to the "low" mode.

Discussion
The aim of this study was to determine the MH of AC and VLC RMGICs, as well as a giomer and to determine a dependence of the curing type (AC/VLC) and storage time on the MH as well as a dependence of three different curing modes ("low", "soft", "high") of the LED curing unit and storage time.
In previous studies, the dependence of the MH of composite materials on the applied polymerization modes of halogen curing units was demonstrated. In a study by Soh and Yap (32) the polymer structure of composite materials was curing mode dependent. Pulse delay curing mode resulted in lower crosslink density and lower Knoop hardness. In da Silva's research (33), higher values of Knoop MH of composite materials were found by exposing the material to high polymerization mode instead of standard and gradual.
A high power LED LCU was chosen in this study since such a curing unit type is the most frequently used in current clinical practice. Hardness was evaluated after four postirradiation times, starting immediately after manufacturer recommended curing time and followed by artificial aging for 1, 7 and 14 days. For the first measurement, the sam- U ovom radu odabran je LED polimerizacijski uređaj jake snage zato što se takav uređaj najčešće koristi u suvremenoj kliničkoj praksi. Tvrdoća je određena četiri puta nakon ples were without contact to storage media, thus simulating a clinical situation in which a restoration is isolated from saliva and water during the placement and light-curing. Afterwards, the restorations were continuously exposed to wet environment throughout their whole service life. Although it is known that MH varies throughout the recommended layer thickness (23,31), the present study focused on the effect of different light-curing modes on MH measured on the specimen surface, in accordance to previous studies (27). Among all tested materials, B2 showed the highest values of MH which were nearly two times higher than these of the RMGICs tested. Such a finding was expected because of the giomer composition close to composite resins, despite the difference in inorganic filler.
The MH values of B2 generally ranged from 70.4±2.5 to 74.4±2.2 (Figure 1). These findings are comparable to the results reported by Mobarak et al. (22) in a study on the influence of different LCU (halogen 350 mW/cm 2 , LED 1400 mW/cm 2 , LED 1100 mW/cm 2 ). The highest MH values of Beautifil, which is compositionally similar to B2, measured fifteen minutes after curing were achieved by Bluephase LCU (LED 1100 mW/cm 2 ), which agrees with our MH values obtained in high curing mode immediately after curing (72.9±0.9, 71.7 ± 2.4 respectively).
During the 14 days of artificial aging, MH values for B2 showed a small but continuous increase. Yap et al. (34) evaluated the hardness of Beautifil, after the 30 days storage in distilled water and reported MH of 72.7±3.8, which is comparable to our results after 14 days, especially to those obtained in low mode because they used LCU with the mean intensity of 460±4 mW/cm 2 . Unfortunately, the cited study did not evaluate MH at earlier time points, thus providing no data for comparison with our results.
VLC RMGICs exhibited a similar influence of curing modes on MH as it was in B2.
A statistically significant effect of the curing mode identified in the omnibus test suggests that MH was positively related to the total radiant energy received by specimen surfaces. Radiant energy (J) delivered by each curing mode (low 9.9, soft 14.7, high 17.5) concurs with the ranking of MH values: low < soft < high (35). The inability of the statistical analysis to detect significant differences within each material x time combination was due to a very small effect size. When the curing mode "high" was used instead of "low", the differences in MH amounted to about 1-3, which represents an improvement in the range of 2-7%?
A similar influence of the curing modes in the polymerization process of two different material groups tested is an interesting finding since the setting mechanism of the RMGICs involves acid-base reaction and polymerization of resinous part of the cement. Both reactions are diffusion-controlled; therefore their rates decrease as the network formation impairs the diffusion of reactive species. Since the freeradical-mediated polymerization reaction is inherently faster than the acid-base reaction, the light activation causes the resinous network formation to occur ahead of the acid-base reaction, thus inhibiting it (11,36). In a Fourier-transform infrared (FTIR) study of visible light-cured RMGICs Fuji osvjetljivanja -prvi put neposredno nakon proizvođačeva preporučenog vremena stvrdnjivanja i zatim nakon umjetnog starenja od 1, 7 i 14 dana. Za prvo mjerenje uzorci nisu bili u doticaju s medijem za pohranjivanje, tako se oponašalo kliničku situaciju u kojoj je restauracija izolirana od sline i vode tijekom postavljanja i svjetlosne polimerizacije. Nakon toga restauracije su trajno izložene vlažnim uvjetima tijekom njihova trajanja u ustima. Iako je poznato da MT varira kroz preporučenu debljinu sloja (23,31), ovo istraživanje usredotočeno je na učinak različitih polimerizacijskih programa na MT površine uzorka, u skladu s ranijim istraživanjima (27).
However, our results showed that MH of VLC RMGICs continued to increase for 14 days post-irradiation. These findings correspond to Ellakuria et al. (26). They found increasing MH up to 15 days, after which MH decreased up to 12 months.
Young (38) investigated conventional GIC (Fuji IX) and RMGIC (Fuji II LC Improved) by FTIR investigation and observed two separate diffusion mechanisms for acid neutralization. The earlier-occurring, faster reaction stopped after 30 and 150 min respectively, suggesting that the water required for its progress was depleted after a certain time period. The slower reaction was also an acid-base neutralization which was triggered by absorbed water, therefore occurring with a delay. Thus, the post-irradiation increase in MH may be mainly addressed to the continuation of the acid-base reaction.
There are several materials related factors that may influence the mechanical properties of RMGIC such as resin monomer content, type of resin monomers, particle volume and size, polymeric acids molecular weight, number and size of voids and water content. From the point of MH, particle volume and size are of particular interest.
Xie et al. (40) evaluated mechanical properties and microstructures of GICs and found that the mechanical properties were closely related to their microstructures. The Knoop hardness (KHN) obtained in their study was significantly greater for Fuji II LC Improved than for Photac Fil, as it was in our study comparing F2LC and PFQ. Based on the SEM fractography, the author suggested that in the case of Fuji II LC Improved a better indentation resistance might be attributed to homogeneously distributed small glass particles, while on the fractured surface of Photac Fil less exposed glass particles well-integrated in the resinous matrix were identified.
MH values for PFQ significantly increased from 0 d to 1 d, followed by a less pronounced trend of increase towards 14 d. This evidence might be explained by research of Kakaboura et al. (41) who used FTIR analysis and found that acid-base reaction of F2LC, Photac-Fil Aplicap (earlier formulation of PFQ) and Vitremer was significantly slowed by the light polymerization. Between the tested materials, the smallest extent of the acid-base reaction was found in Photac-Fil Aplicap. The authors attributed these findings to the majority of its polyacrylic acid added to powder component, which required more water infiltration for full reaction. This could explain significantly lower MH values measured for PFQ in our study at 0 d in all three curing modes when samples were without contact to storage media. After 1 day aging in de-smolaste mreže prije kiselo-lužnate reakcije, čime je inhibira (11,36). U istraživanju s pomoću infracrvene Fourier-Transform (FTIR) analize svjetlom stvrdnjavajućih SMSC-ova Fuji II LC i Photac Fil Quick Aplicap, brzina kiselo-lužnate reakcije bila je najveća tijekom aktivacije svjetlom (37). Ti su rezultati objašnjeni padom u procesu kiselo-lužnate reakcije kontroliranom difuzijom te stvaranjem mreže metakrilata (gelacije) i dodatno doprinosom zagrijavanja koji se pojavljuje zbog egzotermičke reakcije polimerizacije te zagrijavanja polimerizacijskog uređaja (37).
AC RMGICs, FP, and FVIII, although for different clinical use (luting of indirect restorations and restorative treatment, respectively) exhibited MH values in the range of VLC RMGICs. Also, the influence of aging time was similar to that observed in VLC RMGIC products. It can be assumed that despite the monomer polymerization in AC RMGICs is longer than in VLC RMGICs, monomer polymerization influenced the acid-base reaction in the same way in both material groups, resulting in a continuation of the and acid-base reaction in tested time period, which was reflected as an increase in MH.
MH values attained by different curing modes were positively related to radiant energy delivered to the specimen surface. Despite being statistically significant, the differences among curing modes can be considered negligible from the clinical standpoint. Material composition exerted a stronger effect on MH values, suggesting that the material choice had more impact on the mechanical properties than the curing mode. An improvement in MH was identified in all materials after an artificial aging of 14 days, reflecting the long-term continuation of the setting reaction.

Conclusions
All curing modes investigated can be effectively used in clinical work. Material type had more impact on microhardness than the curing mode.