INTRODUCTION
An increase in the demand for improved esthetics and the fluoride-releasing ability of restorative materials has resulted in the development and widespread use of tooth-colored restorative materials in pediatric dentistry. One such material, a glass ionomer cement (GIC), has been advocated for use because of various reasons, including its physical–chemical bonding to the tooth structure, acceptable esthetic properties, biocompatibility, continuous fluoride release to the adjacent structures over a long period, inhibition of bacterial acid metabolism and activity, similar coefficients of thermal expansion to that of the tooth structure, and ease of clinical application.[123] However, conventional GICs also have a number of drawbacks, such as dehydration, initial moisture sensitivity, a prolonged setting reaction time, and a rough surface texture, which can negatively affect the mechanical properties of the restoration and lead to clinical failure.[45]
In an attempt to overcome these disadvantages, several types of glass ionomer material formulations have been developed, in addition to different types of GIC-based restorative materials, to enhance handling characteristics, increase working times, and improve esthetic properties.[67] These include high-viscosity glass ionomers, resin-modified glass ionomer cements (RMGICs), and polyacid-modified composite resins (PMCRs).[67] The most recent innovations are based on nanotechnology and consist of high-viscosity, faster setting GICs that contain nanofluoroapatite/hydroxyapatite particles and resin-modified GICs.[89] The incorporation of nanosized filler particles into glass ionomer–based materials may improve their mechanical properties, wear resistance, color stability characteristics, and resistance to biomechanical degradation.
Although a few studies of nanofilled GICs have been performed,[89] given the increasing use of GICs and recent trends toward increased consumption of acidic beverages, additional research is needed on the effects of these beverages on the surface texture of GICs. The clinical success of restorations depends on their long-term durability in the oral environment. Thus, information is also needed on the performance of GICs as a restorative material. Therefore, the purpose of this study was to compare the influence of exposure to various children's drinks on the color stability and surface roughness of four types of GIC-based/-containing material and the water sorption/stability of these materials. The tested null hypotheses were that (i) exposure to various beverages would not affect the color and surface roughness of the four different tooth-colored materials and (ii) there would be no statistically significant differences in the water sorption/solubility, color stability, and surface roughness of these restorative materials.
MATERIALS AND METHODS
Beverages and restorative materials
Four different beverages (distilled water, cola, orange juice, and chocolate milk) and four different restorative materials were tested in this study: a high-viscosity conventional GIC (Equia); a high-viscosity conventional GIC, with nanofluoride/hydroxyapatite (GCP Glass Fill); a resin-modified GIC, with nanoparticles (Ketac N100); and a polyacid-modified composite resin (Glasiosite) [Tables 1 and 2]. A digital pH meter (inoLab pH/ION 7320; WTW GmbH, Weilheim, Germany) was used to measure the pH of the solutions.
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Table 1: Chemical composition and batch numbers of the tested materials
Table 2: Immersion media used in the study
Specimen preparation
A total of 130 disc-shaped specimens (N = 520) of each material were prepared using a Teflon ring (7 × 2 mm). The ring was placed on a microscope glass slab, and the materials were inserted into the mold. After pouring the restorative material into the mold, a polyester strip was placed on the top, and a second glass slab was positioned over the strip to obtain a flat surface without bubble formation. The chemically activated high-viscosity GICs (Equia and GCP Glass Fill) were allowed to set for a period of time, according to the manufacturers' instructions, whereas the others were polymerized using a VALO LED unit (Ultradent Products, Inc., South Jordan, UT, USA). The light tip of the unit was held over the glass slide (1 mm away from the specimens) for 20 s to each surface, for a total of 40 s. The top surfaces were then polished using 1000-grit silicon carbide paper to obtain uniform surfaces. For the chemically activated materials, surface coatings were applied as recommended by the manufacturers. Then, the diameter and thickness of all specimens were measured using a digital electronic caliper (Mitutoyo Corporation, Tokyo, Japan) to ensure standardization, and the volume of each specimen was calculated in cubic millimeter. The specimens were then stored under moist conditions at 37 ± 1°C for 24 h.
Color measurements
After preparing the specimens, baseline color measurements were performed using a noncontact-type spectrophotometer (SpectroShade Micro; MHT Optic Research AG, Switzerland) against a white background, according to the Commission Internationale de l'Eclairage (CIE) L*a*b* system. This system provides objective information about the location of object color in a uniform three-dimensional color space. It quantifies the color in terms of three coordinate values, L*, a*, and b*, denoting lightness–darkness, red-green, and yellow-blue, respectively. 10L* is achromatic, with 0 = black and 100 = white. In contrast, a* and b* represent chromatic coordinates, with +a = red, −a = green, +b = yellow, and −b = blue. Color change is described quantitatively in delta E (ΔE*) units, which combines changes in each of the individual parameters L*, a*, and b* into a single value. Different ΔE* values have been proposed to determine a “clinically acceptable” color change value, with most available studies applying a value ΔE* ≥3.3 as the threshold for the clinical acceptability of a color change.[1011121314] Prior to all measurements, the spectrophotometer was calibrated. The measurements were repeated three times for each sample.
Sixty specimens of each material were immersed in four different solutions (four subgroups of 15 specimens each) during a 28-day test period. The specimens were immersed for 3 h a day and thereafter placed in distilled water. The immersion media were renewed daily. Color measurements were obtained after 1st, 7th, and 28th days. Color changes (ΔE) of the specimens were calculated at baseline and after 1, 7, and 28 days.
ΔE was calculated as follows:
ΔE* = [(L1* − L0*)2 + (a1* − a0*)2 + (b1* − b0*)2]1/2
Water sorption and solubility
Forty disc-shaped specimens (10 of each material) were incubated in a lightproof desiccator with anhydrous self-indicating silica gel at 37 ± 1°C for 22 h. Then, the specimens were transferred to another desiccator at 23 ± 1°C for 2 h. After each 24-h period, the specimens were weighed using an electronic analytical balance (0.01 mg precision). The measurements were repeated until the mass change of each specimen is not more than ±0.1 mg in any 24-h period, and the baseline constant mass (m1) was recorded. All specimens were kept in 10 mL of distilled water for 28 days, and they were weighed after 1, 2, 3, 4, 5, 6, 7, 14, and 28 days. Prior to weighing, the specimens were gently dried on filter paper until free from visible moisture, waved in the air for 15 s, weighed 1 min later to ± 0.01 mg, and returned to the containers. The recorded mass was denoted as m2. After a total immersion time of 28 days, the specimens of each material were dried to a constant mass (m3) in desiccators for 28 days, using the same cycle described above. The water sorption and solubility of the tested materials were calculated after 28 days. The water sorption (WSP) and solubility (WSL) (in μg/mm3) were calculated as follows:
A. WSP: m2− m3/V
B. WSL: m1-m3/V
Surface roughness test
After preparing the standardized specimens, the mean baseline surface roughness of the tested materials (Ra, μm) was measured with a surface profilometer (Mitutoyo Surftest SJ-310; Mitutoyo, Japan), using a tracing length of 4 mm, cut-off of 0.8 mm, and a measuring speed of 0.5 mm/s. Each specimen was measured five times around the center of the specimen, and the mean roughness was calculated. A calibration block was used periodically to check the performance of the profilometer, and a single operator performed all the test procedures. Then, 60 specimens of each of the four restorative materials were distributed into four groups (n = 15) and immersed in one of the four solutions for 3 h a day. The surface roughness measurements were repeated after 1, 7, and 28 days. The differences among the baseline and 1-, 7-, and 28-day measurements were calculated.
Statistical analysis
Kruskal–Wallis test was used to compare the color stability results of the different restorative materials. The color changes of the materials in four different solutions were tested using Mann–Whitney U-test, and comparisons between the evaluation periods for each group were performed using Wilcoxon's signed-rank test (P < 0.05).
For the surface roughness analysis, baseline and 1-, 7-, and 28-day results within each group and the differences between the measurements were compared using paired samples t-tests. Intergroup comparisons were performed using a multivariate analysis of variance, followed by Tukey's test (P < 0.05).
One-way analysis of variance, followed by Tukey's post hoc tests, was used to compare the water sorption and solubility of the tested materials after 1, 7, and 28 days (P < 0.05). Paired sample t- test was used to detect differences between water sorption of different materials, according to the storage times (P < 0.004). All the statistical analyses of the data were conducted using SPSS statistical software, version 21.0 (SPSS Inc., Chicago, IL, USA).
RESULTS
Color stability
The mean color differences (ΔE) and standard deviations of the four restorative materials after 1, 7, and 28 days are presented in Table 3. After 28 days, the color changes in all the tested materials immersed in the various solutions were significantly higher than baseline scores, regardless of the immersion solution (P < 0.05).
Table 3: Mean values and standard deviations of color change (ΔE) of composite resins after immersion in different solutions
The differences in ΔE values of the various materials were influenced by different immersion solutions. In distilled water group (control group), there were no significant changes in ΔE values of tested restorative materials (P > 0.05). The color changes in all restorative material groups, except for Glasiosite, were significantly higher after 28 days of immersion in chocolate milk compared with those of the control group. Similarly, when compared with the control solution, the level of staining was significantly higher in the GCP Glass Fill and Ketac N100 materials immersed in cola. When compared with the control solution, there was no significant difference in the staining of the Equia and Glasiosite materials immersed in orange juice for 28 days, but there was a significant staining in GCP Glass Fill material immersed in orange juice for 28 days (P < 0.05).
At the end of the 28-day period, among all the tested materials and immersion solutions, GCP Glass Fill had the lowest color stability, and GCP Glass Fill immersed in chocolate milk had the highest ΔE* value (P <0.05). In addition, numerically higher color change scores were calculated after immersion in other solutions. Otherwise, Glasiosite exhibited significantly less color change (P < 0.05) than any of the other restorative material, even after immersion in chocolate milk and cola, both of which have a high staining ability (P < 0.05).
Staining was considered clinically unacceptable when the ΔE value was equal to or greater than 3.3. At the end of the 28-day immersion period, none of the samples in Glasiosite group exhibited color change values above the acceptable clinical limits. However, the specimens in Equia, GCP Glass Fill, and Ketac N100 groups revealed clinically unacceptable color changes, especially after immersion in cola and chocolate milk.
Surface roughness
The mean values and standard deviations of surface roughness in each group were measured at different time intervals, and the mean Ra value differences (ΔRa) among baseline and posttreatment (1, 7, and 30 days) are presented in Table 4. All the tested materials showed significantly higher Ra values compared with the baseline scores after immersion in various solutions, regardless of the type of solution (P < 0.05). Of the four restorative materials, the surface roughness changes were highest in the specimens immersed in orange juice, and the differences in the values of Equia and GCP Glass Fill groups were statistically significant compared with those of the other groups (P < 0.05). However, there were no significant differences in surface roughness values of the specimens immersed in any of the other solution, with similar ΔRa values recorded.
Table 4: Mean (SD) values of water sorption and solubility for all groups
Water sorption and solubility
The water sorption and solubility of the tested restorative materials are presented in Table 5. In all the samples, most of the water absorption occurred during the first 24 h. The 24-h and 28-day water sorption of each test material was significantly different. The water sorption of all the tested materials was also significantly different. Glasiosite had the lowest water sorption (16.75 μg/mm3) among the tested materials, and Ketac N100 (155.41 μg/mm3) and GCP Glass Fill (161.01 μg/mm3) had the highest after 1, 7, and 28 days. There were also significant differences in water solubility of the restorative materials (P < 0.05).
Table 5: Means and standard deviations for surface roughness measurements for the specimens immersed in different beverages
DISCUSSION
In this study, the effects of four different beverages commonly consumed by the pediatric population on the surface roughness and color stability of two high-viscosity glass ionomer restorative materials and an RMGIC and a PMCR, both used for anterior and posterior restorations, were evaluated. In addition, water sorption/solubility of the tested materials was investigated during the 28-day period. According to the results, the null hypotheses that “different solutions would not affect the surface roughness and color stability of tooth-colored restorative materials and that there would be no statistically significant differences in the water sorption/solubility, surface roughness, and color stability among these restorative materials” were rejected.
Visual techniques and specific instruments, or both, can be used to measure color changes in dental materials.[1011] The use of instrumental methods, such as spectrophotometers and colorimeters, to quantify tooth color can potentially eliminate subjective aspects of color assessments.[1112] The CIELAB system, developed by the International Commission on Illumination, is recommended by the American Dental Association for assessing chromatic differences. It is well suited to the determination of small color changes and has various advantages, such as objectivity, sensitivity, repeatability, and an ability to detect small differences in color.[101113] Therefore, in this study, the color evaluations were performed using a spectrophotometer and the CIELAB color system. Based on the results of the color measurements, the tested restorative materials showed significant color changes after 28 days of immersion in four types of solutions. The immersion in different solutions had the least effect on Glasiosite. GCP Glass Fill was more prone to color changes, followed by Equia and Ketac N100. This result could be attributed to high staining susceptibility due to high water absorption rate or surface texture of the materials. According to the water sorption results, among the tested materials, Glasiosite had the lowest level of water sorption, and Ketac N100 and GCP Glass Fill had the highest. Previous studies suggested that restorative materials with hydrophilic properties can absorb water, as well as other fluids, which result in discoloration.[111415] The surface roughness of restorative materials also determines the materials' color stability.[111617] After a standard polishing procedure, among the test materials, Glasiosite had the smoothest surface at baseline and after 28 days of immersion in all the tested solutions, whereas GCP Glass Fill had the roughest surface texture at baseline and after 28 days. The surface roughness values of the GICs in this study may be explained by the deposition of fine-colored particles in the pits of the restorative materials. Such particles would have caused rapid discoloration of the surface of the materials. The maturity of the GIC could also explain the higher surface roughness values. Despite leaving the cement to set for a day, it might not have matured fully, resulting in significant water or other fluid sorption.
In this study, test solutions induced various degrees of discoloration after 1, 7, and 28 days. As immersion time increased, discoloration became more intense, as reported in previous studies.[7101115] The staining potential of drinks and solutions varies according to their composition and other characteristics.[1118] In this study, for all the restorative materials tested, the greatest changes in color were observed following immersion in chocolate milk and cola. The color changes after immersion in the various staining solutions could not be attributed to pH-related surface changes alone because chocolate milk, which is only mildly acidic (pH: 6.4), produced the greatest discoloration, whereas orange juice, which is very acidic (pH: 3.6), produced the lowest discoloration. When materials were immersed in distilled water, the color differences were clinically acceptable (ΔE = 2.02–2.49) after 28 days. This observation confirmed that water sorption per se did not alter the color of the composites to a considerable extent. As distilled water contains no colorants, clinically acceptable color changes might be attributed to water sorption by the organic matrix over time. These findings indicated that discoloration of restorative materials is multifactorial in nature, and that a range of factors, including the composition of the immersion media, titratable acidity, degree of maturation, and food colorant absorption/penetration, may contribute to the amount of staining observed.
According to many studies, a limit of ΔE* of ≤3.3 represents a clinically acceptable color change.[1011121314] In this study, all the restorative materials, except for Glasiosite, showed color changes above this limit after 28-day cola and chocolate milk immersion. In addition, GCP Glass Fill showed visually perceptible, as well as clinically unacceptable color changes, after immersion for 7 and 28 days in orange juice. The specimens in this study were not in continuous contact with the staining solutions tested. Similar to previous studies, which closely replicated actual clinical situations,[1119] the specimens were immersed in one of the beverages for 3 h a day and then immersed in distilled water for the rest of the day. In this study, the roles of saliva (dilution or buffering) or oral clearance could not be mimicked. Tooth brushing and the intermittent nature of exposure to beverages could also affect the level of staining. Therefore, determining the discoloration effects of these beverages in the oral cavity would very likely require a longer period.
In restorative procedures, surface roughness is one of the main criteria to determine the success of the restorative material. Increased surface roughness may reduce esthetics and the longevity of restorative materials due to increased dental plaque accumulation and gingival irritation.[20] In this study, among all the materials tested, the greatest surface roughness occurred in GCP Glass Fill and Equia groups after 28 days of immersion in orange juice. In one systematic review and meta-analysis, the authors concluded that higher roughness was observed in the RMGIC/GIC when compared with resin composites in all follow-ups of the clinical studies evaluated.[21] Although there is no resin composite group in this study, GIC-based materials showed much more roughness values than the resin-containing ones (RMGIC and PMCR). In addition, after immersion in cola, high-viscosity GICs (Equia and GCP Glass Fill) showed marked deterioration at the end of the immersion period. These results are in concordance with those of many previous studies, which concluded that conventional glass ionomers immersed in acidic media have the highest surface roughness, followed by resin-modified glass ionomers and compomer restorative materials.[222324] In acidic solutions, H+ ions of citric or phosphoric acid diffuse into components of the glass ionomer and replace metal cations in the matrix.[25] These free cations then diffuse outward and are released from the surface. As the metal cations in the matrix decrease, more cations are extracted from the surrounding glass particles, causing them to dissolve.[25] The prolonged exposure of the glass ionomer materials in this study to acids points out the higher Ra values recorded by the profilometer.
In this study, the surfaces of the nanofiller GIC Ketac N100 specimens were smoother than those of the other tested GICs and similar roughness scores with Glasiosite both before and after immersion. These findings may be explained by the nanosized filler of 0.005–0.025 μm. Some in vitro studies demonstrated that the addition of nanofillers provided enhanced surface wear and polish relative to other commercially available dental materials.[2627] The relatively higher ΔRa values of Ketac N100 compared with those of Glasiosite may be due to the large volume of water absorption. PMCRs are anhydrides, which react with water in storage medium, resulting in the development of a carboxylate-rich surface on the uppermost layer, rendering this material more resistant to degradation than conventional and resin-modified glass ionomers.[28]
This study compared the water sorption behavior of different glass ionomer–based materials. The amount of water gain increased continuously in all the materials throughout 28 days. The maximum amount of water increase occurred during the first week. These findings are in agreement with those of previous studies, which indicated that among hydrophilic materials, the maximum amount of water gain occurs within the first week.[2930] After 28 days of water immersion, the water sorption of the tested restorative materials was significantly different. The differences may be related to the different compositions of the materials. The basic constituents of GICs, polycarboxylic acids, and ion-leachable glasses bind water molecules. Initially, water sorption transports calcium and aluminum ions, which react with polyacrylic acid.[31] However, over time, water sorption can remove ions and adversely affect the physical properties of the material. In this study, the water sorption of GCP Glass Fill was much more than that of Equia. The different compositions of these GIC materials may explain the variation in their water sorption capacity. Internal and surface crack lines were also observed in many of the samples of the GCP Glass Fill groups. Similarly, in an in vitro study, researchers investigated the microleakage scores of GCP Glass Seal material and they concluded that observed crack areas in many of the samples resulted in higher rates of dye penetration.[32] This situation seen in the in vitro study may also have been partly responsible for the excessive water sorption.
According to the results of this study, the water sorption values of Glasiosite were lower than those of Ketac N100. This finding may be due to the Glasiosite matrix, which is more hydrophobic than Ketac N100 matrix. Previous studies suggested that materials, such as bisphenol A glycidyl methacrylate (Bis-GMA) (Glasiosite), containing hydrophobic monomers are more water sorption resistant than materials, such as hydroxyethyl methacrylate (HEMA) (Ketac N100), containing hydrophilic monomers.[111415] According to the literature, the higher water sorption of RMGICs in comparison to that of compomers may be due to the rapid water sorption of HEMA, a significant resin component found in RMGICs.[1433] Although the amount of hydrophilic HEMA in Ketac N100 is much less than that found in conventional PMCRs, it still seems to be sufficient to enable high water sorption. Also, resin content of compomer materials (%) is greater than that of RMGCIs, and the more glass ionomer content, the more water absorption.
In this study, the solubility scores of the tested restorative materials were significantly different, with Ketac N100 and Glasiosite having similar solubility patterns. The negative solubility values of Equia and GCP Glass Fill may be attributed to incomplete dehydration of these materials. They do not indicate that no solubility occurred in these materials but may hint to their solubility. Negative solubility results were reported in previous studies of different types of restorative materials.[34353637] Mustafa et al. found that a resin-modified composite resin material showed lower solubility compared with a high-viscosity conventional GIC material same as our results and researchers explained this difference by the presence of a more stable polymeric structure of resin-modified composite resin material.[38] Further long-term studies are needed to investigate the solubility characteristics of these restorative materials.
CONCLUSION
Within the limits of this study, the following conclusions can be drawn:
- The compositions of restorative materials play key roles in their color stability, surface roughness, and water sorption/solubility
- The degree of color stability and surface roughness of the restorative materials varied, according to the beverage in which they were immersed
- For all the materials, immersion in chocolate milk and cola resulted in higher rates of color change, whereas immersion in orange juice led to increased surface roughness
- The polyacid-modified composite resins showed the most resistance to staining, the least roughness, and low water sorption.
Financial support and sponsorship
Nil.
Conflicts of interest
There are no conflicts of interest.
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