Erosive Potential of Cola-Type Soft Drinks on the Surface of Restorative Materials: In Vitro StudyABSTRACT
Objective: To evaluate the Knoop hardness of two types of glass ionomer cement - VitroMolar (VM) and Riva Self Cure (RSC), as well as the composite resin Z350 (CR), when exposed to two Cola-type soft drinks.
Materials and Methods: Forty specimens were prepared using a matrix. After placing a polyester strip, the materials were manipulated according to the manufacturers' guidelines. They were kept in an environment with a relative humidity of approximately 100% and an ambient temperature of approximately 23ºC. After 24 hours, the polyester strips were removed from the matrix. The specimens received no protective layer or underwent surface finishing and polishing, and their surface hardness was evaluated. ANOVA with repeated measures was performed, followed by Tukey's test and a t-test.
Results: Regarding the erosive potential of the drinks for each material, cola-type soft drinks (Coke Regular and Coke Zero) caused a significant variation in hardness for VM and the CR (p<0.05). As for RSC, only Coca-Cola Zero reduced the hardness of the material (p<0.05).
Conclusion: Despite the composite resin exhibiting superior initial and final hardness, the material presented the most significant variation in hardness when compared to glass ionomer cement, regardless of the beverage used.
Keywords:
Dental Materials; Tooth Demineralization; Tooth Erosion; Mechanical Phenomena
Introduction
In recent years, dental erosion has been increasingly studied due to the need to prevent tooth structure loss in adults and adolescents [1]. Erosive wear is a mechanical-chemical process that leads to the cumulative loss of dental tissue without bacterial involvement, thereby classifying it as a non-carious dental lesion [2].
Increased longevity, changes in dietary habits, and a decline in tooth loss have led to the greater prevalence and severity of dental erosion [3]. Epidemiological studies show that erosion prevalence in children ranges from 7.2% to 90% [3,4].
Dental erosion is a multifactorial disease [4] in which the contact of tooth surfaces with non-bacterial acids results in irreversible tissue loss due to a chemical process. Intrinsic factors that cause this pathology include chronic vomiting and gastroesophageal reflux [5]. Extrinsic agents, on the other hand, are those not produced by the human body and are associated with the ingestion of acidic foods (such as citrus fruits), medications (like ascorbic acid, acetylsalicylic acid, and vitamin C supplements), and beverages (including industrial juices, soft drinks, and isotonic drinks) [4].
Acidic drinks, with a pH of approximately 3.5 and containing citric, phosphoric, and ascorbic acids, can significantly cause dental erosion when consumed frequently [6]. Factors such as salivary flow rate, buffering capacity, and the duration and intensity of acid exposure on the dental surface can influence the demineralization of dental tissue [4].
Dental erosion initially occurs in the enamel, but dentin exposure may occur over time. In these cases, oral rehabilitation is necessary, commonly performed with glass ionomer cement (GIC) and composite resins (CR), to restore the dental structure, aesthetics, and function, control dentin hypersensitivity, and protect exposed areas [6-8]. Many studies have reported that exposure of these materials to acids with pH levels below the critical threshold can cause erosion on the surface of restorations [9-11]. Hardness, related to resistance to abrasion and wear, is crucial in determining the clinical behavior of restorative materials [12]. The loss of hardness in dental materials can contribute to anatomical deformation and deterioration in the oral cavity [13].
The increased use of GICs, alongside the rising consumption of acidic beverages, highlights the need for studies that assess the potential effects on the surface of these restorative materials. Therefore, the objective of this in vitro study was to evaluate the Knoop hardness of two types of glass ionomer cement and a composite resin when exposed to two commercially available Cola-type soft drinks.
Material and Methods
Study Design and Materials
The restorative materials evaluated in this in vitro study were Vitromolar (VM), a chemically activated glass ionomer cement (DFL Indústria e Comércio S/A, Rio de Janeiro, RJ, Brazil), Riva Self Cure - RSC (SDI Limited, Victoria, Australia), and composite resin - CR Z350 (3M Brazil, Sumaré, SP, Brazil).
Specimen Preparation
For each material, 40 specimens were prepared using a matrix with a cavity 4.0 mm deep and 6.0 mm in diameter. After placing a polyester strip (K-Dent, Quimidrol Comércio, Indústria e Importação Ltda, Joinville, SC, Brazil) at the end of the matrix cavity, the materials were manipulated according to the manufacturer's guidelines at room temperature and relative humidity. Then, they were inserted into the cavity until filled, covered by another strip of polyester [14-16], and placed under a glass plate to remove excess material and flatten the surface at the same level as the upper surface of the matrix [16-19], with the composite resin light cured for 40 seconds (CL-K50, 650 mW, Kondortech Ind. e Com. Ltda, São Carlos, SP, Brazil). Five indentations were made in each sample, and their arithmetic mean was used for statistical analysis.
These were kept in an environment with a relative humidity of approximately 100% and an ambient temperature of roughly 23 °C. After 24 hours, the polyester strips were removed from the matrix, and the specimens did not receive any protective layer or finishing and polishing system to evaluate the behavior of the materials without any surface layer [16]. The surface hardness of these materials was then read.
Initial Surface Microhardness Reading
For the surface microhardness analysis, a microhardness tester (Digital Display Microhardness Tester - Model HVS-1000A, 2008 - Importécnica Ltda., São Paulo, SP, Brazil) and a Knoop indenter with a static load of 25g for 30 seconds dwell time were used. Five indentations were performed on each sample, using their arithmetic mean for statistical analysis [7,18,20].
Surface Treatment
Initially, the pH of the selected beverages was measured using a pH meter (Orion Isometer model 710A, Thermo Fisher Scientific Inc., Waltham, MA, USA) immediately after opening the package at room temperature. This procedure was repeated 10 times, and at each measurement, the drink was dispensed into its respective beaker. During the interval between evaluations, the device's electrode was washed with distilled water. The average of the 10 measurements resulted in the average pH of each solution (Table 1).
Then, for the surface treatment of the specimens, they were individually immersed in 50 mL of each substance for 24 hours, without agitation, at an ambient temperature of approximately 23 °C. They were washed with deionized water for 15 seconds [18], and the surface microhardness was read.
Microhardness Reading After Surface Treatment
After the surface treatment of the specimens, a new reading of the surface microhardness was performed, as previously described.
Statistical Analysis
The results were evaluated using descriptive and inferential statistics. The data showed a normal distribution, allowing for the use of parametric tests. The Shapiro-Wilk test was used to test the normality of the data. The following tests were performed: ANOVA (Analysis of Variance), Tukey's, and t-test. Correlations between variables were conducted using Pearson's correlation test. A significance level of 95% (p<0.05) was considered. The analyses were performed using the Statistical Package for the Social Sciences - SPSS for Windows, version 19.0 (IBM Corp., Chicago, IL, USA).
Results
All materials presented different initial hardness (p<0.05), with the highest mean initial hardness observed for CR Z350 (39.13 ± 7.44), followed by RSC (27.70 ± 3.98) and VM (23.48 ± 4.47). After the trip, CR presented hardness (30.77 ± 9.05) significantly higher (p<0.05) than RSC (25.81 ± 6.50) and VM (22.87 ± 4.09), it was the only material that showed variation before/after immersion (p=0.000), regardless of the drink (Table 2).
Mean surface hardness values of glass ionomer cement and composite resins, before and after acid challenge, and hardness variation.
Regarding the erosive potential of the drinks for each material, cola-type soft drinks (Coke Regular and Coke Zero) caused a significant variation in hardness for VM and the CR (p<0.05). Regarding RSC, only Coca-Cola Zero demonstrated a substantial difference in the hardness of the material (p<0.05).
Discussion
CR Z350 showed higher initial hardness compared to the other materials. Regarding the erosive potential of the drinks, cola-type soft drinks (regular Coke and Coke Zero) caused a significant variation in hardness for VM and CR. In the case of RSC, the material's hardness was reduced only with Coke Zero.
The oral cavity plays a crucial role in the behavior of restorative materials. However, due to the complexity of oral conditions, it is essential to conduct in vitro studies to better understand the fundamental mechanisms of biodegradation [4,7]. In advanced cases of erosion, restorative treatment is necessary, and material selection depends on aesthetic properties, resistance to biodegradation, adhesive capacity, and fluoride release [14]. Therefore, studies that evaluate the effects of erosive substances on these restorative materials are highly relevant, as most patients have at least one restoration and are exposed to a contemporary diet rich in erosive substances [15].
The degradation of glass ionomer cement and composite resin, before and after the erosive challenge, was evaluated by measuring surface microhardness. This property, defined as the resistance of a restorative material to penetration or indentation [21,22], results in its plastic deformation. Therefore, microhardness can help predict the clinical performance of dental materials and their interaction with the oral environment [23] and provide insight into the functional parameters of resistance and wear of the restorative material [24].
Statistical analysis of the data revealed that acidic solutions altered the surface microhardness of the restorative materials, with CR being the material that showed the most remarkable changes after immersion in the two cola-type soft drinks. The findings of this study are consistent with previous research [7,25,26], which reported a decrease in surface microhardness values of composite resin after exposure to soft drinks. Despite this, the resin had the highest hardness value compared to the other restorative materials evaluated.
For resins, this reduction may be related to the loss of filler particles and the degradation of both the binding agent (silane) and the resin matrix [16]. The solvent-polymer interaction is influenced by the action of acids on composite resins. After acid contact, the polymers weaken, reducing hydrogen bond interactions, which decreases the material's microhardness due to a decrease in interaction between polymer molecules [17]. Another factor that may contribute to the reduction of microhardness is the solubility and water absorption of the materials, as water diffusion into the matrix causes degradation and reduces mechanical properties [18-19].
Furthermore, titratable acidity refers to the total amount of acidic species in the medium and determines the hydrogen ions available for interaction. Coca-Cola Zero contains sodium citrate and citric acid, which act as buffering agents, helping to maintain acidity levels in the soft drink without significant variations [27].
In the present study, the GIC showed lower microhardness values after immersion in acidic beverages. It is suggested that sodium citrate, present in Coca-Cola Zero, may have interacted with some components of RSC, increasing titratable acidity levels and, consequently, the drink's erosive potential, thereby reducing the surface microhardness of the restorative material. In the case of conventional GIC, the loss of hardness can be attributed to its excellent solubility in an acidic medium [24].
The evaluated cola soft drinks (Coca-Cola and Coca-Cola Zero) showed statistically significant CR and VM microhardness value variations. The high erosive potential of both drinks, which in the first minutes of exposure depends primarily on their pH [28], may be intensified by the low concentrations of calcium and fluoride ions present, possibly explaining this variation [29]. Additionally, previous research has demonstrated the remineralizing effects of certain compounds on hard tissues. Fluoride [30], casein phosphopeptide-amorphous calcium phosphate [31], and biomimetic hydroxyapatite [32] have shown promising results. It would be interesting in the future to test glass ionomers and composite resins under acid challenges in combination with remineralizing agents to understand whether these substances also contribute to the mechanical characteristics of metallic materials.
The continuous release of fluoride by glass ionomers provides some protection to the adjacent enamel, helping to mitigate the adverse effects of acidity. However, the protective impact of fluoride may be limited when the material is exposed to high and prolonged levels of acidity [33]. According to a study by Szalewski et al., acidic beverages significantly adversely affect composite resins' mechanical properties and surface integrity. Frequent consumption of these beverages can compromise the durability and aesthetics of composite restorations. It is recommended that the consumption of acidic beverages be limited and that oral hygiene practices be adopted to help neutralize the acidic impact [34] and preserve the integrity of composite and glass ionomer restorations.
Toothpaste is the most widely used product, along with the toothbrush. It is known that numerous commercial brands and compositions exist; however, toothpaste containing stannous fluoride forms calcium fluoride deposits, in addition to creating an acid-resistant layer that provides even more excellent protection against subsequent acid challenges, potentially being part of a preventive plan. It is worth noting that this can vary according to the product's characteristics, such as abrasiveness [35].
The results of this study revealed alterations that can occur in restorative materials when exposed to acidic pH. However, this is an in vitro study with some limitations that do not accurately simulate the actual conditions of the oral cavity. Therefore, further studies are needed, primarily using in vivo methodologies, to obtain more reliable results.
Conclusion
Despite composite resin showing higher initial and final hardness, it exhibited the most significant variation in hardness compared to glass ionomer cement, regardless of the beverage used. Dental professionals should educate patients about the risks of consuming acidic beverages and the importance of maintaining good oral hygiene to protect glass ionomer and composite resin restorations.
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Financial Support
None.
Data Availability
The data supporting the findings of this study can be made available upon request to the corresponding author.
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Academic Editor:
Alessandro Leite Cavalcanti
