Evaluation of the flexural strength and surface roughness of dental material after immersion in coffee

This study was done in order to evaluate the changes in flexural strength and surface roughness in dental material restorations after immersion in coffee. Bars specimens (2mm x 2mm x 25 mm) of Z100 3M/ESPE were made according to ISO 4049 using aluminum molds. After curing, the specimens were then randomly divided into five groups and conditioned at 37°C in physiological serum. In all groups, except control, samples were immersed in coffee for ten minutes daily. In group A, the samples were immersed in coffee. For group B, after to be immersed in coffee, samples were immersed in distilled water for one minute. For group C, samples were immersed in mouthwash for one minute. For group D, samples were brushing for ten minutes, with load of 250g and 4250 cycles after all treatments the samples were stored in physiological serum again. In control group, samples were only stored in physiological serum. The five groups were then divided into 2 subgroups according storage time, six and nine weeks, respectively. Flexural strength was determined using three point bending test in a universal testing machine. All the tests were carried out at a room temperature and samples were maintained in physiological serum during the test. Surface roughness measurements were made using a surface roughness tester. All data were analyzed using one-way analysis of variance (ANOVA) followed by Dunnet tests. The ANOVA indicated no difference in the means of surface roughness and significant was observed for flexural strength between control group and one group (coffee).


Introduction
The increase of the life expectancy associated to modern life habits, as fast food, ingestion of pigmented drinks, as coffee, the use of cigarette, etc., has generated new challenges for the dentists and the professionals of this area. Nowadays, the dentists come across functional and aesthetic factors that challenge the materials and the techniques that must adapt to this new reality. Besides, along the life time there is the physiologic wear of the teeth, in other words, the teeth decrease due to the masticatory attrition, and this fact is inevitable. The dentist needs to give back the original format of the teeth, being careful in preserving the remaining dental element.
The composed resin is used since 1960 however, it was starting from the acid of the enamel conditioning, the development of the adhesive system and of the BisGMA molecule in the decade of 1970 that these aesthetic restoring materials had significant progresses. The adhesive system made possible that the cavities to be restored got smaller, because they don't need retention and resistance forms.
Dental composites may be defined as combinations of at least two chemically different materials with a distinct interface. Dental resin composites consist of a resin matrix (organic phase), inorganic filler particles (dispersed phase), fillermatrix coupling agent (interface), and minor additions including polymerization initiators, stabilizers and coloring pigments.
Similar to other composite structures, the type and composition of the resin matrix as well as the filler particles have strong influence on the mechanical properties, which ultimately determines the clinical performance of these materials (Tsujimoto et al., 2018).
Several authors have evaluated the resin properties and others verified that for some resins, the immersion means may alter their mechanical properties such as: hardness, flexural strength, and fracture toughness. However, due to the aesthetic importance of these materials, studies evaluating the influence of color stability has been accomplished. (Alvanforoush et al., 2017b;Camilotti et al., 2021b;Rodrigues et al., 2020) (Alvanforoush et al., 2017a;Camilotti et al., 2021a;Lorenzetti et al., 2015).
Then, some studies have concluded the immersion od the resin in acid environment alter some of their properties.
The purpose of this study was to compare the flexural strength of resin composite after immersion in different media.
The hypothesis tested was the media would adversely affect composite flexural properties.
The coffee was chosen, because Brazil is one of the largest producers of the world and its population is a big consumer of this drink. In the literature, the period of immersion in a general way corresponds to thirty days, and the option for nine weeks is justified once it corresponds to the double of the time; a plastic sieve was used to remove and to put the samples back in the pot with physiologic serum and a tweezers to put them in the brushing machine to avoid the manual contact with the samples. The immersion period in the coffee was ten minutes, because it was taken into account that the ingestion of a cup corresponds to one minute of contact with the teeth/resin, the average ingestion is ten cups a day. The methodology developed looked for a way to diminish the possible effects caused by the coffee, they were: water, mouthwashes and brushing.

Methodology
For this study one hundred samples were manufactured using an aluminum molds, bars (25mm x 2mm x 2 mm) were fabricated from a dental composite restorative (Z100, 3M ESPE, USA) according to the norm ISO 4049. The manufacturing of the samples was carried following the methodology: molds were positioned over a glass slide. After application of resin into one, a second glass slide was pressed over the surface in order to extrude excess material. The surfaces of the samples were then light polymerized in three overlapping irradiations of 40 s and output intensity of 470 mW/cm2 (Figure 1). After curing the specimens were then randomly divided into five groups and conditioned at 37 C in physiological serum. The following cycles was taken for every sample group daily: The A B C and D groups were immersed in 100 ml of coffee for ten minutes aiming to simulate the daily ingestion of coffee. However, for each group samples were submitted for one condition after coffee. For the group A, the samples were immersed in coffee and stored in physiological serum again. For the group B, after the immersion in coffee the samples were immersed for one minute in 200 ml of distilled water and taken back in aquarium. For the group C, the same procedure of group B was followed but a mouthwash. For the group D, after the immersion in coffee the samples were brushed (Figure 2). To simulate the daily tooth brushing of the patient a standard machine was applied. The machine consisted of a base on which there were two round devices where the matrix was inserted, two arms held two Oral B tooth brushes with a load of 250 gr on the bristles. The sample were submitted to 4250 daily cycles, 10 minutes, at 140 cycles/minute (Figure 3). A Colgate toothpaste was used added to water at a 1/1 rate. For every group the samples were taken out of the aquarium on the 6th and 9th weeks. The coffee use was prepared using a standard procedure of 24 gr of ground coffee and 400 ml of water.

Three point bending test
To accomplish the flexural strength by the three point bending test was used with a device developed by Dias (2002) (Figure 4) (Oksman et al., 2002). Such device allows the storage of the samples in physiologic serum during the Research, Society andDevelopment, v. 10, n. 3, e30510312486, 2021 (CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v10i3.12486 5 accomplishment of the test. A piece with supports is fit in its interior on which the sample is placed during the test, the distance between supports is 7 mm. The test was performed with a Universal Testing Machine (Versat 2000, Panambra, Brazil) at a crosshead speed of 0.5mm/min; the load at fracture was recorded and maximum flexural strength was calculated using the following formula: where  is the flexural strength, L the span length, h the specimen thickness, b the specimen width and F is the maximum load applied.

Surface roughness measurement
Surface roughness measurements were made using a surface roughness tester (Mitutoyo). Five roughness average were made for each sample after 6 and 9 weeks.

Statistical analysis
These data were analyzed using one-way analysis of variance (ANOVA) followed by Tukey's multiple comparison tests.

Results and Discussion
The influence of the coffee on the flexural strength and the surface roughness were appraised. The samples were submerged for a period from 6 to 9 weeks in physiologic serum and submitted the immersion in Group A, B, C, D and control during ten minutes daily. The flexural tests were accomplished according to the norm ISO 4049 and during the test care was Research, Society andDevelopment, v. 10, n. 3, e30510312486, 2021 (CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v10i3.12486 6 taken to keep the samples in the physiologic serum, to avoid the dehydration.
The results obtained after the accomplishment of the flexural resistance test for groups A (coffee), B (coffee and water), C (coffee and mouthwash), D (coffee and brushing) and control group can be observed at Table1. In the analysis control group was the one with the greatest dispersion in 6 weeks (SD = 17.26) and the Group C the least dispersed (SD = 4.32); in 9 weeks, the Group D group had the highest dispersion (SD = 10.66) while the Group A group (SD = 1.45) the lowest dispersion. ANOVA showed no statistically significant difference between groups for 6 weeks (F = 1.35; p> 0.05). For 9 weeks, ANOVA showed a statistically significant difference between the groups (F = 4.71; p <0.05).
Dunnett's test was selected to compare between which groups there is a difference, and revealed that the statistical difference was between the Group control X Group A (p <0.05). There was no significant difference between all groups for 6 weeks.
However, a significant decrease in flexural strength was identified for coffee group compared control group after 9 weeks. The results are the according with another author's study.
The mean surface roughness values (Ra e Rz) for specimens after different periods of immersion are summarized in Table 2.  Table 2 shows the means of the roughness of each group at the times of 6, and 9 weeks. Ra roughness the Control group showed the greatest dispersion both in 6 weeks and in 9 weeks (SD = 4.70 and SD = 3.63, respectively), the lowest dispersion was observed in the Group A in 6 weeks and in the Group C at 9 weeks. ANOVA showed no statistically significant difference between groups for either 6 or 9 weeks (F = 0.31; p> 0.05) and (F = 1.06; p> 0.05,) respectively. For the Rz roughness, the Group C showed the highest dispersion in both 6 and 9 weeks (SD = 12.53 and SD = 11.40, respectively), the lowest dispersion was observed in the Group A at 6 weeks and in the Group B in 9 weeks. ANOVA showed no statistically significant difference between groups for either 6 weeks or 9 weeks (F = 1.56; p> 0.05) and (F = 0.78; p> 0.05), respectively.
For the surface roughness tests significant differences were not verified among the groups. Those results were contrary the those obtained other studies (Badra et al., 2005;Camilotti et al., 2021b;Hwang et al., 2018;Phanestu & Syafiar, 2018). However, those authors used artificial saliva associated to the coffee as mean of immersion and, besides, it was observed in their studies that the isolated effect of the saliva increased the roughness, therefore, the coffee didn't influence that property, but it is believed that in the mentioned studies the influence was due the saliva and not to the coffee.

Conclusion
Based on the results we can conclude there were no significant statistical changes after 6 weeks. There were significant statistical changes after 9 weeks between the coffee and control groups and the means used to diminish the coffee effects (distilled water, mouthwash and brushing) showed significant statistical results after 9 weeks.