Evaluation of bond strength of cinnamon essential oil-modified composite resin to dentin: an in vitro study

Introduction

Dental caries, a global problem induced by many factors [1], is characterized by the demineralization of enamel and dentin, with vast amount of consequences on general health, diet, and quality of life. Therefore, required to be treated using suitable dental materials [2]. Secondary caries refers to a new caries lesion that develops at a margin or adjacent to an existing dental restoration [3]. Many clinical trials and reviews confirmed that secondary caries is the substantial cause of failure [4, 5]. In a previous Delphi survey, secondary caries was reported to be one of the most important problems over a 20-year time period [6]. Furthermore, this type of caries raises the oral health care spendings [7], leading to subsequent loss of tooth structure [8].
Flowable composites have many distinguished advantages, including simple manipulation and good marginal seal; therefore, they can be used in restoring many types of cavities, such as small class I, II, and V [9]. Many synthetic compounds have been used to enhance the antimicrobial properties of composite resins (CR), such as silver-releasing fillers [10], amorphous calcium phosphate [11], calcium fluoride [12], and chlorhexidine [13].
Plant products have been introduced as novel the­rapeutic agents incorporated into CR to prevent secondary caries and reduce the side effects associated with synthetic compounds, such as altered taste and hyperkeratosis [14] as well as developing bacterial resistance and allergic reactions [15], providing safer alternatives.
Phytodentistry is the domain of applying phytothe­rapy in numerous aspects of dentistry, including essential oils and extracts [16], as essential oils are considered a source of natural products, which are famous for their antimicrobial activity [17].
Szram and colleagues found that the incorporation of cinnamon essential oil in the composition of flowable composite did not statistically affect the mechanical pro­perties, whereas it could enhance the antimicrobial activity, reducing the risk of secondary caries formation [18]. Cinnamon essential oil has one of the highest antimicrobial activities among the ten essential oils utilized in the study of Lapinska et al. [19]. Moreover, according to Basha and Haddad, the modification of flowable CR with 5 µl of cinnamon essential oil has significantly improved the shear bond strength to enamel [20].
Nowadays, lack of research conducted to explore the impact of the application of cinnamon essential oil on the dentin bond strength, has provoked researchers to assess the profits of using essential oils in contemporary restorative dentistry. Therefore, further studies should be conducted to evaluate the bond strength of cinnamon oil-modified CR.

Objectives

The present study aimed to investigate the shear bond strength of cinnamon oil-modified CR to dentin.

Material and methods

Sample collection and preparation

A total of eighty molars with sound dentin were extracted for periodontal or prosthetic purposes, and stored at 4°C in water with 0.02% thymol. The cusps were completely removed by cutting perpendicularly to the long axis of the tooth, 2-3 mm from the tip of the cusp, to expose superficial midcoronal dentin using a low-speed diamond disc under constant water flow [20]. The roots were then sectioned at the cement-enamel junction to obtain 2-mm-thick dentin slices, to be further embedded in slow-setting acrylic resin blocks. The occlusal side of each disc was ground under running water with 600-grit paper (silicon carbide papers, TOA, Thailand) for 30 seconds to create a standardized smear layer. The surfaces were finally observed under a light micro­scope to eliminate the presence of cracks, residual enamel, coarseness, or pollutants. All unnecessary areas were covered with a nail varnish.

Specimen grouping

Specimens were divided into four equal groups (each, n = 20): group 1 (negative control), group 2 concentration (1 μl/2 g CR), group 3 concentration (2 μl/2 g CR); and group 4 concentration (5 μl/2 g CR).

Preparation of cinnamon oil-modified composite resin

Three different concentrations (1 µl, 2 µl, and 5 µl) of fully concentrated cinnamon essential oil, which was gently transferred by by the oil company deputy (Center Farma Group, Turkey), were added and delivered by an adjustable micro­pipette (TopPette pipettor, Dragon Lab, China) to 2 g of flowable CR (Tetric N-Flow Bulk Fill, Ivoclar Vivadent, Germany), and measured by a pocket scale (pocket scale, Zhejiang Junkaishun, China) (Figure 1). Three concentrations of modified CR were obtained: group 2 (1 μl/2 g CR), group 3 (2 μl/2 g CR), and group 4 (5 μl/2 g CR), whereas group 1 was oil-free (negative control). Mixing was done using a glass jar and rod to acquire a homogenous consistency of the mixture [18, 19], which was delivered into black tape-covered syringes to protect it from light exposure.

Bonding procedure

The dentin was etched for 15 seconds using 37% phosphoric acid etching gel (N-Etch, Ivoclar Vivadent, Germany) according to the manufacturer’s instruction. The etching gel was then rinsed by a pressed water-air stream for 10 seconds. The etched dentin surface was dried intermittently using a pressed air stream for 5 seconds, while a bonding agent (Tetric N-Bond, Ivoclar Vivadent, Germany) was massaged using a bonding brush for each specimen and then dispensed gently with a pressed air stream. A partially transparent plastic mold (2 mm inner diameter, 2 mm height) measured by a vernier caliper (vernier caliper, Lezaco, ART, China) (Figure 2), was fixed on the bonded area and light-cured for 15 seconds (curing light LED.C, Woodpecker, China) [21]. Subsequently, a 2-mm-thick increment of flowable composite was injected and cured for 10 second. Finally, the mold was carefully disposed of, and the composite post was cured for another 10 seconds. The specimens were immersed in distilled water at room temperature for 24 hours before testing to ensure complete setting of the specimens [22].

Shear bond strength testing

The shear bond strength test was conducted by attaching the specimens to universal testing machine (S.A.E. Ibertest, Spain) (Figure 3). The shear force was applied parallel to the dentin post interface at a crosshead speed of 0.5 mm/min using a knife-edged tip until failure occurred [20]. The following equation was chosen to calculate the shear bond strength:
Shear bond strength (MPa) = Force (N)/Area (mm2) [23].

Statistical analysis

Normality of distribution and homogeneity of variances were assessed using Kolmogorov-Smirnov test and Levene’s test, respectively. Since there was normality and homogeneity between the groups, one-way ANOVA was employed to compare SBS means. Finally, since the sample size in all four groups was equal (n = 20), Tukey’s post hoc test was performed for multiple comparisons to determine groups responsible for significance, if pre­sent. All tests were set at a significance level of p < 0.05. Statistical analysis was done with SPSS version 22 (SPSS for Windows, Chicago, SPSS Inc., USA).

Results

The descriptive statistics, including the means, standard deviations, standard errors, and statistical significance, are demonstrated in (Table 1). The Mean SBS value of group 1 was 22.07 ± 2.95, in group 2 was 21.63 ± 2.45, in group 3 was 21.78 ± 2.23, and in group 4 was 19.81 ± 2.47. One-way ANOVA showed a significant difference in SBS values between the four groups (p = 0.027) (Figure 4): group 1 showed the highest SBS, whereas group 4 revealed the least SBS. For multiple comparisons, using the post hoc test, the statistical analysis demonstrated that the significance in SBS values was only between group 1 and group 4 (p < 0.05), whereas no significant differences were observed between group 1 and group 2 and group 3 or between group 4 and group 2 and group 3 (p > 0.05) (Table 2).

Discussion

This study assessed the effect of cinnamon essential oil on the shear bond strength of Tetric® N-Flow Bulk Fill composite to dentin, and was confirmed to be a pure essential oil extracted from the bark of cinnamon (Cinna­momum zeylanicum) by steam distillation, with no organic solvents, fragrances, or any additives.
To date, only two studies have evaluated the mechanical properties of cinnamon essential oil-modified CR [18, 20]. The present findings demonstrate that the addition of 5 μl of cinnamon oil to 2 g of flowable CR remarkably decreased the bond strength. These findings contradict the findings of Basha and Haddad [20], where the modification of flowable CR with 5 μl of cinnamon oil remarkably increased the bond strength. This may be attributed to the more complex and hydrophilic structure of dentin, or to the oil droplets coating the resin intervening with the bonding process. However, the other two concentrations (1 μl and 2 μl) presented no significant effect on the bond on both enamel and dentin. According to this study, higher concentrations of cinnamon oil adversely affected the shear bond strength, which may be attributed to the fact that some compounds found in essential oils possess hydrophilic properties [24]. Unfortunately, the current literature lacks evidence concerning the possible chemical reactions between essential oils and resin materials.
Szram and colleagues have modified the SDR flow bulk-fill composite with cinnamon oil of three concentrations, and concluded that the CR modified with 1 µl and 2 µl of cinnamon oil had clinically acceptable physio-mechanical properties [18]. Neither Szram et al. [18] nor Lapinska et al. [19] have reported the reason for selecting different concentrations of each oil, but the results showed that these amounts were sufficient against oral pathogens and did not compromise the physio-mechanical properties. Moreover, these studies have not determined whether the mixing was manual or automatic. Unfortunately, the literature lacks studies investigating the bond strength of essential oil-modified CR to dentin for appropriate comparison to this study.
For clinical trials, which are time-consuming and expensive, most practitioners rely on in vitro studies to evaluate restorative materials and related bonding systems to dentin, where tensile and shear tests are the most common tests used [25].
There are many factors that affect the shear bond strength to dentin, among which are the modulus of elasticity, diameter of bonded area, thickness of bonding agent, crosshead shape, and speed [26]. The success rate of bonded restorations to dentin depends on the hybrid layer formed after etching [27]. Acid etching demi­neralizes more than 3-5 μm depth, exposing collagen fibers [28]. In addition, a higher organic content of dentin and the presence of dentinal fluid make the bonding procedure more challenging than that of enamel [29].
In the current study and that of Basha and Haddad [20], data were normally distributed, which is in line with other studies [30, 31], which assessed SBS to enamel but contradicted ISO standards [32]. The signi­ficant decline in SBS values of the modified CR may be attributed to the oil droplets prohibiting functional carboxyl groups from both the bond and composite to form a stable bond. Due to its low viscosity, a flowable CR allows producing a homogenous mixture [18, 19]. In this study, the mixing was performed in a glass jar and using a rod to eliminate undesirable chemical reactions [20].
SBS tests are not recommended in terms of evaluating the bond strength of dental materials to hard tissues, as unrealistic stress is generated within the oral cavity. Nevertheless, shear bond tests have been vastly and are still used to evaluate the bonding capacity to dental structure [33, 34].
Cinnamon oil has antimicrobial, antioxidant, cardio­protective [35], antianxiety, and antidepressant proper­ties [36], having a positive effect on bone generation and pulpal stem cells [37], with no cytotoxic effect on human fibroblast cells [38]. In addition, cinnamon oil (C. zeylanicum) has only a few amounts of coumarin, which is responsible for the anticoagulant, carcinogenic, and hepatotoxic properties [39]. Moreover, both in vitro and in vivo studies demonstrate that the active compounds of the oil reduce the anti-inflammatory phase, control collagen type I synthesis, and cell prolife­ration [40, 41]. On the contrary, reports on intraoral hypersensitivity to cinnamon oil barely exist [42, 43]. According to Ganić et al. [44], cinnamon essential bark oil (C. zeylanicum) present less cytotoxicity than cinnamon oil emulsion when applied to MRC-5 fibroblast cells with a concentration of up to 1 mg/ml (0.002 μl/2 g). Whereas, in our study, the lowest concentration used was 1 μl/2 g, as the oil was fully concentrated without being diluted. LeBel et al. [45] examined the cytotoxic effect of cinnamon oil (C. verum) bark on oral keratinocyte cell line B11, concluding that cell viabi­lity diminished to nearly half (48%) when the oil was used in a concentration of over 0.125% [45]. However, in our study, we used full concentration (100%) cinnamon oil, which could lead to more cytotoxic restorations and harm the bonding unnecessarily.
Nevertheless, this study had some limitations. Firstly, this study employed modified Tetric® N-Flow Bulk Fill with cinnamon essential oil instead of SDR flow bulk used in previous studies [18, 19] due to strict economic sanctions. Secondly, the mixing was conducted manually, which could compromise the homogeneity of the mixture and may result in producing voids, nega­tively affecting the mechanical performance. Thirdly, the sample size could have been better estimated using a statistical software such as G*power (Düsseldorf University, Germany). Finally, the mechanical properties of cinnamon oil-modified CR have barely been investigated in the literature, which could negatively influence the findings of the present study.
Before any in vivo studies can be performed, researchers are recommended to conduct research regarding the safety of topical application of essential oils [46]. Long-term performance, aesthetic features, any potential allergic reactions, and cytotoxic effects of essential oil-modified CR, must be investigated strictly. A study reported that essential oils have specific cytotoxic effects, and the severity of these effects depends on the type and concentration of the oil [47]. The authors recommend conducting experimental studies assessing the microleakage and type of failure of essential oil-modified CR.

Conclusions

Within the limitations of this study, CR modified with less than 5 μl of cinnamon oil demonstrated acceptable shear bond strength, suggesting potential for use in restorative applications. Essential oils seem promising agents to replace synthetic compounds in contemporary dentistry, but require further investigations.

Disclosures

1. Institutional review board statement: Not applicable.
2. Assistance with the article: The authors are grateful to Dr. Yousef Deeb for conducting the SBS test by the universal testing machine, and would like to express sincere gratitude to Dr. Haytham Kosheh for purchasing the micropipette. Last but not least, the authors would like to thank the Dean of the College of Dentistry at Al-Andalus University for Medical Sciences, Prof. Dr. Basem Salim, for the approval of using the light microscope.
3. Financial support and sponsorship: None.
4. Conflicts of interest: None.

References