Comparative Evaluation of Microleakage of Two Variables of Glass-Ionomer Cement: An In vitro Study

2.1. Materials

In this study, two types of glass ionomer cements were used to compare in vitro the microleakage at the interfaces:

• A conventional GIC Ketac ™ Fil Plus (3M ESPE),

• A nano-charged GIC Ketac ™ Universal (3M ESPE) (Fig. 1 and Table 1).

2.2. Methods

Forty freshly extracted, caries-free human teeth (twenty maxillary and mandibular molars and twenty premolars) were used for this study. These teeth were stored in distilled water and in a refrigerator until the operation process. These teeth were cleaned using a brush head with water and pumice powder to remove tartar, soft tissue, and other debris. Each tooth was numbered.

On each tooth, two box-type enamel-dentinal cavities were prepared: one on the mesial face and the other on the distal face. Each cavity is located at the cement-enamel junction and has the following dimension:

• Height: 3mm;
• Width: 2 mm in the vestibulo-lingual direction;
• Width: 1.5 mm in the mesiodistal direction (Fig. 2).

These measurements were verified with a periodontal probe to ensure uniformity of preparations, and the cervical angles were rounded. The cavities were prepared with a high-speed handpiece, using a round bur and a cylindrical tungsten carbide bur (diameter 1.2 and 1mm) under heavy water spray. Each bur was used to prepare five cavities; after each manipulation, the teeth have been stored in distilled water. The teeth were randomly divided into two groups of 20 teeth each: Group 1 and Group 2, as shown in Fig. (3).

A mounted metal matrix and a matrix port were used as formwork to replace the missing wall and avoid any risk of the GIC overflowing during sealing. The preparations were gently dried with a blower and a cotton ball.

Ketac™ Universal Nanofilled GIC (3M ESPE) was prepared according to the manufacturer's recommendations. The GIC Ketac ™ Universal (3M ESPE) was inserted into the cavity with a mouth spatula. The formwork system was removed 7 minutes after filling the cavity and setting the material. The restoration was then finished and polished with medium and fine Sof-Lex™ discs (3M ESPE) and silicone tips and kept at 37° and 90% humidity for 24 h. The root tips of the teeth were sealed with a transparent orthodontic resin pellet (ORTHOSELF). Additional protection was provided by applying two coats of clear nail polish to each crown, leaving 1mm around the restoration.

The samples were stored in distilled water for 3 days at 37°C and then thermocycled in water baths. After removing them from the dye solution, the teeth were air-dried and cleaned with abrasive discs to remove all traces of the dye. The teeth were then stored in a 0.9% KCl solution.

Daily cycle: 45 min at 6° + 45 min at 60°, repeated four times in 16h at room temperature. The daily cycle was repeated for five consecutive days [14, 15]. A second application of two coats of varnish to each crown was performed. The crowns of the teeth were subsequently immersed in a 1% methylene blue solution for 48 hours at room temperature (Fig. 4).

A 0.4 mm thick diamonded chain saw Isomet® (Buehler) was used at 7 rpm speed to separate the roots of each tooth from the crown and split each crown longitudinally in the mesiodistal direction at the middle of the mesial and distal restorations. After having separated the roots of each tooth from the crown, two sections were thus obtained, one vestibular and one lingual (Fig. 5).

Each section was photographed using a stereo microscope (Carl Zeiss™ STEMI 2000C ZOOM 6, 5 stereo microscope) under a magnification of 2 and with a digital camera. The degree of infiltration of methylene blue (micro infiltration) in each restoration was established according to a score; it was noted separately for the cervical wall and the axial wall. The observation was carried out separately by two observers.

The infiltration scores used at the axial wall were as follows:

• Score 0: Perfect adaptation/no penetration of the dye;
• Score 1: Infiltration affecting only the enamel.
• Score 2: Infiltration affecting enamel and dentin (without reaching the cervical wall).
• Score 3: Penetration of the dye beyond the cervical wall.

The infiltration scores used at the cervical wall were as follows:

• Score 0: Perfect adaptation/no penetration of the dye;
• Score 1: Infiltration affecting dentin only (without reaching the axial wall);
• Score 2: Penetration of the dye exceeding the axial wall (Fig. 6).

3.1. Statistical Analysis

Chi-square test with SPSS 20.2 software was used to study the distribution of scores. The results were found to be significant if P <0.05. Microsoft office Excel 2013 was also used to carry out some numeric functions. The chi-square test showed a significant difference (P <0.05) between nanocharged GIC and conventional GIC. The nanocharged GIC presented less infiltration; it was more waterproof than the conventional GIC (P = 0.002 at the cervical level and P = 0.007 at the axial level). The chi-square test showed the conditioning operation to have no effect on the tightness since the test was not significant (P> 0.05).

A comparison of the cement and enamel wall within the same group provided an insignificant chi-square test result. So, there was no difference found between cementitious and enamel sealing (Table 4).

4.1. Evaluating the Tightness of GICs

The evaluation of the tightness of a material requires the study of the factors on which this property depends, such as adhesion to calcified dental tissues, dimensional variations, the coefficient of thermal expansion, and the solubility of the material in the oral environment. Sealing is also operator-dependent [21].

Sealing is one of the most important properties of coronary restorative materials. Micro-infiltrations are the most common causes of failure of coronary restoration (Manhart et al., 2004), sensitivity, recurrence of caries, and inflammation of the pulp [22].

There are currently several methods for studying the micro-infiltration of a material (Alani and Toh, 1997) [23]. The most common and simplest methodology uses a dye as the infiltration solution (methylene blue, Fushine) [23]. It is a reliable and inexpensive technique (Raskin et al. 2001) [24]. The only downside is that the assessment is qualitative but the use of a non-parametric scale turns this technique into a semi-quantitative assessment method [25].

Our study showed that the two materials used presented micro infiltration. However, nanocharged GICs exhibited less infiltration on both the axial (enamel and dentin) and cervical (cementum) walls than conventional GIC. This is in agreement with the study by Moheet et al. [25]. A nanostructured material exhibits better properties compared to a conventional material [16]. Its properties are attributable to the nanosize of the components, which varies from 1 to 100 nm in dimension [26]. The glass-ionomer nanostructure allows excellent wetting and, therefore, excellent adaptability to the tooth surface, thus improving the chemical bond between the material and the cavity [27]. Polymerization shrinkage and coefficient of thermal expansion are lower, also improving long-term bonding to tooth structure [28].

The work of Shruthi [29] and Gorseta [30] showed the same results as our study: nanocharged and conventional GICs presented infiltration. These infiltrations are less significant for the nanocharged GIC. Nevertheless, Gorseta et al. have highlighted the effectiveness of GICs in daily dental practice [29]. Hussin's study [31] demonstrated the opposite result: conventional GICs are more airtight than nanocharged GICs, where several factors could be considered to have caused the infiltration: thermal cycling and improper handling.

Infiltration can be explained by cracks in the enamel that appear after extraction. We applied several coats of varnish before thermal cycling and infiltration with methylene blue to avoid this bias. The samples underwent several cycles of thermal cycling to simulate extreme temperatures similar to those in the oral cavity. The varying temperature produces thermal stresses at the tooth/restoration joint, which can lead to fatigue of this joint and, therefore, to infiltration [32].

4.2. Adhesion of Nanocharged GICs to Dental Walls

We chose the box cavities to study the infiltration both at the level of the cervical wall and at the level of the axial wall, thus approaching as much as possible the clinical conditions of site 2. The occlusal interface is enamel-dentinal, and the cervical interface is cemento-dentinal. Our study showed an insignificant result regarding the comparison between the axial wall (p = 0.42> 0.05) and the cervical wall (p = 0.17> 0.05). The majority of published works have studied micro-infiltrations at the level of the enamel-dentinal walls. It is the cavities of sites 1 and 3, which are most often studied.

The adhesion of GIC to the tooth is physicochemical and not mechanical; the bond is with calcium ions [33]. The chemical bond is obtained by the ionic interaction of the carboxyl groups of polyacrylic acid with the calcium of hydroxyapatite in dental tissues (COO-, Ca ++) [34]. Enamel is formed by a volume of 92% hydroxyapatite, dentin from 45 to 50%, and cement from 40%. Thus, the adhesion of GIC to enamel is superior to the adhesion to dentin, which is superior to the adhesion to cementum [35]. This would explain the greater infiltrations observed in the cervical wall.

4.3. Pre-treatment of the Cavity

In our study, the use of conditioning with polyacrylic acid did not give better results (P> 0.05). The protocol of the manufacturer of the Ketac™ Universal mentions that this step is not necessary. The nanocharged GIC can therefore be used without packaging, thus saving time and making handling easier. The free orthophosphoric acid contained in these glass ionomers is sufficient to dissolve the layer of dentin slurry [36]. Pretreatment (conditioning) of enamel and dentin with polyacrylic acid (20s, rinsing, drying) improves the adhesion of GIC.

4.4. Product Handling

The use of the powder-liquid version for the two materials (manual mixing) allowed us to draw the following observations:

• The handling of nanocharged GICs is more difficult than conventional GIC.

• The setting time of the nano-charged GIC is shorter than the conventional GIC.

Manual mixing requires strict adherence to the protocol and dosage mentioned by the manufacturer. The downside to this type of mixture is that it is operator-dependent. Improper handling of the material by the operator can cause poor marginal adaptation and, therefore, micro-infiltration. An inconsistent mixture of powder and liquid can result in a material that is either too liquid or too hard that will adhere poorly to calcified dental tissue. The presence of moisture when handling the material can also promote micro-infiltration [31].

Manual powder/liquid mixing can promote the incorporation of air bubbles inside the seal that would cause surface defects. Hussin et al. [31] explained the difficulty of manual manipulation of nanofilled GICs by the absence of tartaric acid because it induces a latency period in the setting process followed by an increase in viscosity. Thus, tartaric acid has been shown to have a dual effect on the setting, first inhibiting gelation and then accelerating it [31].

De Caluwé et al. [37] have shown that nanofilled GICs exhibit shorter setting time, better compressive strength and modulus of elasticity. Decreasing the setting time of direct restorative materials also allows easier handling and reduced treatment time, benefiting both the clinician and the patient [38].

Conventional GICs are not water stable. They are very sensitive to moisture during setting [39], hence they need to be protected with a varnish [40]. Consequently, the application of a coat of varnish would result in less infiltration. Nanocharged GICs are less sensitive to humidity according to some studies [40], so the application of varnish is not essential [40]. In our study, we did not use the varnish for both types of CVI to have less bias and the same conditions.

4.5. Finishing the Obturation with Nanocharged GIC

Finishing of the filling is a step that should not be overlooked. A polished restoration is more aesthetic and more biocompatible (does not promote the retention of dental plaque) [41]. Nanoparticles provide a better finish [13]. The restoration of a polished GIC is smoother and more aesthetic. In this, fluoride release and adhesion to enamel and dentin are better [26]; also, lower sensitivity to humidity and dehydration have been observed [39]. Clinically, removal of the outermost surface by finishing-polishing procedures would tend to make nanofilled GICs more resistant to biodegradation and, therefore, more aesthetically stable [41].

4.6. The Limitations of the Study

The sample size was one of the limitations of this in vitro study because using a statistically incorrect sample size may lead to inadequate results in both clinical and laboratory studies, and result in time loss, cost, and ethical problems. A larger sample is preferable [42]. Further, a single brand of GIC (3M ESPE) was used for both nanocharged GIC and conventional GIC. Comparing different brands of GIC would have been more interesting. Also, GIC modified by the addition of resin was not compared to nanocharged GIC.