Confocal laser scanning microscopic analysis of microleakage in class II restorations: zirconia-incorporated glass ionomer versus bulk-fill composite 

Introduction

Dental caries is a global issue, with different incidence rates across diverse regions and populations. Factors, such as socioeconomics, oral hygiene practices, dietary habits, access to dental care, and other health determinants, all influence dental caries. Developing an ideal restorative material with optimal physical pro­perties and long-lasting durability is an ongoing focus of research and innovation in the field of dentistry. Different materials, such as composite resins, ceramics, and amalgams, have been used for dental restorations, each with its advantages and limitations. Given its high compressive strength and marginal integrity, amalgam has been a popular choice among dental professionals since the 1890s [1]. However, the use of amalgam has declined in recent years due to excessive loss of tooth structure [2] and the potential threat of mercury toxicity, as assessed by the Food and Drug Administration (FDA). In the recent past, a variety of tooth-colored materials have emerged to replace amalgam [3]. Dental professionals now have access to a wider range of restorative materials, such as composite resins and ceramics, which offer improved aesthetics while retaining good clinical performance.
Resin-based composites (RBCs) have gained significant popularity due to their superior aesthetics and other attributes. However, RBCs have certain limitations, including higher technique sensitivity than in other materials, polymerization shrinkage, and sensitivity as well as a higher risk of wear and staining over time, particularly in high-stress areas. Bulk-fill composites differ from traditional composites in that they have a greater depth of cure, primarily due to increased translucency [4]. The high filler load renders the surface more wear resistance and due to the associated viscous consistency, the surface is sculptable [4]. Glass ionomer cement offers unique advantages, such as chemical adhesion to moist teeth, fluoride release, anticariogenic property, excellent adhesion to dentin, close thermal expansion to the tooth, and biocompatibility [5], making it a valuable restorative material in specific clinical situations and applications in dentistry. It is imperative to understand that glass ionomer cement has certain limitations. In stress-bearing areas, such as the occlusal surfaces of posterior teeth, the use of glass ionomer cement may be limited due to its relatively lower fracture strength and wear resistance [6]. Various strategies have been implemented to improve mechanical properties, including the incorporation of zirconia, hydroxyapatite, N-vinyl pyrrolidone, fluo­rapatite, and hydroxyapatite/zirconium oxide [7]. Zirconomer Improved has nanozirconia fillers in the glass component, which help strengthen the structural stability of a restoration. It also provides superior mechanical properties, excellent marginal adaptation, and outstanding abrasion resistance. Other features of Zirconomer Improved include sustained fluoride release and higher translucency for a closer match to natural tooth [8]. The durability and biocompatibility of a material in an oral environment are indicators of its successful implementation. Microleakage refers to the detectable movement of bacteria, fluids, molecules, or ions between the restorative material and the cavity wall it is applied to. It is an important measure used by clinicians and researchers to predict the performance of a restorative material [8]. Preventing or minimizing marginal microleakage is a crucial goal in restorative dentistry to maintain the long-term success of dental restorations, as it can lead to discoloration or staining along the restoration margins, affecting the aesthetics of restoration and overall appearance of the tooth.
Dye penetration is a commonly used method to assess the extent of microleakage in restorative materials. The dye helps visualize the extent of microleakage by staining the areas penetrated. The confocal laser scanning microscope offers several advantages for evaluating microleakage and investigating restorative interfaces, such as control of depth of field, elimination of background, improved resolution, and minimum specimen preparation [9].
Therefore, a confocal laser scanning microscope (CLSM) may provide more accurate detection of microleakage. Research on the comparative evaluation of the microleakage of Zirconomer is lacking in the current literature. Hence, this study aimed to compare the microleakage of zirconia-reinforced GIC with a nano­hybrid bulk-fill resin composite at both occlusal and cervical levels.

Material and methods

Sample preparation

This study was conducted after obtaining ethical approval from the Institutional Ethics Committee, Dr. D.Y. Patil Vidyapeeth, Pune, India (Ref. No. DYPDCH/DPU/EC/581/75/2023).
A total of forty-six freshly extracted human permanent molar teeth with closed apices were included in the study. Teeth, which were single-rooted, and had root caries, cracks, undergone endodontic treatment, exhibited internal resorption, or showed calcification, were not included. To ensure consistency and reduce the risk of operator performance bias in the experiment, the procedure for preparing and restoring cavities was standardized across all groups and carried out by a single operator. Standardized mesial class II box cavities were prepared on each tooth using an SF-41 diamond bur (Mani Inc., Japan). The dimensions of the preparations measured 3 mm mesiodistally, 2 mm buccolingually, and with an axial depth of 4 mm (Figure 1A).

Randomization and sequence generation

Every sample received a distinct identification number. The distribution of samples to each group was done with a computer-generated random sequence table. This method involved using sequentially numbered, opaque, sealed envelopes to maintain confidentiality. The samples were then randomly split into two groups, each consisting of 23 teeth, according to the restorative material applied (n = 23):
• Group 1: Zirconia-reinforced GIC (Zirconomer Improved, SHOFU Inc., Kyoto, Japan),
• Group 2: Tetric N-Ceram Bulk Fill composite (Ivoclar Vivadent) (Figure 1B).

Restoration process

For group 1, predetermined amounts of powder and liquid were dispensed onto a paper pad in a ratio of 3.6 : 1.0 (two scoops of powder to one drop of liquid). The powder was divided into two equal portions. The first portion was thoroughly mixed into the liquid using an agate spatula, while the second portion was incorporated into the remaining liquid. All samples were then finished and polished after 24 hours using a finishing and polishing kit (SHOFU Inc., Japan).

Thermocycling

In order to replicate the oral conditions, the specimens underwent a thermocycling process consisting of 500 cycles alternating between 5°C and 55°C, with each temperature maintained for 30 seconds, over the course of 24 hours in distilled water.

Assessment of microleakage

The root apices of all teeth were sealed with modeling wax (Figure 1C). Two layers of nail polish were applied to all specimens, ensuring a one mm space around the cavity margins to prevent the penetration of dye through other microfissures and cracks. The teeth were immersed and stored in a 0.5% solution of rhodamine B dye (HiMedia, Mumbai, India) for 24 hours, followed by washing under running water. The specimens were subsequently cut in a mesio-distal direction through the center of restorations using a low-speed diamond disc (Figure 1D).
The sections were analyzed with CLSM (Zeiss LSM 700) at 10x magnification to assess the depth of dye pe­netration at the occlusal and cervical margins, and were scored. The highest score was taken into consideration for each sample.
The depth of dye penetration was assessed according to criteria given by Wahab et al. [10], by assessing the distance from the cavosurface margin extent to the greatest depth of dye penetration at occlusal and cervical levels (Figure 2).

Statistical analysis

Analysis of microleakage was performed, and descriptive statistics were expressed as mean ± standard deviation (SD) for each group. Data were obtained and entered into Microsoft Excel version 13, and analyzed using IBM SPSS version 21. Since the data were observed to be categorical, frequency percentage of the data was obtained. To compare different proportions of grade of microleakage in occlusal and cervical levels for Zirconomer Improved and Tetric N-Ceram Bulk Fill composite, χ2 test of proportion was employed. To compare between microleakage of Zirconomer Improved and Tetric N-Ceram Bulk Fill composite at occlusal and cervical levels, Mann-Whitney U test was used. All statistical tests were conducted with a 95% confidence interval, and a significance level of p < 0.05 was applied.

Results

In a comparison of microleakage between Zirconomer Improved and Tetric N-Ceram at the occlusal level, it was found that Zirconomer Improved showed no grade 0 microleakage specimens. The difference in the proportion of grade 3 microleakage was statistically significant (p < 0.05) (Figure 3).
In a comparison of microleakage between Zirconomer Improved and Tetric N-Ceram at the cervical level, it was observed that 6.5% of the specimens showed grade 0 microleakage. Additionally, 65.2% of the specimens exhibited grade 1 microleakage, and 28.3% showed grade 2 microleakage. No grade 3 microleakage was detected in either group, and the difference in proportion was statistically significant (p < 0.05) (Figure 4).

Discussion

Restorative dentistry governs on the principle of adhesion and bonding. In order to adhere to the tooth structure, a restorative material should closely mimic the phy­sical properties of the tooth structure. Failure of this interfacial bonding can lead to a sequel of challenges, such as microleakage, secondary caries, sensiti­vity, etc. [11].
Zirconomer Improved has been proven to be a strong, durable, and fluoride-rich material that is appropriate for structural cores and bases as well as patients with high caries risk [12]. The implementation of novel nanosized zirconia fillers enhances the translucency of the material, facilitating a more accurate shade match to the natural tooth. It also ensures superior maneuverability, simplifying the process of bulk placement [13].
Microleakage has been seen with practically all of the restorative materials created so far, as stated by Gladys et al. [14]. Since microleakage stands out as a major issue, being linked to pulpal changes, sensitivity due to interfacial hydrodynamic phenomenon, and secondary caries, all of which result in restoration failure, this study was conducted to assess the microleakage associated with a modified restorative material, i.e., Zirconomer Improved, and compare its performance with bulk-fill composite resin. The relevance of microleakage studies is questionable as standardization is impossible for accuracy. According to Singh et al. [15], utilizing two or more different models to analyze the same material might be considered an option to improve its sensitivity and enhance the research outcome. Nevertheless, it can still be stated that microleakage tests may be acceptable and definitive variables to indicate the in vivo performance of restorative materials [16].
Class II cavities have demonstrated a greater level of relative microleakage persistence. It is impacted by a material’s polymerization shrinkage, which is further influenced by several parameters, some of which are within the control of manufacturer, and others within the control of clinician [17].
The present study evaluated the microleakage of class II cavities to determine the bonding capacity of the given materials in both enamel and dentin.
A box-shaped cavity design was proposed to decrease polymerization stresses and most nearly reflected the clinical scenario, resulting in a C-factor of roughly 3, as shown in the study by Küçükeşmen and Sönmez [18]. The proximal box was created over the cemento-enamel junction in this study, since it has been shown that there is significantly more microleakage at the level of 1 mm below the junction compared with 1 mm above it. The reason for this is that acid etching bonds more effectively to enamel than to cement, owing to the enamel’s higher inorganic content (95%) and reduced moisture levels. A comparison shows that cement adhesion is not as strong as that of mantle dentin, which can be explained by mantle dentin’s thicker collagen fibers (range, 0.1-0.2 µm) and greater concentration of hydroxyapatite, leading to a 20-24% difference between these tissues [19].
Leprince et al. [20] investigated the physico mecha­nical characteristics of the majority of commercially available bulk-fill composites. The authors stated that the decrease in time and increased convenience associated with bulk-fill materials is a clear benefit of this material over traditional composites. Therefore, Tetric N-Ceram was selected for the current study for the gap detection both cervically and occlusally. The nanohybrid resin compo­site features camphorquinone as its primary photoactivator, capable of absorbing a blue wavelength between 420 and 495 nm, with a peak absorption of 468 nm [21].
Bajabaa et al. [22] conducted a study to evaluate microleakage in five different resin composites. The authors reported that nanohybrid resin composites, such as Tetric N-Ceram, showed the least microleakage as compared with other resin-based composites. This finding was consistent with the present study, with the result being statistically significant. The observed effect can be attributed to the application of filler technology, which serves to mitigate microleakage by enhancing strength and minimizing interstitial gaps between particles, thereby leading to a decrease in marginal microleakage. Yu et al. [23] demonstrated that polymerization stresses differ significantly between conventional and bulk-filled materials, depending on the product composition, particularly filler volume fraction, cavity depth, and degree of conversion of the RBCs.
In the present study, Zirconomer showed the highest microleakage both cervically and occlusally as compared with bulk-fill composite. Similar results were observed in a study by Albeshti et al. [24], who evaluated microleakage of Zirconomer with Ketac Silver, Filtek Z350 under a stereomicroscope. This result was also seen in accordance with a study done by Sardana et al. [25], who assessed microleakage of Cention-N, Zirconomer Improved, and Solare Sculpt. This was attributed to the broad filler particles, which prevented perfect adaptation of the material to the tooth surface. In a study by Makkar et al. [26], microleakage was observed to be higher in composite compared with Zirconomer. They found that the polymerization shrinkage in composite caused stress on the network and its bonding system, leading to microleakage.
However, no restorative material employed in the studies could avert microleakage entirely. Hence, it is essential to recognize the materials, which show lesser microleakage from the commercially available materials, and optimally utilize them.
Our study replicated the oral environment by storing samples in water, followed by thermocycling to simulate their artificial clinical aging. The detection of microleakage provides valuable insights into the durability of materials and has been approached using various techniques, including radioisotopes, dyes, neutron activation analysis, air pressure, pH changes, and scanning electron microscopy [22]. Dye penetration, although an older method, remains a traditional metho­dology to assess the bond failure between restorations and tooth surfaces. Despite its limitations, this method continues to be favored due to its low cost, simplicity, and ease of implementation [27].
Rhodamine B, a fluorescent dye, possesses a smaller particle size and a greater number of surface-active molecules in comparison with the more widely utilized methylene blue dye. Furthermore, in the presence of calcium oxide-rich materials, methylene blue undergoes modifications, resulting in higher pH that might induce discoloration of the surfaces labeled with it [28]. Kim et al. [29] in their study concluded that rhodamine B was beneficial as a tracer, because it successfully flowed through the gaps surrounding amalgam fillings, causing leakage observable at extremely dilute concentrations. Therefore, all samples were stored in a 0.5% aqueous solution of rhodamine B.
CLSM excludes the need for sample sectioning, dehydration, and polishing artifacts, which may impede dye penetration.
The specimens were assessed with predetermined scales using a qualitative scale, according to the criteria given by Wahab et al. [10]. This test resulted in the mean values of zirconia-reinforced glass ionomer cements being greater than bulk-fill resin composite, showing statistically significant results.
This short-term in vitro study did not simulate the oral cavity environment as the dynamic nature of conditions found in the oral cavity, such as salivary flow charac­teristics, presence of plaque, oral hygiene, and dietary habits utilized by the patient, which can lead to results that may be different from that found in the current study. Therefore, long-term studies and clinical trials are required to completely understand the effectiveness of these materials.

Conclusions

The results of this in vitro study confirmed that neither Tetric N-Ceram nor Zirconomer Improved can completely eliminate microleakage. While the bulk-fill resin composite showed the least microleakage at both occlusal and cervical levels, Tetric N-Ceram exhibited superior performance in microleakage, making it a preferable choice for class II restorations in the posterior teeth due to its advantageous properties.

Disclosures

1. Institutional review board statement: This study was conducted after obtaining ethical approval from the Institutional Ethics Committee, Dr. D. Y. Patil Vidyapeeth, Pune, India (Ref. No. DYPDCH/DPU/EC/581/75/2023).
2. Assistance with the article: None.
3. Financial support and sponsorship: None.
4. Conflicts of interest: None.

References