Microleakage Assessment of Composite Restorations in Class V Cavities by Using Different Techniques – An In Vitro Study

Introduction

The goal of operative dentistry is to restore the tooth to its form, function and aesthetics while maintaining physiologic integrity of teeth with adjacent hard and soft tissues. One of the requisite of ideal restorative material is to adapt itself to the cavity wall.¹ Nowadays, composite resins are frequently used as an aesthetically pleasing and cost-effective replacement for direct and indirect restorative materials. Due to advancement in material aspect and adhesive resin technology, composites have become an increasingly popular alternative to amalgam restorations.² Placement of composite resin restorations will continue to increase in clinical practice with ever increasing patient expectations coupled with improvements in physical properties of resin materials and bonding technologies.

Polymerization shrinkage of resin composites impairs the restoration longevity due to unrelieved stress around the margins of tooth and restoration interface.³ Teeth subjected to heavy occlusal stresses during normal function and parafunction results in generation of tensile and shear stresses in cervical region of tooth due to flexion.⁴ Proper marginal seal or adaptation of restorative material to cavity walls is essential for long term performance of any restoration. Longevity of Class V restoration can be affected by stresses in the cervical area caused by mechanical, thermal factors. The interface between the restoration and tooth surface must be properly sealed to restore form, function and aesthetics as insufficient sealing can result in discoloration of tooth at the margins, secondary caries, postoperative sensitivity, pulpitis, restoration failure, pulp necrosis, complete or partial loss of restoration.⁵

Some studies have shown placement of thin layer of flowable composite under main restorative material causes better adaptation of restoration to cavity walls due to its favorable wetting properties and fewer cavity wall stresses, and subsequently less microleakage.^6,7 New techniques as well as new methods of placement of resin restorative material may eventually offer the opportunity to eliminate marginal leakage around dental restoration.

Studies conducted by Opdam NJ et al. (2003) showed that least voids were found when flowable composite was placed and cured simultaneously with conventional resin-based composite as a single unit.⁸ Current studies conducted showed that use of flowable composite under main bulk composite and co-curing them together (Snow plow technique) showed an increased microleakage rate. However, these studies show conflicting results.⁸

Hence the purpose of this experimental study was to compare and evaluate the microleakage of composite restorations in Class V cavities by using two different techniques using dye penetration method to identify the method with less microleakage scores for treating cervical lesions.

Materials and Methods

An in vitro study was carried out at Bapuji Dental College and Hospital, Davangere. The protocol for the study was approved by the Institutional Review Board and had granted ethical clearance. A total of 30 noncarious extracted premolar teeth were selected from the Department of Oral and Maxillofacial Surgery. All the collected teeth were disinfected and stored for a maximum period of two months in saline. Technical part of study was performed in the Department of Conservative Dentistry and Endodontics, followed by stereomicroscopic examination in the Department of Oral Pathology and Microbiology.

The samples were collected for the study using following criteria:

Inclusion criteria:

• Healthy caries free teeth
• Teeth free of restorations
• Teeth with no cracks on the crown
• Teeth with no cement remnants on the buccal surface as a result of previous orthodontic treatment.

Exclusion Criteria

• Teeth subjected to any form of dental treatment
• Teeth with fluorosis, hypoplasia or abnormalities of crown morphology
• Teeth with fluorosis, hypoplasia or abnormalities of crown morphology

Solare Sculpt composite material (GC Dental Products) was chosen for this study (Figure 1).

The teeth to be prepared were stabilized in modelling wax. Class V cavities were prepared on buccal surfaces of 30 extracted premolar teeth using high-speed handpiece. The size of cavity was approximately 4 mm wide X 2 mm high X 2 mm deep. The outline preparation was done using straight fissure bur and in the end tapered fissure bur was used to form the angles to the walls of prepared tooth (Figure 2). Class V cavities prepared (n= 30) were randomly divided into two equal groups -

Group A [n=15]: Flowable composite was placed under the main composite restoration and were co-cured together for 20 seconds (Snow plow technique).

Group B [n=15]: Flowable composite was cured for 20 seconds and the main composite was built up over it and again cured for 20 sec (Conventional technique).

Restorative Procedure

Group A [Snow plow technique]

Phosphoric acid etchant gel (37%) was applied to the prepared tooth surface and left undisturbed for 15- 20 seconds and then the area was washed with water and dried. Using an applicator tip, bonding agent was applied on the floor and walls of all the cavities and were light cured for 20 seconds. A thin film of flowable composite was placed as an intermediate layer between bonding agent and compactable composite. The main composite restorative material was placed to fill the entire cavity and the material was contoured with the help of teflon coated plastic filling instrument. Flowable composite and main composite were co-cured for 20 seconds.

Group B [Conventional technique]

Steps followed for etching and application of bonding agent were same as discussed above for Group A samples. A thin film of flowable composite was placed as an intermediate layer between bonding agent and compactable composite. Samples were cured for 20 seconds after the placement of flowable composite. The composite restorative material was placed to fill the entire cavity and the material was contoured with the help of teflon coated plastic filling instrument and it was again cured for 20 seconds.

Contouring and polishing

Sof-lex [3M ESPE] was used for contouring and polishing of the restoration. Disks were used sequentially from coarse to fine grit to create a smooth surface. These disks were rotated at low speed in constant shifting motion.

Evaluation of Microleakage

Before performing the dye penetration test, every sample had been subjected to thermocycling and cyclic loading.

Thermocycling

The purpose of thermocycling was to determine the resistance of exposure of tooth samples to alternating extremes of high and low temperatures. Thermocycling of specimens was done according to ISO standards. The teeth were subjected to thermocycling for 500 cycles in a water bath at 5° C and 55° C with dwelling time of 30 seconds.

Cyclic loading

All the teeth were subjected to cyclic loading with the help of Universal testing machine (Figure 3). After thermocycling, the specimens were mounted onto aluminium block with the help of acrylic resin (Figure 4).

At 45° angle to the long axis of tooth, load was applied to the specimen simulating the direction of lateral occlusal forces on premolars. The teeth were loaded with 250 N. The adjustable parameters were number of cycles (10,000), diameter of the tip (3 mm), duration of the loading (500 milliseconds), relaxation time (200 millisecond).

Application of nail polish varnish

Two different colours of nail polish were chosen to differentiate group A samples (yellow) from group B samples (red). Two layers of nail polish varnish was applied all over the tooth surface, except for restoration and 1 mm from the cavo surface margins. To avoid dye penetration through the apices, root apices were sealed with wax.

Dye immersion

The samples were soaked for 24 hours at room temperature in 1% methylene blue solution, after which they were rinsed under running water.

Sectioning of samples

The samples were buccolingually sectioned into two halves in a vertical plane to long axis of tooth using diamond disc and were decoded during analysis.

Microscopic examination

Sectioned restoration was subjected to stereomicroscopic investigation at X30 magnification (Figure 5). Depth of dye infiltration was analysed according to 0 – 3 scale scoring system with the help of an expert (Oral Pathologist) who was blinded to the technique.

Results

The depth of dye infiltration of the samples was analysed using 0-3 score criteria as shown in Table 2. Table 3 shows the detailed description about the experimental groups with related technique of restoration followed.

Table 4 shows the frequency and percentage of the microleakage scores in each of the groups with Chi square value of 6.048 and a p value of 0.109, which is less than 0.05. This demonstrated that there was no statistical difference found in the microleakage scores between Group A and Group B and between the two techniques followed for restoration (Graph 1 and 2).

Table 5 shows mean values and standard deviation of two groups.

Discussion

Much research has been conducted to overcome the problem of microleakage specially in Class V cavities, but till today there has been no dental restorative material or technique that can increase the longevity of restoration and eliminate microleakage completely.⁹ Hence an in vitro study was conducted to compare the snow plow technique with the conventional one.

Stresses generated due to polymerization shrinkage of composites results in adhesive failure and microcracking of restorative material. Extensive studies in the past have proven that technique of placement of restorative material is an important factor in the modification of stresses. Teeth that are subjected to thermal stresses and occlusal stresses to simulate intraoral conditions also play a significant role in modification of shrinkage stresses.¹⁰

Solare Sculpt composite material (GC Dental Products) was chosen for this study because of its excellent material properties exhibited in clinical dentistry. The material can be easily adapted, finished, it is aesthetic and tough with improved wear resistance. Strontium glass fillers are homogeneously dispersed in the material to overcome the drawbacks like failure of composite because of high flexural stresses and wear resistance. Solare Flo (GC Dental Products) has a viscosity of a perfect flowable composite ensuring excellent adaptation without voids and wettability to the cavity walls. Solare Universal bond (GC Dental Products) material demonstrated superior bond strength to enamel and dentin which is attributable to dimethacrylate monomer which increases permeability into enamel and dentin compared to other adhesives.

In this study, thermocycling was done to simulate the intraoral changes in temperature induced by routine eating or drinking^.6,7. The cyclic loading was done in this study to simulate the occlusal stresses that are generated in cervical region during normal function and parafunction.⁴ Load was applied at 450 to the specimen simulating the direction of lateral occlusal forces on premolars.¹¹

Methylene blue dye was accustomed as a tracer to evaluate and score the microleakage rate due to its low cost, ease of application and low molecular weight. The molecule size of dye which was less than an average bacterial cell was helpful in tracing even the smaller microleakage and narrow marginal gaps.¹²

According to study by Labella et al., potential predictors for bond failure of adhesive restorations includes elastic modulustogether with magnitude and kinetics of polymerization shrinkage.¹³ The authors concluded that flowable composites generally showed higher shrinkage than traditional non flowable composites. In literature, it seems that many variables might have an influence on the polymerization shrinkage of flowable composites i.e., filler content, filler size, type of monomers, organic matrix type thickness of the material, power intensity of curing unit, technique of placement of the material.^14,15 Marginal integrity of flowable composites is still controversial and further studies are required to confirm their performance.

In theory, it seems that using a thin film of flowable composite as an intermediate layer improves wettability resulting in better adaptation of restorative material to cavity walls which will lead to fewer cavity wall stresses and subsequently less microleakage.¹⁶ Some studies have indicated that use of flowable composite as an intermediate layer did not reduce the microleakage rate in composite restorations.^13,15

In snow plow technique, thin film of flowable composite was co-cured together with main composite. It was hypothesized that stress absorbing ability of flowable composite maximizes when co-cured with main composite as elastic modulus develops concomitantly with curing of both flowable as well as main composite.¹⁷

In the present study, Group A samples (Snow plow technique) showed an increased microleakage rate with a score of 2 and 3. Some studies conducted in the past on snow plow technique have shown increased microleakage rates.¹⁷ Co-curing of flowable composites along with main composites did not improve the marginal seal or adaptation of restoration to cavity walls. The reason behind increased microleakage of restorations might be due to displacement of flowable composite into main composite which leads to heterogenous increase of resin contents of main composite restorations and subsequently an increased polymerization shrinkage which may lead to contraction forces which dislodges the uncured flowable composite from cavity walls.¹⁸

Considering the limitation of in vitro studies, the results of the present study showed that placement of thin film of flowable composite under the main composite did not improve the leakage rate. There was no statistical variance found in the microleakage scores between Group A and Group B and between the two techniques followed for restoration.

Limitations

Considering the limitations of in vitro studies, many potential variables for loss of marginal integrity of composite restorations include type of restorative material, filler content, filler size, thickness of flowable composite placed, depth of curing composite, technique followed in placement of material, contouring of composites, finishing and polishing procedures, thermocycling and force applied during cyclic loading.

Conclusion

This experimental study demonstrated that all the samples restored using snow plow technique (Group A) showed increased microleakage rates compared to Group B samples. Placement of thin film of flowable composites under main bulk-fill composite did not decrease the microleakage rate of restoration. Microscopic evaluation of both Group A and Group B samples showed no significant differences between the two groups. Samples of Group B showed less microleakage around toothrestoration interface compared to Group A samples.

However, the following study was performed in vitro. The results of same study performed in vivo may or may not be the same. In a clinical study, all potential variables that vary from patient to patient are taken into consideration which includes masticatory forces, type of food, oral temperature, humidity variations, presence of salivary enzymes. Therefore, further studies are required to establish factual clinical worth of the two techniques followed to validate their in vitro established results.

Financial support and sponsorship

This study was supported by UG research grants from Rajiv Gandhi University of Health Sciences in the year 2020-2021.

Conflict of interests

The authors of this manuscript declare that they have no conflicts of interest.

Acknowledgment

I want to thank my teachers for their assistance in getting this study done successfully. I would like to thank the HOD of Department of Oral Pathology and Microbiology and Department of Conservative Dentistry and Endodontists of Bapuji Dental College and Hospital and also HOD of Department of Textiles of Bapuji institute of Engineering who has given me permission to conduct the technical part of the study.