Could Marginal Adaptation of Composite Resin Restorations Be Influenced by a Different Polymer Using Different Techniques?

Abstract

Background: Marginal adaptation is one of biggest challenges in restorative dentistry, mainly in restoration of composite resin. Even with development of lower shrinkage materials, like those which use the polymer silorane, it is necessary a faithful clinical protocol in order to obtain better results and more clinical durability of restorative procedure. The purpose of this study was investigated if different polymers can influence on marginal adaptation of different composite resin restorations. Material and methods: Twenty class V cavities were confectioned in twenty human molars in a standardized way. Ten dentists received one tooth each and a questionnaire to describe their own clinical protocol. The other 10 teeth were restored by one researcher as control group. The cavities received the adhesive system than increments of composite resin, and so the restorations were finished with finishing bur (KG Sorensen, São Paulo, SP, Brazil) to remove the excess, followed by sof-lex discs (3M ESPE). After this stage, all of specimens were embedded in 1% Methylene blue (Prolabo, Paris, France) during 48 h. The evaluation of pigment penetration into the interfaces was performed after specimens were washed in distilled water and longitudinally sectioned with steel Diamond disc in low velocity. Results: There was significant statistical difference between the different techniques using compose resin. Conclusions: it was possible conclude that the clinical protocol to perform dental restoration interfere dramatically in the final results of restorative procedure.

1. Introduction

Restorative dentistry, like the world around us, has evolved significant over time. Dental composites, first introduced by Bowen, R.L., have played a crucial role in advancing restorative techniques that merge function with aesthetics [1]. Composite resin is widely regarded as the preferred choice in restorative dentistry [2], celebrated for its ease of application and outstanding aesthetic qualities [2,3]. However, there are certain properties of composite resins that require improvement [4].

The investigation of factors predisposing to the formation of marginal gaps remains an important area of research. Fissures or gaps at the interface between the tooth structure and the adhesive material of the restoration can compromise the marginal integrity and longevity of the restoration [5,6,7] (Figure 1). In a previous study [8], it was demonstrated that the percentage of dentinal gaps in vitro composite restorations ranged from 14% to 54% across the entire interface, depending on the materials and techniques used. These gaps can contributed to complications such as microleakage, postoperative sensitivity, or secondary caries [9].

The mismatch in thermal expansion coefficients between the dental structure and the restorative material, combined with polymerization shrinkage, contribuites to microleakage, which has been responsible for several failures in restorative dentistry. Microleakage accelerates material damage and reduces the longevity of restorations [10]. Consequently, restorative techniques have been developed to account for factors such as cavity configuration and material volume in order to minimize shrinkage and damage to the tooth-restoration interface [11]. Moreover, advancements in the composition of dental composites have benn made to improve their properties [12,13,14].

As a result, manufacturers have introduced an innovative system that replaces methacrylate with silorane [15]. Silorane exhibits low shrinkage during polymerization. Silorane is derivative of oxirane and siloxane molecules, which polymerize themselves with an opening of cationic ring, in maximum overcoming to the clinical inconvenience of material polymerization shrinkage related to methacrylate-based composite resins. Furthermore, this material has shown good mechanical properties and fluid stability in oral cavities simulators. While conventional methacrylate systems is still widely used, a combination of both materials occurs. The methacrylate initiates the initial polymerization, while silorane forms a cationic ring that is insensitive to oxygen. This innovative approach effectively mitigates the clinical challenges related to polymerization shrinkage observed in traditional systems [15].

Experienced professionals often face technical challenges when performing cavity restorations, particularly in posterior teeth. Issues such as the adherence of restorative materials to clinical instruments can also contribute to the formation of gaps during the procedure [16]. Furthermore, the absence of a standardized protocol for cavity restorations leaves practitioners to rely on personal experience, which may not consistently ensure the durability of restorative procedures, regardless of the materials employed. This lack of uniformity poses a significant concern for both the scientific community and society, underscoring the importance of this study [16].

The distance from the activating light source to the material should be a factor of great care to the professional because, higher the distance, shorter the intensity of light that the resin receives; in this way, time of exposure to light should be higher in order to obtain a satisfactory activation. Sakaguchi, R.L. et al. declare that when the distance between the specimens and the light source is higher than 4 mm, 25% of light is lost [17]. It can be a very influent variable, because 40% of dentists performed the activation in a distance of near 10 mm from the activator tip to the restoration, other 40%, approximately 10–15 mm and 20% less than 10 mm, approximately. This factor associated to reduced time of activation can have influenced drastically in the leakage of specimens.

Therefore, the objective of the this in vitro study was to evaluate the marginal microleakage of restorations performed by dentists using their individual clinical protocols in class V cavities located on the buccal and lingual/palatal surfaces of human molars. The restorations were conducted using Filtek P90 (3M ESPE) a silorane-based restorative material. The null hypothesis tested was that the clinical protocol does not influence the marginal microleakage of class V restorations restored with the silorane-based Filtek P90 composite.

2. Materials and Methods

2.1. Specimen Selection

Fourty recently extracted, healthy human molars were collected based on periodontal or orthodontic indications. The molars were stored in a 10% thymol solution at a temperature of 4 °C ± 1 °C to preserve their integrity until use. After a seven days period, the teeth underwent a thorough cleaning process using a rotating device and prophylactic paste. They were then examined under a binocular microscope (X10, model S2H, Olympus Corp., Tokyo, Japan) to ensure that they were free from any signs of deterioration, gaps, or previous restorations.

2.2. Specimen Preparation

The teeth were embedded in polystyrene resin, and standardized class V cavities were prepared on both the lingual and vestibular surfaces (Figure 2). The cavities were shaped quadrangular, with dimensions of 3.00 mm in width, 3.00 mm in extension, and 2.00 mm in depth [18]. A high-speed rotating diamond bur (#2143, KG Sorensen, São Paulo, Brazil) was used for cavity preparation, with abundant air/water cooling. Afterward, the cavities were refined using a diamond bur of the same diameter but with a lower grit size (#2143F, KG Sorensen, São Paulo, Brazil), also with cooling. The dimensions of the cavities were confirmed using a precision caliper ruler (Digimatic, 500-181U, Mitutoyo Corp., Kanagawa, Japan).

2.3. Restorative Clinical Protocol

The twenty teeth were randomly divided into two experimental groups (n = 20). Group N1 served as the control group, while group N2 was the experimental group. Both groups were restored using a silorane-based resin (Filtek P90, 3M ESPE, St. Louis, MO, USA) (Figure 3). In the control group, a researcher performed the restorations following the manufacturer’s specifications. In the experimental group, experienced practitioners restored the teeth according to their individual clinical protocols. The restorative procedures employed a single bond system: Filtek P90 (Silorane Adhesive system) (a self-etching bond system specifically developed for silorane-based restorative systems), and a single restorative system: Filtek P90 (a silorane-based restorative system) (Figure 4).

In the group of control, the bonding agents were applied in a single layer and photo activated for 10 s with 9 mm diameter light-curing (XL 3000, 3M ESPE, St. Louis, MO, USA). The activation energy was monitored at regular intervals to ensure it remained between 680 mW/cm² and 700 mW/cm². The distance between the light-curing tip and the specimens was maintained at 1 mm. The resin was applied using the incremental technique, with three oblique increments placed. The third increment was carefully added to fully fill the cavity and was then photo-polymerized for 20 s using the same device and method as the adhesive. The restorations were finalized using a finishing bur (KG Sorensen, São Paulo, Brazil) to remove any excess material, followed by polishing with Sof-Lex discs (3M ESPE, St. Louis, Lake Bluff, MO, USA).

The specimens from the experimental group were distributed to dental professionals for restoration. Each professional was assigned one specimen, along with the bond agent (Filtek P90; 3M ESPE, St. Louis, MO, USA), the restorative material (Filtek P90; 3M ESPE, St. Louis, MO, USA), and a questionnaire for data collection regarding the procedure performed. After restoration, all specimens from both groups were immersed in a 1% methylene blue solution (Prolabo, Paris, France) for 48 h. Following this, the evaluation of pigment penetration at the interfaces was evaluated. The specimens were rinsed with distilled water and then longitudinally sectioned from the vestibule to lingual direction using a low-velocity diamond steel disc on a cutting machine with water cooling (IsoMet^® 1000 Precision Saw, Buehler Ltd., IL, USA).

2.4. Analysis of Marginal Microleakage

The microleakage analysis was performed using a binocular microscope (X10, model S2H, Olympus Corp., Japan). The evaluation criteria involved assigning scores ranging from 1 to 5 as adapted Bud, M.G. et al. [19]. with score (1) indicating complete absence of microleakage, (2) indicating leakage up to half of the surrounding wall, (3) indicating leakage along the entire surrounding wall, (4) indicating leakage in both the surrounding and axial walls, and (5) indicating leakage in both the surrounding and axial walls extending towards the pulp. Figure 5 illustrates an example of score 3.

2.5. Statistical Analysis

The data collected in this study were recorded in an Excel spreadsheet (Microsoft Windows 2010) and analyzed using the BioEstat^® software (version 5.3). The Shapiro-Wilk test was uses to assess the normality of the data distribution. Since the data did not follow a normal distribution, statistical analysis was conducted using the Mann-Whitney U test (p < 0.05).

3. Results

Table 1. Classification of scores for specimens of each group.

Group	Score 1	Score 2	Score 3	Score 4	Score 5
Control	7	11	2	0	0
Experimental	0	1	11	8	0

and Figure 6 displays the mean values of marginal microleakage scores for both the control and experimental groups. A statistically significant difference was observed between the groups (p < 0.05). In the group of control, 55% of specimens received a score of 2, while 35% had a score of 1, and 10% a score of 2. In the experimental group, 40% of specimens received a score of 4, and 55% received a score of 3, and 5% a score 3.

A questionnaire was used to gather data on the clinical protocol followed during the restorative procedure. According to the responses, 100% of the participants reported performing an acid etching on the enamel. Of these, 50% etched for 30 s, 10% for 20 s, 20% for 15 s, and 10% did not provide information regarding the duration. Regarding acid etching on dentin, 90% of participants confirmed performing this step, while 10% did not. Among those who performed the dentin acid etching, 70% did so for 15 s, 10% for 20 s, 10% for 10 s, and 10% did not provide the duration or claimed not to have performed the acid etching on dentin.

Continuing with the protocol, all participants reported rinsing the cavities to completely remove the acid etch. Among them, 10% rinsed for 25 s, 50% for 30 s, 20% for 15 s, 10% for 20 s, and 10% did not specify the duration. A similar question regarding the drying of the cavity was also asked, and all participants confirmed performing this step. Of these, 40% used cotton for drying, 20% Sused a triple syringe, and 40% used both cotton and a triple syringe. Regarding drying time, 30% dried for 2 s, 10% for 3 s, 20% for 5 s, 30% for 10 s, and 10% did not provide the duration.

As for the application of the bond system, 100% of the participants reported performing this procedure. Among them, 80% applied one layer, while 20% applied two layers. For photoactivation, 40% performed it for 10 s, while 60% performed it for 20 s. Regarding the number of increments for restorative material placement, 90% used three increments, and 10% used four increments. Photoactivation was performed for 20 s in 80% of the cases, 10% for 10 s, and 10% for 40 s. When asked whether they calibrates the potency of photoactivator equipment, 100% of the participants answered no. In response to the question about the distance between photoactivator equipment and the restoration, 40% reported a distance of approximately 10 mm, 40% reported between 10 and 15 mm, and 20% reported a distance of less than 10 mm.

4. Discussion

In the present study, a significant difference was observed between evaluated groups, leading to the rejection of the null hypothesis. Issues related to dental composites, such as polymerization shrinkage, remain key areas of focus in studies like this one. Dental companies continue to invest in the development of materials that exhibit reduced polymerization shrinkage [20].

The ongoning evolution of restorative materials aims to achieve durable restorations that can remain functional and intact in the oral cavity for many years without requiring repairs [21]. However, despite the advancements and improvements in restorative materials, the polymerization shrinkage remains an inherent characteristic. This phenomenon occurs due to the reduction in distance between monomers as they react through Van der Waals forces, which encompass the sum of all attractive and repulsive interactions [22]. As demonstrated in this study, polymerization shrinkage negatively impacts the integrity of restorations.

Demarco, F.F. et al. [23]. stated that factors directly under the control of the dental professional have a significant impact on the final outcomes of restorative procedures compared to factors solely dependent on the manufacturer, such as matrix composition, type and quantity of inorganic particles, and material color saturation. This assertion could be observed in the present study, where the same materials were used for both the control and experimental group, with variations observed only in the techniques employed.

Regarding the thickness of the increments, the manufacturer of Filtek P90 resin recommends increments up to 2.5 mm when using a photoactivation device with an intensity of 500 mW/cm² for 40 s. However, other studies suggested limiting the increment thickness to 1.0 to 1.5 mm to optimize polymerization and improve the material’s mechanical-physical properties. In this studt, 90% of participants used three increments, while 10% reported using four increments. This factor may not have direct influenced the results, as 10% of the specimens in the control group also received three increments [24].

Lima R.B.W. et al. observed that increments greater than 2 mm of thickness do not achieve sufficient polymerization [25]. For thicker layers to receive adequate polymerization, longer lightr exposure is required [26]; In this study, 80% of participants reported photoactivating the increments for 20 s, 10% for 10 s, and the remaining 10% for 40 s. This suggests that the combination of increment thickness and light exposure may significantly influenced the results.

The cavity configuration also plays a crucial role in contributing of polymerization shrinkage, due to the C-Factor [27]. The C-Factor refers to the radio of bonded to unbonded surfaces. It is estimated that achieving a satisfactory bond of dentin depends on on maintaining a Factor-C value of 1 or less, which minimizes polymerization shrinkage. Under adverse conditions, such as bonding to all the cavity walls, shrinkage stress can exceed the tensile strength of composites, contributing to microleakage. Although this variable was challenging to evaluated in the present study, it is known that class V cavities can present a C-Factor as high as 5, making them particularly prone to shrinkage-related issues.

Studies have emphasized the importance of assessing the light-curing unit to ensure that the photosensitizer receives the required wavelength, which should range between 468 nm and 480 nm. This evaluation is crucial for achieving a uniform and thorough polymerization process, as it ensures the proper wavelength and intensity are delivered [22]. However, it is notable that none of the participants in this study reported evaluating the photoactivation devices. This oversight could have significantly influenced the results obtained.

The moment when the resin transitions from a fluid to a viscous state is referred to as the gel point. At this stage, the resin acquires a high modulus of elasticity, losing its ability to flow and beginning to transfer the stress generated by polymerization shrinkage to the tooth-restoration interface. Shrinkage that occurs before the gel point is called pre-gel contraction. During this fase, the resin’s molecules can move, reposition, and reorient, compensating for polymerization shrinkage. However, after gel point, the resin’s ability to flow becomes restricted, and all the stress generated in transferred to the bond interface between the tooth and the restoration [28]. To address this, it has been proposed that polymeration shrinkage stress can be minimized by extending the pre-gel stage. This is achieved using initial photoactivation with low-intensity light for a specified duration, followed by high-intensity fo another specific duration-a technique known as soft-start polymerization [29]. The effectiveness of this method in improving marginal integrity has been demostrated [30]. In this study, all participants used a constant-intensity light activation method. While this approach may cause the restorative material to lose its capacity of flow quickly, it was also the method employed in the group of control. Therefore, it is unlikely to have been a significant factor influencing the results.

The importance of proper photoactivation, along with other factors directly controlled by the clinical professional, plays a vital role in the clinical durability of restorative procedures. Future studies addressing these themes are necessary to provide valuable guidance to professionals regarding materials and techniques that promote clinical success.

While research is making strides in developing new materials with reduced shrinkage potential, this study highlights that clinical outcomes are significantly influenced by the professional’s approach. Therefore, adherence to appropriate protocols, combined with the use of suitable materials and techniques is essential for achieving enhanced durability and successful treatment outcomes. Clinical success is not determined solely by the materials used but is largely dependent on the techniques applied. This study had certain limiations, including the absence of a sample size calculation, the lack of compartion with other types of polymer, and the variability in techniques used by the experimental groups. These factors may have influenced the results and should be considered in future research.

5. Conclusions

Considering the experimental and control groups tested, it can be concluded that the clinical protocol used for performing dental restorations significantly influences the final outcomes of the restorative procedure.