Effect of silorane-based adhesive system on bond strength between composite and dentin substrate.

CONTEXT: The complexities of the oral environment, the dentin substrate, and the different bond and composite resin systems represent a challenge to the maintenance of reasonable bond between the composite resin and the tooth structure. AIMS: To evaluate the effect of the adhesive system on bond strength between silorane-based composite resin and dentin.
MATERIALS AND METHODS: Fourteen human molars extracted were selected and vertically cut into 3 dentin fragments, randomly divided among the experimental groups and restored with Z250 and P90 composite resin using different adhesive protocols (Adper Single Bond 2, Silorano primer, Adper SE Plus, and Scotchbond Multiuse). Two composite resin cylinders were built up on each dentin surface (n = 10) and subjected to a micro-shear bond strength test. STATISTICAL ANALYSIS USED: Kruskal-Wallis one-way analysis of variance and Tukey test (P = 0.05).
RESULTS: According to the results, Kruskal-Wallis test evidenced at least one statistical significant difference (P = 0.001). The Tukey test showed statistically significant differences among the group (P < 0.05). Group PSM8 (P90 + SM) showed statically significant higher results when compared with groups PSP4 (P90 + SP), PSB2 (P90 + SB), and ZSE5 (Z250 + SE).
CONCLUSION: The results evidenced that the monomer of the adhesive system has an effect on bond strength between the composite resin and dentin.

INTRODUCTION

Composite resin, due to its high esthetic potential, was initially recommended for the restoration of anterior teeth. However, after the improvement of this group of materials, resulting in better mechanical properties and wear resistance, its use could be recommended for posterior teeth.[1]

The development of composite resins with methacrylate matrix leads to its acceptance as a restorative material for several clinical applications. Nevertheless, this restorative system still presents limitations such as greater abrasion potential when compared to silver amalgam, possible pulpal damage associated with the bond system application,[23] and the polymerization shrinkage.[4]

As a restorative alternative for different clinical conditions, a silorane-based restorative system was developed, obtained by the combination of oxirane and siloxane molecules. During the polymerization of this monomer, molecules become larger with increasing conversion degree and reducing postoperative sensitivity and microleakage, due to the absence of gaps at the interface.[5]

In clinical applications, a silorane-based composite is used with its specific self-etching adhesive system, aiming to provide resistant and durable bonding to enamel and dentin, besides excellent marginal integrity. To ensure maintenance of integrity at the adhesive interface, it is important to combine lower polymerization contraction and high bond strength values.[56] However, the complexities of the oral environment, the dentin substrate, and the different bond and composite resin systems represent a challenge to the maintenance of reasonable bond between the composite resin and the tooth structure.[789]

When dentin is compared to enamel, it represents a challenge to obtain the maintenance of bond durability and stability, once the composition is characterized by a collagen matrix involved by inorganic material, with approximately 20% water.[78] Dentin also represents physiological and morphological varieties; in other words, tooth age related to the aggression of dental element has been exposed in the oral cavity, changes its characteristics, and makes it less resistant to acid etching.[1011]

Therefore, this study aims to evaluate the effect of the adhesive system on bond strength between the silorane-based composite resin and dentin. The null hypothesis of this research is that the monomer of the adhesive system does not affect bond strength between the composite resin and dentin.

MATERIALS AND METHODS

Fourteen sound human molars extracted no later than 90 days and stored in 10% formalin[12] had their roots cleaned with periodontal curette (S.S. White/Duflex, Rio de Janeiro, RJ, Brazil) to remove the soft tissue, and dental prophylaxis using prophy brush, pumice and water was performed on occlusal surface. Teeth were maintained in formalin solution until their usage. The research project was reviewed and approved by the Research Ethics Committee of University of Southern Santa Catarina.

Teeth were vertically cut in a mesiodistal direction to obtain 2 mm thickness slices. Dental fragments were embedded in circular acrylic resin holders to be polished up to a standard roughness. The exposed surface was submitted to abrasion using a silicon carbide sandpaper (180 μm grit) under water cooling until a 0.5 mm deep flat area of dentin was exposed; 320 and 400 grit silicon carbide papers were used to reduce the surface roughness. Dental fragments were cleaned in ultrasound bath with distilled water for 10 min.

Prior to the bonding procedure, samples were submitted to abrasion using 600 grit sandpapers under water cooling for 30 s to form a smear layer. Three dentin fragments were obtained from each tooth (n = 42) and randomly divided (by drawing lots) among the 8 experimental groups [Table 1]. For the total etch groups (groups 1, 2, 7, and 8), 37% phosphoric acid (3M ESPE, Saint Paul, MN, EUA) was applied to the dentin surface for 15 s, and thoroughly rinsed off by water/air spray for 15 s. Excess water was removed with moist cotton pellet, and the adhesive system was applied. Those samples that did not need previous etching (groups 3, 4, 5, and 6) were cleaned using water spray for 10 s and the excess water was removed as previously described. After corresponding adhesive systems were applied following the manufacturers’ directions, a polyethylene matrix with 1 mm internal diameter and 3 mm length was used to build a micro-shear composite resin sample using incremental insertion technique. Two composite resin cylinders were built up on each dentin surface (n = 10). Photoactivation of increments was performed for 40 s from the top of the matrix (750 mW/cm−2, Optilight Plus; Gnatus, Ribeirão Preto - SP, Brazil) until the cavity was completely filled. After the careful removal of the polyethylene matrix, specimens were stored at 37°C distilled water for 24 h.

Table 1

Experimental groups according to materials/techniques employed

Each composite resin cylinder was individually involved by steel wire (0.2 mm diameter) as close as possible to the restorative interface, which was attached to the shear device in an Universal testing machine (DL2000, EMIC, São José dos Pinhais, PR, Brazil), equipped with a 50 N load cell. The loading was applied at a crosshead speed of 0.5 mm/min until the failure of the specimen. The results were obtained in kg f/cm2 and converted into MPa by considering the surface area at the adhesive interface.

The mean bond strength value for each experimental group was calculated using the load necessary to failure divided by the bonded surface area.

RESULTS

The normality test (Shapiro–Wilk) failed (P < 0.05). Results were compared among the groups employing Kruskal–Wallis one-way analysis of variance and Tukey test, using an overall level of significance of 5%.

According to the results obtained, Kruskal–Wallis test evidenced at least one statistical significant difference (P = 0.001). The Tukey test showed statistically significant differences among the group (P < 0.05) as shown in Table 2.

Table 2

Kruskal-Wallis test and Tukey test results for all groups

DISCUSSION

Based on the results of the present study, the null hypothesis may be rejected. The results of this study evidenced that the monomer of the adhesive system has an effect on bond strength between the composite resin and dentin.

Micro-shear bond strength test was selected for this research proposal instead of microtensile bond strength because the first one shows more faithful results based on the characteristics of the adhesive interface, whereas the application of orthodontic wire allows for forces to occur at the interface between the tooth surface and the cylinder of resin.[13] Additional advantages of the micro-shear bond strength test are lower cost of equipment required for samples preparation and simplicity of preparation and standardization of the testing area by using microtube matrices,[14] which reduces stress accumulation at the adhesive interface. Thus, the results from micro-shear bond strength are more realistic than the ones presented by microtensile bond strength.[15] Furthermore, microtensile testing presents cohesive failures in the dental substrate due to microfractures in the dentin caused by stress generation during specimen preparation, while the micro-shear specimens show prevalence of adhesive failures.[15]

The possibility of chemical compatibility between silorane-based and methacrylate-based materials was first reported by Santini and Miletic.[16] The authors mentioned that there was a chemical affinity between the silorane bond and the conventional methacrylate-based composite resins. When all groups from Z250 groups were compared with P90 groups using the same adhesive system, there was no statistical difference among them. These results were not predicted and are not in agreement with the manufacturer's recommendations, which assert that the silorane-based adhesive system was developed with a two-step self-etching technique, exclusively for restorations using Filtek P90 composite resin. However, Van Ende et al.[17] say that the composition of the adhesive system recommended for silorane-based composites is methacrylate-based and could be safely employed with conventional composites.

When the self-etching bond system and the total-etching adhesive system were compared to each other using the same composite resin, there was statistically significant difference between groups PSP4 (P90 + SP) and PSM8 (P90 + SM). Like previously described, it seems that the chemical compatibility between the conventional adhesive system and the silorane-based resin is similar to the one that occurs between the silorane composite resin and the particular “silorane-based” adhesive system due to the presence of methacrylate in the adhesive's composition.[161819]

The best results founded by group PSM8 (P90 + SM) could be explained by the combination of two different conditions: First of them is the fact that the molecules of silorane-based materials become larger due to polymerized increase of the conversion degree, reducing shrinkage and gaps at the interface.[5] The second is that the use of a three-step conventional adhesive systems has shown the best adhesive strength values in clinical and laboratory tests.[20]

According to the present study, some of the associations employed between adhesive and composite resin presented similar results to the materials with the same composition; however, more studies are necessary to prove its effectiveness in order to encourage its use in a safe way in clinical daily.

CONCLUSION

Within the limitations of this study, the following conclusions may be drawn:

When all groups from Z250 groups were compared with P90 groups using the same adhesive system, there was no statistically difference among them;

When self-etching bond system and total-etching adhesive system were compared to each other using the same composite resin, there was statistically significant difference between groups 4 (P90 + SP) and 8 (P90 + SM).

Financial support and sponsorship

The authors thank PIBIC/CNPQ and PUIC/UNISUL for the financial support of this study.

Conflicts of interest

There are no conflicts of interest.