Influence of two different desensitizers on the shear bond strength of two newly introduced bonding agents following the total etch strategy: an in vitro study

Introduction

Today composite resins are considered the material of choice because of the increasing demand for high quality esthetic restorations in everyday practice.¹ The quest for an ideal esthetic restorative material for the built up of teeth has resulted in significant improvements in their properties and techniques of application.²

However, 25-30% of the study population report postoperative sensitivity following posterior composite restorations. Brannstorm states the etiology of dentinal sensitivity to be chemical, thermal, or osmotic stimuli, which in turn can result in the fluid movement inward or outward. This creates a mechanical disturbance that excites nerve fibers in the pulp and elicits a pain response.^2,3

Desensitizers are agents used to reduce the dentin permeability by sealing the dentinal tubules.⁴ Tay et al.⁵ found that, application of oxalate desensitizing agent to etched dentin surface resulted in subsurface tubular occlusion. Also, desensitizing agents like Gluma and I-Bond precipitate the proteins in the dentinal tubules in addition to resin occlusion.^6-9 As total-etch strategy gives higher bond strength and increased post-operative sensitivity compared to self-etch, the former technique was used for evaluation in the present study. However, with benefits of decreased microleakage, certain studies report that the application of desensitizers significantly reduced the shear bond strength.^10,11 This study was done to evaluate the effect of two different desensitizing agents and adhesive systems on the shear bond strength of composite to dentin surface.^12,13

Materials and methods

60 freshly extracted caries free human premolars were used in the current study. The teeth were cleansed well and storage was done in distilled water till further use (Fig. 1). 

Preparation of the surface

Flat dentin surfaces were made on extracted teeth with diamond disk under water coolant, after which it was embedded in a block of chemically cured acrylic resin in such that that there was 3- 4 mm of coronal dentin exposure.Dentinal surface was ground using 600 grit silicon carbide paper.

The study specimens were divided into three major groups with two subgroups in each group containing 10 teeth each. In control group 1, etching was done on the dentinal surface with 37% phosphoric acid for 15 seconds, followed by rinsing with water for 20 s, after which they were blotted dry with cotton pellets, with appearance of a glistening surface. Two coats of All bond Universal was applied. A 3mm high cylindrical polyethylene tube with an diameter of approximately 2mm was seated on the flat dentin surface. Fill up composite resin was condensed into the mold with a Tefloncoated instrument. A Mylar strip was placed over the mold after which light curing was done for 40 s. The cured cylindrical button of 3‑mm diameter and 2‑mm thickness was removed out of the mold with a ball burnisher.

In control group 2, Dentin surface was etched similar to group 1. After that, with the help of a disposable brush, One Coat 7.0 was applied as per manufacture instructions. Then the teeth were restored similar to group 1. In experimental group 3, Gluma desensitizer was applied following manufacturer instruction prior to application of bonding agent with an applicator swab. All bond universal was then applied and teeth were restored similar to group 1. In experimental group 4, Dentin surface was etched and pretreated with Gluma desensitizer similar to group 3. Over that, One Coat 7.0 bonding agent was applied similar to group 2 and the teeth were restored similar to group 1.

In experimental group 5, After etching of dentine surface similar to group I, MS Coat One was applied with the help of a paint brush and allowed to dry for 2 min. Finally, All bond Universal was coated on the surface and restored similar to group 1.

In experimental group 6, Dentin surface was etched and pretreated with MS Coat One desensitizer like group 5. One coat 7.0 bonding agent was then applied and the teeth restored like the group 1 specimens.

Shear Bond Test

Universal testing machine was used for the test. The long axis of the study samples were kept perpendicular to the direction of the force. The crosshead speed was 1 mm/min and the maximal load to debond the samples was measured in Newtons (N). An equation was used to calculate the shear bond strength in mega Pascals (MPa), by the ratio of the maximum load (in Newtons) to the cross‑sectional area of the debonded interface (in mm2).

Statistical analysis

For statistical analysis, One‑way ANOVA and Tukey’s multiple comparison post hoc test was applied. SPSS (version 22.0) was used to carry out the statistics.

Results

Group 1 and 2 in which no pretreatment was done with any desensitizing agents exhibited highest bond strength as opposed to the experimental groups. In the groups 3 and 4 treated with Glumadesensitizer, group 4 which used One Coat 7.0 bonding agent showed higher bond strength than that shown by All bond Universal. In the experimental groups 5 and 6 pretreated with MS Coat Onedesensitizing agent, group 6 which used One Coat 7.0 showed a greater bond strength than group 5 which used All bond Universal, both in which values were lesser than the control group with no desensitizer pretreatment. (Table 2, 3 and Fig. 1)

Among the bonding agents indicated, One Coat 7.0 bonding agent showed marked increase in bond strength than All bond Universal, in both the groups pretreated with Gluma& MS Coat One desensitizer. Among the desensitizers Gluma showed greater bond strength than MS coat one desensitizer in the experimental groups. Thus, One coat 7.0 with Gluma desensitizer showed the highest mean bond strength in comparison to all bond universaland the MS Coat One desensitizer (TABLE 4).

Discussion

The desensitizers under evalution for the study were chosen in accordance to Salz and Bock ( 2010) ¹⁴, who delineated that the most common method to decrease the permeability of the exposed dentinal tubules and sensitivity are agents which form precipitate like potassium oxalate and the protein coagulants (glutaraldehyde, chlorhexidine, and fluoride). Desensitizing agents have been seen to influence the bonding of restorative materials to the surface of the tooth. Numerous studies have stated that the desensitizing agent currently being used may interfere with the bond strength to dentin.¹⁵ Hence, the aim of the present study was to assess the influence of two desensitizer'son the shear bond strength of composite bonded to dentinal surface, in vitro. [Glutaraldehyde desensitizing adhesive system (Gluma) and a potassium oxalate desensitizer solution (MS Coat One)]

Group 2 was found to have the highest shear bond strength values in the current study. Cavalcanti et al (2013)¹⁶, in a study which elucidate this result revealed that removig the smear layer by total etching facilitated the adhesive to diffuse freely into the dentinal tubules resulting in an increased bonding. The features of a more simplified etch-and-rinse bonding agent evaluated in the the present study, could give reason for itsincreased bond strength under both experimental conditions. The filler load of nanopartilces in one coat 7.0 adhesive system was10% by weight in its composition, which could account for the strength of the adhesive layer, creating an adequate thickness over the hybrid layer thereby protecting it against stresses.

The group 5 exhibited the least shear bond strength value. This result conforms to the findings of Perdigão (2010)¹⁷, who brought to light that some dentin desensitizers seals the dentinal tubules which in turn blocks the tubular fluid leading to the deterioration of bonding for some current adhesives.

Observation from the current study about group 5 comply with those of others^18,19 studies that stated that, there was a elimination of the smear layer and depletion of crystals of calcium oxalate from the dentinal surface, facilitating the formation of a hybrid layer if an acid etching procedure was carried out prior to the application of oxalate desensitizer. The deposition of calcium oxalate on the dentinal surface could inhibit the hybrid layer formation, resulting in lower bond strength and formation of gaps at the bonded interface in the areas of stress. Christensen GJ (2002; 2006)^20,21 confirmed that the application of oxalate desensitizing agent occluded dentinal tubules and reduced bond strengths greatly, possibly due to the presence ofmonohydrogenmonopotassium oxalate in this desensitizer. This might get precipitated and seals of dentinal tubules, thus affecting the bond strength of the bonding agents.^22,23

Gluma desensitizer after etching of dentin (group 3 and group 4) has shown better bond strengths. Higher bond strength with gluma has been reported in a few other studies.^24,25 Greater bond strength could be because of the use of glutaraldehyde and 2-hydroxyl ethyl methyacrylate (HEMA). Glutaraldehyde is a wellknown fixativeas well as a flocculating agent that crosslinks collagenous biomaterials. The aldehyde group of glutaraldehyde cross-links principally with the ε-amino groups of lysine and hydroxylysine residues in dentin collagen. This results in protein fixation, bonding glutaraldehyde to the dentin collagen fibrils. This development could possibly stabilize the collagen layer and thus contribute to enhanced bond strengths. HEMA plays an imperative role as a stiffening agent averting any subsequent shrinkage and undergoes a potential reaction (chemical) between its ester functional group and dentin collagen. It also encourages dentin adhesion and helps in the diffusion of monomer from the resin for the development of a hybrid layer.^26,27

Dentin bonding agents currently available, contains hydrophilic monomers as primers along with a solventlike acetone, ethanol and an adhesive resin. Ethanol and acetone performs as a carrier and water chaser, providing the functional monomeric unit for the hybrid layer. Volatility of acetone is more than ethanol.²⁸ When acetonebased adhesives agents are applied to an etched or wet substrate there is and evaporation of acetone and water, leaving the monomer that covers the exposed collagen network. Ethanol works in a similar way, but it has a lesser capacity for dissolving monomer and a lower vapor pressure than acetone. This is the reason why All bond universal, an ethanol based bonding agent gives a lower bond strength when used on the dentinal surface compared to the solvent free adhesives like one coat 7.0.²⁹

Conclusion

From the evaluations of the present study, a conclusion that the Gluma with One Coat 7.0 can be used to reduce the post-operative sensitivity without compromising bond strength between the resin and the tooth surface can be derived