Effect of saliva contamination on the shear bond strength of a new self-etch adhesive system to dentin.

AIMS AND
OBJECTIVES: To evaluate the effect of saliva contamination on the shear bond strength of a new two-step self-etch adhesive (P90 system adhesive) to dentin and to determine the effect of contaminant removing treatments on the recovery of bond strengths.
MATERIALS AND METHODS: The buccal surfaces of 40 human premolars were ground to expose dentin. The specimens were randomly divided into four groups. Group 1 is uncontaminated and serves as the control group. Further groups were divided based on the step in the bonding sequence when the contamination had occurred as follows: Group 2 (primer, saliva contamination, rinse and dry), group 3 (after procedure of group 2, reapplication of primer), and group 4 (after procedure like in control group, saliva contamination, rinse and dry). Filtek P90 composite resin cylinders of 3 mm diameter and 3 mm length were fabricated on the surfaces. Shear bond strength testing was done in an Instron Universal Testing Machine and the data were subjected to one-way analysis of variance (ANOVA) and Student's t-test.
RESULTS: With P90 system adhesive, group 2 and group 4 showed lower shear bond strength than group 1 (control) and group 3 (P < 0.05).
CONCLUSION: Saliva contamination significantly decreased the shear bond strength of the adhesive to dentin.

INTRODUCTION

With increased demand and use of aesthetic restorations, contamination control has become an important topic since dental adhesives and composites are highly vulnerable to contamination. Moisture such as gingival fluid, blood, hand piece oil, and in particular, saliva can affect the quality of the bond leading to microleakage at the interface. As a result, loss of the restoration, recurrent caries, postoperative sensitivity, and discoloration may occur.[1] Therefore, proper isolation and control of contamination are essentially required during bonding procedure.[2] Controlling saliva in the field of operation is difficult in adhesive dentistry, especially in those cases where cavity margins extend below the gingival tissues or when indirect restorations are seated or in newly erupted molars or when patients have problem in mouth opening.[3]

Dentin bonding is extremely complex when compared to enamel bonding because dentin is a more heterogeneous substrate with much higher organic and water content than enamel. Consequently, studies related to the bonding efficacy of saliva-contaminated dentin bonding agents are controversial.[4]

Recently, self-etch adhesive systems have been introduced in an effort to simplify the bonding procedure.[5] Two-step system combines a weak phosphoric acid and primer in one bottle and an adhesive in a second bottle. One-step system, also called all-in-one adhesives, combines conditioner, primer, and bond in one bottle.[6] These adhesives eliminate the rinsing and drying steps, thereby reducing the possibility of over wetting or over drying, both of which can negatively influence adhesion.[78] Siloranes are a totally new class of methacrylate-free compounds for use in dentistry. This monomer type can be chemically explained as a combination of siloxanes and oxiranes, thereby combining the properties of both, i.e., low polymerization shrinkage due to oxirane monomers and increased hydrophobicity due to the presence of siloxane species in the composition.[910] The polymerization occurs via a cationic ring opening reaction which results in a lower polymerization contraction. As the resin matrix of silorane composite differs from that of conventional methacrylate-based composites, a new adhesive needs to be designed and developed to enable bonding of the highly hydrophobic silorane composite to tooth. P90 system adhesive, a two-step self-etch adhesive, is the one and only self-etch primer and bonding system available to bond Filtek P90 Silorane-based composite. It consists of a P90 self-etch primer which is hydrophilic and bonds to tooth and a P90 adhesive bond which is hydrophobic and adheres to the resin.

Until now, studies are not available evaluating the effect of saliva on the bond strength of self-etch adhesives used to bond composites based on silorane chemistry to tooth. The present study was undertaken to evaluate the effect of saliva contamination on the shear bond strength of self-etch primer (P90 system adhesive) to dentin and to determine the effect of contaminant removing treatments on the recovery of shear bond strengths.

MATERIALS AND METHODS

Forty caries-free extracted permanent human premolars, after thorough preparation, were mounted in self-cure acrylic resin; the buccal surface of each tooth was ground on wet 240-grit silicon carbide disks to remove enamel and to expose 5-6 mm area of middle depth dentin. The surface was then subsequently polished with 600-grit abrasive for 30 sec. A double-sided adhesive tape with a hole (3 mm in cross section) was attached, so as to limit the surface area for bonding. The prepared teeth were stored in distilled water until preparation of the specimens for bonding. Prior to bonding, fresh whole saliva was collected from a single female donor in a sterile beaker and was used immediately.

All the teeth were randomly divided into four groups of 10 teeth each. Subsequently, saliva contamination and bonding procedure with P90 system adhesive (3M ESPE, Seefeld, Germany) [Figure 1] was carried out as follows:

Figure 1

Schematic representation of the control and experimental protocol

Group 1 (control): In this group, the specimens were not subjected to saliva contamination. Self-etch primer and bond were applied to the dentin of each specimen according to the manufacturer's instructions [Table 1]

Table 1

Materials used in the study

Group 2: Self-etch primer was applied as in the control group, and dentin surfaces were subjected to contamination with saliva for 15 sec using a microbrush. The surfaces were rinsed for 10 sec with water stream from an air-water syringe, followed by air drying for 5 sec. The bonding agent was applied as in the control group

Group 3: The same procedure as in group 2, but after rinsing saliva and drying, the self-etch primer was re-applied. The bond was applied as in the control group

Group 4: Self-etch primer and bond were applied as in the control group. The surfaces were then subjected to contamination with saliva for 15 sec and then rinsed and dried as in group 2.

A Teflon tube of 3 mm inner diameter and 3 mm length was applied on the surfaces. The Teflon tube was filled with silorane composite resin (Filtek™ P90, shade A2, 3M ESPE) in two increments and each increment was compressed firmly and light cured (Elipar 2500, 3M ESPE, St. Paul, MN, USA) for 40 sec. The Teflon tube was removed and additional curing of resin cylinder was done. The prepared specimens were stored in distilled water at 37°C for 24 h and shear bond strength testing was done in an Instron Universal Testing Machine (Unitek, 9450 PC, FIE, India) at Central Institute of Plastic Engineering Technology (CIPET), Hyderabad, Andhra Pradesh, at a crosshead speed of 0.5 mm/min. A knife edge chisel-shaped shearing rod was used for debonding of resin cylinders. The shear bond strength data were subjected to one-way analysis of variance (ANOVA), while comparison of means between groups was done using “t” test. P < 0.05 were considered significant for all statistical analyses. Statistical analysis was performed using the Statistical Package for Social Sciences (SPSS) software for Windows version 17.

RESULTS

The maximum load in Newtons required to debond the cylinders of the above restorative materials was recorded to calculate the stress at fracture, i.e., the shear bond strength in MPa. Data were summarized as Mean ± SD. The mean shear bond strength for each group is shown in Table 2.

Table 2

Mean, standard deviation

DISCUSSION

Dentin bonding systems are sensitive to contamination by saliva, blood, and excess water.[11] Therefore, adhesive systems capable of tolerating contamination are highly desirable. This study was designed to investigate their effect during different stages in the application of a new self-etch primer P90 system adhesive. This adhesive system differs from a typical two-step self-etch adhesive. Firstly, it involves the application of two resin solutions, i.e., P90 SE-primer, which is rather hydrophilic to bond to tooth tissue, and P90 bond is, on the contrary, quite hydrophobic in order to bond the hydrophilic substrate with the hydrophobic silorane composite. Secondly, the primer and bond need to be light cured separately, whereas in typical two-step self-etch adhesive, successive application of primer and adhesive is finalized by only one light curing step. The primer contains relatively high amount of hydroxyl ethyl methacrylate (HEMA) which keeps this resin solution homogenous, preventing phase-separation effects that have been documented for typical HEMA-poor one-step adhesives.[1213]

In the present study, good bond strength values were documented for the P90 adhesive system to uncontaminated dentin (group 1). De Munck et al.[14] and Van Ende et al.[15] have shown that these adhesives provide nano-interaction to dentin and also provide potential for additional primary chemical interaction with hydroxyapatite that remained available at the dentin interface, thereby contributing good bond strengths.

The present study showed a significant reduction in the shear bond strength values when contaminated with saliva after light curing the P90 primer (group 2). The reason might be that despite its two-step application technique, its mechanism of adhesion resembles more that of a one-step adhesive because the actual bond to the tooth surface is realized by the P90 self-etch primer only.[16] So, the separately light-cured P90 primer can actually be regarded as a one-step self-etch adhesive.[12] Several saliva contamination studies[1217] concluded that with one-step adhesive systems, glycoproteins in saliva adsorb onto the poorly polymerized adhesive layer, thereby preventing adequate co-polymerization.

On reapplication of primer for 10 sec (group 3), the values were restored close to the control. These findings were consistent with previous study[8] where the bond strengths were recovered owing to the fact that the hybrid layer reformed after the removal of the unstable contaminated primer layer.

When contamination occurred following light curing of the P90 bond (group 4), there was a statistically significant decrease in the shear bond strength. This might be due to the following reasons:

Adsorption of glycoproteins to the poorly polymerized adhesive surfaces, thus preventing adequate polymerization

Rinsing of the contaminant after adhesive application could have disrupted the oxygen inhibited and unpolymerized layer, thereby compromising the co-polymerization with the subsequent resin layer.[171819]

The limitations of the present study are that the bond strength after the reapplication of adhesive bond after rinsing and drying of contaminant and before composite restoration was not studied. Also, the bond failure by microscopic method was not evaluated, therefore necessitating further studies.

CONCLUSION

Within the scope of the current study, the following conclusions were drawn with P90 system adhesive.

Saliva contamination after the application of self-etch primer significantly reduced the bond strength of the adhesive

Rinsing saliva did not restore while reapplication of primer restored the bond strengths to that of the control group