Influence of Cleaning Solutions on Shear Bond Strength of Resin Cement to Saliva Contaminated Zirconia: an In-Vitro Study

Introduction

Dental ceramics are extremely aesthetic restorative materials that mimic natural teeth better. Translucency, fluorescence, chemical stability, biocompatibility, high compressive strength, and a coefficient of thermal expansion similar to that of tooth structure are all desirable qualities of dental ceramics.¹

Adhesion in dentistry

Despite zirconia’s excellent mechanical qualities, a major issue arises due to the bonding of ceramic restoration to resin cement.² When bonding ceramic to the tooth structure, the final bond strength of the restoration is determined by two interfaces: dentin-resin cement and ceramic-resin interfaces. As a result, it is critical to achieve adequate bond strength on these surfaces. The wettability of the conditioned adherent surface with resin cement is critical for ceramic bonding, regardless of the mechanism of bonding.^3,4,5

According to one study, resin cement provides better bond strength than conventional water-based cement such as zinc phosphate and glass ionomer, as well as resin-modified glass ionomer cement.^6,7 Another study looked at the bonding of zirconia with cement with and without artificial aging and discovered that the resin cement might result in the creation of long-lasting and strong bonds even after treatment with under storage and thermocycling.⁸

Cleaning contaminated zirconia

One of the primary causes of poor bond strength of restorations to tooth substrate is saliva contamination. During the try-in procedure, it was nearly impossible to avoid. As a result, before adhesive cementation, efforts were conducted to eliminate any inorganic or organic impurities.

Yang et al. discovered that saliva contamination and cleaning procedures had a significant impact on resin bonding to zirconia and its durability. The authors found that during saliva immersion, non-covalent adsorption of salivary proteins on roughened, “activated” airborne particle abraded surface occurred, which could not be eliminated by a water rinse, as demonstrated by X-ray photoelectron spectroscopy (XPS). The study on XPS indicated that an organic layer resisted complete removal with a water rinse, isopropanol, or phosphoric acid after saliva contamination.⁹

The wettability of ceramic by adhesive resin, the roughness of the ceramic surface, the composition of the adhesive resin, the handling performance of adhesive resin, and probable contamination during bonding procedures are all the factors impacting resin bonding to zirconia ceramic. Several studies have demonstrated different strategies for removing contamination, however, none of the methods proved to be the best.

The purpose of this study was to compare and investigate the efficacy of three different cleaning agents on the shear bond strength of resin-zirconia bonding following a simulation of try-in with saliva exposure.

Materials and Methods

As per the statistical analysis, a sample size of 75 was obtained.

Fabrication of acrylic blocks to embed the zirconia discs:

A cuboid-shaped, wax pattern measuring 30×30×30 was made using modeling wax. Cold cure acrylic powder and liquid were then mixed in a porcelain jar and poured into the modeling wax mould. A cold cure acrylic block was made out of it, then a putty impression of this block was taken. Using this putty mould, fifteen acrylic blocks were made.

Preparation of samples:

Zirconia disc specimens of diameter 25mm and thickness of 2mm were fabricated from zirconia blocks. One surface of all the specimens was polished with 600 grit carbide paper, air-abraded with 50micron aluminum oxide (Al₂O₃ ) at 0.3MPa for 15secs at a distance of 10mm, and rinsed with water and air-dried. The prepared discs were embedded in acrylic blocks. The air abrasive surface was then treated according to the study design.

Contamination protocol and Experimental design: Following air abrasion treatment, specimens were immersed in artificial saliva for 1min except for the control group and divided into five experimental groups according to the cleaning methods, as follows:

a) Grouping of samples and cleaning protocol:

Group A: Control group (con) – no saliva contamination

Group B: Water Rinse (WS) – After saliva contamination, samples were rinsed under water for 15secs and then air-dried with oil-free air for 10secs.

Group C: Ivoclean (IC) – After saliva contamination, samples were cleaned with Ivoclean for 20secs, followed by rinsing with water for 30secs, and then air-dried for 10secs. (fig.1) 

Group D: 5% sodium hypochlorite (SHC) – After saliva contamination, samples were cleaned with 5% sodium hypochlorite for 20secs, followed by rinsing with water for 30secs and then air-dried for 10secs. (fig. 2)

Group E: 1% hydrogen peroxide (HP) – After saliva contamination, samples were cleaned with hydrogen peroxide for 20secs, followed by rinsing with water for 30 secs, and then air-dried for 10 secs.

b) Fabrication of resin cylinder and bonding procedure:

Resin cylinders were built in each sample by packing the resin cement (RelyX Unicem 200 automix) in cylindrical-shaped plastic matrices with an internal diameter of 2.5mm and then irradiated for 30secs using halogen curing light. In this manner, 5 cylinders were made on one zirconia disc and a total of 15 resin cylinders were prepared for each group (fig. 3).

c) Shear bond strength testing

The bonded specimens were left at room temperature for 30mins and then stored in water for 24hrs before shear bond strength testing using the universal testing machine. (fig. 4)

Results

The study observed that the mean shear bond strength in the control group was 15.18 ± 1.79, the water rinse group was 3.89 ± 1.22, the Ivoclean group was 14.88 ± 2.59, the NaOCl group was 14.71 ± 2.27, and the H₂0₂ group was 5.89 ± 0.70. This difference in mean shear bond strength between the 5 groups was statistically significant at P<0.001.

Table 1 represents multiple comparisons of the mean difference in the shear bond strength values between the 5 groups. The control group showed significantly higher mean shear bond strength as compared to H₂0₂ ; P<0.001. The Ivoclean group showed significantly higher mean shear bond strength as compared to H₂0₂ and Control group; P<0.001. 

NaOCl showed higher mean shear bond strength as compared to H₂0₂ and Control group; P<0.001. The H₂0₂ group showed significantly higher mean shear bond strength as compared to the water rinse group; P=0.03. However, no significant difference was observed between control and Ivoclean group [P=0.99], control and NaOCl group [P=0.96], and Ivoclean and NaOCl group [P=1.00]. The control group shows significantly higher mean shear bond strength followed by the ivoclean group, NaOCl group, and H₂0₂ group with the water rinse group showing the least mean shear bond strength. [Refer graph 1] 

Discussion

In prosthodontics, a strong adhesion provides high retention, improves marginal adaptation, prevents micro infiltration, and increases the fracture strength of the restored tooth. This kind of bonding is based on micromechanical interconnection and chemical adhesion of the adhesive to the ceramic surface. The challenge in promoting a strong and reliable bond between the internal surface of zirconia restoration to the resin luting agents lies in achieving a bonding surface free of contaminants that often results from intraoral try-in procedures. Saliva contaminations adversely affect the resin bonding because organic deposits remain on the restorative material after a few seconds of exposure to the saliva. Saliva contaminants occur during the tryin procedures from inner surfaces of zirconia before adhesion. Therefore, the removal of these contaminants plays an important role in the longevity of restorations. Previous studies reported that water rinsing may not be effective to remove some saliva contaminants from the zirconia surface. It prevents chemical bonding to zirconia ceramics while thermocycling and further interfere with the formation of a durable bond. Lower bond strength and a high percentage of failure of adhesive modes can be explained by the fracture phenomena at the surface area of zirconia ceramics.¹⁰

In the present study, water rinsing of specimens after saliva contamination decreased the bond strength (3.89±1.22 MPa) compared to the controls (15.18± 1.79 MPa). This shows that surface contamination of zirconia ceramic with saliva is related to the decreased bond strength. This result is in agreement with the study done by Quaas et al. who designed the study to test the resin-ceramic bond strength and its durability related to the cleaning methods of contaminated ceramic bonding surfaces. They found that no cleaning after the contamination group showed the lowest bond strength.⁹

Sodium hypochlorite is the most commonly used root canal irrigant. It is an inexpensive lubricant and is used as a deproteinizing solution. It is used in dilutions ranging from 0.5% to 5.25%. It is known that NaOCl can remove the organic materials and proteins in the smear layer of dentin as a non-specific proteolytic agent. The clinically used concentration of NaOCl is 0.5% to 5.25%. Therefore, 5% NaOCl was used in this study. Cleaning with NaOCl showed good cleaning results and shear bond strength (14.71±2.27) similar to the control group. This result is in agreement with the study done by Jaeyong Kim et al. Their study compared the shear bond strength of zirconia crown after cleaning the crown contaminated by saliva or blood.¹⁷

Ivoclean is an alkaline suspension of zirconium oxide articles. According to the manufacturer’s scientific documentation, Ivoclean contains zirconium oxide, water, polyethylene glycol, sodium hydroxide, and other additives. Due to the size and concentration of particles in the medium, phosphate contaminants from saliva are more likely to bind to the particles in the Ivoclean than ceramic surfaces, leaving behind a clean zirconium oxide surface. Kim et al. assessed the ability of Ivoclean to decontaminate the zirconia surface using the shear bond strength test. They found it to be effective in removing contaminants and improving bong strength.¹⁷ In a similar study by Feitosa et al. Ivoclean was found effective in the decontamination of zirconia ceramics with saliva. In the present study, the ivoclean group showed bond strength results (14.88±2.59 MPa), comparable to the control group (15.18±1.79 MPa).¹²

Hydrogen peroxide is one of the principal reactive products of oxygen metabolism and is often used as a bleach or cleaning agent due to its oxidizing properties. It is used in mouth rinses and dentifrices in a concentration of ≤1%. A 1% hydrogen peroxide concentration has no adverse soft tissue and hard tissue side effects.24 In the present study, the hydrogen peroxide group showed bond strength results (5.89±70 MPa) comparable to the water rinse group.

During the try-in procedure of zirconia restoration, the surfaces to be bonded may be additionally contaminated by blood or silicone indicators, which might also compromise resin bonding. These contaminants were not included in the present study and further experiments are needed to determine whether the cleaning solutions tested are also effective for such contamination.

The limitations of this study include the specimen design as crown-shaped specimens are cemented, ideally. Slight variations are seen in angulations of resin cylinders when bonded to zirconia discs. Qualitative analysis was not conducted to determine the level of contamination inside the zirconia. Artificial saliva is used in this study which differs from natural salivary constituents. Blood and silicone indicator contaminants are not included in this study. The sample size is less.

Conclusion

Hence, it can be concluded that saliva contamination significantly reduces the resin shear bond strength to zirconia. Zirconia ceramics cleaning protocol must be considered after exposure to saliva during intraoral try-in procedures.

Ivoclean and 5% sodium hypochlorite are effective cleaning methods for removing saliva contaminants from the zirconia surface. 5% sodium hypochlorite is an economical alternative to Ivoclean for cleaning the saliva-contaminated zirconia.

Conflict of interest

None