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Zirconia is a white crystalline oxide of zirconium that has been used in dentistry since the end of the 1990s (Manicone et al., 2007; Vagkopoulou et al., 2009; Thompson et al., 2011; Ӧzcan et al., 2012). Zirconia-based restorations are used widely in dentistry because of their excellent optical and mechanical properties (Manicone et al., 2007; Vagkopoulou et al., 2009; Ӧzcan et al., 2012). In addition, the introduction of computer-aided design/computer-aided manufacturing (CAD/CAM) systems has contributed to the increased popularity of zirconia restorations in dentistry (Manicone et al., 2007; Vagkopoulou et al., 2009).

Adhesion between restorations and resin-based cement is a crucial factor that affects the longevity of restorations. When the adhesion is insufficient, the adhesion itself can be weakened by microleakage and secondary caries can occur (Thompson et al., 2011; Ӧzcan et al., 2012).

Silica-based ceramics, which are still used widely, are subjected to phosphoric acid or hydrofluoric acid treatments to roughen their surface and are then treated with silane, which increases their durability by enhancing the mechanical interlocking and chemical bonding with the resin-based cement (Thompson et al., 2011).

Unlike silica-based ceramics, however, phosphoric acid and hydrofluoric acid cannot be used effectively on zirconia. Many zirconia surface treatments, such as airborne-particle abrasion with alumina, selective infiltration etching and tribochemical silica coatings, have been introduced to achieve durable cementation (Thompson et al., 2011). Kern and Wagner reported that the application of resin-based cement containing 10-methacryloyloxydecyl dihydrogen phosphate (MDP) resulted in high bond strength (Kern et al., 1998). Since then, many researchers have focused on MDP-containing resin cement or bonding/silane coupling agents (Gargari et al., 2010; Thompson et al., 2011). Many studies reported that the application of a MDP primer improved the bond strength of resin-based cement to zirconia due to chemical bonding between the MDP and zirconia (Koizumi et al., 2012; de Souza et al., 2014; Yi et al., 2015; Pilo et al., 2016). Primers containing MDP have been used widely in zirconia cementation procedures.

During the try-in procedure, the zirconia surface is contaminated by saliva. Some studies reported that saliva contamination affects the bond strength and durability of resin bonding to zirconia by reducing the adaptation of the restoration materials to the bonded surface (Yang et al., 2008; Phark et al., 2009; Aladağ et al., 2014; Feitosa et al., 2015; Ishii et al., 2015; Kim et al., 2015; Angkasith et al., 2016; Tunc et al., 2016).

Phosphoric acid is effective in removing organic contaminants, which led to various experiments aimed at applying phosphoric acid to remove saliva contamination from the zirconia surface. On the other hand, the results showed that the cleaning procedure with phosphoric acid weakened the bond strength. The residual phosphorous from phosphoric acid bonded with zirconia, which hindered bonding between MDP and zirconia and had a negative effect on adhesion (Yang et al., 2008; Phark et al., 2009; Feitosa et al., 2015; Ishii et al., 2015; Angkasith et al., 2016; Tunc et al., 2016). Previous studies showed that air-abrasion appears to be an effective cleaning method to remove saliva contaminants but many studies suggested that additional air-abrasion may affect the zirconia phase transformation and weaken the zirconia ceramic adhesion (Yang et al., 2008; Phark et al., 2009; Chintapalli et al., 2013; Feitosa et al., 2015; Ishii et al., 2015; Tunc et al., 2016).

Recently, a commercial universal cleaning solution (Ivoclean; Ivoclar Vivadent, Schaan, Lichtenstein) was launched. The manufacturer claimed that this cleaning solution could be used effectively in ceramic and metal restorations.

In dental clinics, sodium hypochlorite (NaOCl) is used widely and is very effective in removing organic materials (Dikmen et al. 2015). One study reported that NaOCl was effective in cleaning zirconia surfaces (Kim et al., 2015). On the other hand, another study reported that the effectiveness of NaOCl in cleaning zirconia was uncertain (Aladağ et al., 2014).

Therefore, this study evaluated the effects of various cleaning methods on the shear bond strength of zirconia to resin cement after saliva contamination.

Tables 2 and 3 list the results of the shear bond strength test. The results showed significant differences in shear bond strength among the groups. Group 7 (positive control) showed the lowest bond strength. Groups 1 and 3 showed low shear bond strength and did not show significant difference with group 7. Group 2 showed significantly higher shear bond strength than groups 1 and 3, and did not show significant difference with the group 8 (negative control). A comparison of the single coating of MDP and re-coating of MDP showed a significant difference between groups 1 and 4 and between groups 3 and 6 but there was no significant difference between groups 2 and 5.

This study examined the effect of the cleaning methods on the shear bond strength of self-adhesive resin cement to saliva-contaminated zirconia surface. Saliva contains organic materials, such as saliva protein, food debris, bacteria, and enzyme molecules, as well as inorganic materials in a water solution (Yang et al; 2008). Prior to cementation, contamination by saliva inside the restoration is unavoidable and removal of this contamination is crucial for the long-term durability of the restoration. Organic contaminants, including saliva proteins, are difficult to remove, and if they remain, they have negative effects on adhesion by altering the surface state of zirconia (Yang et al., 2008; Phark et al., 2009; Aladağ et al., 2014; Feitosa et al., 2015; Ishii et al., 2015; Kim et al., 2015; Angkasith et al., 2016; Tunc et al., 2016). The results of this study confirmed that the saliva contamination had a negative effect on the resin cement and zirconia surface.

Previous studies showed that water rinsing was not effective in removing saliva contamination from the zirconia surface zirconia (Yang et al., 2008; Phark et al., 2009; Feitosa et al., 2015; Ishii et al., 2015; Tunc et al., 2016). On the other hand, Angkasith et al. reported that after the application of MDP primer prior to salivary contamination, water rinsing is effective in recovering the bond strength (Angkasith et al., 2016). The results of the present study showed that water rinsing is ineffective in removing saliva contamination even if MDP primer is applied prior to saliva contamination. Although the zirconia surface becomes more hydrophobic after applying MDP, the adhesion of saliva to the surface could not be avoided due to its amphiphilic property (de Souza et al., 2014; Hanning, 1997). In a previous study, XPS analysis revealed the existence of the remaining carbon residue even after cleaning with water, suggesting that contaminants, such as saliva protein, cannot be removed completely with water once they adhere to the surface of the restoration material (Yang et al., 2008; Phark et al).

A previous study showed that NaOCl is effective in removing saliva contamination from the zirconia surface (Kim et al., 2015). In this study, cleaning with NaOCl was not effective in removing saliva contamination from the zirconia surface. The results showed that the zirconia surface cleaned with NaOCl has no significantly different adhesion strength compared with the zirconia surface cleaned with water. Although NaOCl is effective in dissolving organic material and proteins, it has been reported that in cases where MDP was applied prior to saliva contamination, the cleaning process by microbrush could remove the MDP coating, resulting in decreased adhesion (Tunc et al., 2016). Moreover, when NaOCl remained on the surface, it hindered polymerization of the adhesive, and decreased the bond strength of the resin cement (Dikmen et al., 2015).

Recently, a new commercial cleaning solution called Ivoclean was launched on the market. Ivoclean is an alkaline suspension of zirconium dioxide particles in water that absorbs phosphate contaminants like a sponge in the media, leaving a clean surface. In this study, the Ivoclean group showed a higher bond strength than the other cleaning methods. According to the manufacturer’s instructions, MDP should be re-coated after cleaning with Ivoclean and then rinsed with water (Ivoclean Product Brochure 2011). However, re-coating of MDP did not show significant difference with single coating of MDP. Therefore, Ivoclean maintained the bond strength regardless of the MDP re-coating.

After the cleaning procedure, a re-coating of MDP produced a higher bond strength, and recovered the bond strength to near its original value. As mentioned previously, MDP is removed partially during the cleaning procedures (Tunc et al., 2016). Therefore, the chemical bond recovered through re-coating with MDP. Moreover, re-coating of MDP will enhance the bond strength by increasing the surface wettability, which was lowered by contact with saliva or moisture (de Souza et al., 2014; Yi et al., 2015; Ishii et al., 2015).

There are no reports on the application of MDP both before and after the cleaning procedure. Re-coating of MDP in clinical examples may be an effective method for recovering the shear bond strength.

One limitation of this study was that the aging values were not taken. Therefore, further study will be needed to compare the cleaning methods under aging conditions.

Within the limitation of this study, water rinsing and NaOCl were found to be ineffective in removal saliva contaminants from the zirconia surface. On the other hand, Ivoclean was found to be effective regardless of the re-coating of MDP. After cleaning, re-coating MDP resulted in a higher shear bond strength than the case where water or NaOCl was used without a MDP re-coating. Therefore, when water or NaOCl is used to remove surface contaminants, a re-coating of MDP has a positive effect on cementation.