Since the acid-etch technique was first introduced in dentistry by Buonocore1) in 1955, phosphoric acid has been routinely used to etch the enamel surface in order to create micromechanical retention with resin composites. Formation of tag-like resin extensions into the enamel microporosities were considered the predominant bonding mechanism of resin to etched enamel2,3).

Recently, in an effort to simplify the dentin/enamel bonding systems, a single procedure using self-etching adhesive system has been proposed. These acidic primers include a phosphorated resin molecule that performs two functions simultaneously that are conditioning and priming of tooth.

Several self-etching adhesive systems are on the market today, and the acidity of self-etching systems allows mineralized tissue to be etched in a single treatment step. However, there are some controversies on the etching effect of the acid of the self etching adhesives. Some studies have reported that weaker acids etch enamel as effectively as 30% to 40% phosphoric acid4); but data from other studies indicate that weaker acids provide significantly lower shear bond strengths when the manufacturers' recommended application times are used to etch enamel5). Self-etching primer may exhibit insufficient decalcification of enamel if it is a weak acid6) and the priming time may influence the morphologies of the tooth-resin interfaces or bond strength7,8).

Up to now, many self-etching adhesive systems vary in their acidity by differences in the composition and concentration of polymerizable acids and/or acidic resin-monomers in these systems9,10). Therefore the purpose of this study was to evaluate the influence of additional etching on shear bond strength of self-etching adhesive systems to enamel.

Ninety extracted sound bovine permanent anterior teeth were chosen. They were sectioned at cemento-enamel junction and embedded in autopolymerizing acrylic resin (Orthodontic Resin, Dentsply/Detrey, Konstanz, Germany) molds so that the prepared enamel surface was 2 mm above acrylic resin cylinders. Specimens were placed in tap water to reduce the temperature rise from the exothermic polymerization reaction.

After the resin have completely polymerized, the labial surfaces of the teeth were ground with model trimming wheel and 600-grit abrasive paper under wet conditions to expose enamel surfaces. After ultrasonic cleaning with distilled water for 3 minutes to remove any surface debris, these surfaces were washed and dried with oil-free compressed air (Hotman, Dentro, Tokyo, Japan). The teeth were randomly divided into six groups of 15 teeth each and treated in the manner of Table 1. Composition of each system is shown in Table 2.

Each etching group was pretreated with 32% phosphoric acid with benzalkonium chloride (UNI-ETCH, Bisco, Inc., Schaumburg, IL, USA) for 15 seconds while non-etching groups received no pretreatment. After the specimens were fully rinsed with running water for 10 seconds and dried in Clearfil groups, Clearfil SE Bond® (Kuraray Co., Ltd., Osaka, Japan) primer was applied to the tooth surface with Microbrush (Int'l, Co., Ltd., Dungarvan, Waterford, Ireland). In Adper groups, after the two reservoirs were sequentially squeezed to mix the respective components, Adper™ Prompt L-Pop (3M ESPE AG., Germany) was applied and scrubbed on the bonding site of enamel for 15 seconds and then gently dried for 3 to 5 seconds with compress air. In Tyrian groups, Tyrian SPE™ (BISCO, Inc., Schaumburg., IL USA) primer were applied to the tooth surface with Microbrush.

After primed tooth surface was dried with oil-free compressed air for 5 seconds, the bonding agent was applied following the manufacturer's instruction, and air-thinned, and irradiated using a curing light (Optilux 501) for 10 seconds with the intensity of 400 mW/cm².

A mounting jig (Ultradent Product Inc., South Jordan, Utah, USA) with an internal ring of 2.38 mm in diameter and height of 2.0 mm was placed against the tooth surface and stabilized with an alignment tube. Hybrid resin composite (Clearfil AP-X, Kuraray Co., Ltd., Osaka, Japan) was packed into the mold and light-cured for 40 seconds. After polymerization, the alignment tube and mold were removed and the specimens were placed in 37℃, 100% humidity. Twenty-four hours after storage, the specimens were tested in shear mode using a chisel-shaped rod in an Instron testing machine (Type 4202, Instron Corp., Canton, Massachusettes, USA) at crosshead speed of 1 mm/minute.

The data for each group were subjected to independent t-test at p < 0.01 to make comparisons between the groups.

The results of the shear bond strength tests to enamel were shown in Table 3.

In Clearfil SE Bond®, additional phosphoric acid etching group showed higher shear bond strength than priming only group (p < 0.01). However, there were no significant differences between additional etching group and non-etching groups in both Adper™ Prompt L-Pop and Tyrian SPE™ (p > 0.01).

Currently, clinical use of self-etching adhesive systems are increased because it is convenient and less technique sensitive. However, enamel bonding of self-etching adhesive systems is still a point of discussion. In the present study, additional etching procedure improved the bond strength of Clearfil SE Bond® and did not improve in Adper™ Prompt L-Pop and Tyrian SPE™.

Several studies have shown that self-etching adhesive with a resin composite did not provide as good bonding to the enamel surface as phosphoric acid etching system11,12). However, several studies reported good bonding performances of self-etching adhesive system on enamel12). Etching of enamel with 30% to 40% phosphoric acid gives bond strength about 20 MPa13,14), and the results of this study showed good enamel bond strength comparable to these previous results.

In the present study, the shear bond strength of etching group was significantly higher than priming only group in Clearfil SE Bond®. This can be explained by the acidity of each self-etching bonding systems. Clearfil SE Bond® had a pH value of 2.6, while Adper™ Prompt L-Pop and Tyrian SPE™ had a pH value of 1.3 and 1.0 each15). According to Barkmeier et al.16), primer with a pH around 1.4 presumably allows mineralized tissues to be conditioned and primed in one treatment step.

In the self-etching primer adhesive system, the acidic part of the primer may be neutralized partly by calcium and phosphate ions released during demineralization. Demineralization is therefore self-limiting, in that the high concentration of these ions tends to limit further dissolution of apatite. Thus the acidity of self-etching system is very important part of demineralizaiton of enamel, and it is coincident with the result of this experiment.

Clearfil SE Bond® contains a functional monomer, 10-methacryloyloxydecyl dihydrogen phosphate (MDP) and it may also bind to calcium. Recently, Yoshida et al.17) reported that MDP tightly adheres to hydroxyapatite and its calcium salt hardly dissolves in water. MDP is considered a more favorable adhesive monomer since it has two hydroxyl groups that chelate to calcium ions of enamel18). Acidic monomers in primer, such as 4-acryloxyethyl trimet (4-AET) and 2-hydroxyethyl methacrylate (HEMA), may promote the penetration of bonding resin into dentin and enamel and/or may increase the wettability of bonding agent on tooth surface, resulting in an effective seal of flowable composite19). The difference in bonding capacity to tooth substrates among MDP (Clearfil SE Bond®), Bis-GMA (Adper™ Prompt L-Pop) and Bis(2-(methacryloyloxy) ethyl) phosphate (Tyrian SPE™) is unclear, but it is very interesting that these three adhesives show similar bonding performance for enamel without pretreatment of 32% phosphoric aicd. If the etching effect of Clearfil SE Bond® is less effective than other groups, this result may indicate that other factors - e.g., filled bonding layer - of the bonding system can affect the final bond strength.

In conclusion, additional phosphoric acid etching on the enamel surface can improve shear bond strength of mild acidic self-etching system. However, it does not affect on the strong acidic self-etching system.

Further investigation is needed to evaluate the clinical significance of additional etching on the other self-etching systems which have different pH acidic primer.