## 4 Discussion
Therefore, the second null hypothesis, that the enamel bond strength would not differ by pre-etched area, was rejected. Although enamel HAP is considered less amenable to chemical reaction with functional monomers than dentin HAP [23,24], the functional monomer in the orthodontic adhesive may enhance enamel bond effectiveness. However, it may be difficult to reach the minimum enamel bond strength requirement without phosphoric acid pre-etching, even when orthodontic adhesive pastes contain functional monomers. [PERSON] et al. investigated the bond effectiveness of five self-adhesive flowable resin composites without any pre-treatment, and recommended selective enamel etching for these materials [25]. Based on previous investigations of self-adhesive materials and the outcomes of this study, it may be necessary to pre-etch at least half of the bracket base area in the case of UB. It is difficult to compare the outcomes of in vitro experiments and clinical situations directly. In addition, the bond strength value is influenced by many factors, such as the type of bond strength test, adherent substrate, material selection, adherent conditions, caries prophylaxis treatment, etc. [26, 27, 28]. In this study, the adherent enamel surface was ground flat to standardise conditions, but this introduces an important difference from the clinical situation. The outer surface of intact enamel, i.e., the prismless layer, has stronger resistance to acid than a prismatic one. In a previous investigation of self-etching adhesive systems for intact enamel using different etching strategies, unground enamel without pre-etching showed significantly lower bond performance than ground enamel, but no significant difference was observed between ground and unground enamel with pre-etching [29]. It was suggested that mechanical interlocking is more important for intact enamel adhesion than chemical bonding. Hence, phosphoric acid pre-etching is necessary for securing brackets during treatment, even when using orthodontic adhesive pastes marketed as self-adhesive. Although there has been a study showing the enamel bonding effectiveness of UB, little information is available on this material [30]. On the other hand, TB has been used in many studies, often as a control material [31, 32, 33]. In this study, to standardise the bonding protocol between TB and UB, we omitted the priming step of TB. The primer composition of TB is similar to typical bonding agents and does not contain any functional monomers. In a previous study, SEM revealed no ultrastructure differences in the etched enamel region according to primer application [34]. In addition, previous laboratory studies demonstrated comparable tensile bond strength with and without primer application [35, 36, 37]. These findings suggest that primer application may not influence enamel bond effectiveness or morphological features in the vicinity of the interface of TB, and that the bonding procedures used in the baseline and partially pre-etched groups did not cause problems. Regarding the ARI, a significant difference was detected in UB, but not TB, according to the pre-etched area. UB tended to show an increase in the rate of ARI scores of 2 and 3 with increasing pre-etched area. Remnant adhesive paste on the enamel surface should be removed with rotary rather than hand instruments [38]. Although grinding and polishing the adhesive remnant on the tooth surface may be time-consuming, this method is safe and gentle [39]. Therefore, adhesive paste containing functional monomers with partial pre-etching may be preferable to functional monomer-free adhesive paste because of adaptation and improved treatment of the debonded enamel surface.