## 4 Discussion  
In this study, bovine enamel was employed as a substitute for human enamel because of the difficulty of collecting the latter in the same condition [13]. To maximise the similarity to clinical orthodontic treatment with direct bonding of brackets, it would have been best to use intact enamel in the experiment. However, the undulations of bovine enamel are deeper and broader than those of human enamel. Therefore, the adherent enamel surfaces were ground flat to standardise the methodology and ensure both an appropriate adherent area for the bonded assembly and a uniform stress distribution [14,15]. Various types of functional monomer have been employed in different types of adhesive systems, such as phosphoric acid ester, carboxylic acid and alcohol functional monomers [16]. The functional monomers 10-methacryloyloxydely dihydrogen phosphate (MDP) and 4-methacryloxyethyl trimellitate anhydride (4-META) have often been employed in orthodontic adhesive systems [17,18]. In this study, UB contained MDP as a functional monomer, but there were no significant differences in SBS between UB and the functional monomer-free orthodontic adhesive paste TB, regardless of the pre-etched area. Therefore, the first null hypothesis, that the enamel SBS in different orthodontic adhesive pastes would not differ according to the presence or absence of functional monomers in the adhesive paste, was not rejected. An optimal orthodontic adhesive system would not fail during treatment, and would allow easy debonding of the brackets on completion of treatment [19]. Previous studies have reported that the ideal enamel bond strength of orthodontic brackets ranges from 6 to 13.7 MPa [5,6]. Excessive enamel bond strength may cause damage to the intact enamel surface when debonding brackets [20]. In this study, the baseline and 3 mm groups were within the range of ideal bond strength for both materials. Although UB showed higher SBS values than TB, the difference was not significant in any pre-etched group. This can be explained by the lower mechanical properties of  
Figure 3: Representative SEM images of the orthodontic adhesive paste/enamel interface in UB. The whole interface (arrow range) region showed excellent adaptation ((**a**): magnification 30\(\times\)). In the pre-etched region ((**b**): magnification 1000 \(\times\), (**c**): magnification 10,000\(\times\)), a typical etching pattern and resin enamel tags were clearly observed (arrowheads). The non-etched region ((**d**): magnification 1000\(\times\), (**e**): magnification 10,000\(\times\)) showed neither the typical etching pattern nor resin enamel tags. UB due to the presence of functional monomers. The incompatibility between residual acidic monomer and the other resin monomers, and the low pH of functional monomers, may inhibit polymerization, resulting in lower mechanical properties [21,22]. This was supported by the results of the KHN and ARI evaluation. UB showed a significantly lower KHN and marked increases in the rates of ARI scores 2 and 3 than TB. However, on SEM images, although UB in the 2 mm group showed excellent adaptation in both pre-etched and non-etched regions, TB in the 2 mm group showed gap formation between the enamel and adhesive paste in the non-etched region. This gap formation between the adhesive paste and enamel substrate may be a cause of caries, staining and bracket failure during orthodontic treatment. Therefore, orthodontic adhesive paste containing functional monomers may improve the integrity of the interface even in non-etched regions due to its chemical bonding ability. On investigation of the influence of the pre-etched area on the enamel SBS, no significant differences were observed in SBS with UB between the 3 mm and baseline groups. On the other hand, the functional monomer-free adhesive paste TB showed significantly lower SBSs in the partially pre-etched groups than the baseline group, regardless of the pre-etched area.