Influence of Pre-Etched Area and Functional Monomers on the Enamel Bond Strength of Orthodontic Adhesive Pastes  
[PERSON]  
1 Department of Orthododonics, Nihon University School of Dentistry, Tokyo 101 8310, Japan; [EMAIL_ADDRESS] (Y.T.); [EMAIL_ADDRESS] (Y.U.); [EMAIL_ADDRESS] (M.I.); [EMAIL_ADDRESS] (M.M.)  
[PERSON]  
1 Department of Orthodonics, Nihon University School of Dentistry, Tokyo 101 8310, Japan; [EMAIL_ADDRESS] (Y.T.); [EMAIL_ADDRESS] (Y.U.); [EMAIL_ADDRESS] (M.I.); [EMAIL_ADDRESS] (M.M.)  
[PERSON]  
3 Department of Oral Structural and Functional Biology, Nihon University Graduate School of Dentistry, Tokyo 101 8310, Japan; [EMAIL_ADDRESS] (A.U.); [EMAIL_ADDRESS] (K.W)  
[PERSON]  
3 Department of Oral Structural and Functional Biology, Nihon University Graduate School of Dentistry, Tokyo 101 8310, Japan; [EMAIL_ADDRESS] (A.U.); [EMAIL_ADDRESS] (K.W)  
[PERSON]  
1 Department of Orthodonics, Nihon University School of Dentistry, Tokyo 101 8310, Japan; [EMAIL_ADDRESS] (Y.T.); [EMAIL_ADDRESS] (Y.U.); [EMAIL_ADDRESS] (M.I.); [EMAIL_ADDRESS] (M.I.); [EMAIL_ADDRESS] (M.M.)  
[PERSON]  
1 Department of Orthodonics, Nihon University School of Dentistry, Tokyo 101 8310, Japan; [EMAIL_ADDRESS] (Y.T.); [EMAIL_ADDRESS] (Y.U.); [EMAIL_ADDRESS] (M.I.); [EMAIL_ADDRESS] (M.M.)  
[PERSON]  
4 Department of Operative Dentistry, Nihon University School of Dentistry, Tokyo 101 8310, Japan; [EMAIL_ADDRESS] (Y.T.); [EMAIL_ADDRESS] (Y.U.); inaba.mizuki@nihon-u.  
there are adhesion concerns, due to low elastic modulus and gap formation at marginal regions, about using these materials on enamel without any pre-treatment, because they have weaker etching capabilities than previous adhesive systems [4; 5]. Therefore, selective etching is recommended for retention of restorations. In orthodontic treatment using bonding brackets, phosphoric acid pre-etching is the standard bonding procedure. On the other hand, when the pre-etched area extends beyond the area covered with resin bonding agents, there are concerns about the risk of caries, progression of enamel deficiency and staining during and after orthodontic treatment [6].  
Bonding orthodontic brackets does not necessarily require extremely strong adhesive, because the brackets are physically removed after treatment. A previous study reported that moderate bond strength (6-8 MPa) may be necessary to maintain orthodontic brackets [7], but a bond strength of 13.7 MPa or more may lead to cracking of the enamel when debonding the brackets [8]. Therefore, pre-etching a limited area of enamel, in combination with the use of an adhesive containing functional monomers, may be helpful to reduce the risk of over-etching while ensuring sufficient retention of brackets. However, little information is available about the enamel bonding effectiveness of such techniques.  
This study was performed to investigate the influence of the pre-etched area and functional monomers in orthodontic adhesive pastes on enamel bond strength and the ultrastructure at the adhesive paste/enamel interface. The null hypotheses were as follows: the enamel bond strength of different orthodontic adhesive pastes would not differ according to the presence or absence of functional monomers, and the enamel bond strength would not differ according to the pre-etched area. Therefore, in order to compare their adhesive effects, adhesion strength was examined by shear bond strength test and adhesive state at the enamel-adhesive interface was evaluated by adhesive remnant index, Knoop hardness test and scanning electron microscopy observation.