Effect of dentin primers containing N-methylolacrylamide or N-methylolmethacrylamide on dentin pretreatment

The effect of experimental dentin primers containing N-methylolacrylamide (MEAA) or N-methylolmethacrylamide (MEMA) on the bonding of three commercial light-cured resin composite systems [Restorative Z 100 (Scotchbond), Palfique Estelite (Macbond) and Photo Clearfil A (Clearfil Photobond)] to etched dentin was investigated. Water solutions of 35% hydroxyethyl methacrylate (HEMA), 50% MEAA or 30% MEMA were used as dentin primers. The dentin etched with 10% phosphoric acid solution was pretreated with dentin primers for 30 s. The resin composite systems were applied in a Teflon tube positioned onto pretreated dentin surfaces. After water immersion for 1 day, the shear bond strengths were measured. The thicknesses of hybrid layers at dentin–resin interfaces treated with 6 mol/l HCl and 1% NaOCl were measured by scanning electron microscopy. The dentin primer pretreatment increased the bond strengths of all resin composite systems. For Macbond and Clearfil Photobond, the bond strengths (14.2–26.5 MPa) with MEAA and MEMA were higher than those (10.5 and 17.8 MPa) with HEMA. All hybrid layer thicknesses were 1–1.5 μm after HCl immersion. The hybrid layers after NaOCl immersion become narrower. The main fracture pattern of specimens exhibiting high bond strengths (more than 14.2 MPa) was dentin cohesive fracture after bond test. For Macbond and Clearfil Photobond, the layers of specimens pretreated with MEAA and MEMA were clearly thicker than those pretreated with HEMA after NaOCl immersion. MEAA and MEMA solutions were more effective in improving the bond strength of Macbond and Clearfil Photobond to etched dentin than was HEMA. Macbond and Clearfil Photobond created good hybrid dentin layers which could resist NaOCl-attack when MEAA and MEMA solutions were used as dentin primers.