2009 Volume 52 Issue 1 Pages 27-38
The objective of this study was to investigate the ultra-morphological smear layers produced by various dental instruments, and to evaluate the effect of the smear layers on the short-and long-term bonding effectiveness of two commercially available one-step adhesive systems bonded to dentin. Smear layers were produced on flattened mid-coronal dentin surfaces of extracted human molars by one of the following materials: two types of diamond burs (regular-grit or superfine-grit) or two types of SiC papers (#600 or #2,000). Thickness of the smear layers and size of the particles of the four materials were measured by SEM. TEM images of the smear layers were taken, as well. Two types of dentin surfaces produced using a regular-grit diamond bur and #2,000 SiC paper were conditioned with either Clearfil S3Bond (Kuraray, S3) or Absolute (Dentsply-Sankin, AB) without light-curing, rinsed with acetone, and observed by SEM. Moreover, resin composites (Z-100, 3M ESPE) were bonded with either S3 or AB, and incrementally built-up on two types of the dentin surfaces. The specimens were sectioned into slabs and further trimmed into an hourglass shape with an interface area of approximately 1mm2. They were left untouched or thermo-cycled (100,000 cycles). Micro-tensile bond strength (μTBS) testing and ultra-morphological observation by TEM were performed. The larger the particle sizes were, the thicker the width of the smear layer became, although TEM images of smear layers produced by the four materials revealed no differences in ultra-morphology. For S3, thick smear layers were not removed completely, whereas AB removed the smear layers despite their thickness. Whether the specimens were thermo-cycled or not, the μTBS of S3 on the dentin surface with thick smear layers was significantly lower than that of the thin, smear layer covered dentin surface. However, the μTBS of AB was not affected by the thickness of smear layers. For both adhesives, 100,000 thermo-cycles significantly decreased μTBS despite the thickness of the smear layers, and TEM images revealed scattered voids, which appears to be an indication of interfacial degradation. It is concluded that 1) the thickness of the smear layer produced on the dentin surface was dependent on the materials used; 2) thick smear layers affected the bonding effectiveness of S3 which had a low ability to dissolve the smear layers; 3) smear layers did not always affect the bonding effectiveness because AB completely removed the smear layers regardless of their thickness; and 4) the bonding effectiveness of one-step adhesive systems decreased after long-term thermo-cycling.
