microleakage in class ii composite restorations ... - ePrints@USM

a complex substrate. In addition, the effect of the tooth type, the bonding location (deepversus superficial dentin, crown versus root dentin), and tooth qualities (young, old orsclerotic dentin) on adhesion must be taken in account (Van Landuyt et al., 2005).Most commonly, the tooth to which the bond will occur is covered with a smearlayer. The smear layer is an adherent layer of debris on tooth surface when they are cutwith rotary or hand instruments. The smear layer is revealed by scanning electronmicroscope (SEM) as a 1-2 micron layer of debris with a mainly granular substructurethat entirely covers the dentin. The orifices of dentin tubules are obstructed by debristags, called smear plugs, which may extend into the tubule to a depth of 1-10 microns.These smear plugs are contiguous with the smear layer (Van Landuyt et al., 2005).The thickness and morphology of the smear layer probably varies with the methodused for producing the smear layer and with the location within dentin in relation to thepulp. But the smear layer is believed to be consisted of shattered and crushedhydroxyaptite, as well as fragmented and denatured collagen. However, early smearlayer incorporating non acidic adhesives, applied without prior etching, did notpenetrate deeply enough to establish a bond with the intact dentin. Such bonds wereprone to cohesive failure of the smear layer. Therefore, two options were established toovercome low bond strengths due to the limited strength of the smear layer. This isthrough removal of the smear layer (totally or partially) or incorporation bymodification of the smear layer in the hybrid layer. However, it has been suggested thatadhesive techniques that require smear layer removal are associated with more postoperative sensitivity than systems that leave the smear layer in situ (Van Landuyt et al.,2005).The first adhesives achieving clinically acceptable results were based on the smearlayer removal, but recently new smear layer incorporating adhesives have regained its22