Figure 3. Fractographic failure analysis of Scotchbond MP and Scotchbond 1 (3M Espe). (a) Fe-SEM overview photomicrographs of the fracture surfaces (left = dentin side; right = composite counterpart) of a representative µTBS sample prepared with Scotchbond MP that was stored for 4 yrs with the resin-dentin interface directly exposed to water (4yr-DE). Scratches remaining from smear-layer preparation confirmed that the interface failed adhesively (A) at the level between dentin and the bonding layer for an area of 0.84 mm² or 84% of the total surface area. A small area of 0.16 mm² or 16% of the total surface area represents a cohesive (C) failure in the bonding resin. (b) Magnification of the adhesive failure area at the composite side of the same sample as in (a) shows a typical pattern of islands of hybrid layer (H) fragments still attached to the composite (Comp.) and detached from dentin. (c) TEM photomicrograph (non-demineralized, unstained section) of the adhesive failure area sectioned from the same sample as in (a). The thin black line covering the fracture plane (arrows) and underneath the embedding resin (E) represents the gold coating applied for the Fe-SEM examination conducted beforehand. The hybrid layer (H) was pulled from unaffected dentin (U) either at the base (left) or close to the top (right). (d) High-magnification Fe-SEM photomicrograph of the composite site of a fractured four-year-stored Scotchbond 1 sample with direct exposure of the interface to water (4yr-DE). The sample failed within the hybrid layer, part of which remained attached to the composite. A resin tag (T) within a dentinal tubule is surrounded by loosely organized collagen fibrils (Coll.), the typical cross-banding of which can be observed. This suggests either that this hybrid layer collagen was inadequately enveloped by resin or that resin was eluted.