Histological features of initial secondary caries adjacent to composite resin restorations.

The aim of this study was to analyse morphologically the initiation of the secondary caries lesion. Thirty-three proximal surfaces adjacent to Class II resin restorations were examined by stereomicroscope (x40) for the presence of caries and gap. Sixty sites (39 sound, 7 with lesions, 14 adjacent to lesions) were prepared for examination using polarized light microscopy in water and after air drying. Lesions were observed in 52.7% of the wet sections and 85.0% of the dried sections. The dried ground sections displayed larger lesions. Further analyses were performed on the dried sections. One hundred per cent of the ground sections from the stereomicroscope carious sites, 87.2% from sound sites, and 71.4% from sites adjacent to lesions displayed demineralized areas. Morphological analysis indicated that caries lesion initiates at the external enamel surface in the interface between tooth and restoration. Two distinct patterns of caries development were observed, related to the direction of enamel rods: (1) the direction of the rods in the adjacent enamel was parallel to the cavity wall (the deepest part of caries penetration was located at the cavity wall), and (2) the direction of the rods ran in the apical direction (the deepest part of caries penetration was oblique to the cavity wall). The pattern 1 could be mistaken for a wall lesion. The prevalence of lesions adjacent to restorations was similar in the sites with (85.7%) and without (84.0%) gaps. The findings support the assumption that secondary caries initiation in vivo is not related to the presence of a gap and follows the direction of the enamel rods.