Variable polymerisation shrinkage and the interfacial micropermeability of a dentin bonding system

Adhesive bonds are subjected to competing stresses whilst a restoration is setting. This study aimed to determine the effect of these stresses on a chemically- and light-activated dentin adhesive (Scotch Bond MultiPurpose Plus: SBMP+) by assessing the morphology and micropermeability of the interfacial region.

Restorative materials and techniques were selected to impart differing setting stresses to the adhesive interface. An amalgam (Dispersalloy), a light-activated composite (Z100) with a high- or low-intensity curing light, and chemically-activated composite (Adaptic) were placed either in bulk (4 mm thickness) or in one thin layer (2 mm) in occlusal cavities in 55 freshly extracted third molars. In addition, thin composite restorations were placed on 15 horizontally sectioned teeth using a glass slide as a rigid clear matrix. The pulp chambers of all samples were filled with rhodamine B dye solution for 4 hours. Samples were longitudinally sectioned and the interfacial dye leakage measured using a confocal optical microscope. Data were analysed using the Kolmogorov-Smirnov test.

In occlusal cavities, Z100 had significantly more leakage than the amalgam and Adaptic. There was no significant difference in micropermeability between large and small increments, or curing light intensity for Z100, but when placed in bulk, there was evidence of incomplete composite polymerisation near the cavity floor, especially with the low intensity curing light. Cohesive failure of the deep resin composite and gap formation were a frequent finding with the high-intensity light curing. The least leakage for Z100 was achieved with a thin layer on flat dentin. In contrast, the chemically-activated composite performed well in bulk when the surface of the restoration was freely exposed, but badly when constrained by the glass matrix.

Restoration setting stress is critical in the microscopic integrity of the adhesive bond to dentin.