Ultrastructural study of the resin infiltration zone in acid-treated human enamel

Abstract

A method was devised for clearly defining the infiltration zone of resin into partially-dissolved enamel at the ultrastructural level. Enamel spicules displaying prisms in cross-sectional and longitudinal aspects on adjacent sides were treated with 1 per cent TCA, given an outer coating with Au-Pd, and examined by scanning electron microscopy. After embedment, the normal enamel was totally removed to some depth, revealing the inner aspect of the resin infiltration. The normal enamel was replaced with resin after an inner coating of Au-Pd had been applied to the inner aspect of resin infiltration. Transmission electron microscopy of the area above the remaining spicule revealed that (1) resin infiltration, as clearly delineated by the inner and outer coatings, varied with the orientation of the enamel prisms; (2) resin infiltration closely followed the dissolution pattern of the acid in the enamel and, therefore, the furthest resin infiltration was at the arcuate prism peripheries; (3) enamel crystals were present in a solid zone of subsurface resin. The resin formed a tight interface with the crystals as it both surrounded the crystals and penetrated the dissolved crystal cores. This solid zone of subsurface resin gives prolonged protection to the normal enamel and might also enhance the bonding between resin coating and enamel.