Enamel structural changes induced by hydrochloric and phosphoric acid treatment

The aim of this study was to evaluate enamel acid-induced structural changes after 2 different treatments, by means of Raman and infrared (IR) spectroscopy analyses, and to correlate these findings with permeability measured as fluid discharge from outer enamel.

Two different treatments were investigated: 10 enamel slices were etched with 15% hydrochloric acid (HCl) for 120 seconds and 10 slices with 37% phosphoric acid gel (H₃PO₄) for 30 seconds, rinsed for 30 seconds and then air-dried for 20 seconds. Powders of enamel treated as previously described were produced. Replicas of enamel subjected to the same treatments were obtained to evaluate the presence of fluid droplets on enamel surface.

Raman and IR spectroscopy showed that the treatment with both hydrochloric and phosphoric acids induced a decrease in the carbonate content of the enamel apatite. At the same time, both acids induced the formation of HPO₄^2- ions. After H₃PO₄ treatment, the bands due to the organic component of enamel decreased in intensity, while they increased after HCl treatment. Replicas of H₃PO₄ treated enamel showed a strongly reduced permeability. Replicas of HCl 15% treated samples showed a maintained permeability. A decrease of the enamel organic component, as resulted after H₃PO₄ treatment, involves a decrease in enamel permeability, while the increase of the organic matter (achieved by HCl treatment) still maintains enamel permeability.

The results suggested a correlation between organic matter and enamel permeability. Permeability was affected by etching technique and could be involved in marginal seal, gap and discoloration at the enamel interface, still causes of restoration failure.