Factors affecting the differential dissolution of human enamel in acid and EDTA. A scanning electron microscope study

Abstract

Surfaces of sound, human permanent teeth, either natural surfaces or surfaces cut in various planes, were etched with a variety of agents. In general, acids preferentially removed prism centres and ethylene diamine tetra-acetic acid (EDTA), at neutral pH, preferentially removed prism peripheries. A relationship between time and concentration was suggested by the observations that application of very dilute EDTA solutions resulted in the formation of narrow clefts at prism junctions which progressed to destroy the prism peripheries. Short exposures to dilute acids produced similar clefts as well as shallow concavities within the body of the prism; these concavities deepened with length of exposure leaving the prism cortex relatively unaffected. Etching with organic acids at neutral pH, where they might be expected to manifest some chelating activity, and etching with EDTA at acid pH, produced variable results with features typical of both acid and neutral EDTA attack. When the ionization of acids was inhibited by preparation in non-aqueous solvents, prism junctions appeared to be preferentially attacked. The role of crystal orientation and regional differences in chemical composition of enamel are discussed as possible explanations for the findings. It is suggested that the major factor which determines the differential etching by different agents is the greater permeability of prism junctions, which admits the large EDTA molecule and acid anions more readily than the bodies of the prisms. The pattern of tissue destruction seen in enamel caries could be explained on the basis of acid dissolution alone, or by a mixture of acid dissolution and calcium complexation.