Some notes on the diffusion of acidic and alkaline agents into natural human caries lesions in vitro

Abstract

The aim was to obtain information on the penetration of ions, in particular H + and OH −, into a wide variety of natural caries lesions in human teeth. Thirty-seven permanent teeth were slit in half and soaked in a 1% neutral (blue) litmus solution for at least a week. The cut face of the tooth was painted with a transparent, colourless nail varnish before the tooth was immersed in either of 0.1 M hydrochloric acid or 0.1 M phosphoric acid. In a reverse experiment the samples were soaked in acid (red) litmus solution and then immersed in 0.1 M sodium hydroxide after varnishing the cut face. All solutions were saturated with enamel apatite to avoid the possibility that the solutions might etch their way into the enamel. The colour change was recorded photographically. Several days of external acid/alkali exposure were required to change the litmus colour in the body of most of the lesions, suggesting that diffusion of acidic and alkaline species through the natural surface into the body was slow. The presence of a well-mineralized surface layer covering the lesion postponed the colour change. In contrast, the colour change of two root surface lesions was rapid, occurring within an hour. Thus, because the lesion body is relatively isolated and because lesion fluid is in intimate contact with the enamel apatite, we conclude that lesion fluid is close to saturation with respect to this mineral. Further, a pH change of lesion fluid in vivo paralleling that in plaque after a sucrose intake is unlikely. These results imply that therapeutic remineralization with calcium phosphates of the deeper part of the caries lesion will only be possible when the mineral-rich surface layer has been removed.