PHYSICAL CHEMISTRY OF ENAMEL DISSOLUTION 249 



fusing into the enamel through the enlarged pathways, will begin, 

 at some point, to have a larger and larger effect in reducing the 

 attack within the enamel lesion as more and more exposed apatite 

 crystals become covered with the polymeric substances. For this 

 reason, it is also very doubtful whether the organic component can 

 ever be completely omitted in an in vitro system for reproducing 

 the typical natural lesion. 



The protection of the surface layer is increased by the presence 

 of acid-insoluble inorganic compounds, particularly those resulting 

 from reaction with enamel, such as calcium fluoride or stannous 

 phosphate. 



Diffusion within the Enamel 



In a system which restricts interaction with the enamel surface, 

 hydrogen ions will tend to diffuse into any region lower in acid 

 concentration. That such regions must exist and are accessible to a 

 solution phase has been demonstrated by the diffusion of radioactive 

 isotopes through enamel under in vivo conditions (Sognnaes et ah, 

 1955). It is obvious from experimental results that although the sur- 

 face apatite crystals are protected by the organic polymers, the dif- 

 fusion of acid into enamel is not blocked by this coating. Hence, any 

 acid penetrating beyond the limits of the organic protective coating 

 will react to dissolve subsurface enamel. At the point of reaction be- 

 tween acid and hydroxyapatite, the principles involved in the sur- 

 face dissolution of enamel in acid, as determined in a previous study 

 (Gray, 1962), can be invoked. As the acid reacts at specific sites in 

 the enamel, calcium and phosphate accumulate and retard or arrest 

 the dissolution reaction. Then, acid will tend to diffuse farther into 

 enamel for reaction at advanced unprotected sites, but it is limited 

 by the available diffusion pathways and the depleted acid concen- 

 tration. Diffusion of soluble calcium and phosphate out of enamel 

 is favored by the wide diffusion pathways already established and 

 by the favorable concentration gradient to the exterior solution. At 

 the same time, any increase in pH, either ahead of or behind the 

 reaction site, will cause reprecipitation of calcium phosphate, the 

 particular phase depending on the existing conditions. As calcium 



