PHYSICAL CHEMISTRY OF ENAMEL DISSOLUTION 221 



The major reaction products — calcium and phosphate — also were 

 incorporated into the decalcification medium at appropriate stages 

 in the study. Carbonate, another reaction product, was not con- 

 sidered in this preliminary investigation, but may have to be ac- 

 counted for in later, more sophisticated systems. 



It is obvious that if a homogeneous solid sample of hydroxyapatite 

 is exposed to acid, attack will begin at the surface. It is also obvious 

 that if diffusion pathways within such a sample are available, the 

 acid will tend to diffuse into those regions of the sample having 

 lower acid concentrations. The presence of diffusion pathways in 

 normal sound enamel has been amply demonstrated (Pickerill, 1912; 

 Sullivan, 1954 ) . Therefore, the logical expectation for a system con- 

 sisting of enamel exposed to acid is that acid will diffuse both to 

 the enamel surface and into the enamel. If the acid is consumed very 

 rapidly at the surface, then little or no acid will remain to diffuse 

 into the enamel. If, on the other hand, the surface reaction is re- 

 tarded, more acid will diffuse farther into the enamel until subsur- 

 face reaction occurs. Thus, conceivably, there could be competition 

 between ( 1 ) reaction at the surface and ( 2 ) diffusion into enamel 

 with subsequent subsurface reaction. In the event that the rate of 

 the first step could be greatly reduced relative to the second step, 

 then the result would be an incipient caries-like lesion. The studies 

 by Hals et al. (1955) support this idea of a competition between 

 surface and subsurface reaction. 



The wide variety of conditions encountered in the formation of 

 incipient carious lesions in vivo and in vitro poses the problem of 

 how the surface reaction can be uniformly retarded in all these 

 systems. A review of both the natural and the artificial systems which 

 have been reported indicates, as far as can be determined, that in 

 every case organic material was present. This conclusion is based 

 on the following considerations. In the oral cavity there are present 

 the enamel cuticles, bacterial plaque, and saliva. In vitro systems 

 using bacteria contain proteins, polysaccharides, mucoproteins, or 

 mucopolysaccharides. The enamel surface as it occurs in the oral 

 cavity is less soluble than subsurface enamel, and this property has 

 been ascribed, in part at least, to the presence of certain inorganic 



