248 -T. A. GRAY AND M. D. FRANCIS 



Important Factors in Incipient Carious liEsiON 

 Formation 



Preservation of Enamel Surface 



In a previous section, it was indicated that the enamel surface is 

 protected principally by the presence of polymeric organic material. 

 The exact nature of this protective mechanism is not understood, 

 but a protective coating resulting from adsorption of the organic 

 compound on the apatite crystals appears most logical. The higher 

 organic content of incipient carious lesions as compared with normal 

 enamel demonstrates that organic substances diffuse into these en- 

 amel defects. Therefore, protective coatings of organic origin can 

 be expected within the enamel near the surface, and thus could 

 account for the relativelv sound outer layer overlying the main de- 

 calcified region. Indeed, electron micrographs do show that this 

 outer layer has the highest residue after demineralization. 



Obviously, some dissolution occurring in this relatively sound 

 outer layer would promote the diffusion of acid and organic material 

 within the enamel. The reduced density of the outer layer of a lesion 

 produced in vitro can be seen in microradiographs. 



In previous studies (Soni and Brudevold, 1959), it has been shown 

 quite clearly that this outer layer in natural lesions has a lower 

 density than normal enamel. The hardness is also slightly reduced 

 within this layer (Gustafson, 1957), consistently with the reduced 

 density. In electron micrographs, the channels between the rods 

 going through this layer are clearly apparent. The organic material 

 apparently diffuses into the enamel as the acid creates voids and 

 enlarges the interrod spaces. 



In order for the organic material to be protective (Table I), it 

 must be water soluble or in a colloidal suspension so as to gain access 

 to the hydroxyapatite surface, and it must be adsorbed. Polymeric 

 saccharides and proteins are known to adsorb on many substrates, 

 and these materials are highly effective in protecting the enamel 

 surface during subsurface decalcification. The monomers have little 

 or no such effect. The organic material, constantly present and dif- 



