DENTAL HARD TISSUE DESTRUCTION 133 



Irrespective of the type of hard tissue involved, the surface versus 

 subsurface attack depends on whether the tissue is exposed to a 

 simple demineralizing solution or to a mixture also containing an 

 organic polymer. In the former case the dissolution is most intense 

 superficially; in the latter case, there is present a thin relatively in- 

 tact surface layer with considerable subsurface demineralization. 



The typical pattern of incipient enamel caries — in which there is 

 initially a relatively intact enamel surface overlying a considerably 

 demineralized subsurface zone — can be duplicated in vitro by ex- 

 posing untouched extracted teeth to a pure and simple decalcifying 

 solution. In part, this may be due to the higher fluoride content and 

 lower carbonate content of the outer enamel surface. But, in part, 

 it appears to be caused by organic, natural surface coatings that in- 

 fluence the physical chemistry of the enamel, and in part by rede- 

 position of some other mineral phase with different solubility proper- 

 ties (Sognnaes, 1962; Gray and Francis, this volume, chapter 8). 



Even if the enamel surface is removed, the remaining enamel will 

 exhibit the typical opacity and the relatively intact surface layer as 

 well as the subsurface demineralization zone, provided the specimen 

 is exposed to a demineralization solution containing an organic poly- 

 mer simulating the mucoid of saliva (Fig. 25B and C). In other 

 words, this pattern can be reproduced irrespective of the presence 

 of the high-fluoride layer in the intact enamel surface. Indeed, this 

 same subsurface demineralization (even though not visible to the 

 naked eye ) can be produced in relation to other hard tissue surfaces 

 such as dentin, provided it is exposed to the polymer-containing de- 

 mineralizing solution ( Fig. 25E ) . 



Fig. 25. In vitro demineralization of human teeth exposed for 96 hours to 

 a buffered reagent (0.04 m lactic acid, 0.03 m CaCL adjusted to pH 4.0 with 

 NaOH) containing an organic polymer (6 per cent hydroxyethyl cellulose). 

 Typical opacities have developed (A, B, C, X 10) whether the exposed white 

 enamel "window" was produced on an originally intact tooth surface (A), on 

 a pumiced subsurface (B), or on a deep internal layer of enamel (C, top), 

 whereas no opacity is seen in the exposed dentin surface (bottom of C). How- 

 ever, in microradiographs (X 100) prepared from specimen C it is noted that 

 a subsurface leaching out of minerals has occurred, both in enamel (D) and 

 in dentin (E). Also note (D and E at right) the thin (white) zone of relatively 

 less demineralized surfaces. 



