214 J. A. GRAY AND M. D. FRANCIS 



the reactaiit or solute to the sohd surface. The diffusion rate is de- 

 termined h\ the concentration of the reactant, and, therefore, tlie 

 rate of reaction with the sohd is directly proportional to the concen- 

 tration of the reactant. Equilibrium solubilitv must also be con- 

 sidered because it mav be attained frequentb, and studies at 

 equilibrium provide information which is difficult or impossible to 

 obtain from rate data. These phvsical-chemical principles provide 

 the basis for examining the formation of the carious lesion. 



The following presentation can be regarded as a study of a model 

 system for producing caries-like lesions irrespective of the chemical 

 agents which cause caries. The primarv reactant examined in this 

 discussion is acid or acidic buffer. The model based on such an acid 

 reactant is considered most promising because ( 1 ) adequate quanti- 

 ties of acid have been shown to exist in the immediate en\ ironment 

 of the teeth, (2) an adequate substrate, hvdrox\'apatite, exists on 

 which the acid can operate, and (3) caries-like lesions have been 

 produced in vitro with acid. 



The only other agents ever considered extensively as the primarv 

 reactants for enamel are proteolvtic enzvmes and complexing com- 

 pounds. However, several attempts to produce a proteolvtic enzyme 

 effect in enamel resulting in caries-like lesions have not been success- 

 ful (Burnett and Scherp, 1952; Jenkins, 1961). Furthermore, the 

 rather small supply of substrate (i.e., organic content of enamel) 

 upon which the proteolytic agents can act ( Orban, 1957; Schatz et 

 al., 1956) makes this approach less logical. It does appear conceiv- 

 able that compounds which can form strong soluble complexes with 

 calcium, such as ethylenediaminetetraacetate, could be used to pro- 

 duce caries-like lesions in enamel (Ravnik and Loe, 1961), although 

 no major attempts have been made to complete such a study. Many 

 complexing agents have the necessary capability for dissolving the 

 substrate, the inorganic portion of enamel, just as does acid. How- 

 ever, this concept becomes untenable in vivo because complexing 

 agents of sufficient strength and in adequate concentration in the 

 vicinity of the enamel surface (other than hydrogen ion) seem 

 scarcely possible on the basis of a quantitative consideration of po- 

 tential sources such as the organic moietv of enamel, bacteria, or 



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