PHYSICAL CHEMISTRY OF ENAMEL DISSOLUTION 233 



methods of observation, including histochemistry (Opdyke, 1962), 

 the incipient carious lesion produced by the in vitro system is iden- 

 tical in appearance with natural lesions. 



Variables Affecting Rate of Carious Lesion Formation 



The rate of formation of the lesion produced in vitro was studied 

 by quantitative measurements of calcium and phosphate released 

 from enamel as a function of pH, buffer concentration, calcium con- 

 centration, and temperature. 



The dissolution of enamel during incipient carious lesion forma- 

 tion was measured as a function of exposure time. Both an integral 

 method and a differential method for determination of reaction 

 rates were used (Laidler, 1950). The amount of enamel dissolved 

 during exposure to the various mediums was determined by an- 

 alyzing chemically for calcium (Welcher, 1958) and phosphate 

 ( Lucena-Conde and Prat, 1957). The calcium and phosphate ana- 

 lytical results were converted to weight of enamel, using its reported 

 composition (Orban, 1957), and rechecked by analyzing known 

 weights of dissolved enamel. By way of an example, the first medium 

 to be discussed contained 0.05 m lactic acid, and hydroxyethyl cellu- 

 lose was added in an amount of 6 gm per 100 ml of the lactic acid 

 solution (hereafter referred to as 0.05 m lactic acid plus 6 per cent 

 hydroxyethyl cellulose). The initial pH was adjusted with concen- 

 trated sodium hydroxide or hydrochloric acid as required. The 

 amounts of enamel dissolved, determined by both phosphate and 

 calcium analyses of the same solution (Fig. 25), were in good agree- 

 ment, indicating no appreciable preferential dissolution of one com- 

 ponent over the other during incipient carious lesion formation. This 

 correspondence held during all subsequent measurements, and only 

 the results of the phosphate analyses are presented in the figures to 

 follow. The coeflBcient of variation in the determination of dissolved 

 enamel was ±11 per cent based on phosphate analysis and ±13 

 per cent based on calcium analysis across different enamel samples 

 and experimental conditions. The enamel samples used in these 

 studies were examined for surface damage with a metallurgical 

 microscope using top illumination, and then were sectioned with 

 a special dental saw (Gray and Opdyke, 1962) for observation by 



