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



and phosphate concentration is reduced by precipitation and/or 

 diffusion, the reaction with hydrogen ion is renewed and the lesion 

 below the surface slowly progresses. 



Concent rat io)i Gradients in the Solution Phase 



In the course of incipient carious lesion formation, different solu- 

 tion compositions must exist inside and outside the enamel with a 

 gradient extending from one to the other. For the in vitro system 

 the external or bulk solution is defined bv the initially prepared 

 composition. At or near the enamel surface, however, there will be 

 a region of different composition as solution components diffuse 

 into the enamel and products diffuse out. The hydrogen ion and 

 undissociated acid concentrations will certainly be lower and the 

 calcium and phosphate concentrations ( from the reaction products ) 

 will be higher at the surface than in the external bulk solution. In- 

 side the enamel, the hvdrogen ion concentration will be lower than 

 in the bulk solution, owing to reaction of hvdrogen ions with enamel. 

 Defining the concentrations of reaction products relative to the ex- 

 terior bulk solution is not alwavs so straightforward. When such 

 materials are not incorporated initially in the decalcification medium, 

 the concentration within the enamel will obviously be higher. When, 

 however, calcium and phosphate are added initially as part of the 

 decalcification medium, sometimes in large amounts, the concentra- 

 tions in the exterior phase may approach or even exceed those in 

 the solution phase inside the enamel. As the concentration of cal- 

 cium, for instance, is raised in the external decalcification medium, 

 the rate of enamel dissolution decreases owing to the decrease in 

 rate of removal of the reaction products; it will be recalled that 

 diffusion rate is controlled principally bv the difference in concen- 

 tration from one site to the other. At sufficiently high calcium con- 

 centrations, the enamel dissolution reaction stops altogether. 



The amount of calcium required in the decalcification medium to 

 inhibit incipient carious lesion formation is greater than would be 

 expected on the basis of enamel solubility rate measurements (Gray, 

 1962). The existence within the enamel of other species of calcium 

 and phosphate in the form of soluble complexes, for example acid 

 phosphate and calcium lactate, would permit a greater rate of out- 



