One of the changes resulting in carious infection of the dental tissues is considered to be decalcification by acids which bacteria produce. The decalcification would proceed under the neutralization process of a small quantity of acids by dissolution of the dental tissues surrounding the infected site.
The reaction was investigated using equipment capable of recording pH changes in suspension of dental tissue powder after addition of a certain amount of acid. Dentin or enamel powder was suspended in 100 ml of CO₂-free distilled water, 1 ml of acid was added, the mixture was stirred at 37°C and the pH movement of the suspension was measured. The pH value dropped immediately after addition of the acid, and thereafter increased slowly to return to the constant value. According to the curve of pH change, the pH value at 2 hrs. afer acidification was defined as this final constant pH. Effect of powder size, the concentration of suspension, kind and amount of acids on the pH change was investigated, and dissolved Ca and P in the suspension were estimated.
The results were as follows:
Smaller particles of tooth powder and higher concentration of the suspension caused a slighter drop and a faster recovery of pH. Lactic acid had the strongest decalcification capacity as compared with other acids: acetic, formic, propionic, succinic, and butyric acid. The solubility of enamel is less than that of dentin because the final pH of enamel is lower and recovery time is longer than that of dentin.
Assuming that acids were produced at the dentino-enamel junction, acids would be immediately neutralized by the dissolution of dentin which dissolves ealier than enamel and enamel would hardly be affected. These results also explain the formation of the half-moon shaped decalcification which is observed only at the dentin site of rat molars in the early stage of experimental caries described in a previous report. (Onisi et al. 1968).