Time Studies of the Penetration of Extracted Human Teeth by Radioactive Nicotinamide, Urea, Thiourea, and Acetamide

Abstract

Rapid diffuse penetration of grossly intact enamel by isotope-labeled solutions of nicotinamide, urea, thiourea and acetamide was determined radioautographically in 23 extracted human teeth. Eight teeth were formol fixed. No effect of fixation, age, sex, or tooth location was observed. Nicotinamide penetration was the slowest. The earliest diffuse penetration observed was through 0.4 mm of enamel within 79 minutes. Urea, thiourea and acetamide penetrated more rapidly. Urea penetrated 1/2 thickness of the enamel (0.5 mm) within 20 minutes. Thiourea (a toxic substance not suitable for oral use) and acetamide penetrated the entire thickness of the enamel- (1.1 and 0.3 mm, respectively) within 34 minutes. After a slightly longer period, all 4 compounds penetrated the dentoenamel junction at intervals and went into the dentin. In a few teeth the pulp was reached. Chromatographic examination of dentin extracts from 2 teeth, to the enamel surfaces of which C14 had been applied, identified the compound which penetrated as the urea molecule.