TIME STUDIES OF THE PENETRATION OF EXTRACTED HUMAN TEETH BY RADIOACTIVE NICOTINAMIDE, UREA, THIOUREA, AND ACETAMIDE

I. DIFFUSE PENETRATION FROM THE ENAMEL SURFACE

¹ Radiobiology Section, Los Alamos Scientific Laboratory of the University of California, Los Alamos, N. M.

1. Rapid diffuse penetration of grossly intact enamel by isotope-labeled solutions of nicotinamide, urea, thiourea, and acetamide was determined radio-autographically in twenty-three extracted human teeth.

2. Eight of the teeth were formol fixed. No effect of fixation, age, sex, or tooth location was observed.

3. Nicotinamide penetration was the slowest. The earliest diffuse penetration observed was through 0.4 mm. of enamel within seventy-nine minutes.

4. Urea, thiourea, and acetamide penetrated more rapidly. Urea penetrated one-half the thickness of the enamel (0.5 mm.) within twenty 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 thirty-four minutes.

5. After a slightly longer period, all four compounds penetrated the dentoenamel junction at intervals and went into the dentin. In a few teeth the pulp was reached.

6. Chromatographic examination of dentin extracts from two teeth, to the enamel surfaces of which C¹⁴-urea had been applied, identified the compound which penetrated as the urea molecule.