The water in human dental enamel and its diffusional exchange measured by clearance of tritiated water from enamel slabs of varying thickness.

Tracer clearance studies were used to measure the amount and mobility of the water in human dental enamel. Slabs of enamel whose thickness ranged from 0.4 to 0.8 mm were equilibrated with tritiated water in dilute neutral buffer at 35 degrees C for 1 week, rinsed, and allowed to exchange at the same temperature with stirred unlabelled buffer. This was sampled and counted periodically to allow calculation of diffusion coefficients from the clearance rate. Exchangeable water amounted to 2.0 +/- 0.4% w/w (6.0 +/- 1.2% v/v). Some samples were masked on one side to double the diffusion distance. Diffusion calculations assuming homogeneous diffusion gave anomalous coefficients varying with sample thickness, indicating a more complex system was involved. In contrast, analysis assuming a biphasic distribution of diffusion pathways was much more successful. This assumed slow exchange from an inner pore system 'A' (probably associated with the prism cores) followed by much faster diffusion along a minority of larger pores 'B', such as exist at the prism boundaries. The 'B' pore-volumes were based on past studies of porosity in enamel, and the 'A' volumes on this work. Analysis gave an overall effective diffusion coefficient (De) for thick samples of 4.3 x 10(-7) cm2 s-1. The results were consistent with a DB of 0.9 x 10(-5) cm2 s-1 and a rate-constant (kAB) for exchange between the two pore systems which, if operating over a half-prism width (say 2 microns) would correspond to an 'inner' (DAB) diffusion coefficient in the region of 6 x 10(-12) cm2 s-1.