In vitro root caries progression measured by 125I absorptiometry: comparison with chemical analysis

Radiation from a 125I source and a non-image-forming detector was used for non-destructive measurements of root caries progression. Blocks were cut parallel to the cementum surface of unexposed human roots. These blocks were then individually demineralized in under-saturated calcium phosphate solutions over an 84-hour period. In order for the in vitro root surface demineralization to be followed, the changes in transmission (delta T) through the blocks were measured, by 125I absorptiometry, eight times during the course of the experiment. Chemical analyses of the calcium output (delta Ca) from the blocks into the demineralizing solutions were also performed, and the rate of demineralization (Vdem) was calculated from these values. The precision of 125I absorptiometry was calculated from 176 duplicate transmission measurements, and the coefficient of variation was found to be 0.20%. The correlation coefficient between delta T and total delta Ca for each of 22 cementum/dentin blocks ranged between r = 0.934 and r = 0.998. The progression of root hard-tissue lesions observed by these two methods and by the calculated Vdem was found to be proportional to the square and cubic roots of time. The study shows that 125I absorptiometry can be used for continuous non-destructive measurements of root hard-tissue demineralization in vitro.