Demineralisation of Permeable Hydroxyapatite with Alternating Water and Acidic Buffer: Scanning Microradiographic Study of Effect of Switching Period

Permeable hydroxyapatite (HAP) blocks were exposed for equal times alternately to pH 4.0 buffer and water for 237 h. Rates of HAP loss with time (determined from changes in X-ray attenuation) were measured as a function of switching period tau (the time for a complete cycle) from 0.5 to 6 h and with a continuous buffer flow. The mean rate of HAP loss decreased markedly as tau increased, and for large tau was about half the rate for continuous buffer flow. We propose that demineralising conditions through the depth of the HAP are influenced by the extent of retention of buffer within its pores which will depend on tau. A mathematical model with parameters R(0) and Deltat was developed, where R(0) is the rate of demineralisation for continuous flow, and Deltat a time added to each tau/2 buffer exposure to account for its retention in the HAP pores. Experimental data fitted the model with Deltat approximately 8 to approximately 10 min and with R(0) close to the rate observed for continuous buffer flow. The model predicts that the rate decreases and approaches R(0)/2 as tau --> infinity, as was found experimentally to be the case. This type of study could potentially give information about subsurface porosity and transport processes during acidic dissolution of permeable solids, for example in dental caries and dental erosion.