No abstract

The mineral in bones and teeth is an impure form of hydroxylapatite (HAP), the principal impurity being 2—5 wt.% carbonate. This mineral dissolves during remodelling of bone and also in dental caries as a result of the action of acids produced by osteoclasts and by bacteria, respectively. In enamel, demineralization proceeds with preferential loss of carbonate relative to phosphate. Surprisingly, in the early stages, the demineralization is subsurface. In order to facilitate the understanding of physical chemical aspects of these processes, we have undertaken studies of demineralization in model systems. We give three examples here. The first two used scanning microradiography in which the specimen is stepped across a 10—30 μm diameter X-ray beam. Intensity measurements allow calculation of the mineral mass per unit area in the X-ray path through the specimen. In the first experiment, porous HAP sections were separated from a reservoir of acidic buffer by a column initially filled with water (the diffusion length) and scanned with the X-ray beam perpendicular to the axis of the diffusion length. The rate of total loss of mineral along each profile was calculated from the scans. The rate of demineralization fell as the diffusion length increased. We believe the explanation is that the rate-controlling step is the diffusion of dissolved HAP away from the solid to the buffer reservoir. In the second experiment, demineralizing solution and water were pumped alternately, for equal lengths of time, past blocks of porous HAP or enamel. The X-ray beam was perpendicular to the exposed surface. As the rate of switching between solutions decreased, the mean rate of demineralization also fell. We propose that this effect is due to retention of acid in the pores of the HAP during the time when water flows, allowing further demineralization to take place during this time. The third study used X-ray microtomography, a form of 3D microscopy, to study the loss of mineral in compacted carbonate apatite powders. The powders were packed in six 10 mm internal diameter acrylic cylinders to a depth of 4 mm (after pressing). One end was covered with a porous polyethylene disc and each tube placed in acidic buffer for 70 days. Periodic examination by microtomography showed the development of subsurface demineralization. Infrared spectroscopy of the dissected-out surface layers showed preferential loss of carbonate over phosphate by comparison with deeper layers. Rietveld analysis of X-ray powder diffraction data showed changes in the crystallographic structures of the apatites between the initial and dissected-out apatite.

Subsurface demineralization in enamel caries is known to entail diffusion of reagents and products both within the lesion and within the plaque biofilm external to the lesion. However, development of a predictive mathematical model for subsurface demineralization is hindered by limited quantitative understanding of the effects of these diffusion processes. The purpose of this quantitative study was to investigate and understand the effect of external diffusion length on the rate of demineralization in a simple model system. Ten, 500-microm thick sections cut from a porous hydroxyapatite (HAP) pellet were inserted in scanning microradiography (SMR) cells. The exposed thin edges of the sections were initially separated by columns of water (diffusion lengths) of 0-0.9 cm from a 1-l reservoir of demineralizing buffer (pH 4). Buffer was found to diffuse from the reservoir through the increasing diffusion lengths to the exposed HAP surface, whilst dissolved product diffused along the reverse path. Rates of HAP loss (from SMR measurements) decreased as the diffusion length increased. Experimental data were fitted to a general diffusion-reaction model. This showed that the solution near the HAP surface was almost completely saturated with HAP, and that the diffusion of dissolution products, rather than of buffer species, was rate limiting.

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 τ (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 τ increased, and for large τ 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 τ. A mathematical model with parameters R₀ and Δt was developed, where R₀ is the rate of demineralisation for continuous flow, and Δt a time added to each τ/2 buffer exposure to account for its retention in the HAP pores. Experimental data fitted the model with Δt ∼8 to ∼10 min and with R₀ close to the rate observed for continuous buffer flow. The model predicts that the rate decreases and approaches R₀/2 as τ → ∞, 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.