Characterization of a synthetic apatite sinter for study of diffusion processes during acidic dissolution of dental enam
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J. C. Elliott and P. Anderson Department of Biochemistry, The London Hospital Medical College, London El 2AD, England (Received 25 August 1986; accepted 23 December 1986) A sintered aggregate of hydroxyapatite, Ca ] 0 (PO 4 ) 6 (OH) 2 , designed for in vitro simulation of some of the physicochemical processes of dental decay is described and characterized. Diffusivity at 35 °C (by tritiated water effusion) was (2.9 + 0.6) X 1CT6 cmVs. The apparent density (2.50 + 0.04) g/cm 3 gave a porosity of 20.8 + 1.3 vol % based on the theoretical density of hydroxyapatite. Estimates of the volume percent porosity from water vapor desorption and tritiated water effusion were 19.2 + 2.9 and 20.7 + 1.4, respectively. Pore-size distribution measurements using mercury porosimetry showed that most of the porosity (approximately 17% of the sample volume) was concentrated between 30 and 60 nm equivalent pore radius. The material, which has been successfully used to simulate some aspects of the dental decay process, should find further application in this and related fields.
I. INTRODUCTION
II. EXPERIMENTAL
Tooth enamel consists principally of small crystals of hydroxyapatite, with a small amount of organic matter and water between them. In order to elucidate some of the physicochemical factors that affect acidic demineralization processes during the progress of caries (tooth decay), a great deal of effort is expended on in vitro studies. Samples of enamel, masked except for a window area on the natural surface, are placed in acidified solutions or gels, or in contact with acid-producing bacteria that are known to be the main causative agents in the caries process. The progress of demineralization in many of these systems seems, at least in the early stages, to be histologically identical with that of a natural carious lesion in which an intact surface zone obscures mineral loss from the underlying enamel. However, there are many problems associated with such studies, some of which come down to uncertainty about which of the characteristics of these caries-like lesions are the result of intrinsic factors (e.g., existing gradients of porosity, trace elements, or small amounts of organic material), and which result from the basic physicochemical interaction of acid buffers with the porous tooth mineral. Recently, with these and other considerations in mind, Anderson and Elliott1 reported a scanning x-ray microradiographic study of the formation of caries-like lesions in sintered synthetic apatite aggregates. Here we characterize the density, porosity, and permeability of the aggregates to help our understanding of the processes involved during these experiments and their computer simulation.2
The preparation of samples was as described previously. ' In brief, the calcium hydroxyapatite was prepared by dropwise addition of ammonium orthophosphate solution to calcium nitrate at 70 °C andpH 11. The precipitate was ground to a fine powder, compressed isostatically in an oil bath at 1500 atm (152 MPa) in the fingertip of a surgeon's glov
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