Quantitative Study of Fluoride Transport During Subsurface Dissolution of Dental Enamel

Abstract

Previous studies using bovine dental enamel as a model have shown that surface and subsurface dissolution of enamel may be governed by micro-environmental solution conditions. We have now investigated the demineralization phenomenon more rigorously with the primary objective of developing a method for deducing solution species concentration profiles as a function of time from appropriate experimental data. More specifically, in this report, a model-independent method is described for determination of the pore solution fluoride gradients in bovine enamel during subsurface demineralization. Microradiography was used to determine the mineral density profiles, and an electron microprobe technique to determine total fluoride (F) profiles associated with the enamel. In each case, matched sections of bovine enamel were exposed to partially saturated acetate buffers at pH = 4.5 containing 0.5 ppm F for various periods of time (from six to 24 hours). The treated enamel was found to have an intact surface layer and subsurface demineralization. The extent of the demineralization and the depths of the lesions increased with time in all cases. The data were farst used to calculate (a) the total F gradients in the enamel at various times, and (b) the local uptake rate of F as a function of time and position. Then, by manipulation of the equations describing the uptake and transport of F, we calculated the pore diffusion rate of F and the micro-environmental solution F concentration in the aqueous pores as a function of time and of distance from the enamel surface. It was also possible to calculate an intrinsic F diffusion coefficient in the pores, which was about 1.0 × 10^-5 cm²/ sec, in good agreement with reported values. ¹⁴C-sucrose uptake and release experiments with identically prepared demineralized enamel sections were also conducted to provide an independent check on the assumed dependence of porosity on mineral density. The results of this investigation, especially the outcomes relative to this new method for determination of pore solution F gradients during acid attack of the dental enamel, should be valuable in future studies of the mechanism(s) of the action of F in inhibiting dental enamel demineralization.

Get full access to this article

View all access options for this article.

References

Bergstrom, D.H. ; Fox, J.L. ; and Higuchi, W.I. (1984): Quantitative Microradiography for Studying Dental Enamel Demineralization and Remineralization, J Pharm Sci 73:650-653.

Bergstrom, D.H. (1985): Inter-crystalline Solution lon Activity Product Basis of Subsurface Dissolution in a Hydroxyapatite System, Ph.D. Thesis, University of Utah, Salt Lake City, Utah.

Carslaw, H.S. and Jaeger, J.C. (1959): Conduction of Heat in Solids, 2nd ed., Oxford: Clarendon Press , p. 326.

Chu, J.S. ; Fox, J.L. ; Higuchi, W.I. ; and Nash, W.P. (1989): Electron Probe Microanalysis of Subsurface Demineralization and Remineralization of Dental Enamel, J Dent Res 68:26-31.

Crommelin, D.J. ; Higuchi, W.I. ; Fox, J.L. ; Spooner, P.J. ; and Katdare, A.V. (1983): Dissolution Rate Behavior of Hydroxyapatite-Fluorapatite Mixtures, Caries Res 17:289-296.

Faxen, H. (1922): Widerstand gegen die Bewegung einer Starren Kugel in einer zahen Fliissigkeit, die Zwischen zwei Parallellen Ebenen Wanden Eingeschlossen ist, Ann Phys 68:89-119.

Fawzi, M.B. ; Fox, J.L. ; Dedhiya, M.G. ; Higuchi, W.I. ; and Hefferren, J.J. (1978): A Possible Second Site for Hydroxyapatite Dissolution in Acidic Media, J Colloid and Interface Sci 67:304-311.

Fox, J.L. (1977): Diffusional Controlled Processes in Complex Chemical Systems, Ph.D. Thesis, University of Michigan, Ann Arbor, MI-

Fox, J.L. ; Higuchi, W.I. ; Fawzi, M.B. ; and Wu, M.S. (1978): A New Two Site Model for Hydroxyapatite Dissolution in Acidic Media, J Colloid and Interface Sci 67:312-330.

10.

Fox, J.L. ; Bergstrom, D.H. ; and Higuchi, W.I. (1988): Physical Model for Lesion Formation in the Presence of Low Levels of Solution Fluoride, J Pharm Sci (submitted for publication).

11.

Fox, J.L. and Chen, J.S. (1988): Laplace Transform Solution of Multi-Laminar Non-Steady-State Diffusion Problems, Int J Mass and Heat Trans (submitted for publication).

12.

Gee, A. ; Domingues, L. ; and Dietz, V. (1954): Determination of Inorganic Constituents in Sugar , Anal Chem 26:1487-1492.

13.

Goldberg, A.H. and Higuchi, W.I. (1968): Improved Method for Diffusion Coefficient Determination Employing the Silver Membrane Filter, J Pharm Sci 57:1583-1585.

14.

Higuchi, W. t.; Gray, J.A. ; Hefferren, J.J. ; and Patel, P.R. (1965): Mechanism of Enamel Dissolution in Acid Buffers , J Dent Res 44 :330-341.

15.

Higuchi, W.I. ; Mir, N.A. ; Patel, P.R. ; Becker, J.W. ; and Hefferren, J.J. (1969): Quantitation of Enamel Demineralization Mechanisms: III. A Critical Examination of the Hydroxyapatite Model, J Dent Res 48 :396-409.

16.

Levich, V.G. (1962): Physical Hydrodynamics, New York: Prentice-Hall, Inc., p. 71.

17.

Liang, Z.S. (1971): Kinetics and Mechanism of Fluoride Uptake by Hydroxyapatite , Ph.D. Thesis, University of Michigan, Ann Arbor, MI.

18.

Ludwig, A. ; Dave, S.C. ; Higuchi, W.I. ; and Fox, J.L. (1982): Influence of Fluoride Ions on the Dissolution Kinetics of Apatite Powders, J Pharm Belg 37:141-149.

19.

Ludwig, A. ; Dave, S.C. ; Higuchi, W.I. ; Fox, J.L. ; and Katdare, A. (1983): Dissolution Rates of Apatite Powders in Acidic Fluoride Solutions and the Relationship to Hydroxyapatite Disk and Bovine Enamel Behavior, Int J Pharm 16:1-10.

20.

Mir, N.A. ; Higuchi, W.I. ; and Hefferren, J.J. (1969): The Mechanism of Action of Solution Fluoride upon the Demineralization Rate of Enamel, Arch Oral Biol 14:901-920.

21.

Patel, M.V. ; Fox, J.L. ; and Higuchi, W.I. (1987): Physical Model for Non-steady-state Dissolution of Dental Enamel, J Dent Res 66:1418-1424.

22.

Piessens, R. and Huysmans, R. (1984): Automatic Numerical Inversion of the Laplace Transform, Algorithm 619, ACM Trans Math Softw 10 :348-353.

23.

Press, W.H. ; Flannery, B.P. ; Teukolsky, S.A. ; and Wetter-Ling, W.T. (1986): Numerical Recipes: The Art of Scientific Computing, New York: Cambridge University Press.

24.

Stineman, R.W. (1980): A Consistently Well-behaved Method of Interpolation , Creative Computing 6:54-57.

25.

Van Dijk, J.W.E. ; Borggreven, J.M.P.M. ; and Driessens, F.C.M. (1983): Diffusion in Mammalian Tooth Enamel in Relation to the Caries Process, Arch Oral Biol 28:591-597.

26.

Wu, M.S. ; Higuchi, W.I. ; Fox, J.L. ; and Friedman, M. (1976): Kinetics and Mechanism of Hydroxyapatite Crystal Dissolution in Weak Acid Buffers Using the Rotating Disk Method, J Dent Res 55:496-505.

27.

Zahradnik, R.T. and Moreno, E.C. (1975): Structural Features of Human Dental Enamel as Revealed by Isothermal Water Vapor Sorption, Arch Oral Biol 20:317-325.