The Influence of High- and Low-molecular-weight Inhibitors on Dissolution Kinetics of Hydroxyapatite and Human Enamel in Lactate Buffers: A Constant Composition Study

Abstract

The kinetics of dissolution of hydroxyapatite (HAP) and human enamel (HE) powders in lactate buffers was investigated by means of a Constant Composition technique. The rates were interpreted in terms of a surface-controlled dissolution mechanism. It was shown that the overall dissolution of both HAP and HE crystallites in lactate buffers resulted from an interplay of the calcium chelating action of lactate ion, its interaction with surface calcium sites, and the retardation effects of both ionized (L^-) and un-ionized (HL) lactic acid due to adsorption at phosphate surface sites. The presence of lactate in demineralizing solutions also influenced the "acquired crystallite resistance" for dissolution, and the rates decreased as the reaction proceeded. The influence of surface protective agents [methanehydroxydiphosphonate (MHDP) and polyacrylic acid (PAA)] on HAP and HE dissolution rates was also extensively studied in partially saturated lactate buffer (0.05 mol L^-1). Although the overall kinetic effects of both additives appeared to be similar, there were considerable mechanistic differences. MHDP might interact with lactate by competition for surface calcium sites. In contrast, PAA appeared to act independently of lactate under the experimental condition used. This would tend to support the recommendation of White (1987), that PAA was more suitable as a surface protective agent for artificial caries lesion preparation.

Get full access to this article

View all access options for this article.

References

Arends, J. (1982): Mechanism of Dental Caries. In: Biological Mineralization and Demineralization, G.H. Nancollas , Ed., Berlin: Springer Verlag, pp.79-101.

Arends, J. and Christoffersen, J. (1986): The Nature of Early Caries Lesions in Enamel, J Dent Res 65:2-11.

Budz, J.A. ; Lo Re, M. ; and Nancollas, G.H. (1987): Hydroxyapatite and Carbonated Apatite as Models for the Dissolution Behavior of Human Dental Enamel, Adv Dent Res 1 :314-321.

Budz, J.A. and Nancollas, G.H. (1988): The Mechanism of Dissolution of Hydroxyapatite and Carbonated Apatite in Acidic Solutions, J Cryst Growth (in press).

Chen, W.C. and Nancollas, G.H. (1986): The Kinetics of Dissolution of Tooth Enamel-A Constant Composition Study, J Dent Res 65:663-668.

Christoffersen, J. (1980): Kinetics of Dissolution of Calcium Hydroxyapatite; III. Nucleation-controlled Dissolution of a Polydisperse Sample of Crystals , J Cryst Growth 49:29-44.

Davies, C.W. (1962): Ion Association, London: Butterworths.

De Rooij, J.F. and Nancollas, G.H. (1984): The Formation and Remineralization of Artificial White Spot Lesions: A Constant Composition Approach, J Dent Res 63:864-867.

Eanes, E.D. (1979): Enamel Apatite: Chemistry, Structure and Properties , J Dent Res 58(B):829-836.

10.

Featherstone, J.D.B. ; Duncan, J.F. ; and Cutress, T.W. (1979): A Mechanism for Dental Caries Based on Chemical Processes and Diffusion Phenomena During in vitro Caries Simulation on Human Tooth Enamel, Arch Oral Biol 24:101-112.

11.

Gill, J.S. and Varsanik, R.G. (1986): Computer Modeling of the Specific Matching Between Scale Inhibitors and Crystal Structure of Scale Forming Minerals, J Cryst Growth 76:57-62.

12.

Koutsoukos, P.G. ; Amjad, Z. ; Tomson, M.B. ; and Nancollas, G.H. (1980): Crystallization of Calcium Phosphates. A Constant Composition Study, J Am Chem Soc 102:1553-1557.

13.

Margolis, H.C. ; Murphy, B.J. ; and Moreno, E.C. (1985): Development of Carious-Like Lesions in Partially Saturated Lactate Buffers, Caries Res 19:36-45.

14.

McDowell, H. ; Wallace, B.M. ; and Brown, W.E. (1969): The Solubilities of Hydroxyapatite at 5, 15, 25 and 37°C, IADR Prog and Abst 48: No. 340.

15.

Nancollas, G.H. and Mohan, M.S. (1970): The Growth of Hydroxyapatite Crystals, Arch Oral Biol 15:731-745.

16.

Nancollas, G.H. and Tomson, M.B. (1978): Mineralization Kinetics. A Constant Composition Approach, Science 200 : 1059-1060.

17.

Patel, M.V. ; Fox, J.L. ; and Higuchi, W.I. (1987): Effect of Acid Type on Kinetics and Mechanism of Dental Enamel Demineralization, J Dent Res 66:1425-1430.

18.

Pearce, E.I.F. (1983): A Microradiographic and Chemical Comparison of in vitro Systems for the Simulation of Incipient Caries in Abraded Bovine Enamel, J Dent Res 62:969-974.

19.

Perrin, D.D. (1979): Stability Constants of Metal-Ion Complexes; Part B: Organic Ligands. IUPAC Chemical Data Series-No. 22, Oxford: Pergamon Press.

20.

Plasschaert, A.J.M. ; Morch, T. ; and Konig, K.G. (1972): Effect of Sodium Lactate under Conditions of Neutral pH on the Release of Calcium from the Enamel Surface in vitro, Caries Res 6:334-345.

21.

Rolla, G. (1977): Formation of Dental Integuments-Some Basic Chemical Considerations, Swed Dent J 1: 241-251.

22.

Sillen, L.G. and Martell, A.E. (1971): Stability Constants of Metal-Ion Complexes. Special Publication No. 25, London : The Chemical Society.

23.

Voegel, J.C. ; Gillmeth, S. ; and Frank, R.M. (1983): Calcium Release from Powdered Enamel and Synthetic Apatite after Pre-treatment with Various Low Molecular Weight Organic Acids , Caries Res 17:212-220.

24.

White, D.J. (1987): Use of Synthetic Polymer Gels for Artificial Carious Lesion Preparation, Caries Res 21:228-242.

25.

Ziolkowski, F. and Perrin, D.D. (1977): Dissolution of Urinary Stones by Calcium-chelating Agents; A Study Using a Model System, Investigative Urology 15:208-211.