Chemical Aspects of Dental Caries

Abstract

The chemical processes which lead to the phenomenon of subsurface demineralization of hydroxyapatite-like human tooth enamel have been studied. This phenomenon is known as the early carious lesion, and is the precursor of carious cavity. The early lesion characteristically has an apparently intact surface layer overlying subsurface demineralization. Tooth crowns were cleaned and immersed for 0.5 to 60 days at 37°, without agitation, in a range of lactate buffers with pH from 4.0 to 7.0, with or without diphosphonate (MHDP). Examination by optical microscopy, microradiography, and scanning electron microscopy (SEM) showed that artificial carious lesions with the characteristics of natural caries were produced with MHDP present. Similar lesions were produced in enamel where the 'matured' outer enamel had been removed. Direct dissolution occurred without MHDP. Crystallographic changes in the surface layer during lesion formation were followed by X-ray diffraction. Synthetic hydroxyapatite tablets showed similar phenomena. Artificial lesions in human enamel were further examined by X-ray powder diffraction, SEM, electron microprobe analysis, high resolution lattice imaging, and hardness testing. Selected demineralizing solutions were analysed for calcium and phosphorus. Surface layer thickness was related to [MHDP] and to calcium diffusion, but independent of pH and lactic acid concentration. Middle zone depth was related to unionised lactic acid concentration [HL], and inner zone depth to [HL] and [H+]. The inner zone showed interprismatic dissolution and the middle zone both inter- and intraprismatic attack. Apparent diffusion coefficients of the order 10-10 cm2s-1 for middle and inner zone boundary movement are comparable with those for diffusion of small neutral molecules in water-filled polymers. A mechanism for dental caries formation is proposed which is based on the work reported in this thesis and the reports in the literature on phosphate and enamel chemistry: (i) Reversible adsorption of suitable species partly protects the outer enamel surface (in vivo, the acquired pellicle). (ii) Lactic acid (HL) and subsequently H3PO4, in unionised form, diffuse into enamel initially between prisms and then between crystals, through the water-filled pores of the organic matrix. (iii) H+ and L- both attack the more soluble enamel fraction, leaving well-formed apatite. (iv) Ion movement is inhibited and calcium and phosphorus diffuse outwards as aqueous unionised species Ca(H2PO4)2, CaHPO4, CaL2, CaCO3 and Ca(OH)2. (v) CaHPO4 precipitates in the inner zone and redissolves as the middle zone forms. (vi) An apparently intact surface layer is formed as calcium and phosphate, from subsurface dissolution, repair existing crystallites maintaining an equilibrium between rate of loss to the exterior and deposition in the surface layer. Subsurface dissolution continues. (vii) Fluoride may be transported as HF, CaF2, CaFPO3.