Dynamics of Carious Lesion
Caries involves the interaction over time of bacteria capable of producing acids, a substrate metabolizable by these bacteria, and host factors, including the tooth and its salivary environment.
The hypothesis of a caries-specific plaque, involving certain particularly cariogenic acidogenic species (and not all acidogenic bacteria) is now accepted (TenCate 2006).
Demineralization/Remineralization Process
The carious lesion results from a succession of ionic diffusion reactions and dissolution-precipitation of mineral elements which occur in an open system comprising different environments (Silverstone 1973, Hennequin 1999).
Schematically, the appearance of an initial lesion successively involves:
The presence of carbohydrates in saliva;
The diffusion of these carbohydrates in the biofilm and their transformation by bacteria,
Metabolic production of acids in the biofilm with release of H+ protons;
The diffusion of these ions through the biofilm to the enamel surface,
Their penetration into the aqueous gel which occupies the network of pores of the enamel;
Disruption of ionic balances at the interface (dental tissue/aqueous gel).
Subsequently, the ionic concentrations of hydrogen, calcium and phosphate in the biofilm will govern the exchanges. They will orient the balances towards demineralization or remineralization.
Demineralization is governed by the fact that mineral compounds obey the solubility product principle, with the compound with the lowest dissociation constant (pK value) controlling the reaction.
The dissolution of apatite is a complex phenomenon that results in the destruction of hydroxyapatite and the formation of phosphate ions, Ca²+ AND OH-.
The chemical reaction and the consumption of protons (H+) degrade the surface of the crystal; the ions produced, resulting from the degradation of the crystal, will diffuse towards the surrounding environment.
At a critical pH of 5.5, HA reacts on the surface with hydrogen ions.
We then observe, depending on the pH, a conversion of PO4³- into HPO4²-, H2PO4- or H3 PO4 and a simultaneous buffering of H+. In this situation, the equilibrium is broken and the HA crystal is dissolved.
When the enamel dissolves, a precipitate is formed from the surrounding aqueous solution which contains several ions, which precipitate tends to have the composition of the less soluble compound so as to re-establish an equilibrium.
Calcium phosphates are metastable, as soon as they are dissolved in different states, generating new reactions and so on.
The absorbed layer would tend to form a new surface layer that is less permeable and less soluble, promoting the slowing down and stopping of dissolution.
Thus, as soon as an initial lesion appears, it intrinsically tends towards passivation and reversibility, with local factors being able to counteract this tendency if they are aggressive or contributing to amplifying it when they are favorable.
Calcium and phosphate ions from saliva and plaque diffuse to the enamel where they can participate in remineralization.
This process is favored by the presence of fluorine ions at the reaction site.
This phenomenon of demineralization-remineralization is similar for dentin and cementum.
However, differences in histological structures and organic proportions of the tooth composition will considerably modify the nature and progression of the carious lesion.
1-3: Effects of fluorides
The presence of fluoride ions at the mineral reprecipitation site will promote the remineralization of the tooth, on the one hand by allowing the formation of fluorinated apatites and, on the other hand, by constituting reserves of calcium fluorides in the biofilm.
The interaction of fluorine with the HA surface occurs in several ways:
Acid dissolution of apatite is controlled by the diffusion of ions at the solid interface. During the initial times, fluorine accelerates the dissolution, leading to the accumulation at the interface of calcium and phosphate which will secondarily reduce the dissolution of apatite.
The inhibition of apatite dissolution is mainly explained by two phenomena : the adsorption of fluorine ions on the surface and the formation of CaF2.
Inhibition is more effective when fluorine is present at the interface and in solution. In solution, it saturates the aqueous phase relative to hydroxyapatite.
The reduction of acid dissolution is identical for organic fluorine ions (fluorinated amines) and mineral fluorine ions (NaF).
Dynamics of Carious Lesion
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