Surface layer of enamel caries: a new theory for its features

   The surface layer of enamel caries has been shown to present high gray levels in micro radiography and or negative birefringence in polarized light microscopy. It is well known, however, that such features are not universal. Enamel caries lesions, artificial and natural, can present a surface layer whose mineral content is similar or lower than that of the body of the lesion. They can also lack negative birefringence. Despite of this, it is commonly reported to present a permeability lower than that of the body of the lesion. This lower permeability is considered as a limiting factor for both remineralization and resin infiltration. Here it is discussed one possible explanation for the low permeability of the surface layer that is not dependent on either a higher mineral content (relative to they of the body of the lesion) of the surface layer or on negative birefringence. 

Permeability is the product of solubility and diffusion coefficients. When a single liquid infiltrating in a single solid, both solubility and diffusion are similarly affected by the same factors. This is the case when enamel is infiltrated by a single material (e.g..: remineralization solution, resin infiltrant). Different layers of enamel caries can be compared with regard to the infiltration of a single material. The proportion of enamel pores infiltrated by a given material can be reasonably considered as an indication of enamel permeability. Two layers of enamel caries presenting the same pore volume, say 15%, but different permeabilities are expected to present different proportions of their pores infiltrated by a given material. Thus, permeability can be estimated from the ratio of infiltrated pore volume (ALPHAinf) by the total pore volume (V2):

ALPHAinf/V2        (1).

  The actual infiltrated pore volume depends on quantitative data of volume constituents (mineral, water and organic) at a specific enamel layer. Recently, it was reported a methodology of measuring such volume constituents in enamel caries (Barbosa de Sousa et al., Caries Res 2013; 47(3): 183-92; DOI: 10.1159/000345378). The same paper reports a measure of permeability:

(ALPHAxALPHA)/V2     (2)

which is the total water volume (ALPHA) squared divided by V2. Barbosa de Sousa et al. report example of surface layer with low mineral content and low permeability (similar to that of normal enamel). This provides an explanation for the low permeability of the surface layer even in the cases when its mineral content is similar to that of the body of the lesion (the location of the higher permeability in enamel caries).  Surface layer of all lesions can be considered. The content found in De Medeiros et al. (J Microsc, 2012, 246(2): 177-89; doi: 10.1111/j.1365-2818.2012.03609.x) and Barbosa de Sousa et al. (Caries res, 2013) provide an explanation on how the surface layer can present a low mineral content combined with relatively high grays levels (in micro radiography) and negative birefringence (in polarized light microscopy). 

  Features of the surface layer, such as permeability and aspects under micro radiography and polarized light microscopy, are explained taking into account, in addition to the mineral volume, the water and organic volumes. Incorporation of organic matter from the oral cavity follows prism paths, and the surface layer is the closest one to the start of prisms paths on the enamel surface. These are the new parameters that make the new explanation possible.

In summary, the new explanation of the surface layer of enamel caries is that such layer presents a relatively low permeability. Such low permeability is a result of the combination of all enamel constituents (mineral, water and organic matter). This is supposed to be a universal feature of the surface layer. The theory predicts that identification of the contributing factors allows implementation of strategies that can improve permeability and, hence, improve remineralization and resin infiltration.