Surface layer of natural enamel caries: PLM features under water and air immersion

  It is commonly considered that the surface layer fo natural enamel caries (NEC) is relatively unaffected, similar to normal enamel. Most of the basis come from features in polarized light microscopy (PLM) (negatively birefringent under water immerison)  and transverse microradiography (TMR) (radiopaque). Almost all textbooks and review articles on histopathology of enamel caries deal with the surface layer as an evidence of remineralization, and as a layer realtively similar to normal enamel. This is the main message propagated in the academic field.  However, the surface layer is not a universal feature of NEC. In addition, NEC with surface layer showing negative birefringence under water immeriosn do not behave in the same way as normal enamel when the immersion medium is changed. Some examples follow below.

Fig. 1. Three NEC lesions (A and B; C and D; E and F), in the midcrown region and occlusal region, photographed in a PLM with a Red I interference filter of 540 nm (red background) under water (A, C and E) and air immersion (B, D and F). Lesions in A and C show continuous surface layer, while lesion E show a discontinued one. After dehydration, surface layer of all lesions became either opaque (dark colour) ou positively birefringence (orange colour). 

     When immersed in air, all lesions had either a reversal of the sign of bireferingence (from negative to positive) of became opaque (dark). Positive bireffringence in air is not a feature of normal enamel in that regions of the tooth. And normal enamel does no get opaque (dark against a red background) when dried. The dark colour cannot be considere as isotropy. Isotropy means the same colour of the background. The dark colour is consistent with light being reflected backwards to the base of the microscopy. This is a feature of highly porous enamel or normal enamel heated to 500º C. Thus, surface layer cannot be considered always as relatively unaffected and similar to normal enamel. Is it correct to propagate the message that surface layer is similar to normal enamel? Those trying to learn histopathology of enamel caries are being erroneously educated by current textbooks.