Dark and translucent zones of natural enamel caries: new article

  A new article was published on the histopathological features of the dark and translucent zones: http://onlinelibrary.wiley.com/doi/10.1002/jemt.22962/full#article-nav. New explanations for the dark and translucent zones are reported. This has implications on enamel caries remineralization and infiltration.

  THEORIES ON THE DARK AND TRANSLUCENT ZONES

1) CLASSICAL DESCRIPTION AND THEORY - DARLING A.I. (The selective attack of caries on the dental enamel. . Annals of the Royal College of Surgeons of england, 1961:354-369)
1.1    DARK ZONE:
     Location: always between the body of the lesion and the translucent zone.
     Mineral loss = 2-4%
     Pore sizes: large and small sizes, but most of them (~ > 50%) are small (innaccessible to quinoline)
     Interpretations: (a) results of further mineral loss relative to the translucent zone (Darling, 1961);
                          (b) result of remineralization of larger pores (Silverstone; Caries Res, 261-274, 1967)

1.2 TRANSLUCENT ZONE

     Location: at the fornt of the enamel caries lesions (between the dark zone and normal enamel).
     Mineral loss = 1%
     Pore sizes: large sizes (accessible to quinoline)
     Interpretations: (a) results of loss of organic material (Darling, 1961);
                          (b) result of early demineralization (Hallsworth, Robinson, Weatherell, Caries Res, 156-168, 1972).

2) NEW DESCRIPTION AND THEORY

2.1 DARK ZONE (de Mattos Brito et al., Micros Res Tech, 2017. Doi: 10.1002/jemt.22962)

     Location: within the body of the lesion and or between the body of the lesion and the translucent zone.
     Mineral loss = > 2-4%; mean mineral volume of 73% (± 8.92%; ranging from 55.15 to 91.14%)
     Pore sizes: appearance of enamel caries immersed in quinoline depends on the pore volume components:  a mixture of organic matter, firmly bound water, air, and quinoline.
     Interpretation: results of the combination of the enamel pore volumes in histological zones where transport of quinoline occurs mainly paralell to prisms' paths.

Example of component volumes at a histological site in the dark zone:
Mineral volume = 74.893%
Organic volume = 13.526%
Firmly bound water = 6.671%
Air volume = 0.982%
Quinoline volume = 3.928% (corresponds to 15.6% of the pore volume)
Total enamel volume = 100%

2.2 TRANSLUCENT ZONE

     Location: deep to the positvely birefringent (under water immersion) body of the lesion, but not necessarily at the fornt of the enamel caries lesion (lesion continues beyond the translucent zone).
     Mineral loss = > 1%; mean mineral volume of 77.83% (± 8.08%, ranging from 62.68% to 92.26%)
     Pore sizes: appearance of enamel caries immersed in quinoline depends on the pore volume components:  a mixture of organic matter, firmly bound water, air, and quinoline
     Interpretations: results of the combination of the enamel pore volumes in histological zones where transport of quinoline occurs mainly paralell to prisms' paths.

Example of component volumes at a histological site in the translucent zone:
Mineral volume = 74.684%
Organic volume = 15.094%
Firmly bound water = 6.321%
Air volume = 1.237%
Quinoline volume = 2.664% (corresponds to 10.5% of the pore volume)
Total enamel volume = 100%
 

The differences between the dark and translucent zones above are mainly due to a higher organic volume in the translucent zone. Different combinations of component volumes could result in pseudoisotropy (of the dark zone) and negative birefringence (of the translucent zone).

2.3 LATERAL CONNECTION OF PRISMS' SHEATHS

    Replacement of air by the infiltrating liquid (quinoline) is also influenced by the existence of lateral connection of prims' sheaths (the main pathways for transport of materials). Figure 1 shows two anatomical lateral relationships between prisms' sheaths: without lateral connection (A and B) and with lateral connection (C and D). Both can be found in different histological zones of the enamel caries lesion. Lateral connection facilitates the replacemt of air by quinoline, resultng in negatively birefringent zone. The lack of lateral connection results in more entrapment of air in the pores, reducing the penetration of quinoline and, thus, favoring formation of a dark zone (pseudoisotropic of positively birefrngent zone). Dark zone can be formed in areas of the body of the lesion where there are no lateral connections of prism's sheaths.
   Translucent zone lacks lateral connection, but present higher mineral and organic volumes than the dark zone, resulting in a negatively birefringent zone.

 

Fig. 1 Two types of anatomical relationships between prisms' sheaths. A and B, without lateral connection, where the main transport pathway is paralell to the axes of the prisms. C and D, with lateral connection of prisms' sheaths, resulted of increased pore sizes, where the main transport pathways run both paralell and perpendicularly to the axes of the prisms. The later configuration facilitates the replacement of air by quinoline.

    Figure 2 shows histological features of one natural enamel caries lesion under water, air, and quinoline immersion media.

Figure 2. Histological zones of natural enamel caries lesion (proximal surface) under various immersion media (water, air, and quinoline), showing dark zone involving most of the body of the lesion. 

Reference

De Mattos Brito CS, Meira KRS, De Sousa FB: Natural enamel caries in quinoline: volumetric data and pattern of infiltration. Microsc Res Tech. 2017; doi: 10.1002/jemt.22962.