Caries Management - Science and Clinical Practice. Группа авторов

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СКАЧАТЬ At the lesion front is a zone that can be seen in about 50% of all samples.¹⁰ It appears brighter than healthy, sound enamel and shows hardly any structural features in transmitted light microscopy using an index matching fluid (a fluid with a refractive index close to enamel, i.e., quinoline with n = 1.625 at 589nm). This translucent zone contains porosities larger than the size of quinoline molecules that are mainly located at the prism boundaries.¹¹ The pore volume in sound enamel is around 0.1%. In the translucent zone the pore volume is increased up to 1%¹² (Fig. 3.11). Examined with polarized light microscopy, the translucent zone, like sound enamel, shows negative birefringence. In the translucent zone, crystals of slightly smaller size than in sound enamel (40nm diameter) can be found, indicating initial demineralization.

      The next zone after the translucent zone toward the enamel surface is commonly named the dark zone and can be found in most sections through enamel lesions.¹⁰ It appears dark-brownish in transmitted light microscopy using quinoline as an index matching fluid. This indicates the presence of porosities smaller than the size of quinoline molecules. Because these small porosities remain filled with air, light scattering is increased in this area leading to more backscattering, which explains the dark appearance in transmitted light and bright appearance in reflected light microscopy. Presence of small porosities, besides larger porosities that can be filled with quinoline, is also confirmed by polarized light microscopy. Due to the presence of these air-filled small porosities, negative birefringence, typical for sound enamel and the translucent zone, turns into positive birefringence in the dark zone. Using an index matching fluid of smaller molecular size (Thoulet solution, n = 1.62), the dark zone becomes bright in transmitted light microscopy and negatively birefringent in polarized light microscopy. The pore volume of the dark zone has been estimated to be 2%–4%. Demineralization in the dark zone was also found to affect prism boundaries, but to a greater extent than in the translucent zone. There is some evidence that the small porosities (that cannot be penetrated by quinoline) are partly remineralized larger porosities.¹⁰ An indication that remineralization is part of the processes taking part within the dark zone is the presence of crystals of larger diameter (50–100nm) as compared with the crystals of sound enamel (40nm).¹³ Recently, the presence of exogenous lipids and proteins in the porosities within the dark zone has been reported and it was concluded that they may hamper mineral precipitation and thus remineralization.¹⁴

      The next zone toward the enamel surface is the body of the lesion, which shows the highest degree of demineralization within the carious lesion. The body of the lesion is present in all samples prepared from enamel caries lesions. This zone is clearly visible by microradiography (see below) but also with clinical radiography when not superimposed by too high an amount of sound dental hard tissue (see Chapter 6). Using imbibition in quinoline as an index-matching fluid, the body of the lesion appears bright, similar to sound enamel in transmitted light, meaning the presence of porosities with a relative large diameter. In polarized light microscopy the body of the lesion shows positive birefringence (as opposed to negative birefringence of sound enamel). Using water as an imbibition medium that has a refractive index different from enamel mineral (n_water= 1.333), the body of the lesion stays positive birefringent using polarized light microscopy (sound enamel is negative birefringent under these conditions). In this zone striae of Retzius are clearly visible. Crystals become partly or completely dissolved along the prism boundaries but crystal dissolution can also be observed within the prisms (see TEM below). The pore volume of all zones within enamel is highest in the body of the lesion, being 5%–25% and higher.¹⁵

      The outermost zone of enamel caries is the surface zone, sometimes referred to as the intact or pseudo-intact surface layer with a typical thickness of 10–50μm, but it may extend up to 120μm.¹⁶ Its unique property is a relatively high mineral content and a pore volume that can be below 5% due to remineralization processes (see Chapter 2). Therefore, crystal size is somewhat larger (40–80nm) than in sound enamel. The surface zone is clearly visible using microradiography (see below). Using polarized microscopy, the surface zone is negative birefringent, being the same as sound enamel using water as the imbibition medium (n = 1.333) and also (in most cases) using no additional imbibition medium (air with n = 1.000).

      Fig. 3.10a–c Light microscopy of a histological section through an enamel caries lesion not reaching the enamel–dentin junction.

      a Transmitted light microscopy shows (starting at the outer surface) an intact surface layer (SL), the body of the lesion (BL), the dark zone (DZ), and the translucent zone (TZ).

      b Polarized light microscopy of the same specimen. The translucent zone and parts of the surface zone show positive birefringence. The body of the lesion is negative birefringent.

      c Polarized light microscopy using a lambda-plate. The translucent zone and parts of the surface zone show positive birefringence. The body of the lesion is negative birefringent.

       NOTE

      The classical caries zones of enamel caries with an intact surface have been identified on the basis of differences in the amount and size of porosities using transmitted light microscopy (starting at the outer enamel surface):

      • Surface zone

      • Body of the lesion

      • Dark zone

      • Translucent zone

      Using imbibition media of varying optical refractive indices with transmitted light microscopy or polarized light microscopy, information on the size of the porosities and the pore volume within sound and carious enamel can be obtained.

      Fig. 3.11 Schematic drawing of the different zones of enamel caries not reaching the enamel–dentin junction, porosities (lower part of the drawing), and pore volume (in parentheses). SL: surface layer; BL: body of the lesion; DZ: dark zone; TZ: translucent zone.

       Fig. 3.12a, b

      a Microradiograph (detail of a TMR x-ray exposure) of a surface-perpendicular cut through an artificial, early caries lesion of the enamel. The dotted line encloses the area that was analyzed and is represented in (b). The dotted square encloses 265 × 277μm. A thin reasonably well mineralized surface layer and the lesion body are clearly visible.

      b Microradiogram of the microradiograph in (a). The mineral profile shows a typical artificial enamel caries with the mineral-rich surface layer (SL), the body of the lesion with the lesion body point (BL), the lesion depth (LD) at 73μm, the sound mineral content at 87vol%, and the integrated mineral loss (ΔZ) which is the blue area above the mineral profile. Natural caries lesions in their early state show the same features, albeit with a lesion body which is less demineralized, and a wider and higher mineralized surface layer.

      Transverse Microradiography СКАЧАТЬ