The White Spot Lesion
The first clinical signs (visible to the eye of the dental professional) of caries are the so-called “white spot” lesions. These lesions can be seen when plaque is removed from the enamel surface and this surface is dried with compressed air for a few seconds (Fig. 3.5). At a more advanced stage of disease, white spot lesions are visible also when the enamel surface is still wet. At this stage the caries process may have advanced through the entire enamel layer and into the dentin.2–4 White spot lesions appear white, because a greater proportion of the incoming light is being backscattered as compared with the surrounding sound enamel. This is due to an increase in pore volume, which is an increase of porosities in size and number when enamel becomes demineralized, and the difference of the refractive indices of air (or electrolyte) filling those porosities and adjacent enamel.
Activity of White and Brown Spot Lesions
When located interproximally, natural white spot lesions typically appear as oblong shapes below or around the contact point, where they cannot be viewed directly under normal conditions (Fig. 3.6). When located at the vestibular or oral aspect of the tooth just above the gingival margin, they are spread out in a line. In some instances, a whitish, sometimes discolored band can be found parallel to, but at some distance from, the gingival margin in teeth of the permanent dentition (Fig. 3.7). In this case, most likely, the lesion has developed when the tooth was not fully erupted, which also is a period during which tooth cleaning is often neglected. Following complete eruption, the local ecosystem may have changed due to better cleaning, which is why the demineralized band stabilizes, remaining an inactive white spot lesion, sometimes turning into a brown spot.
Fig. 3.8 Section through an interproximal (approximal) caries lesion exposed by operative treatment procedures. The enamel caries (arrow) is typically cone-shaped in appearance.
Fig. 3.9 Histological section through the crown of a premolar tooth with fissure caries without cavitation viewed by reflected light microscopy. Lesion development follows the prism orientation, leading to the shape of an inverted cone (“butterfly”). The deepest demineralization of this long fissure occurred at the fissure entrance, most likely due to a better nutritional situation for the plaque bacteria at this site.
When located occlusally, white spot caries lesions develop locally in particular in the fossa areas and grooves (Chapter 1).5–7 Quite often white spot lesions incorporate exogenous pigments from food, which adds a brownish tint (brown spot lesion). It is believed that exogenous pigments are incorporated with time, indicating that brown spot lesions have existed for longer already than white spot lesions. This would mean that brown discoloration is a sign of slow progression or low activity. Certainly, the pigmentation of white spot lesions relies on several factors like nutritional habits, surface porosity, and so on. In some instances, white spot lesions remain white for many years and therefore it cannot be assumed that the absence of brown discoloration in enamel lesions is a sign of high caries activity and fast progression.
A more reliable marker for lesion activity is the appearance of the surface of a white spot lesion following removal of the biofilm. While active lesions are in a state of nett loss of minerals, they have more porosities at the surface than lesions that are not active. Therefore, an active white spot lesion shows a matt, dull surface, while an inactive lesion presents a shiny surface.1,8 White spot lesions that are not covered by plaque are clinically almost certainly shiny and inactive, mainly due to incorporation of minerals (remineralization), and abrasion and polishing related to plaque removal.1
Lesion progression of white spot lesions within enamel roughly follows the rod/prism direction. The three-dimensional morphology of small white spot lesions on the approximal surface resembles a cone with its base at the enamel surface and its tip toward the enamel–dentin junction (EDJ) (Fig. 3.8). The three-dimensional morphology of the occlusal fissure lesion is with the tip at the surface and the base toward the EDJ due to the directions of the prisms in the fissure area (Fig. 3.9).
NOTE
The first clinically visible sign of caries-affected enamel is the white spot lesion, which can be as deep as several hundred micrometers. This lesion is composed of a porous body covered by a well-mineralized surface zone. The appearance and porosity of this surface layer is influenced by the lesion activity. Lesion development follows the direction of the enamel prisms.
Transmitted and Polarized Light Microscopy
Many of the findings from histology of the caries lesion were obtained using transmitted and polarized light microscopy. A caries lesion that is well developed within enamel and has not reached the dentin, and thus likely is not cavitated, has typical histological features. These features can also be found in part within lesions in a more advanced state with a cavitated surface.
BACKGROUND
In transmitted light microscopy thin sections of dental hard tissue can be viewed making use mainly of differences in scattering and absorption. Changes within enamel due to carious demineralization lead to a higher degree of scattering and backscattering, whereby these areas appear darker compared to the surrounding, sound areas. Additional incorporation of pigments leads to increased absorption that adds to the effect of backscattering.
Polarized light microscopy makes use of the optical anisotropy of crystals, that is, the ability of a crystal to split incident polarized light into two parts composed of waves that oscillate perpendicularly to each other and travel at different speeds, due to different refraction indices. This property is called birefringence. It occurs in many but not all crystalline materials, and also in regularly constructed organic materials, like some proteins. One of the two refractive indices is called the extraordinary refractive index (ne) which is defined as parallel to the optical axis of the crystal as opposed to the ordinary refractive index (no) which occurs perpendicular to the optical axis of that crystal.9 The crystal is optically isotropic only for light propagating parallel to the optical axis. For all other directions of incoming light the crystal is anisotropic, meaning that the incoming light wave is split into two waves that travel under the influence of two different refractive indices, no and ne, with their oscillating planes perpendicular to each other. For ne > no a crystal or object is referred to as being positive birefringent, for ne < no it is negative birefringent. In the presence of porosities alongside such crystals, the matter that fills these porosities (the imbibition medium, e.g., air, Canada balsam, quinoline, or other fluids) adds a third refractive index. The relationship between the three refractive indices and the pore volume in the object determines the maximum amount of light passing through the second polarizer that, within a polarization microscope, sits on the ocular side of the object. By variation of the imbibition media having different refractive