Clinical and histological aspects of caries lesions on occlusal approximal and smooth surfaces

The occlusal surface is characterized by the pit and fissure systems, a favorable biofilm stagnation area where the bacterial accumulations receive the best protection against functional/mechanical wear (mastication, attrition, abrasion from brushing, flossing or toothpicks). Those aspects contribute to the high prevalence of caries on occlusal surfaces both in the primary and permanent dentition (Kidd & Fejerskov, 2004).

The complex anatomy of the occlusal surfaces requires professional special attention and deep understanding of how lesions develop on this surface. It is known that the deepest part of the fissure usually harbors non-vital bacteria or calculus (Ekstrand & Bj0rndal, 1997). An enamel caries lesion begins along the pits and fissures through acids diffusion from bacterial metabolism in the biofilm. This diffusion occurs through the side walls of the pits and fissures, guided by prisms direction and striae of Retzius. Histologically, the lesion forms in three dimensions and assumes the shape of a cone, with its base toward the enamel-dentin junction. Acids lead to the demineralization underneath the enamel surface and there is an enlargement in intercrystalline spaces, increasing its permeability. Over time, the surface porosity has increased and leads to a considerable increase of the lesion body (a subsurface lesion starts to form). Occlusal enamel breakdown is the result of further demineralization, thus leading to cavity formation (Nyvad et al., 2008).

The lesions on smooth surfaces result from an accumulation of biofilm along the gingival margins. Characteristically, those lesions follow the form of the gingival contour and can progress to form a cavity in enamel, and subsequently, that can extend through the dentin (Nyvad et al., 1999). In section, the smooth-surface lesion is conical as a result of systematic variations in dissolution along the enamel prisms. The conically shaped lesion represents a range of increasing stages of lesion progression, beginning with dissolution at the ultrastructural level at the edge of the lesion (Bj0rndal & Thylstrup, 1995).

On the approximal surfaces, the biofilm accumulation occurs in the region below the contact point between the contact face and the gingival margin. The lesion may extend to the buccal and lingual directions, following the gengival contour (Nyvad et al., 1999). Histologically, the initial lesion in the approximal surface has a triangular shape with its base toward the outer surface and the apex facing the enamel-dentin junction. As mentioned earlier, this is because the acid diffusion from the bacterial metabolism is determined by the distribution of the biofilm and follows the direction of the enamel prisms (Nyvad et al., 2008).

Caries lesion on approximal surfaces in primary teeth presents a rapid rate of progression due to the morphologic characteristics of these teeth, making its detection difficult. Primary teeth have thinner enamel and dentin, lower mineralization rate, large dentinal tubules and larger contact proximal areas, which allow greater biofilm accumulation, and consequently, leading to initiation and progression of dental caries (Mortimer, 1970; Pitts & Rimmer, 1992).

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