Central Dark Line Research Articles

Intracrystalline structure of enamel crystals affected by caries.

Enamel crystals in the demineralized zones in early caries lesions of human teeth were observed by high-resolution electron microscopy. The enamel crystals frequently exhibited perforations in their centers and defects of various sizes on their lateral surfaces. There were a number of small electron-lucent spots, suggesting that the dissolution of crystals had taken place there. These spots were in especially large numbers near the central dark line. The central perforations, the lateral defects, and the small spots had a common habit which formed regularly along the crystalline a- and b-axes. In many cases, when the central dark line was seen, the perforations were located a few unit cells away from the line. The perforations seem to result from a fusion of small spots, which enlarge by involving other small spots. The lateral defect seemed to enlarge by removal of unit cells and progression along the a- and b-axes. In the regions where the small spots were present, however, the enlargement of the defects also progressed involving the spots. The central dark line seems to be rather resistant to dissolution. One of the main factors for the central perforation of the crystals is thought to be the presence there of especially large numbers of defective sites.

HRTEM of hydroxyapatite in carious human tooth enamel

Human dental enamel is composed 98% by weight of small, densely packed carbonated hydroxyapatite (HAP) crystals. These crystals have an unusual morphology and often contain a “central dark line” (CDL) defect. The HAP crystals in carious enamel are preferentially dissolved at the CDL defect. Bres et al. have identified three types of disorder in human tooth enamel crystallites near to the crystal midline: a) regions of lattice buckling and departure from hexagonal symmetry, b) dislocations and c) grain and twin boundaries. Nelson et al. have obtained very clear images of planar defects, similar to the CDL defects in human enamel, at the centers of crystals of certain synthetic carbonated apatites. In this present study HRTEM images have been obtained from carious regions of tooth enamel, and an analysis of the results is attemped in the light of previous work.

TEM study of the central dark line in enamel crystallites.

The dark line which is observed in enamel crystallites represents a planar defect involving a single 100 plane of the hydroxyapatite structure. It may occur in the majority of crystals throughout the enamel, although it is only observed in specific diffraction conditions. Its presence may be related both to the formation and growth of crystallites in the developing tooth and to the manner in which the crystallites dissolve during caries. TEM studies show clearly that the central defect is not a dislocation, stacking fault or lattice twin boundary. They further indicate that it cannot represent a structural twin boundary. The remaining possibilities are that it represents a substitution in the HAP lattice, most likely involving carbonate ion, or a separate compatible calcium phosphate phase.