Demonstration of the central dark line in crystals of dental calculus

Using an electron microscope and Fourier transform infrared (FTIR) microspectroscopy, we studied the lattice images of crystallites of dental calculus to demonstrate the presence of the central dark line (CDL) in its crystallite and to compare this CDL with that of bone and synthetic hydroxyapatite crystals. Ultrastructural observations revealed clearly a number of crystallites, which displayed a proper lattice image and CDL similar to that of bone, in the dental calculus. FTIR microspectroscopy revealed that the dental calculus displayed a set of major spectra analogous to that of bone. These results suggest that the formation process of hydroxyapatite crystals with CDL in dental calculus, which is considered to be an unusual type of calcified structure in association with microorganisms, is basically similar to that of the ordinary calcifying hard tissues (bone, enamel, etc.).