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High resolution electron microscopy of the junction between enamel and dental calculus.

Newly formed dental calculus grows on the enamel surface after the tooth crown is exposed to the oral environment. In order to demonstrate the crystal coherence, the junction between enamel and dental calculus was examined in the high resolution electron microscope. Ultrathin sections were also used for selected area electron diffraction to reveal any newly formed mineral phase. High resolution electron microscopy (HREM) revealed that lattice fringes of dental calculus crystals directly coincided with those of enamel crystals. Two types of coherence were identified at the junction: in one type, a dental calculus crystal contacted the side of an enamel crystal; in the other type, the former contacted the tip of the latter. Tilt boundaries between lattice striations showed varying degrees of curvatures in contact levels. A dislocation of lattice striations was also observed at the junctional regions. Selected area electron diffraction patterns of dental calculus were almost consistent with hydroxyapatite (OH-AP). These findings indicate that the elongation and/or enlargement of lattice fringes of calculus crystals during their growth brings direct coherence with enamel crystals. This suggestion is supported by the fact that it is clinically difficult to completely remove the dental calculus without the loss of the superficial layer of enamel.