High resolution electron microscopy of the interface between dental calculus and denture resin

Dental calculus may grow on the denture surface. In order to demonstrate the mechanism of deposition, the interface between calculus and denture resin was investigated using a high resolution electron microscope. Ultrathin sections were also used for electron diffraction of selected areas to reveal any mineral phase. The mineral layers without mineralized bacteria adjacent to the denture surface revealed a marked variation in thickness and crystal shape. Three types of crystal shape were observed at the junction: needle-like, rod-like and plate-like crystals. High resolution electron microscopy (HREM) showed that both rod-like and plate-like crystals were an aggregation of fine crystallites. The lattice fringes of the fine crystallites were observed among the near atomic structures of resin polymer at the interface in all three types of crystals. The electron diffraction patterns of selected areas revealed that needle-like and rod-like crystals were composed of hydroxyapatite (OH-AP), while plate-like crystals were composed of a mixture of OH-AP and whitlockite. These findings indicate that, after saliva penetrates through the acrylic resin, calcium and phosphate ions in the saliva are trapped in the molecular chains of the resin polymer, while the local ion concentration then increases to reach supersaturation, whereas a spontaneous precipitation would occur at the superficial layer of the denture resin. Furthermore, a thin intermediate layer of crystallites might be indispensable for the scaffolding process in the calculus formation on the denture surface.