An ultrastructural study of bacterial invasion and tissue breakdown in human experimental root-surface caries

This study describes the structural features of the interface between microbial deposits and root cementum in actively progressing root-surface caries lesions developed experimentally in six elderly individuals. A total of 18 specimens was examined by microradiography, and a further 18 by light and transmission electron microscopy after intra-oral periods of one, two, and three months. All specimens showed various degrees of subsurface dissolution of mineral and bacterial invasion of the cementum. Although the microradiographic pattern of mineral loss was subsurface in nature, transmission electron microscopy showed dissolution of crystals in the outermost layers of the cementum, with a distinct gradient inward. Bacterial invasion occurred along the borders between bundles of relatively well-mineralized extrinsic collagen fibers in which the characteristic cross-banding remained intact. The pattern of bacterial invasion was influenced by the incremental lines and the cemento-dentinal junction. The invading bacteria were almost exclusively Gram-positive, of various shapes, and possessed thick, moderately electron-dense cell walls and electron-lucent "vacuoles" in the cytoplasm. It is concluded that because of pronounced mineral loss of the outermost cementum, accompanied by bacterial invasion, the surface of an active cementum caries lesion, as observed by transmission electron microscopy, is not identical to that seen in microradiograms.