The tangentially polished surfaces of the outer enamel under the projections of the marginal ridges and the basal tubercle in the lingual side of maxillary incisor teeth, extracted from elderly periodontic patients, were observed by scanning electron microscopy after EDTA etching. In general, EDTA selectively erodes the prism sheath regions; however, our observations reversed this. It indicates that the prism sheath regions may have been penetrated by organic materials with minerals derived from saliva due to an exposure of the old teeth to the oral cavities for a longer duration since eruption and also the attrition for a longer period. The etched surfaces more or less showed a disordered orientation of enamel prisms. At the junctions of such disorders, there were scattered abnormal-shaped prisms including a spiral, a circle, and double borders. Moreover, the 'inner' prism-like sheaths of the double bordered prisms occasionally showed a circle or a dotted remnant other than an arcade, whereas the circular prisms frequently contained an arcade, a spiral, or a smaller circle in the 'outer' sheaths. These abnormal-shaped prisms are probably formed by the prism orientation disorders following the variable morphological changes of Tomes' processes during the formation of their enamel projections.
The purpose of this study was to observe and compare the morphological changes of human enamel and dentin irradiated by Er : YAG, Nd : YAG and CO2 laser. Three enamel sections, two dentin sections of coronal portion and three dentin sections of root portion were prepared and each section was irradiated by three types of laser at the energy level of 300 mJ/pulse, 8 Hz; 2 W, 20 pps; and 1 W, CW respectively. The lased areas were observed by a telescopic light microscope and a scanning electron microscope. Er : YAG laser irradiation produced a white color zone and Nd : YAG laser irradiation produced a slight melt zone on each enamel surface. The dentinal tubules of coronal and root portion dentin sections were completely sealed by Er : YAG or Nd : YAG laser energy. The irradiated area of each enamel surface was changed into a white spot and severe carbonization was observed around the white spot in case of CO2 laser irradiation. Each dentin surface was melted and recrystalized with CO2 laser irradiation. Although some minor morphological heat damage changes were observed on the lased enamel and dentin surfaces irradiated by Er : YAG laser, these results suggested that the Er : YAG laser had the ability to remove enamel and dentin depending on laser conditions.