[Effect of different hand instruments on the shape of the root canal]

Six different types of root canal instruments have been tested in vitro for alterations of the original shape of the canal during the manual preparation of curved root canals. Blocks of transparent Plexiglas with simulated curved canals were prepared using a modified conical canal preparation method. The alterations were documented with the aid of superimposed photographs. Without exception, all types of instruments resulted in a straightening of the canals. Zones of increased loss of tooth structure were found mesially in the apical and coronal thirds. In the middle third this zone was on the distal side. The more curved the canals, the more pronounced was the alteration of the original canal shape. Marked differences in enlarging, straightening and perforations were observed between the instruments tested in the apical and coronal thirds.