Crown-down tip design and shaping.

The shaping affects of three types of stainless-steel files that differ only in tip shape were studied using curved canals in acrylic blocks. Ninety curved canals were divided by degree of curvature into three equal groups of 30. The canals in each group were shaped using a crown-down technique and files possessing one of the three different tip shapes. The tip shapes were biconical tip (Flex-R), conical tip (Mor-Flex), and pyramidal tip (Flex-O). The canals were photographed before and after shaping and the finished shape analyzed. The incidences of transportation, ledging, and instrument damage were recorded during instrumentation. National Institutes of Health image software was used to measure the longitudinal cross-sectional areas and cross-sectional diameters at specified canal depths. Statistical analysis showed a significant difference in the preparations between the three tip shapes. Biconical tip files (Flex-R) produced the least transportation and no ledges. Pyramidal tip files (Flex-O) produced the most transportation, ledges, and damaged files. Only pyramidal-shaped tips developed ledges and experienced file separation. The participants reported less difficulty shaping with a biconical-tipped file and the greatest technical difficulty with a pyramidal-shaped tip. This study suggests that during crown-down rotational instrumentation, biconical file tips maintain the original canal curvature better and more frequently than do conical and pyramidal file tips.