Objective To compare the effect of different nickel-titanium rotary files on fracture resistance of teeth by observing status of dentinal microcracks after root canal preparation with.Methods 6 5 mandibular anterior teeth with single oval canal were randomly divided into three experimental groups (n=20)and a control group (n=5),SAF、ProTaper Universal and ProTaper Next system were used to prepare root canal in experimental groups,while the control group were left unprepared.Roots were sectioned from 3,6,9 mm to the apex,its status of dentinal microcracks was evaluated under a stereomicroscope.Results Root fracture inci-dence rate of group ProTaper Universal was significantly higher than that of group SAF and group ProTaper Next (P<0.05). There were no statistically significant differences in incidence rate of dentinal microcracks between group SAF and group ProTaper Next (P>0.05).Conclusion Roots with oval shaped canal prepared with SAF and ProTaper Next produce less dentinal microc-racks,which decrease risk of vertical root fracture.

BACKGROUND:Amount of debris extruded apicaly is an important indicator to evaluate the effect of root canal preparation instrument. Studies have shown that debris extruded apicaly can cause post-treatment pain, and thus, reducing the amount of debris extruded apicaly can decrease the incidence of post-treatment pain. OBJECTIVE:To compare the root canal cleanness and the amount of apicaly extruded debris during root canal preparation with Self-adjusting files, ProTaper Next and ProTaper Universal. METHODS:Forty-five premolars with single canal were divided into three groups randomly, and then prepared by Self-adjusting files, ProTaper Next and ProTaper Universal, respectively. Debris extruded apicaly was colected, dried and weighed. Then roots were longitudinaly sectioned and observed under scanning electronic microscope to evaluate the cleanness of the root canal. RESULTS AND CONCLUSION:The amount of apicaly extruded debris in the ProTaper Universal group was significantly higher than that in the Self-adjusting files group and ProTaper Next group (P < 0.05), but there were no statisticaly significant differences between the groups of Self-adjusting files and ProTaper Next in apicaly extruded debris (P > 0.05). Scores on debris and smear layer were lower in the group of Self-adjusting files than the groups of ProTaper Next and ProTaper Universal (P < 0.05), but there was no difference between the latter two groups (P > 0.05). These findings indicate that the Self-adjusting files and ProTaper Next can extrude less debris beyond the apical foramen, which may decrease the incidence of post-treatment pain. Cleaning ability of Self-adjusting files is better than ProTaper Next and ProTaper Universal.