الفهرس 
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Abstract
Summary
Root canal preparation using NiTi instruments could damage the dentinal wall. Dentinal defects are in the form of cracks either complete or incomplete cracks, these cracks could lead to areas of stress concentration and vertical root fracture.
The aim of this study was to compare the incidence of dentinal crack formation after root canal preparation with ProTaper Next, ProTaper Gold and TruNatomy rotary instruments. Ninety five freshly extracted human lower first molars were collected for this study, the degree of mesial root curvature was measured for every tooth before they were distributed evenly among the experimental groups. Access cavity was done for each tooth, length was determined before instrumentation and mesio-buccal canals were instrumented by ProTaper Next, ProTaper Gold and TruNatomy files.
Samples were classified as follow:
group I: normal control group (unprepared teeth) 5 teeth, 5 canals.
group II: In which ProTaper Next system was used for mechanical preparation, consisted of 30 teeth, 30 canals.
group III: In which ProTaper Gold system was used for mechanical preparation, consisted of 30 teeth, 30 canals.
group IV: In which TruNatomy system was used for mechanical preparation, consisted of 30 teeth, 30 canals.
Each experimental group was subdivided into two equal subgroups, 15 canal each.
• Subgroup A: Teeth with straight or moderately curved roots.
• Subgroup B: Teeth with severely curved roots.
Teeth were sectioned at 3mm, 6mm and 9mm from the apex and examined under stereomicroscope to determine the presence or absence of dentinal cracks and the extent of the crack.
Teeth prepared with ProTaper Next showed highest percentage of cracks, followed by ProTaper Gold and TruNatomy samples.
Teeth with severely curved roots showed more cracks specially in teeth prepared with ProTaper Next in compared to straight or moderately curved roots.
Conclusion
Based on the results of this study it could be concluded that:
1) ProTaper Next, ProTaper Gold and TruNatomy rotary files may cause dentinal cracks.
2) ProTaper Next and ProTaper Gold rotary files tend to produce more cracks than TruNatomy file.
3) Root curvature might increase the liability for crack formation.
4) The highest percentage of cracks was in middle section followed by apical, while the lowest percentage was in coronal section.
5) For the three files the highest percentage of incomplete cracks was found in the apical section of severely curved roots while the highest percentage of complete cracks was found in the middle section of severely curved roots.