الفهرس 
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Abstract
NiTi instruments are used in rotational & reciprocation motion; might exerts force to the root canal walls,such force may increase the risk of dentinal defects in root dentine in all sections of the root. The dentinal defects can occur as craze lines, incomplete cracks, or complete cracks.
They act as areas of stress concentrations and may affect teeth longevity .
The aim of the present study was to compare the frequency of dentinal defects after root canal preparation with single file systems used in different kinematics. Forty five mandibular first molars were collected & the degree of curvatures were calculated for each tooth before they were evenly distributed among different groups. The access cavity and glide Path was done by manual files Iso K files # (10) & (15). In half of samples for each group. They were prepared by Hyflex EDM file system in all mesiobuccal canals and Reciproc in all mesiolingual canals.
Samples were classified as follows:
Teeth were randomly and equally distributed into three equal groups (n=15)
group 1: Teeth prepared with Hyflex (EDM) rotary system.
group 2: Teeth prepared with Reciproc reciprocating system.
group 3: Teeth left unprepared.
Then groups 1&2 were subdivided according to glide path establishment into two equal subgroups:-
Sub group A (n=15):glide path was established in mesio buccal canals for Hyflex EDM & mesio lingual canals for reciproc R25 using manual K files #10 .
Sub group B (n=15): root canals were prepared directly without establishment of a glide path.
After tooth sectioning and evaluation by stereomicroscope we concluded the following:
Sections prepared by Hyflex EDM and Reciproc both created complete, incomplete cracks, craze lines with no significant difference between them.
Results showed that no significant difference among groups in all different levels of the cut (apical, middle, and coronal) sections and establishment of glide path has no significant impact on the dentinal micro-crack formation.