الفهرس 
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Abstract
Summary and Conclusions
Three different NiTi rotary instruments that were manufactured from conventional NiTi, M-wire, and CM-wire – namely- OneShape, Protaper Next, and Hyflex EDM respectively; were compared for their cyclic fatigue resistance and metallurgical characterization. Study was done on new unused instruments and those used in instrumentation of one and three simulated root canals to reveal the effect of use on the instruments properties.
The study also compared between contemporary and recently invented innovative variations in cross section shape and movement envelop of the mentioned instruments on the canal cleanliness in terms of débris and smear layer using SEM.
The artificial canal used in our study for cyclic fatigue testing was carved in a tempered stainless steel block by laser and the cyclic fatigue testing was done using the static mode of cyclic fatigue.
Simulated canals in acrylic blocks model was used in this study to allow for a standard lengths, curvatures, radii, as well as texture.
Studied parameters for metallurgical characterization were: Fractography, EDS Spectra, microhardness, microstructure, and XRD.
The following conclusions can be drawn:
• HF and PN showed a significant decrease in NCF after three times use denoting a clear evidenced reduction in resistance to cyclic fatigue resistance with clinical use. OS did not show a significant decrease in NCF.
• Fracture surfaces revealed collectively similar general features that pertain to fatigue type failure in all tested instruments irrespective of the material type.
• XRD revealed that all the three tested instruments showed comparable major peaks for the (110) atomic planes in austenite. The pattern of XRD showed slight change after simulated clinical use in HF (EDM) only, while remained nearly unchanged in both OS and PN instruments.
• A significantly lower Ni and Ti were found in HF (EDM) as compared to both OS and PN instruments as revealed from the EDS Spectra.
• An interesting EDS result was the finding of Oxygen impurity in HF (EDM) instruments-possibly related to the Spark finishing procedure.
• No one instrument was able to produce a perfectly clean canal wall free of débris and/or smear layer in all studied canal thirds.
• The least smear score was recorded for PN and the highest one was found in HF instruments with the OS between them in this respect.
• Significant differences were found in both débris and smear layer scores between the three canal thirds for all examined instruments.
• A recommendation should be directed to the manufacturers -specially the single file techniques- of complementing the rotary instrumentation with an adjunctive active copious irrigation especially at the apical canal thirds.
• In face of the recent innovations in NiTi instruments designs; a recommendation for modified scoring system; with a less number of scoring variables may be required.