الفهرس 
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Abstract
The aim of this study was to measure apical root surface strain (RSS) during root-end preparation performed with two different ultrasonic tips (Smooth stainless steel and diamond coated Woodpecker ultrasonic tips) and their impact on root surface temperature rise and root surface micro-cracking.
Fourty recently extracted human maxillary central incisor teeth were selected. Crowns were sectioned away whilst maintaining a 15-mm length of the root. Each root was cleaned, shaped, obturated and numbered. Apical resection was performed 3 mm from the apex with 90-degree angle to their longitudinal axis under water spray cooling. Then the root surfaces were photographed with a stereomicroscope under 400x magnification, to assess the presence of microcracks.
The roots were wrapped with a single sheet of aluminum foil, and embedded centrally in auto-polymerizing hydrophilic vinyl polysiloxane putty impression material that was surrounded by an acrylic tube (9 mm high and 20 mm in diameter). Then, the roots were removed after the putty set, and the aluminum foil peeled off. A light hydrophilic vinyl polysiloxane impression material was placed in the socket and the teeth were immediately replaced, the polysiloxane filled the space created by the foil sheet representing a stimulated periodontal ligament. The apical end of the acrylic tube showed 3 mm of the root apex exposed to allow strain gauge placement.
The samples were then divided into 2 groups: group I: Smooth stainless steel (SS) ultrasonic group (20 roots), group II: Diamond coated (DC) ultrasonic group (20 root).
An electrical strain was fixed on the middle of buccal root surface at a distance of 1 mm from the root end with cyanoacrylate adhesive. The instantaneous RSS induced by each instrument, and the maximum RSSs were recorded.
Teflon insulated type K temperature probe was placed on the root surface 1mm from the root end to measure temperature changes during the rootend cavity preparations.
Data were presented as mean and standard deviation values; the independent t-test was used to compare the two groups. The significant level was set at P ≤ 0.05.
The results showed that retrograde preparation by two tips have strained the tooth structure, where the diamond coated ultrasonic tip caused significantly higher root surface strain than the smooth stainless steel ultrasonic tip. Temprature variations was significantly higher using DC tip than SS tip.
After completion of root-end preparation, the root surfaces was examined and photographed with a scanning electron microscope (SEM) at a 200x magnification for the presence of microcracking.
The results showed that cracks were observed in both groups. Overall evaluation of samples showed that the DC retrotip reduced the number and severity of the cracks formed after preparation.
After obtaining results, correlation between strain and microcrack formation was represented by R square for a sample statistic. After analysis of correlation coefficient of SS group, it was observed that there is a weak positive correlation between strain values and microcrack formation.
After analysis of correlation coefficient of DC group, it was observed that there is a medium positive correlation between strain values and microcrack formation