MATERIALS AND METHODS Forty nearly simulated canals in resin blocks (Dentsply Maillefer) were used; each canal was 12 mm in length with a 40�� curve. One horizontal and one vertical groove were made on each block to achieve exact relocation in image superimpositions. The pre- and post-operative images of the blocks were taken with a digital camera at 8 megapixel resolution and with a special appliance in which distance and angle between the camera and specimen were fixed. The two basic motions in rotary canal preparations are rotary movement of the file and pecking motion. The first was controlled by using a handpiece with a torque- and speed-controlled electric motor VDW Silver (VDW, Munich, Germany) and the second motion, which achieves the short in-and-out movement of the file during canal preparation, was controlled and standardized by using a computer-controlled device and a previously designed program.

The device has five main parts: An electrical stepper motor, a holder and stabilizing arm, a handpiece, a screw bar and holder and the socket tray attached to the base. The stepping motor was controlled by a specially written a computer program that works under LabVIEW 5.0 software (National Instruments Corporation, Austin, TX, USA). The specimen was fixed in the socket tray. The number of movements was then adjusted in the software and the operation was initiated. Linear vertical movement of the file was automatically stopped by the software. One pecking motion step of the device consisted of one short linear in (1 mm) and one short linear out (0.8 mm) movement of the file.

The file tip progressed 0.2 mm in the canal per step movement. When the movement of the file stopped, a rubber stopper was placed adjacent to the flat coronal surface of the resin block and fixed with light-curing resin. The file was removed and the distance between the fixed stopper and file tip was measured with a digital caliper to 0.01 mm accuracy. Simulated canals were continuously irrigated with 2.5% NaOCl during the instrumentation phase. After each instrument was removed, irrigation was repeated and a #10 stainless steel file (VDW; Antaeos, Munich, Germany) was placed until its tip reached the WL. A total of 40 simulated canals were randomly divided into two groups, as the Mtwo group for use with a single-length technique and the ProTaper group for use with the crown down technique. The WL for all specimens was 12 mm. All sequences of the instrument series and master apical files (MAFs) used were according to the manufacturer’s recommendations for use in severely curved Anacetrapib canal preparations. MAF for Mtwo was #25/.06 and for ProTaper was an F2 file.