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 »  Home  »  Endodontic Articles 4  »  A comparison of the shaping characteristics of two nickel–titanium endodontic hand instruments
A comparison of the shaping characteristics of two nickel–titanium endodontic hand instruments
Introduction - Materials and methods.



C. Dobo-Nagy, T. Serban, J. Szabo, G. Nagy  & M. Madlena
Department of Prosthetic Dentistry, Faculty of Dentistry, Semmelweis University, Budapest, Hungary.
Dental School, Medical University of Pecs, Pecs, Hungary.

Introduction.
Cleaning and shaping of root canals is the most important aspect of root canal treatment. Over the years, a number of instruments have been developed for shaping canals. In previous studies (Dobo-Nagy et al. 1997a,b), statistically significant differences were found when comparing the shaping ability of stainless steel K-files and K-Flex files. Post-instrumentation shape of curved canals was superior when prepared with flexible stainless steel instruments than with conventional files (Briseno & Sonnabend 1991, Al-Omari et al. 1992). However, increasing the flexibility of endodontic instruments through the use of Ni–Ti alloy has not always resulted in better shaping ability (Gambill et al. 1996, Harlan et al. 1996, Elliott et al. 1998, Carvalho et al. 1999).
The Ni–Ti S hand file is made by grinding a circular tapered blank into a double helix fluted pattern. The helical angle is closer to an H-file than a K-file and this may explain the improved cutting efficacy of S-files in linear motion (Schafer 1997). Shaping ability of stainless steel files was tested in resin blocks (Briseno & Sonnabend 1991) but no statistically significant differences between the nine instruments were found. However, when taking the ideal values as reference points, statistical differences were found in canals enlarged with S-files at all the three measured levels. In addition, the S-file provided poorer results compared with the H-file at the apical level (1 mm coronally from the end-point) and at the third measured point (11.5 mm coronally). The S-file showed an increased tendency to transport root canals toward the convex side at the apical level and toward the concave side at the middle level, when compared to stainless steel, a K-file or a K-Flex file. To date, no investigation of the shaping quality of Ni–Ti S hand files has been reported.
The aim of this study was to compare the shaping characteristics of the two nickel–titanium instruments in extracted human root canals.

Materials and methods.
A total of 153 extracted human teeth with a single canal and patent apical foramen were selected. The specimens had been stored in phosphate buffer saline solution (pH 7.2) containing sodium azide 0.2% at 4 C. Each root was embedded in a 15 15 20 mm resin block. Access cavities were cut just into the dentine using a high-speed bur. The roof of the pulp chamber was then removed with a slowly rotating round bur. Following pulp extirpation, the root canals were filled with a radiopaque mixture of Lipiodol Ultra-Fluid (Byk, Konstanz, Germany) and Micropaque HD Oral (Guerbet, Schultzbach, Germany) by centrifuging at 1200 g for 30 s. An initial size 15 apical file (IAF) was inserted into the canals and digitized radiographs (RVG, Trophy Radiologie, Paris, France) were taken from bucco-lingual (b-l) and mesio-distal (m-d) directions using a paralleling technique (Forsberg 1992). Dilacerated, bayonet-shaped and extremely narrow canals that could not be penetrated by a file size 10 were excluded. Roots were divided into three groups on the basis of their radiographic images using computergraphic software (Dobo-Nagy et al. 1995). The roots were grouped according to canal shape, namely: straight (I-form), apically curved ( J-form) and continuously curved (C-form). Each group (containing 20 roots) was divided randomly into two equal subgroups for preparation using one of the following two instrument types: (i) Ni–Ti K-file (Beutelrock, München, Germany); (ii) Ni–Ti S-file (Sendoline, Kista, Sweden). Root canal enlargement was carried out randomly by two operators with considerable experience in root canal preparation. The process was carried out with purely tactile sensation, however, the working length and the b-l and m-d configuration of each canal was always available to the operator. The root length was modified by sectioning the coronal end in order to obtain a working length of 18 2 mm. Apical limit was determined 1 mm coronal to the radiographic apex. Files were used in a push–pull motion around the entire circumference of the canal. The step-back preparation technique was adopted for both instruments (Mullaney 1979). Preflaring of orifices was not carried out. The IAF was size 15, the master apical size was 30 and the root canal flared using a size 40 file. Copious irrigation with 10 mL distilled water was used throughout the preparation and was introduced with a 27-gauge needle and a syringe. Two postoperative radiographs were taken of each root using radiovisiography as described previously.

Canal defects.
The apical limit of the root canal preparation located within 1 mm of the radiographic apex was termed the apical stop . The creation of a false channel that exited the root surface during instrumentation at a point other than the anatomic apical foramen was termed a perforation . A zip was defined as an irregular and excessive widening of the canal close to the end-point of the preparation. The narrowest level of the apical third of the canal associated with zip which occurred more toward the orifice was termed the elbow . In the middle part of the canal, excessive removal of dentine from the inner aspect of canal curves was termed the danger zone . Coronal movement of the apical stop and loss of working length due to a buildup of dentine mud was termed a blockage .

Assessment of canal preparation.
The time taken to prepare each canal was recorded in min and s. It included the preparation, as well as changing files, and irrigation time.
Superimposed, standardized 1 : 10 projections of the original canal and the prepared canal in mesio-distal and bucco-lingual directions were used to evaluate the difference in shape between pre- and postoperative canals. Accurate location and superimposition of the radiographs in both views was facilitated by the presence of two metal markers placed at the corners of one side of each resin block.
The prevalence of perforations, blockages, zip and elbow formation were noted. The amount of dentine removed at the most coronal level of the canal was determined and the surface area at this level obtained by calculating the area of an ellipse ( ab). Lateral projections of the elliptic surface were gained by the two directional radiographs. The radii of the ellipse (a and b) were determined by dividing the measured diameters of the canal at this level of the two views (b-l and m-d), respectively.
Canal transportation was expressed by the maximum asymmetry of preparation. The maximum asymmetry of preparation at the apical level and at the danger zone of the root canals was measured on the inner concave and the outer convex sides. Measurements were carried out perpendicular to the axis of the original canal using the method of Dobo-Nagy et al. (1997b). The original canal width was divided into two halves, thus defining a point of the canal axis which served as a reference point for the measurement of asymmetry. Asymmetry (expressed as an absolute value) was calculated by subtracting the left from the right prepared canal contour distances from the reference point.

Statistical analysis.
Analysis of covariance was used to compare the preparation time, the amount of removed material at the most coronal part of canals and root canal form groups. Duncan’s multiple range test was used for statistical analysis of the asymmetry of preparation values. Significant differences were noted at P < 0.05 level.