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 »  Home  »  Endodontic Articles 3  »  Does the first file to bind correspond to the diameter of the canal in the apical region?
Does the first file to bind correspond to the diameter of the canal in the apical region?
Introduction - Materials and methods.

M.-K. Wu, D. Barkis, A. Roris & P. R. Wesselink
Department of Cariology, Endodontology and Pedodontology, Academic Centre for Dentistry Amsterdam (ACTA), Amsterdam, the Netherlands.

In histological sections of infected teeth bacteria were found in the dentinal tubules adjacent to the pulp (Armitage et al. 1983, Ando & Hoshino 1990, Peters et al. 2001). It is therefore considered appropriate to remove the most heavily infected layer of dentine. Since the diameters of the apical canals vary greatly in all tooth groups (Wu et al. 2000), no standard size is advisable for the apical enlargement. One recommended approach is to enlarge the apical root canal to three sizes larger than the first file to bind (Walton & Torabinejad 1996, Weine 1996). The concept behind this approach is that the first file to bind reflects the diameter of the apical canal; by using three successively larger files to the same working length the layer of heavily infected dentine should be removed from all regions of the apical canal wall. Another purpose of this approach is to create an apical stop (apical matrix) that was supposed to facilitate reduced leakage and material extrusion. On the other hand, taking successively larger files to the same length in a curved root canal can predispose to apical lacerations or ledging (Tang & Stock 1989, Briseno & Sonnabend 1991, Nagy et al. 1997, Buchanan 2000).
However, there is no evidence that the instrument that binds first does actually reflect the diameter of the canal in the apical region. Furthermore, since many canals are oval (Wu et al. 2000) it is uncertain whether removing dentine from the wall of the recesses is always possible (Reynolds et al. 1987, Zuolo et al. 1992, Siqueira et al. 1997). Indeed, cleaning in these regions of the canal may have to rely more on irrigation (Hand et al. 1978, Lumley et al. 1993). At the same time, whether removing a layer of dentine is always necessary for successful debridement is also unknown (Lussi et al. 1999). Thus, the concept of widening the apical canal to three sizes larger than the first file to bind is not based on evidence.
The purpose of this study was to determine whether the first file that binds reflects the diameter of apical canal.

Materials and methods.
Twenty mandibular premolars with single curved canals and an apical foramen at the apex were selected. In all cases the curved portion was not longer than the apical half of the root. The curvature of each canal was determined from bucco-lingual and mesio-distal radiographs using the method suggested by Schneider (1971). Following access to the canal system, apical patency was confirmed by inserting a size 10 file through the apical foramen. The teeth were divided equally into two groups on the basis of their bucco-lingual internal diameters as determined from the radiographs. A size 10 file was inserted into the canal until the tip of the file was just visible at the apical foramen. The actual canal length was determined and the working length established by deducting 1 mm. The coronal aspect of each canal was flared using Gates Glidden drills (Dentsply Maillefer, Ballaigues, Switzerland); size 50 (No. 1) was used to a depth where resistance was met, and then sizes 70–110 (Nos 2–4), respectively, to 1, 2 and 3 mm shorter. In all canals the No. 1 Gates prepared approximately the coronal two-thirds of the canals. Each canal was irrigated with 2 mL of a freshly prepared 2% solution of sodium hypochlorite after each Gates Glidden drill.
Since the Lightspeed instruments have no handle, their shanks were embedded in acrylic resin within a piece of silicone rubber tube. Acrylic resin handles were created on removal of the rubber tubes after polymerization. This procedure was repeated for the K-files to ensure that the handles of both instruments would have the same size and shape. The apical half of the cutting head of Lightspeed instrument was removed using wet SiCpaper, resulting in an instrument that had its largest diameter at its tip.
Without seeing the instrument or the tooth, one investigator inserted increasing sizes of instruments into the canals starting with size 10. When binding occurred, this was reported to a second investigator who recorded the length: at the working length or short of the working length. When short, the instrument was inserted to the working length by clockwise and counter clockwise rotation. The files were fixed at the working length in root canals with acrylic resin.
The apical 1 mm of each root was removed horizontally by grinding on wet SiC-paper to expose the canal and instrument at the WL level. Care was taken not to grind into the instrument and no instrument became dislodged. All teeth were then washed in a 0.5% NaOCl solution and distilled water successively for 15 min each. The root surface exposed by apical grinding was then stained with 2% methylene blue and observed under a Wild Photomakroskop M400 microscope with camera (Wild, Heerbrugg, Switzerland) at a 40 magnification. Colour slides of the exposed surfaces were taken and the slides were scanned as TIFF (Tagged Image File Format) images. Using a KS100 Imaging system 3.0 (Carl Zeiss Vision GmbH, Hallbergmoos, Germany) two measurements of canal diameter were made with accuracy of 0.01 mm at right angles for each root; the shorter dimension was recorded as the short canal diameter. The instruments had a circular, square or triangular shape at the working length. The length of the diagonal line of a square, the diameter of a circle or the length of one side of a triangle was measured using the same program and considered as the diameter of instrument. The discrepancy between the canal and instrument diameters in the two instrument groups was compared and analysed statistically using the Mann–Whitney U -test.