Results.
The results are shown in Table 1. There was no significant difference in canal perimeter between the two groups (P = 0.576), confirming that both were balanced in respect of anatomy. The circumferential filing technique removed the inner layer of dentine from agreater proportion of the perimeter of the canal wall than the balanced force technique (Table 1; Figs 3and4). However, the difference was not statistically significant (P ј 0.101).

Table 1. The capability to remove the inner layer of dentine at a level 5 mm from the apex.

Discussion.
Most rotary instrumentation produce a round preparation (Hulsmann et al.2001, Wu & Wesselink 2001). When the balanced force technique was performed in oval canals, the round preparation did not include the recesses, with the result that a portion of the canal wall was unprepared (Wu & Wesselink 2001). In this study, the balanced force technique again prepared less than 40% of the canal wall (Fig. 4; Table 1). The use of larger files in order to scrape more canal walls is not to be recommended, because these unnecessarily weaken the mesial and/or distal walls.
The concept behind circumferential filing is that a small file can move around the oval canal on the outstroke. Thus, it was speculated that the file could contact the whole canal wall without the risk of mesial or distal perforation. In this study, a size 90 Gates-Glidden drill had been used to 8 mm from the apex to facilitate the action of circumferential filing in deeper root canals and the circumferential filing did indeed prepare more aspects of the wall than the balanced force technique (Table 1). However, 40%of canal wall was not instrumented even after the use of circumferential filing. There is no evidence to prove that using a technique to remove dentine from 60% of the canal wall will lead to a higher success rate than using a technique that removes dentine from 40% only.
In the study by Reynolds et al. (1987), circumferential filing scraped 29, 60 and 64% of the wall in the apical, middle and coronal portions of root canals, respectively. It is unclear why this technique scraped more aspects of the wall in the middle and coronal portions than in the apical portion. The results of this study are in line with those of others (Reynolds et al. 1987, Zuolo et al. 1992, Siqueira Jr et al. 1997, Evans et al. 2001), demonstrating that thus far no technique has proved capable of scraping the whole circumference of the wall.
In this study, no stepback procedure was performed. Clinically, larger files are used during the stepback, but large files cannot prepare the narrow recesses in oval canals. However, including stepback may widen the canals in both groups and increase the percentage of prepared wall.
The capability of instrumentation to remove the inner layer of dentine was evaluated using only one section in this study. In cross-section the shape of root canals is not always oval at each level within a root (Wu et al. 2000b). Because the presence of oval canal was con- firmed at the level 5 mm from the apex in this study, evaluation at the same level guaranteed observation of the effect of instrumentation in oval canals, which was the purpose of this study.
In this study, regardless of which technique was used, the instruments did not succeed in contacting 40% or more of the root canal wall. As yet, no technique has proved capable of removing dentine from the entire wall. This indicates that it is not possible to remove the inner layer of dentine from the entire canal wall of oval canals. Nevertheless, clinically high success rates have been recorded, even in the absence of strong disinfectants designed to kill microorganisms in the dentinal tubules (Peters et al.1995). The hypothesis that the mechanical removal of heavily infected dentine is vital to the success of the treatment is being challenged. Therefore, it is not advisable to enlarge canals unnecessarily by means of large-sized instruments; rather the canals should be widened to allow effective irrigation and filling.

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