Introduction

Proper three-dimensional cleaning and obturation of all the root canals of a tooth is a prerequisite for the success of endodontic treatment. The pulp canal system is complex, and the root canals may branch, divide and rejoin. Weine categorized the root canal system in any root into four basic types (1). Others found a much more complex canal system (2, 3); Vertucci identified eight pulp space configurations (2). In both grouping methods (Weine and Vertucci), type II is comprised of two separate canals leaving the pulp chamber and joining short of the apex to form one canal. In one study, the prevalence of this configuration in individual teeth was as follows:

- In maxillary anterior teeth, mandibular premolars, distobuccal and palatal roots of maxillary molars: 0%

- About 0-20% in maxillary premolars,

- About 0-27% in the mesiobuccal canal of maxillary molars

- About 0 -10% in mandibular anterior teeth

- About 0-33% in mesial roots of mandibular molars

- About 0-9% in distal roots of mandibular molars (2)

According to a recommendation, this anatomy is best treated by preparing and obturating one canal to the apex and the other to the point of juncture because the point where the two canals join is a potential point to become transported (4).

Iatrogenic preparation errors such as canal transportation are clearly undesirable and can be broadly defined as any deviation from the natural canal path (5). Thus, techniques and endodontic instruments should be employed that cause fewer errors, provide greater precision, and decrease working time (6).

Various methods, such as double radiographic superimposition technique, have been used to investigate preparation errors and root canal transportation (6). Classical in vitro methods of studying the morphologic characteristics of root canal systems produce irreversible changes in the samples and can yield only a 2-dimensional image (7, 8). More accurate information can be achieved with cone-beam computed tomography (CBCT), which provides quantitative and qualitative evaluation of the root canal in 3 dimensions (9). CBCT is a non-invasive experimental method that allows the comparison of pre- and post-preparation images of root canals.

No published article is available in relation to the evaluation of the root canal transportation in type II canals in the literature. The aim of the current study was to analyze the effects of different instrumentation methods on canal transportation in type II canals using CBCT technique.

Materials and Method

A total of 20 mandibular molars were selected for this study. The inclusion criteria for the study were: type II canals in the mesial root, confirmed by conventional radiography taken in mesio-distal direction, fully formed apices, maximum canal diameter equivalent to K-file #15 (DentsplyMaillefer, Switzerland) and canal curvature less than 25°. The teeth were stored in 10% formalin until use. Prior to the study, the teeth were washed with distilled water to remove residual formalin.

For the initial CBCT scan, every 5 teeth were mounted in foam arches in close contact to each other to allow reproducible orientation in the pre- and post-preparation scans and an acrylic facing was placed on the facial side to mimic soft tissue on the radiographs (fig. 1). Prior to preparation, all the teeth were scanned by a CBCT scanner (Planmeca OY, Helsinki, Finland), with a resolution (pixel size) of 160 µm, and a field of view (FOV) of 5×8×8 cm³. The 1-mm layer of images were taken axially by moving the toolbar from 2 mm above the juncture point of the canals to 2 mm below the juncture, and perpendicular to the long axis of the root. The pre-procedural images were recorded to be later compared with post-preparation images.

After initial scanning, the access cavities were prepared and the root canals were localized and explored with a #10 K-file (DentsplyMaillefer, Switzerland) until they were visible at the apical foramen. Working length (WL) was determined by subtracting 1 mm from a #10 K-file length visible at the apex. At this point, the total samples that were standard according to inclusion criteria, were divided randomly into 2 groups of 10 teeth each.

In the first group (G1), the lingual canal of the mesial root was selected as the main canal and prepared to the WL. The other canal (buccal) was prepared to the juncture point. In the second group (G2), both canals in the mesial root were prepared to the WL.  Canal patency was achieved by k-file #10. The junction was determined by CBCT.

All the canals were instrumented by the student of dentistry on 6^th year with stainless steel K-files (DentsplyMaillefer, Switzerland) using the step-back technique. Apical enlargement was carried out with instrument size up to #30. Anti-curvature technique was applied in all instrumentation steps. During in­strumentation, the root canals of both groups were irrigated with 2 mL of 3% sodium hypochlorite (Whitex, Chemin Chemical Co., Iran) after each file. After preparation was completed by an experienced operator, the samples were mounted in foam arches in the same sagittal position and re-scanned using the same parameters as in the initial scan, for compar­ison against the pre-preparation images.

The degree of canal transportation was calculated by measur­ing the shortest distance from the edge of a non-instrumented canal to the periphery of the root (mesial and distal, buccal and lingual) and then comparing this with the same measurements made on images of instrumented canals. According to Gambilli et al (10), the formula for canal transportation (in mm) is: ((X1-X2)-(Y1-Y2))

where X1 and X2 are the shortest distances between the me­sial (and buccal) root periphery and the canal in non-instrumented and in­strumented canals, respectively, and Y1 and Y2 represent the shortest distance between the distal (and lingual) root periphery and the canal in non-instrumented and instrumented canals, respec­tively (Fig. 2).

Pre- and post-operative calculations were compared to deter­mine the existence of canal transportation from 2 mm above the juncture point to 2 mm short of the apex.

According to this formula, a ‘0’ value indicates no canal trans­portation. Any result other than ‘0’ indicated transportation of the root canal. A negative value represented trans­portation occurring in the distal (and lingual) direction, whereas positive values represented transportation in the mesial (and buccal) direction.

The transportation results were analyzed statistically. Data were distributed normally (Shapiro-Wilk test) and were analyzed statistically using univariate, two-way repeated-measures ANOVA. A 95% level of confidence was used for all the analyses.

Figure 1. Mounting of tooth for CBCT scan

Figure 2. The degree of canal transportation was calculated by measuring the shortest distance from the edge of a non-instrumented canal to the periphery of the root (mesial and distal, buccal and lingual) and then comparing this with the same measurements made on images of instrumented canals.

Results

Regarding the direction of root canal transportation, both preparation techniques caused transportation toward both sides in both directions- buccolingual and mesiodistalwithout any significant differences
(table1, 2). There was no mutual interaction between these two factors: instrumentation technique and increments in both mesio-distal (p=0.26) and
bucu-lingual (p= 0.46) directions(Table 2), however; the distal displacement was more in increments coronal to the junction point and the mesial displacement was more in increments apical to the junction point, in both preparation techniques, without any significant differences. The mesiodistal displacement in junction point was less than apical and coronal points with no significant difference.

 There were no significant differences between the two techniques in causing transportation in both buccolingual (p=0.12) and mesiodistal directions (p=0.99) Table 2). Significant difference was found between different increments in mesio-distal direction (p=0.01) but no significant differences in transportation between different increments (-2 mm to 2 mm) in buco-lingual direction (p=0.06) (Table 2).

Table 1. Canal transportation (mm) in bucco-lingual and mesio-distal direction for each preparation technique at each sector of the canal (n=10)

Table 2. Effect of different parameter on transportation

  Discussion