| Abstract|| |
Aim: To evaluate the discrepancies between the diameter of the canal and the first file to bind at the working length, before and after preflaring in teeth with apical curvatures.
Materials and Methods: Twenty teeth with apical curvatures were selected for the study. Following access, the patency file was fixed at the working length, and the apices were ground perpendicular to the canal at this level. Diameters of canal and the first file to bind at the working length were observed for each tooth, both before and after preflaring, under stereomicroscope at 30X magnification.
Results: ANOVA and post hoc Bonferroni tests showed significant differences in both groups regarding anatomical diameter at working length and the first file to bind. The area of contact of the file at apical region remained the same in both groups.
Conclusion: Using the first file to bind for gauging the diameter of the apical canal was found to be an unreliable method in teeth with apical curvatures, even after preflaring.
Keywords: Apical curvature; apical diameter; preflaring
|How to cite this article:|
Dillon JS, Amita, Gill B. To determine whether the first file to bind at the working length corresponds to the apical diameter in roots with apical curvatures both before and after preflaring. J Conserv Dent 2012;15:363-6
|How to cite this URL:|
Dillon JS, Amita, Gill B. To determine whether the first file to bind at the working length corresponds to the apical diameter in roots with apical curvatures both before and after preflaring. J Conserv Dent [serial online] 2012 [cited 2020 Feb 28];15:363-6. Available from: http://www.jcd.org.in/text.asp?2012/15/4/363/101908
| Introduction|| |
Cleaning and shaping of the root canal system has been considered as the foundation for successful endodontic therapy. For this, the clinician must determine three critical parameters: The length of the canal, the taper of the preparation and the horizontal dimension of the preparation at its most apical extent.  The working length to which the root canal should be instrumented and irrigated with the aid of radiographs and modern electronic apex locators is relatively accepted; , however, the apical working width remains a dimension of controversy with comparatively little scientific basis.  The importance of enlarging the pulp space is well established, but it is still debatable as to how large is large enough in the apical region.  Studies have shown that larger apical preparations promote the removal of infected dentin  and the application of irrigants to apical areas;  however, over preparation can result in more procedural errors or weaken the root.  Thus, this emphasizes on the importance of the accurate determination of the working width at the apical region.
Since the diameters of the apical canals vary greatly in all tooth types, no standardized size is advisable for the apical enlargement. The classic parameter for enlargement of the apical region at the working length has been the use of three file sizes greater than the first file that binds in the apical region. ,, The concept behind this approach is that the first file to bind reflects the diameter of the apical canal. However, there is no evidence that the instrument that binds does actually reflect the diameter of the canal in the apical region.  Histological studies have shown that the canals instrumented to three sizes larger were not thoroughly cleaned.  Further, root canals are frequently elliptical in cross-section having narrow and large diameter, thus it is uncertain whether removing dentin from the wall of the recesses is always possible.
The tactile detection of the apical constriction and apical file size determination is an empirical and inaccurate method (Wu et al., 2002). It could be demonstrated that early preflaring of the canals removes coronal interferences and curvature influence and can thus provide better apical size information, and with this awareness a better decision can be made concerning the appropriate final diameter needed for complete apical shaping. ,, To date, studies for the determination of the apical diameter in the curved canals have shown the effect of preflaring with curvatures in the middle third or apical half of the canal. How far the diameter of the first file corresponds to the apical diameter in the canals having curvatures no more than the apical thirds of the root after preflaring is unknown. Thus, this study was conducted with the aim to determine the discrepancies between the diameter of the first file to bind at the working length and the apical diameter in the canals with apical curvatures, both before and after preflaring in the same teeth.
The null hypothesis is that preflaring in the canals with apical curvatures will not make any significant difference in the discrepancy between the diameter of the canal and the first file to bind at working length, than without preflaring.
| Materials and Methods|| |
For this in-vitro study, twenty extracted permanent posterior teeth with curved root canals (curvatures between 30-40 degree measured using Schneider's method), fully formed apices and patent foramens were selected. Teeth with complicated anatomy and external root resorption were excluded from the study. In all cases, the curved portion was no longer than the apical one third of the root. Standard access cavities were made and patency was confirmed by inserting a size 6 file into the root canal until the tip of the file was just visible at the apical foramen: the working length was set 1 mm short of the apical foramen. The files were fixed at the working length in root canals with acrylic resin. The apical 1mm of each root was removed horizontally by grinding on a wet Arkansas stone to expose the canal and the instrument at the working length. Care was taken not to grind into the instrument and no instrument became dislodged during grinding. After removing the apical 1mm, the instrument was removed and both the working length and reference point of individual canal were recorded. In order to remove the organic debris, all the teeth were washed in a 0.5% NaOCl solution and distilled water successively for 15 mins each.
Sizing of the canals
Group I: before preflaring
Each canal was sized using stainless steel K-files, starting with size 8. File sizes were increased until a binding sensation was felt at the working length, and the instrument size was recorded for each tooth. During sizing care was taken not to force the instrument into the canal, but to use a passive watch-winding action. The first file to bind at the working length was fixed at this position using acrylic resin. The apical region was then observed under a stereomicroscope at 30 X magnification and images were digitally recorded.
Group II: After preflaring
After removing the files from each sample, the coronal and middle portions of the canal were enlarged with Gates-Glidden drills, size 1-4. The length of preflaring was determined by the resistance felt in the middle portion of the canal. Size no.1 was used up to this length, and then sizes 2-4 were used respectively to 1, 2 and 3mm shorter. Irrigation with 3% NaOCl was repeated after each GG drill and patency tested with a small file. After preflaring was completed, a new evaluation of the first file that fit at the working length was completed. This was accomplished in the same manner as previously described and the readings were recorded. The apical region was again observed at the same position as group I under stereomicroscope and the images were recorded digitally.
The analysis of the images for both the groups was performed using AutoCAD software. Two measurements of canal diameter were made with accuracy of 0.01 mm at right angles to each other for every root. The longer dimension was recorded as maximum canal diameter and shorter dimension as the short canal diameter. The difference between the larger diameter of the canal and the file were determined and put to statistical analysis.
| Results and Observations|| |
One-way ANOVA test showed the minimum and the maximum values for the diameter of the canal and the first file to bind in both the groups, I (without preflaring) and II (after preflaring), together with the mean, standard deviation and 95% confidence interval.
Bonferroni Post hoc test was used for multiple comparisons between the mean diameters of the canal with the diameter of the file in both the groups. The percent diversity of the diameters of the file in both groups from the canal diameter was also determined [Table 1].
|Table 1: Multiple comparisons between the diameters of the canal with the diameter of the file in both the groups|
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Significant differences were found in both experimental groups regarding anatomical diameter at the working length and the first file to bind in the canal (P<0.01, 95% confidence level).
In all the canals in group I and group II, the diameter of the instrument was smaller than the major (larger) diameter. The discrepancy was up to 0.1184 in group I and 0.1711 in group II. The discrepancy between the instrument and canal major diameter was similar in both groups (P>0.05).
In 79 % of the canals after preflaring, the diameter of the instrument was smaller than the short diameter of the canal.
In 47% of the canals, the instrument in both the groups bound at one side of the wall only; in 53 % the instrument did not contact the wall.
The area of contact of the file in apical region in both the groups remained the same.
| Discussion|| |
Success of the endodontic therapy depends on the thorough chemo mechanical debridement and three dimensional obturation of the root canal system. Adequate instrumentation not only removes the superficial layer of infected dentin, but also allows sufficient space to act as a reservoir for irrigation and produce a shape that facilitates sealing.  Thorough instrumentation of the apical region has long been considered to be an essential component in the cleaning and shaping process. It was discussed as a critical step as early as 1931 by Groove. Simon later recognized the apical area as the critical zone for instrumentation.  Historically, the final apical size had been related to the initial apical size. ,, Several authors have attempted to quantify the horizontal dimension of the apical constriction and hence the final apical size, but most of the studies could not give the true picture of the horizontal dimensions, as they did not take into consideration all the variables.  Variables such as age, curvatures, apical shape, sectioning of the teeth perpendicular to the long axis of the canal may bring about a discrepancy.  In this study, care was taken to cut the section perpendicular to the canal at the apical region.
Many studies have demonstrated that widely accepted endodontic cleaning and shaping techniques are inadequate and that the inadequate instrumentation is due to the fact that root canal diameter is larger than the instrument caliber used in each preparation.  This may be attributed to the variable anatomy of the root canal in the apical region.  Results of this study showed that the first file that bound at the working length was noticeably smaller than the size of the root canal at the working length even after preflaring. In 79% of the cases, the diameter of the file was even smaller than the minor diameter of the canal. This finding is supported by the study conducted by Paque F et al. (2010) in which the two and three dimensional analysis of μCT revealed that the fit of the initial apical files that bound in the apical area was poor and that the mean canal area filled by the instruments 1 mm coronally from the working length was below 40% in all canal types. Therefore, the aim of removing the infected layer of dentin by enlarging to three sizes larger than the first file to bind may not be achieved.
Another purpose of the approach to use successively three files to the same working length is to create an apical stop.  In our study 53% of the cases, the first file binding at the working length did not touch the walls at the apical region. The false binding sensation may be due to the coronal interferences (as in group I), presence of calcifications or collagen fibres. In 47% of the cases, the file bound at only one side of the wall. These results are in agreement with the study by Wu et al., in which they found that in 25% of the cases the instrument did not contact the wall whereas in 75% of the cases it touched only one side.  However, the variable percentage in their study from ours may be due to the type of teeth (mandibular premolars), the location of the curvatures (apical half) and the type of the instrument (K-file or modified Lightspeed instrument with the largest diameter at the tip) used. Considering this fact that the first file to bind frequently bound at one side of the canal wall, the apical stop may be created only on one side. Although, the small shelf may prevent the master file from moving apically, it is questionable whether it has any function in reducing leakage and material extrusion. Hence, the anecdotal rule of three files up may not be useful without a proven method to judge apical size before instrumentation, and three has yet to be confirmed as the magical number to apical cleanliness. 
More recent studies report that initial flaring before determining the apical size may give a more accurate measurement of the apex.  The apical diameter proved to be at least one file size or two sizes larger once preflaring was done. , But these studies do not give information about the discrepancy between the apical canal diameter and the file diameter before and after preflaring. Our results showed that though the instrument size increased after preflaring in all but one case, the difference in discrepancy between the canal and the file diameter before and after preflaring remained significant. This may be due to the apical curvature interference or variable shape of the apical region.
It is interesting to note from the results of the study that even after preflaring although the size of the file increased, its area of contact in the apical region and hence the discrepancy remained the same. This may be due to the deflection of the file caused by apical canal curvature. The Gates Glidden drill removes the interferences from the coronal two thirds of the root canals in an attempt to reduce binding in the coronal region.  However, apical curvature interferences may still remain as shown by the results of this study. Obviously, even this procedure could not guarantee that the instruments bound only at the working length. Clinically, it is also not always possible to straighten the coronal two-thirds of root canal because sometimes the radius of root curvature is long. 
Although, instrumenting to a larger file size appears to be more effective to a certain extent, many studies have reported that there was no or little difference in intra-canal bacterial reduction with or without apical enlargement. , Further, over enlarging the canals, without knowing the exact dimensions of the canal can lead to iatrogenic complications like ledges, transportations, perforations and fractures.  Hence, questions such as whether the canals are cleaner with larger preparations and whether the roots are weaker if they are enlarged still needs to be answered.
| Conclusion|| |
In conclusion, using the first file to bind for gauging the diameter of the apical canal and as guidance for apical enlargement is not a reliable method in teeth with apical curvatures even after preflaring.
Further research is needed to know the influence of apical enlargement on the success and failure of endodontic treatment. All potential mechanisms to increase irrigation efficacy that can effectively debride the apical area even with minimal preparation should be explored. Efficacy of the newer systems like the SAF to accurately detect the canal diameter should be checked.
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Jaidev Singh Dillon
G-462, BRS Nagar, Ludhiana (Pb)
Source of Support: None, Conflict of Interest: None