Relationship between Files that Bind at the Apical Foramen and Foramen Openings in Maxillary Central Incisors-A SEM Study

Inaccessibility to apical constriction and preoperative presence of periradicular lesion are significant factors related to outcome, increasing the risk of root canal therapy failure (1). Debridement of the root canal by instrumentation and irrigation is considered the most important single factor in the prevention and treatment of endodontic diseases (2). Instrumentation should go apically enough to eliminate or at least reduce bacterial load (3). The presence of microorganisms in the cemental canal and their participation in the development of periapical lesions has been well demonstrated (4,5) and therefore this part of the radicular anatomy should not Relationship between Files that Bind at the Apical Foramen and Foramen Openings in Maxillary Central Incisors A SEM Study


INTRODUCTION
Inaccessibility to apical constriction and preoperative presence of periradicular lesion are significant factors related to outcome, increasing the risk of root canal therapy failure (1).
Debridement of the root canal by instrumentation and irrigation is considered the most important single factor in the prevention and treatment of endodontic diseases (2).Instrumentation should go apically enough to eliminate or at least reduce bacterial load (3).
The presence of microorganisms in the cemental canal and their participation in the development of periapical lesions has been well demonstrated (4,5) and therefore this part of the radicular anatomy should not be overlooked during root canal therapy.
Apical patency consists of the passive use of a small size file through the apical constriction without enlarging this region (6).Despite the fears in the past with apical foramen handling (7), this has been recently proposed (7,8).
While some authors (3,7,9) believe that apical patency promotes cleaning of the cemental canal, others (10,11) raised the question of how can this procedure clean properly using an instrument with smaller diameter than that of the apical foramen.
There is no information on this topic in endodontic literature.The aim of this study was to analyze the relationship between the files that bind at the apical foramen and the apical foramen openings in maxillary central incisors by scanning electron microscopy.

MATERIAL AND METHODS
Fifty human maxillary central incisors with complete root formation from the tooth bank of the Dental School of the Bahiana School of Medicine and Public Health were used.Inclusion criteria were lack of complex anatomy, acute curves, incomplete root formation and apical resorption, observed by means of direct examination and periapical radiographs.This study was approved by the Ethics Committee of the University of Ribeirão Preto (Protocol #124/09).
Access and preparation of the pulp chamber were carried out with a #3 carbide round bur (KG Sorensen, Cotia, SP, Brazil) and Endo-Z bur (Maillefer, Ballaigues, Switzerland).Root canals were explored with a size 15 K-file (MMDF, Burges, France) with watch-winding motion until the tip was visible at the apical foramen and root length was determined at this stage.
Line access was performed with #1 and 2 LA Axxess (SybronEndo, Glendora, CA, USA), which correspond to 20/.06 and 35/.06 file tips, 4 mm short of root length with concomitant irrigation with 1 mL 2.5% sodium hypochlorite (Q-Boa; Indústrias Anhembi S/A, Osasco, SP, Brazil).Foraminal patency was re-established with a #15 K-file, and an ascending size sequence of K-files (MMDF) was inserted within the root canal with gentle watch-winding movements until the file would feel fitting snugly at the apical foramen.Size was recorded, the instrument was removed and the tip cut and flattened at D1 with a double-face diamond disk (KG Sorensen) to allow better conditions of area measurement.Files were maintained close to the respective teeth.
Roots were cross-sectioned 10 mm short of the apex and the respective files re-inserted until they reached the foramen.The files were fixed to the roots with methyl cyanoacrylate and following setting they were cut at the same level of the roots.
Roots were fixed in stubs and gold sputtered, as previously reported ( 12), and a scanning electron microscope (Philips XL-30; Philips, Eindhoven, Netherland) was used at 140× magnification to map the contour of the foramen together with the flattened tip of the file.Images were captured digitally and analyzed by one observer, which performed 3 measurements of each file tip area and other 3 measurements of each apical foramen area, at different day intervals, using the Image Tool software (The University of Texas Health Science Center, San Antonio, TX, USA) (Fig. 1).The mean values were recorded.
Statistical analysis was run using InStat software (GraphPad Software Inc., San Diego, CA, USA), and normal distribution was confirmed by Kolmogorov-Smirnov test.Student's t-test was performed at α=0.01.

RESULTS
The files that fitted at the apical foramina presented great discrepancy in relation to the foraminal openings (Fig. 2).
Table 1 shows the mean values of the area for each apical foramen opening and file tip cross-section and the discrepancy between them (μm 2 ).As these mean values were obtained from 3 measurements for each foraminal area and 3 for each file area, Lin's Concordance Correlation Coefficient was used to verify their reproducibility.High concordance was obtained.The coefficient was superior to 0.99 in all comparisons.
Table 2 shows the mean values for file tips and apical foramina, with standard deviation, as well as the range of values and the medians.The mean area of foraminal opening was 281,341 μm 2 , 3.8 times greater than the file mean area, which was 76,129 μm 2 (Table 2).Comparison of mean values by the Student's t-test demonstrated significant difference (p<0.0001).The mean discrepancy between the areas of foraminal opening and file tip was 205,212 μm 2 ± 80,293.

DISCUSSION
Constriction area or CDC junction is the site where working length is ideally set (13,14).It is also known as minor foramen (15).Beyond CDC junction, cemental canal diverges its walls and ends at the external root portion, the foraminal opening, where it reaches its greater diameter.For this reason, it is also known as major foramen (13,15).The apical foramen is the round or circumferential extreme of the root, acting as a funnel or crater, which differentiates from the terminus of the cemental canal of the root surface (15).
Current literature demonstrates that instruments that bind at the apical constriction actually do not express its real anatomic diameter.On the contrary, a discrepancy will exist between them (16).
If the instruments that bind at the apical constriction (minor foramen) actually show discrepancy with the convergent walls of the dentinal canal, when they reach the cemental canal there is a tendency to enhance this discrepancy because of its divergent walls, particularly at the apical foraminal opening (major foramen).This discrepancy can be observed in Figure 2 and Tables 1 and 2.
The tips of the files were cross-sectioned at D1 in this study to allow more reliable measurements.If the discrepancy was great at D1 (Fig. 2), it can be speculated that it would be even greater at D0.In the clinical condition it is the final 1 mm of the file that will be in the cemental canal.
E n l a r g e m e n t o f the root canal is justified for mechanical (shaping) and biological (cleaning) reasons (17)(18)(19).The apical foramen instrumentation may be more favorable to the healing of chronic periapical lesions (20) because, exerting a mechanical action on cemental canal walls, it probably reduces bacteria load from the apical foramen and promotes a better infection control, the basis for success in Endodontics.
Considering that the files that bind at the apical foramen do not touch the divergent walls of the cemental canal (Fig. 2), it is possible that infection control is not properly carried out in some cases.Therefore, apical patency does not represent an effective foraminal cleaning procedure, as assumed by some authors (3,7,9), and the patency file would not remove significant amount of debris (13).
According to Butler (21), the apical foramen should be instrumented with a file that fits closely the constriction of the canal and be followed by the next two of its type in series and size.However, as in dentinal canal, instrumentation of the cemental canal should not follow rigid pre-established principles, but rather, each clinical situation should be individually examined (22).
The results of this study suggest that it would require 4 files of greater size beyond the one that bound to the foramen in order to allow a better relationship between files and apical openings of maxillary central incisors.However, these results apply to maxillary central incisors and further research including other groups of teeth is needed.

Figure 1 .
Figure 1.Measurement of the apical foramen area using the Image Tool Software.A= Observe the line demarcating the foraminal area; B= Same image as in A seen in detail.

Figure 2 .
Figure 2. SEM micrographs of the files in the apical foramen.A-D = Note the great discrepancy between K-files and foramina (Original magnification 140×).The tips of files were cut at D1 for area measurement.

Table 1 .
Areas of each apical foramen opening and file tip cross-section and discrepancy between them (μm 2 ). .

Table 2 .
Mean values and standard deviations of foraminal area and file tip area (μm 2 ).