Indian Journal of Dental Research

ORIGINAL RESEARCH
Year
: 2016  |  Volume : 27  |  Issue : 1  |  Page : 66--72

Influence of working length and foraminal enlargement on foramen morphology and sealing ability


Juliana Melo Silva1, Gustavo A Brandão2, Emmanuel João Nogueira Leal Silva3, Alexandre Augusto Zaia4,  
1 Department of Endodontics, Federal University of Pará, Belém, Pará, Brazil
2 Department of Public Health, Federal University of Pará, Belém, Pará, Brazil
3 Department of Endodontics, Grande de Rio University (UNIGRANRIO), RJ, Brazil
4 Department of Restorative Dentistry, Endodontics Area, State University of Campinas (UNICAMP), Piracicaba, SP, Brazil

Correspondence Address:
Emmanuel João Nogueira Leal Silva
Department of Endodontics, Grande de Rio University (UNIGRANRIO), RJ
Brazil

Abstract

Aim: The present study evaluated the influence of the working length and foraminal enlargement on the sealing ability and anatomy of the apical region of the root. Materials and Methods: Fifty-five roots were divided into three groups: G1, instrumentation 1 mm short of the major foramen; G2, instrumentation in the limit of the apical foramen; and G3, instrumentation 1 mm beyond the foramen. All groups were prepared using nickel-titanium rotary files and obturated with AH Plus and Gutta-percha. Photomicrographs were taken using a scanning electron microscopy (SEM) before instrumentation, after instrumentation with each file, and after root canal filling. Moreover, bacterial microleakage with Enterococcus faecalis was performed. Results: The results were analyzed using Mann-Whitney, Friedman, Kruskal-Wallis and Kaplan-Meier tests at a significance level of 5%. The cemental canal was uninstrumented in G1. No statistical differences regarding foramen deviation was observed when compared G2 and G3 (P > 0.05). SEM analysis showed that G2 and G3 resulted in good apical foramen obturation. Microleakage showed no statistically significant differences between all of the groups tested. Conclusions: Under the conditions of this study, it can be concluded that foramen enlargement resulted in more apical deviation; however, no differences in bacterial microleakage was observed among the experimental groups.



How to cite this article:
Silva JM, Brandão GA, Leal Silva EJ, Zaia AA. Influence of working length and foraminal enlargement on foramen morphology and sealing ability.Indian J Dent Res 2016;27:66-72


How to cite this URL:
Silva JM, Brandão GA, Leal Silva EJ, Zaia AA. Influence of working length and foraminal enlargement on foramen morphology and sealing ability. Indian J Dent Res [serial online] 2016 [cited 2019 Nov 14 ];27:66-72
Available from: http://www.ijdr.in/text.asp?2016/27/1/66/179834


Full Text

Apical patency is a technique in which the apical portion of the canal is maintained free of debris by recapitulation with a small file through the apical foramen. [1] The most predictable method is to regularly use a so-called patency file during cleaning and shaping procedures. This file can be defined as a small flexible file, which is passively moved through the apical constriction without widening it. [2]

The passive cleaning of the cemental canal with flexible files without widening the apical constriction has been recommended for endodontic treatment. However, in cases of apical periodontitis, recognizing the presence of microorganisms in the apical portion of the canal and even in the lesion itself [3],[4],[5] has contributed to the acceptation of larger apical preparations [1],[2] and cleaning, debridement, and enlargement of the apical foramen [6] during root canal instrumentation because it can overcome the potential limits of irrigation in the apical area, optimizing root canal disinfection. [6],[7],[8],[9],[10]

Achieving a larger apical diameter during foraminal enlargement, however, might lead to apical transportation. This transportation of the apical foramen can lead to complications in subsequent cleaning and filling procedures. [11] To date, numerous studies have examined canal transportation using different file systems. [12],[13] However, to the best of the author's knowledge, no studies have been performed to evaluate the anatomy and sealing ability during foraminal enlargement. The purpose of this study was to evaluate the influence of the working length and apical foraminal enlargement on the anatomy of the apical foramen and the sealing ability after root canal filling with AH Plus and Gutta-percha.

 MATERIALS AND METHODS



The present research was approved by the Ethics Committee in Research of the Piracicaba School of Dentistry, State University of Campinas, protocol number 080/2009. Fifty-five intact human molar teeth were used in this study. Preoperative mesiodistal and buccolingual radiographs were taken for each root to confirm the canal anatomy. The criteria for tooth selection included: No visible root caries, no fractures, no cracks, no signs of internal or external resorption or calcification and a completely formed apex. Roots canals with no more than 5° of curvature were considered straight and were included in this study.

The palatal roots of maxillary molars were decoronated to standardize the root length to 11 mm. The samples were randomly divided into three groups (n = 15) according to the different working lengths: G1, instrumentation 1 mm short of the major apical foramen; G2, instrumentation in the limit of the major apical foramen; and G3, instrumentation 1 mm beyond the major apical foramen. In all groups, electronic tooth length measurements were carried out before root canal preparation using the electronic apex locator Root ZXII (J. Morita, Kyoto, Japan) using a previously described in vitro model. [14] All groups were prepared using the crown-down technique using k3 rotary files (Sybron Endo, Orange, CA, USA). The electric motor was calibrated to run at a speed of 300 rpm with a torque of 2 Ncm according to the manufacturer's instructions. Cervical and middle thirds were enlarged with instrument sizes of 25, 0.10 taper and 25, 0.08 taper. Apical rotary instrumentation was performed by enlarging the root canal to at least three times the size of the first manual file that bound at working length, according to the different groups, with a 0.06 taper.

Before the use of each new instrument, the canals were filled with 0.5 mL of 2% chlorhexidine (Biodinâmica, Ibiporãã, PR, Brazil) as the auxiliary chemical substance. After the use of each instrument, 5.0 mL of saline solution was used as the irrigating solution. Before obturation, a final flush with 10 mL of 17% ethylenediaminetetraacetic acid (Biodinââmica) was performed. Then canals were dried using paper points (Dentsply, Ballaigues, Switzerland).

The canals were obturated with Gutta-percha and AH Plus (Dentsply), using single-cone obturation technique. In G1, a medium nonstandardized master Gutta-percha point (0.06 taper) was selected and cut at its apical third to the size of the master apical file preparation; in G2 and G3 the point was cut two sizes larger than the size of the master apical file preparation in order to prevent apical extrusion of the Gutta-percha. In G1, the point was fitted at the working length and seated in the root canal. In G2 and G3 the point was fitted 2 mm before the apical foramen and seated in the root canal. A Touch 'n Heat plugger (SybronEndo) was selected and prefitted to its binding point at 8 mm short of the working length, with aid of a silicon stop. Touch 'n Heat was used at a power setting of 7. The preheated plugger was inserted, and the apical pressure was maintained for nearly 10 s, allowing the apical segment of the Gutta-percha to cool under this force to compensate for Gutta-percha shrinkage; the plugger was then removed.

For microscopic analysis, the specific parameters of 3 kV, and ×50 magnification were used, in a scanning electronic microscope (JSM 5600 Lv, JEOL, Tokyo, Japan). The specimens were mounted on specific metallic stubs to prevent their movement and to allow the evaluation to be made parallel to the long axis of the foramen. Photomicrographs were taken before instrumentation, after instrumentation of the apical foramen by each file, and after root filling. Consequently, five images were taken for each root. The samples were placed in the same position to record the standardized images of root apex. The teeth was carefully analyzed at scanning electron microscopy (SEM) at a magnification of ×50 to the identification of the major foramen of each root. The opening of the largest diameter found at the root apex identified the major apical foramen. [15] A grid system was used to evaluate the photomicrographs that consisted of a circle divided into four equal segments, with radii projecting to intersect the canal surface; the center of the circle was located in the center of the root canal [16] [Figure 1]. Each segment was measured according to a scoring system [Supplementary file 1]. A single examiner, who was trained and calibrated for the study, performed the blind evaluations of 225 images. Approximately, 10% of the sample was re-evaluated to verify intra-examiner reproducibility. The examiner agreement was >0.89 (89%) according to the Kappa test. All image procedures were performed using AutoCAD 2008 software (Autodesk, Mill Valley, California, USA). Then, the specimens were sterilized in a gamma irradiation chamber for 24 h at 27°C with a 14.5 KGy dose. Glass flask apparatus was prepared for this experiment, and consisted of two separate chambers; the lower part was filled with brain heart infusion broth (BHI) so that only the root apex was in contact with the broth, whereas the coronal part was immersed in BHI containing Enterococcus faecalis (ATCC 29212) to investigate bacterial microleakage. [17],[18] The flasks were then incubated at 37°C in an atmosphere of 10% CO 2 , and microbial growth was checked daily, by the appearance of turbidity in the BHI broth, for 60 days. The colony-forming units were counted, and the purity of the cultures was confirmed by Gram staining, catalase production, colony morphology on BHI agar 1 blood, and by the use of a biochemical identification kit (API 20 Strep, BioMérieux; Marcy-l'Etoile, France). The positive controls (n = 5) were filled with Gutta-percha only and tested with bacteria, whereas the negative controls (n = 5) were sealed with nail varnish and cyanoacrylate to test the seal between the chambers.{Figure 1}

[INLINE:1]

The results were analyzed using BioEstat 5.0 software (Mamirauá, Teféé, AM, Brazil). Mann-Whitney was used to compare differences between different groups regarding apical deviation. Friedman test was used to detect differences in treatments across multiple test attempts. Kruskal-Wallis was used to analysis of bacterial leakage results. Kaplan-Meier test was used to measure the fraction of bacterial living for a certain amount of time after root canal filling.

 RESULTS



The samples in G3 showed the best results for cementum removal after apical enlargement with the first and second sized files beyond the first file that bound at working length. The SEM images of these samples showed more scores of 2 (enlargement by two quarters of the circumference of the circle) and 4 (enlargement by four quarters of the circumference of the circle). However, after the third file was used, there were no differences between G2 and G3 [Table 1]. G3 presented more apical foramen deviation compared to G2, with more scores of 2 (deviation by two-quarters of the circle) and 3 (deviation by three-quarters of the circle) [Table 2] and [Table 3], [Figure 3].{Table 1}{Table 2}{Table 3}

In the bacterial microleakage test, all positive controls showed bacterial microleakage within 72 h. No penetration of bacteria was observed in the negative controls during the observation time of 60 days. The Kaplan-Meier probabilities for the experimental groups are shown in [Figure 2]. No significant differences were observed amongst the experimental groups (P > 0.05).{Figure 2}{Figure 3}

 DISCUSSION



The apical portion of a root canal is often not cleaned as well as the middle and coronal portions after root canal preparation with various instruments and techniques. [9],[19],[20] Bacteria located in anatomic complexities such as dentinal tubules, irregularities, and ramifications, especially in the apical region can be protected from the effects of instruments and chemical substances used in the main canal. [3],[5] Insufficient cleaning of the apical portion might cause periapical inflammation. [19] One explanation for this is that the master apical file used is too small to achieve sufficient apical debridement. [21] It has been reported that increasing the apical enlargement might enhance the debridement of the apical portion of a root canal. [22],[23] In addition, larger apical size preparations have also demonstrated greater microbial reduction in the apical third. [24],[25],[26],[27]

Different working lengths have been proposed but the most commonly accepted working length is 1 mm short of the apex. According to this concept, the cemental canal should not be instrumented and thus, will not be cleaned. [6],[28],[29] Studies on periapical healing associated with teeth presenting periapical lesions showed that the best result was obtained when the cemental canal and the apical foramen were widened more than the patency instrument. [6],[9]

Apical widening may result in a severe periapical inflammatory reaction that is deleterious to the healing process. [28] However, the results of an in vivo histological study involving apical and periapical tissues following root canal therapy showed that apical foramen widening favored the healing outcome of induced chronic periapical lesions in the teeth of dogs. [6] Borlina et al. [29] also reported a high incidence of microorganisms in the groups in which apical foramen widening was not performed, and acute inflammation was also present. Therefore, performing apical widening of the cemental canal and the apical foramen may enhance the potential for apical healing because it removes a greater amount of contaminated cementum and promotes more favorable conditions for healing. [9] Other advantages of this procedure are that it minimizes the risk of loss of length, [30] eases irrigation in the apical third of the canal, [27] and improves the tactile sensing of the clinician during apical shaping. [28] Apical enlargement ensures cleanliness and improves the quality of canal filling. [2],[31],[32],[33]

Despite all the advantages of apical widening, one commonly reported disadvantage is the possibility of postoperative pain related to physical trauma in the periapical region. It is suggested that even the use of small patency files through the apex can cause a periapical acute inflammatory response and severe postoperative pain. [34] However, a recent study demonstrated that a foraminal enlargement instrumentation technique resulted in the same postoperative pain and necessity for analgesic medication when compared to a nonenlargement technique. [35] Corroborating with this study, Arias et al. [31] did not find any statistically significant differences in the incidence of postoperative pain using controlled over instrumentation, a much more aggressive technique than just maintaining apical patency. Moreover, Torabinejad et al. [35] found that unintentional overextension of the files does not affect the incidence of postoperative pain.

In this study, the influence of different working length associated with apical widening on the anatomy of the apical foramen and on the sealing ability after root canal filling was evaluated. The findings of the present study showed that working length, instrumentation in the apical foramen (G2) and instrumentation 1 mm beyond the foramen (G3) had no statistically significant differences on apical widening. After the third file was used 46.7% had touched all walls of the cemental canal in G2, and 53.3% had touched the walls in G3. G1, where the instrumentation was made 1 mm short of the major foramen, served as the control because the files did not touch the canal walls, so there was no widening or deviation in the apical foramen.

Instrumentation 1 mm beyond the apical foramen (G3) can promote more deviations from the original foramen anatomy. G3 had 66.7% apical foramen deviation, but this deviation did not affect the quality of obturation. Many factors have been discussed as being contributors to deviations, such as complex canal anatomy, [36],[37],[38] instrument design, instrumentation sequence, [39] operator experience, [40] and the inadequate use of an irrigant. [41] In this study, the use of files with a 0.06 taper could have caused apical foramen deviations as the great taper of the instrument decreases its bending resistance. However, numerous studies have shown that nickel-titanium rotary instruments can effectively produce a well-tapered root canal for sufficient for obturation, with a minimal risk of transporting the original canal. [42]

Although foraminal widening can promote more deviations, microscopic analysis showed that both widening groups (G2 and G3) resulted in good quality apical foramen obturation. G2 showed that 93.3% of the apical foramen was filled, with 86.7% in G. However, statistical analysis of bacterial microleakage showed no statistically significant differences between all tested groups. One of the major concerns regarding performing foraminal widening is the possibility of increased risk of endodontic sealer extrusion. This extrusion through the apical foramen into the perirradicular tissues can increase the risk of tissue irritation or delayed healing. However, according to the results of previous studies, the toxicity of endodontic sealers tends to decrease with time. [35],[43],[44],[45] Hence, despite the transitory irritability that endodontic sealers may cause to periapical tissues, endodontists should evaluate the advantages and disadvantages of sealer extrusion since the nonwidened and nonsealed areas in the apical region may serve as microorganism niches, initiating, or perpetuating an endodontic failure. [3],[5]

Recent studies suggested that root canal instrumentation at apical foramen and 1 mm beyond the foramen can potentially cause cracks on the apical root surface. [46],[47] However, in the present study, no apical cracks was observed. The contradictory results observed between the present study, and the previously mentioned research may be explained by differences in the methodology. Future randomized controlled trials should be conducted to evaluate the effectiveness of foramen widening on the successful outcome of root canal treatment, recognizing the treatment factors that will increase the predictability of endodontic therapy.

Under the conditions of this study, it can be concluded that foramen enlargement resulted in more apical deviation; however, no differences in bacterial microleakage was observed among the experimental groups.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

References

1Goldberg F, Massone EJ. Patency file and apical transportation: An in vitro study. J Endod 2002;28:510-1.
2Buchanan LS. Management of the curved root canal. J Calif Dent Assoc 1989;17:18-25.
3Nair PN. On the causes of persistent apical periodontitis: A review. Int Endod J 2006;39:249-81.
4Subramanian K, Mickel AK. Molecular analysis of persistent periradicular lesions and root ends reveals a diverse microbial profile. J Endod 2009;35:950-7.
5Ricucci D, Siqueira JF Jr. Biofilms and apical periodontitis: Study of prevalence and association with clinical and histopathologic findings. J Endod 2010;36:1277-88.
6de Souza Filho FJ, Benatti O, de Almeida OP. Influence of the enlargement of the apical foramen in periapical repair of contaminated teeth of dog. Oral Surg Oral Med Oral Pathol 1987;64:480-4.
7Card SJ, Sigurdsson A, Orstavik D, Trope M. The effectiveness of increased apical enlargement in reducing intracanal bacteria. J Endod 2002;28:779-83.
8Albrecht LJ, Baumgartner JC, Marshall JG. Evaluation of apical debris removal using various sizes and tapers of ProFile GT files. J Endod 2004;30:425-8.
9Fornari VJ, Silva-Sousa YT, Vanni JR, Pécora JD, Versiani MA, Sousa-Neto MD. Histological evaluation of the effectiveness of increased apical enlargement for cleaning the apical third of curved canals. Int Endod J 2010;43:988-94.
10Lin LM, Rosenberg PA. Repair and regeneration in endodontics. Int Endod J 2011;44:889-906.
11Weine FS, Kelly RF, Lio PJ. The effect of preparation procedures on original canal shape and on apical foramen shape. J Endod 1975;1:255-62.
12Kunert GG, Camargo Fontanella VR, de Moura AA, Barletta FB. Analysis of apical root transportation associated with ProTaper Universal F3 and F4 instruments by using digital subtraction radiography. J Endod 2010;36:1052-5.
13Yang G, Yuan G, Yun X, Zhou X, Liu B, Wu H. Effects of two nickel-titanium instrument systems, mtwo versus ProTaper universal, on root canal geometry assessed by micro-computed tomography. J Endod 2011;37:1412-6.
14Soares RM, Silva EJ, Herrera DR, Krebs RL, Coutinho-Filho TS. Evaluation of the Joypex 5 and Root ZX II: An in vivo and ex vivo study. Int Endod J 2013;46:904-9.
15Martos J, Lubian C, Silveira LF, Suita de Castro LA, Ferrer Luque CM. Morphologic analysis of the root apex in human teeth. J Endod 2010;36:664-7.
16Tan BT, Messer HH. The effect of instrument type and preflaring on apical file size determination. Int Endod J 2002;35:752-8.
17Gomes BP, Sato E, Ferraz CC, Teixeira FB, Zaia AA, Souza-Filho FJ. Evaluation of time required for recontamination of coronally sealed canals medicated with calcium hydroxide and chlorhexidine. Int Endod J 2003;36:604-9.
18Jacobovitz M, Vianna ME, Pandolfelli VC, Oliveira IR, Rossetto HL, Gomes BP. Root canal filling with cements based on mineral aggregates: An in vitro analysis of bacterial microleakage. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2009;108:140-4.
19Nair PN, Henry S, Cano V, Vera J. Microbial status of apical root canal system of human mandibular first molars with primary apical periodontitis after "one-visit" endodontic treatment. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2005;99:231-52.
20Lorencetti KT, Silva-Sousa YT, Nascimento GE, Messias DC, Colucci V, Abi Rached-Junior F, et al. Influence of apical enlargement in cleaning of curved canals using negative pressure system. Braz Dent J 2014;25:430-4.
21Wu MK, R'oris A, Barkis D, Wesselink PR. Prevalence and extent of long oval canals in the apical third. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2000;89:739-43.
22Lee SJ, Wu MK, Wesselink PR. The efficacy of ultrasonic irrigation to remove artificially placed dentine debris from different-sized simulated plastic root canals. Int Endod J 2004;37:607-12.
23van der Sluis LW, Wu MK, Wesselink PR. The efficacy of ultrasonic irrigation to remove artificially placed dentine debris from human root canals prepared using instruments of varying taper. Int Endod J 2005;38:764-8.
24Shuping GB, Orstavik D, Sigurdsson A, Trope M. Reduction of intracanal bacteria using nickel-titanium rotary instrumentation and various medications. J Endod 2000;26:751-5.
25Marroquín BB, El-Sayed MA, Willershausen-Zönnchen B. Morphology of the physiological foramen: I. Maxillary and mandibular molars. J Endod 2004;30:321-8.
26Weiger R, Bartha T, Kalwitzki M, Löst C. A clinical method to determine the optimal apical preparation size. Part I. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2006;102:686-91.
27Ricucci D, Langeland K. Apical limit of root canal instrumentation and obturation, part 2. A histological study. Int Endod J 1998;31:394-409.
28Holland R, Sant'Anna Júnior A, Souza VD, Dezan Junior E, Otoboni Filho JA, Bernabé PF, et al. Influence of apical patency and filling material on healing process of dogs' teeth with vital pulp after root canal therapy. Braz Dent J 2005;16:9-16.
29Borlina SC, de Souza V, Holland R, Murata SS, Gomes-Filho JE, Dezan Junior E, et al. Influence of apical foramen widening and sealer on the healing of chronic periapical lesions induced in dogs' teeth. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2010;109:932-40.
30Flanders DH. Endodontic patency. How to get it. How to keep it. Why it is so important. N Y State Dent J 2002;68:30-2.
31Arias A, Azabal M, Hidalgo JJ, de la Macorra JC. Relationship between postendodontic pain, tooth diagnostic factors, and apical patency. J Endod 2009;35:189-92.
32Adorno CG, Yoshioka T, Suda H. The effect of working length and root canal preparation technique on crack development in the apical root canal wall. Int Endod J 2010;43:321-7.
33Cailleteau JG, Mullaney TP. Prevalence of teaching apical patency and various instrumentation and obturation techniques in United States dental schools. J Endod 1997;23:394-6.
34Silva EJ, Menaged K, Ajuz N, Monteiro MR, Coutinho-Filho Tde S. Postoperative pain after foraminal enlargement in anterior teeth with necrosis and apical periodontitis: A prospective and randomized clinical trial. J Endod 2013;39:173-6.
35Torabinejad M, Kettering JD, McGraw JC, Cummings RR, Dwyer TG, Tobias TS. Factors associated with endodontic interappointment emergencies of teeth with necrotic pulps. J Endod 1988;14:261-6.
36Pruett JP, Clement DJ, Carnes DL Jr. Cyclic fatigue testing of nickel-titanium endodontic instruments. J Endod 1997;23:77-85.
37Haïkel Y, Serfaty R, Bateman G, Senger B, Allemann C. Dynamic and cyclic fatigue of engine-driven rotary nickel-titanium endodontic instruments. J Endod 1999;25:434-40.
38Iqbal MK, Maggiore F, Suh B, Edwards KR, Kang J, Kim S. Comparison of apical transportation in four Ni-Ti rotary instrumentation techniques. J Endod 2003;29:587-91.
39Yared GM, Bou Dagher FE, Machtou P. Influence of rotational speed, torque and operator's proficiency on ProFile failures. Int Endod J 2001;34:47-53.
40Peters OA. Current challenges and concepts in the preparation of root canal systems: A review. J Endod 2004;30:559-67.
41Schäfer E, Schlingemann R. Efficiency of rotary nickel-titanium K3 instruments compared with stainless steel hand K-Flexofile. Part 2. Cleaning effectiveness and shaping ability in severely curved root canals of extracted teeth. Int Endod J 2003;36:208-17.
42Brackett MG, Lewis JB, Kious AR, Messer RL, Lockwood PE, Brackett WW, et al. Cytotoxicity of endodontic sealers after one year of aging in vitro. J Biomed Mater Res B Appl Biomater 2012;100:1729-35.
43Zmener O, Martinez Lalis R, Pameijer CH, Chaves C, Kokubu G, Grana D. Reaction of rat subcutaneous connective tissue to a mineral trioxide aggregate-based and a zinc oxide and eugenol sealer. J Endod 2012;38:1233-8.
44Silva EJ, Rosa TP, Herrera DR, Jacinto RC, Gomes BP, Zaia AA. Evaluation of cytotoxicity and physicochemical properties of calcium silicate-based endodontic sealer MTA Fillapex. J Endod 2013;39:274-7.
45Silva EJ, Santos CC, Zaia AA. Long-term cytotoxic effects of contemporary root canal sealers. J Appl Oral Sci 2013;21:43-7.
46Adorno CG, Yoshioka T, Jindan P, Kobayashi C, Suda H. The effect of endodontic procedures on apical crack initiation and propagation ex vivo. Int Endod J 2013;46:763-8.
47Adorno CG, Yoshioka T, Suda H. Crack initiation on the apical root surface caused by three different nickel-titanium rotary files at different working lengths. J Endod 2011;37:522-5.