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Incidence of ProTaper Universal System Instrument Fractures - A Retrospective Clinical Study.

INTRODUCTION

The advent of nickel titanium (NiTi) instruments has significantly improved mechanical preparation of root canal systems, minimizing procedural errors such as step formation and apical transportation (1, 2). Nevertheless, fracture continues to be one of the main concerns associated with the use of this type of instrument (3, 4).

NiTi instruments used in continuous rotation are subjected to two types of antagonistic forces. The portion of the instrument that acts on the external part of the curvature undergoes stress, while the portion that acts on the internal part of the curvature undergoes compression. At each rotation a complete cycle of tension and compression occurs, leading to fracture of the instrument due to cyclic fatigue. For fracture by torsion to occur, the tip of the instrument must be immobilized and a rotational force must continue to be applied to the instrument. When the plastic limit of the instrument is broken by virtue of the application of force, fracture occurs (3).

Some factors, such as the instrument design, may seriously influence the mechanism of fracture by torsion, since the cutting blades act as stress concentrators that promote structural defects (3). On the other hand, instruments that have radial surfaces tend to have greater mechanical strength (5).

The ProTaper Universal System (Dentsply/Maillefer, Ballaigues, Switzerland) is still one of the automated systems most used for root canal preparations (6, 7). Due to its triangular cross-section and progressive taper, the rotational friction between the blades of the instrument and dentine is reduced, thus minimizing fatigue, without compromising its cutting capacity (8, 9). However, there are few studies that have clinically evaluated the incidence of fracture of the instruments of this system (10, 11).

Therefore, the aim of this retrospective clinical study was to evaluate the incidence of ProTaper Universal System instrument fractures, associated with observation of the arch, group of teeth, and root thirds in which these fractures occurred. The null hypothesis tested was that these factors would not interfere in increasing the incidence of fracture of the instruments of this system.

MATERIALS AND METHODS

Clinical and Data Collection Procedures

After approval from the Clinical Research Ethics Committee of the Paranaense University (CAAE: 58036016.4.0000.0109), this retrospective clinical study was conducted by means of analyzing charts, clinical record cards and radiographs of patients submitted to endodontic treatments performed by postgraduate students of the second year of the Specialization Course in Endodontics of FUNORTE/SOEBRAS (Florianopolis Nucleus), Florianopolis, Santa Catarina, Brazil, between March 2009 and March 2014. All students received exactly the same laboratory training prior to performing treatments on patients.Therefore, all the patients who participated in this study, or their legal guardians, signed the Term of Free and Informed Consent, in which they authorized the treatment to be performed.

To standardize the sample, the teeth selected were those that had been submitted to identical biomechanical preparation protocols, using the ProTaper Universal System (Dentsply/Maillefer) after anesthesia and placement of the rubber dam: 1) coronal access; 2) localization and initial preparation of the root canal entrances with instrument SX; 3) preparation of the cervical and middle third(s) with Gates Glidden Drills (Dentsply/Maillefer); 4) determination of the working length(s) with the use of an electronic foraminal locator; 5) manual glide path creation with the Flexo-file #15 (Dentsply/Maillefer) instrument up to the working length; 6) use of instruments S1 and S2; and 7) conclusion of the preparation by means of using instruments F1, F2, F3, F4 or F5, depending on the anatomical diameter of the canal. Canals with anatomical diameters of 8 and 10, 15 and 20, 25 and 30, and above to 30 were instrumented until F2, F3, F4 and F5 files, respectively. At each change of instrument, 2.5 mL of sodium hypochlorite solution (Formula & Acao, Sao Paulo, SP, Brazil) was used in the concentrations of 2.5% for cases of biopulpectomy, and 5.25% for the cases of pulp necrosis. As the final irrigation protocol, 3 mL of 17% EDTA (Biodinamica, Ibipora, PR, Brazil) was applied for 3 minutes within the root canals to remove the residual smear layer. Afterwards, the canals were filled by the single cone technique, with AH Plus Cement (Dentsply-DeTrey, Konstanz, Germany).

Engine-driven files were used adapted to an electric motor (X-Smart, Dentsply-Maillefer) with a 16:1 reduction handpiece using recommended torques (1.5 Ncm for S2; 2.0 Ncm for F1; 3.0 Ncm for SX, S1, F2, and F3; respectively) and rotation speed (250 rpm). Instruments were discarded and replaced when they were worn, fractured, or with any other discernible defects observed by using a loupe at 4x magnification (EyeMag Pro S; Carl Zeiss do Brasil Ltda., Sao Paulo, SP, Brazil). Each instrument was used for a maximum of 4 times (10). Instruments that had been used in very complex or severely curved canals were discarded at once (11).

In case of instrument fractures, the occurrence was noted on the patient's clinical record chart, followed by radiographic exam for proof and localization of the fractured instrument (canal and root third). Based on the data collected, 1031 teeth were selected, totaling 2355 root canals.

Statistical Analysis

The general incidence of instrument fractures and their frequency, considering the group of teeth, arch and root thirds, were cataloged and the data obtained were statistically analyzed (Exact Fischer test, with level of significance of 1%) by using the Minitab 17.0 Software (Minitab Inc., State College Philadelphia, PA, USA).

RESULTS

The total number of teeth and root canals evaluated and the incidence of fractures are summarized in Table 1.

The general percentage of fractures, considering the number of teeth and number of root canals may be visualized in Table 2. Instrument fractures occurred more frequently in the first (8.8%) and second (9.6%) mandibular molars, however, without statistically significant difference between them (p=0.81).

In the first and second maxillary molars, the incidence of fracture was 4.7% and 5.1%, respectively, also without significant difference (p=0.81) (Table 3). Considering the dental arches (maxillary and mandibular), the fractures occurred with significantly higher frequency in the mandibular arch (66.7%), in comparison with the maxillary arch (33.3%) (p<0.01).

A significantly higher percentage of fractures occurred in the apical third (84.4%) compared with the middle third (15.6%) (p<0.01). Comparison of the values may be seen in Table 4.

DISCUSSION

Cleaning and shaping of the root canal system has become safer with the advent of instruments made of NiTi, by significantly reducing the incidence of apical transportation, root perforations and zip formation, in comparison with treatments performed with stainless steel instruments (12). The aim of this retrospective clinical study was to evaluate the incidence of ProTaper Universal System instrument fractures, associated with observation of the arch, group of teeth, and root thirds in which these fractures occurred. The null hypothesis was rejected because the group of teeth and root third had significant influence on the occurrence of fractures. Considering the number of teeth (1031) and root canals (2355) evaluated, the general incidence of fracture was 4.4% and 1.9%, respectively. These rates can be considered low, mainly when considering the number of root canals evaluated. These results were in agreement with those obtained in previous studies that also obtained low incidence of NiTi instrument fracture (10, 13-16).

Analyzing the incidence of fracture of ProTaper Universal System instruments, Wolcott, et al. (10) and Di Fiore, et al. (13) found a 2.4% and 0.41% rates of fracture in 4652 and 828 root canals evaluated, respectively. One of the main reasons associated with the higher instrument fracture rates observed in the present study compared with both studies previously cited was the experience of the operators. In the study by Wolcott et al. (10), operators were endodontists with 15 years of experience, on an average. In the study by Di Fiore et al. (13), the operators were students from the Endodontics Residency Program at New York University. In the present study, the operators were also postgraduate students, however, in Brazil, postgraduate courses usually take place once a month. On the other hand, endodontics residency programs of american universities happen in full time. Therefore, students are subjected to a longer training load, which gives them a significantly greater clinical experience. In previous studies, the experience of the operators has been indicated as an important variable for the reduction of fracture incidence when using different NiTi systems (17, 18). Pirani et al. (19) also found a relatively high rate of fracture (5.3%) of ProTaper Universal System instruments when used by postgraduate students of the University of Bologna. However, in this study it is necessary to consider the greater complexity of the interventions because they were endodontic retreatments.

In the present study, 93.3% of the fractures occurred in molars, results that were corroborated by several studies (10, 12, 13). This finding was due to the anatomic characteristics found in these teeth, which imposed greater difficulties during instrumentation, predisposing to a higher incidence of instrument fractures (20-23). Moreover, it is worth pointing out that 66.7% of the fractures occurred in mandibular molars, and of these, 80% occurred in the mesial root. The mesial root of mandibular molars, in addition to the curvature toward the distal direction frequently observed, may also present curvatures in the bucco-lingual direction, which are not visualized radiographically (24, 25). This may have significantly contributed to the high rates of fracture observed specifically in this group of teeth, because this anatomic complexity results in greater rotary flexure of the instrument, thereby concentrating the forces of stress that may cause premature failure of the NiTi alloy (26).

Similarly, when the root third was considered in the analysis, 84.4% of the fractures occurred in the apical third. When NiTi instruments act in the apical third, they are subject to fracture by both torsion and rotary flexure by virtue of the smaller dimensions of the root canal, in addition to the eventual presence of curvatures (11, 26, 27).

In spite of the greater flexibility and elasticity, in comparison with manual instruments made of stainless steel, automated NiTi instruments tend to present a higher rate of fracture during root canal preparations (12). Nevertheless, as observed in the present study, this incidence was low considering the number of teeth and root canals evaluated, demonstrating that they were safe to use.

CONCLUSION

Based on results obtained from this study, it can be concluded that the incidence of rotary ProTaper universal files fracture is low amongst postgraduate students. The arch (mandibular) and root third (apical) have a significant effect on the incidence of these fractures.

Disclosures

Conflict of interest: The authors deny any conflict of interest.

Ethics Committee Approval: CAAE: 58036016.4.0000.0109

Financial Disclosure: The authors declared that this study has received no financial support.

Authorship contributions: Concept--R.M.; Design--R.M.; Supervision--C. de S.J., M.F.C., A.P.P., J.S.J., L.C.-S., L.R.A.; Fundings - R.M., L. da F.R.G.; Materials - R.M., L. da F.R.G.; Data collection &/or processing - R.M.; Analysis and/or interpretation--C.de S.J., M.F.C., A.P.P., J.S.J., L.C.-S., L.R.A.; Literature search -R.M.; Writing - R.M.; Critical review - R.M., L. da F.R.G.

REFERENCES

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(15.) Shen Y, Coil JM, Haapasalo M. Defects in nickel-titanium instruments after clinical use. Part 3: a 4-year retrospective study from an undergraduate clinic. J Endod 2009; 35(2):193-6. [CrossRef]

(16.) Alfouzan K, Jamleh A. Fracture of nickel titanium rotary instrument during root canal treatment and re-treatment: a 5-year retrospective study. Int Endod J 2018; 51 (2):157-63. [CrossRef]

(17.) Yared GM, Dagher FE, Machtou P, Kulkarni GK. Influence of rotational speed, torque and operator proficiency on failure of Greater Taper files. Int Endod J 2002; 35(1):7-12. [CrossRef]

(18.) Mandel E, Adib-Yazdi M, Benhamou LM, Lachkar T, Mesgouez C, Sobel M. Rotary Ni-Ti profile systems for preparing curved canals in resin blocks: influence of operator on instrument breakage. Int Endod J 1999; 32(6):436-43. [CrossRef]

(19.) Pirani C, lacono F, Gatto MR, Fitzgibbon RM, Chersoni S, Shemesh H, et al. Outcome of secondary root canal treatment filled with Thermafil: a 5-year follow-up of retrospective cohort study. Clin Oral Investig 2018; 22(3):1363-73. [CrossRef]

(20.) Briseno-Marroquin B, Paque F, Maier K, Willershausen B, Wolf TG. Root Canal Morphology and Configuration of 179 Maxillary First Molars by Means of Micro-computed Tomography: An Ex Vivo Study. J Endod 2015; 41 (12):2008-13. [CrossRef]

(21.) de Pablo OV, Estevez R, Peix Sanchez M, Heilborn C, Cohenca N. Root anatomy and canal configuration of the permanent mandibular first molar: a systematic review. J Endod 2010; 36(12):1919-31. [CrossRef]

(22.) Silva EJ, Nejaim Y, Silva Al, Haiter-Neto F, Zaia AA, Cohenca N. Evaluation of root canal configuration of maxillary molars in a Brazilian population using cone-beam computed tomographic imaging: an in vivo study. J Endod 2014; 40(2):173-6. [CrossRef]

(23.) Villas-Boas MH, Bernardineli N, Cavenago BC, Marciano M, Del Carpio-Perochena A, de Moraes IG, et al. Micro-computed tomography study of the internal anatomy of mesial root canals of mandibular molars. J Endod 2011; 37(12):1682-6. [CrossRef]

(24.) Cunningham CJ, Senia ES. A three-dimensional study of canal curvatures in the mesial roots of mandibular molars. J Endod 1992; 18(6):294-300. [CrossRef]

(25.) Kartal N, Cimilli HK.The degrees and configurations of mesial canal curvatures of mandibular first molars. J Endod 1997; 23(6):358-62. [CrossRef]

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(27.) Garcia-Font M, Duran-Sindreu F, Morello-Castro S, Mercade-Bellido M, Bueno-Martinez R, Roig-Cayon M. Failure of ProTaper rotary Ni-Ti instruments used by undergraduate students. J Clin Exp Dent 2012; 4(4):e199-203. [CrossRef]

Ricardo MACHADO, Claudemir de SOUZA JUNIOR, Matheus Felipe COLOMBELLI, Ana Paula PICOLLI, Jaci Simi JUNIOR, Leopoldo COSME-SILVA, Lucas da Fonseca Roberti GARCIA, Luiz Romulo ALBERTON

Please cite this article as: Machado R, de Souza Junior C, Colombelli MF, Picolli AP, Junior J, Cosme-Silva L, et al. Incidence of ProTaper Universal System Instrument Fractures - A Retrospective Clinical Study. Eur Endod J 2018; 3: 77-81

Clinical Practice Limited to Endodontics (R.M. [??] ricardo.machado.endo@gmail.com) Navegantes, Santa Catarina, Brazil; Department of Endodontics, (C. S.J., M.F.C.), Paranaense University - UNIPAR, Francisco Beltrao, Parana, Brazil; Department of Endodontics, (A.P.P., J.S.J.), Integrated College from North of Minas Gerais - FUNORTE/SOEBRAS, Florianopolis, Santa Catarina, Brazil; Department of Endodontics, (L.C.S.), Sao Paulo State University - UNESP, School of Dentistry, Aracatuba, Sao Paulo, Brazil; Department of Dentistry - Endodontics, (L.F.R.G.), Health Sciences Center, Federal University of Santa Catarina UFSC, Florianopolis, Santa Catarina, Brazil; Department of Veterinary Medicine and Graduate Program in Animal Science, (L.R.A.), Paranaense University--UNIPAR, Umuarama, Parana, Brazil

Received 03 May 2018, last revision received 28 May 2018, accepted 05 June 2018

Published online: 19 July 2018

DOI 10.14744/eej.2018.30592

HIGHLIGHTS

* The aim of this retrospective clinical study was to evaluate the incidence of ProTaper Universal System instrument fractures, associated with observation of the arch, group of teeth, and root thirds in which these fractures occurred.

* Instrument fractures occurred more frequently in the mandibular first (8.8%) and second (9.6%) molars, however, without statistically significant difference between them (p=0.81). In the first and second maxillary molars, the incidence of fracture was 4.7% and 5.1%, respectively, also without significant difference (p=0.81).

* The fractures occurred with significantly higher frequency in the mandibular arch (66.7%), in comparison with the maxillary arch (33.3%) (p<0.01).

* A significantly higher percentage of fractures occurred in the apical third (84.4%) compared with the middle third (15.6%) (p<0.01).

* The general percentage of fractures, considering the number of teeth and number of root canals evaluated was 4.4% and 1.9%, respectively. However, the arch (mandibular) and root third (apical) had a significant effect on the incidence of these fractures.
TABLE 1. Total number of teeth and root canals evaluated, and incidence
of fractures

Teeth             Quantity                 Maxillary arch
                  of canals
                             No. of teeth  No. of canals     No. of
                                                           fractures

Central incisors      1            51             51           0
Lateral incisors      1            49             49           0
                      2             0              0           0
Canines               1            36             36           0
                      2             0              0           0
First premolars       1             4              4           0
                      2            71            142           1
Second premolars      1            45             45           1
                      2            52            104           1
First molars          1             2              2           0
                      2             3              6           0
                      3           167            501           7
                      4            18             72           2
Second molars         1             3              3           0
                      2             6             12           0
                      3            46            138           2
                      4             4             16           1
Third molars          1             0              0           0
                      2             0              0           0
                      3             3              9           0
                      4             0              0           0
Total                 -           560           1190          15

Teeth                           Mandibular arch

                  No. of teeth  No. of canals    No. of fractures

Central incisors        15            15                 0
Lateral incisors        12            12                 0
                         1             2                 0
Canines                 13            13                 0
                         2             4                 0
First premolars         34            34                 0
                         2             4                 0
Second premolars        56            56                 0
                         2             4                 0
First molars             1             1                 0
                         7            14                 1
                       152           456                13
                        45           180                 4
Second molars            1             1                 0
                        16            32                 0
                        92           276                11
                         6            24                 0
Third molars             0             0                 0
                         5            10                 0
                         9            27                 1
                         0             0                 0
Total                  471          1165                30

TABLE 2. Number of fractures/evaluated teeth (%), fractures/evaluated
canals (%) per arch, and general mean relative to incidence of
fractures

                                      Maxillary arch  Mandibular arch

Number of fractures/evaluated teeth   15/560 (2.6%)    30/471 (6.3%)
Number of fractures/evaluated cana1s  15/1190 (1.2%)   30/1165 (2.5%)

                                       Total   Percentage of fractures

Number of fractures/evaluated teeth   45/1031          4.4%
Number of fractures/evaluated cana1s  45/2355          1.9%

TABLE 3. Ratio, number and percentage of teeth where fractures occurred

Teeth             Endodontically   Incidence of      Percentage of
                  treated teeth     fractures    fractures considering
                                                     total number
                                                       of teeth
                                                       evaluated

First premolars   Maxillary    75        1               0.1%
                  Mandibular   36        0               0%
Second premolars  Maxillary    97        2               0.2%
                  Mandibular   58        0               0%
First molars      Maxillary   190        9               0.9%
                  Mandibular  205       18               1.7%
Second molars     Maxillary    59        3               0.3%
                  Mandibular  115       11               1.1%
Third molars      Maxillary     3        0               0%
                  Mandibular   14        1               0.1%
                  Total                 45               4.4%

Teeth              Percentage of     Percentage
                   fractures in    in relation to
                  relation to the     general
                     group of        number of
                   teeth itself      fractures

First premolars        1.3%             2.2%
                       0%               0%
Second premolars       2.1%             4.5%
                       0%               0%
First molars           4.7%            20%
                       8.8%            40%
Second molars          5.1%             6.7%
                       9.6%            24.4%
Third molars           0%               0%
                       7.1%             2.2%
                        -             100%

TABLE 4. Distribution and percentage of fractures, considering the
position in the arch and root third

Arch          Root third      Total
            Middle  Apical

Maxillary    3      12 (**)   15
             6.7%   26.7%     33.3%
Mandibular   4      26 (**)   30
             8.9%   57.8%     66.7%
Total        7 (*)  38 (*)    45
            15.6%   84.4%    100.0%

(*) Exact Fisher Test (p=0.000)
(**) Exact Fisher Test (p=0.001)
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Title Annotation:ORIGINAL ARTICLE
Author:Machado, Ricardo; de Souza, Claudemir, Jr.; Colombelli, Matheus Felipe; Picolli, Ana Paula; Simi, Ja
Publication:European Endodontic Journal
Article Type:Clinical report
Date:Jul 1, 2018
Words:3636
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