Apical transportation of manual NiTi instruments and a hybrid technique in severely curved simulated canals.

AIM: To evaluate the apical transportation induced by two instrumentation techniques in severely curved simulated canals.
MATERIALS AND METHODS: Forty simulated canals were divided into two groups (n = 20), according to the following instrumentation techniques: ProTaper Universal Manual System and a hybrid technique. The simulated canals in the ProTaper group were prepared following the technique recommended by the manufacturer: SX files in the cervical third of the root canal and S1, S2, and F1 files up to the working length. In the hybrid group, preparation was performed with K-files sizes 15, 20, and 25 to the working length, followed by cervical preparation with Gates Glidden burs 1, 2, and 3. Apical finishing was performed with the ProTaper manual files S2 and F1. To analyze apical transportation, the simulated canals were photographed before and after preparation at ×8. The images of the root canals pre- and post-instrumentation were superimposed to measure the distance between the inner and outer walls along the first 3 mm of the apical third.
RESULTS: The hybrid group presented the highest apical transportation values; however, with no statistically significant difference in comparison with the ProTaper group (P > 0.05).
CONCLUSIONS: Both techniques induced similar apical transportation in the original path of the simulated root canals.

INTRODUCTION

The main objective of endodontic therapy is to clean and shape root canals; however, the procedure is more difficult in severely curved root canals.[1] In addition to the anatomy of curved root canals, their preparation is limited as it is difficult to control the instrument inside the endodontic space.[2] Due to their curvature, force components tend to displace the instrument in the opposite direction, which may cause apical transportation, as well as root perforations.[23]

Apical transportation consists in a change of the original trajectory of the canal in the apical region, and occurs because of the wear of the concave wall of root canals with accentuated degree of curvature.[4]

Despite all innovations, manual instrumentation with stainless steel instruments is still widely used in clinical practice as tactile sensitivity brings confidence to the operator in some particular cases.[4] However, in severely curved canals, the use of such instruments with a diameter larger than size 25 may cause apical transportation from the original path of the root canal. In these cases, the use of NiTi instruments is recommended.[56]

In order to improve root canal cleaning during biomechanical preparation, new instrumentation techniques, such as the hybrid ones, have been developed.[7] These techniques combine different instrument systems for the different root canal thirds, significantly reducing the risk of instrument fracture while maintaining the original morphology of the root canal without becoming ineffective.[67]

The present study evaluated the apical transportation induced by two instrumentation techniques: Conventional (manual) NiTi instruments, and a hybrid technique using both NiTi and stainless steel instruments, in severely curved simulated canals. The null hypothesis tested was that there would be no significant difference in the performance between the different instrumentation techniques.

MATERIALS AND METHODS

Forty severely curved simulated root canals in clear resin blocks (IM Brazil Ltda., São Paulo, SP, Brazil), measuring 16 mm in length and angle of curvature of 40°, were used in the present study. Prior to biomechanical preparation, a size of 10 K-type file was introduced into the canal, until the active tip of the file exceeded the apical foramen, retracting it until it was within the apical limit of the root canal, thereby obtaining the real working length (16 mm). Then, the simulated canals were randomly divided into two groups (n = 20), according to the instrumentation technique:

ProTaper Universal Manual System (Dentsply/Maillefer, Ballaigues, Switzerland): SX file in the cervical third of the canal and files S1, S2, and F1 up to the working length, respectively, according to manufacturer's recommendations.

Hybrid technique: Instrumentation with stainless steel K-files sizes 15, 20, and 25 (21 mm long, Dentsply/Maillefer) to the working length; cervical preparation with Gates Glidden burs 1, 2, and 3, (Dentsply/Maillefer); and apical preparation with the ProTaper universal manual files S2 and F1 in the working length.

The irrigating solution used was 2 ml of 2.5% sodium hypochlorite (Rio Química, São José do Rio Preto, SP, Brazil) at each change of instrument. A new set of stainless steel K-type files and ProTaper universal files was used for every five simulated canals instrumented in both groups. All procedures were performed by a single operator, specialist in endodontics.

To analyze the apical transportation, the simulated canal blocks were photographed before and after preparation with a digital camera (Sony Cybershot 8.1, Tokyo, Japan) coupled to an operative microscope (DF Vasconcelos, São Paulo, SP, Brazil) at ×8 and focal distance of 20 cm.

The images were analyzed using the Image J 1.47 software (National Institutes of Health, Bethesda, MD, USA). The pre- and post-instrumentation images of the simulated canals were superimposed on each other and the distance between the outer wall of the instrumented canal and the inner wall of the noninstrumented canal were measured at three different points to determine apical transportation [Figures 1 and 2]. The first point was measured at 1 mm from the apex, the second point at 2 mm, and the third at 3 mm. The mean value of the measurements performed at the three different apical points was considered to obtain the value of apical transportation for each sample.

Figure 1

(a) Simulated canal from ProTaper group. (b) Preinstrumented canal colored in red. (c) Postinstrumented simulated canal. Note the apical transportation induced by instrumentation (arrow). (d) Postinstrumented canal colored in blue. (e) Superimposed canals for 3 mm-apical area evaluation (indication)

Figure 2

(a) Simulated canal from the hybrid group. (b) Preinstrumented canal colored in red. (c) Postinstrumented simulated canal. (d) Postinstrumented canal colored in blue. (e) Superimposed canals for 3 mm-apical area evaluation (indication)

The normal distribution of data was tested by the Kolmogorov-Smirnov test, and the values obtained for the different tests were statistically analyzed (Kruskal–Wallis, Dunn's Multiple Comparisons test - P < 0.05) using the GraphPad Prism 4.0 program (GraphPad Software, La Jolla, CA, USA).

RESULTS

The mean values of apical transportation in millimeters (mm) are shown in Table 1. The hybrid group presented the highest apical transportation values; however, no statistically significant difference was found in comparison with the ProTaper group (P > 0.05).

Table 1

Mean values (mm) and SD for apical transportation

DISCUSSION

The aim of the study was to evaluate the apical transportation induced by two instrumentation techniques: Manual NiTi instruments of the ProTaper Universal System, and a hybrid technique using both NiTi and stainless steel instruments. Based on the results, the null hypothesis was accepted as the two instrumentation techniques did not present any significant difference regarding apical transportation.

Root canal preparation, particularly in curved root canals, requires care during instrumentation to obtain a continuous rounded shape, with smooth surface and absence of foramen transportation.[46] Therefore, different techniques and different types of instruments have been constantly developed to increase the clinical success of endodontic therapy.[89]

The development of instruments made of NiTi alloys enabled the better preservation of the internal anatomy of root canals, since the great difference of these metal alloys is their mechanical properties, with emphasis on their shape memory and superelasticity.[10]

Among the currently available NiTi files, the ProTaper (Dentsply/Maillefer, Ballaigues, Switzerland) is one of the most popular systems.[6] The system may consist of manual or rotary instrumentation, including instruments for preparation of the cervical and middle thirds of the root canals; and apical finishing.[6] The instruments for cervical and middle thirds preparation are classified into Shaping 1 (S1), Shaping X (SX), and Shaping 2 (S2). The finishing instruments are subdivided into Finishing 1 (F1), Finishing 2 (F2), and Finishing 3 (F3).[6]

To conduct the study, severely curved simulated canals in resin blocks were used to evaluate the behavior of endodontic instruments and observe the changes in root canal morphology during instrumentation. The use of artificial canals made of resin offer several advantages over human teeth due to the standardization of canal curvature and diameter, and the hardness of the walls.[11]

Currently, more accurate methods for apical transportation evaluation, as micro-computed tomography, are used in endodontic research.[412] Micro-computed tomography equipment allows evaluating both external and internal root canal morphology before and after biomechanical preparation in a three-dimensional manner, differently from the radiographic method, which produces a two-dimensional view of three-dimensional anatomical structures.[412] In this study, digital images of the artificial canals made of clear resin were analyzed using a specific software for this purpose. This method allows superimposition of pre- and post-instrumentation images, ensuring an accurate analysis of the root canal morphology alteration induced by the instrumentation techniques.

In the present study, the transportation was measured in the last 3 mm of the apical third, since the higher rates of apical transportation occur in this area.[12] Studies reported that manual instruments with a diameter greater than a size 25 file are not recommended for curved canals instrumentation, as curved canals promote greater contact of the instrument with the canal walls, causing transportation from its original path.[5] In these cases, the use of NiTi instruments is recommended.[5]

During instrumentation with the hybrid technique, a size of 25 K-file was used throughout the entire working length. However, apical finishing was performed with instruments F1 and S2 of the ProTaper Universal System. By using the NiTi files in the apical third, the two techniques induced similar apical transportation.

At lower temperatures, NiTi alloy is presented in the martensitic phase, which provides greater malleability and flexibility; however, it may be easily deformed when stress is applied.[1314] At higher temperatures, above the critical temperature of the alloy, NiTi remains in the austenitic phase, which has the characteristic of being the strongest and hardest phase of the alloy, with properties similar to those of titanium.[1314] When submitted to stress, NiTi alloys may undergo a change of phase, thereby altering their characteristics.[15]

One of the main advantages of the ProTaper Universal System is the cutting capacity, which reduces clinical working time.[16] On the other hand, the benefits of hybrid techniques are significant, because fewer instruments are needed, decreasing the time required during the clinical procedure.

It is valid to emphasize that the increased resistance of the stainless steel files favors instrumentation of curved canals, since the NiTi files are sterilized for reuse, which may compromise the fatigue strength of the alloy if they are used without prior root canal preparation with stainless steel files.[17] Several studies have evaluated and demonstrated the quality of rotary and NiTi manual instruments used in endodontic practice.[181920] However, stainless steel files are still widely used for root canal treatment. New techniques and further research must be developed to assure greater safety, efficiency, and convenience for effective and appropriate treatment.

CONCLUSIONS

Despite the limitations of this in vitro study, it may be concluded that both techniques induced similar apical transportation. The hybrid technique may be used for biomechanical preparation without interfering significantly in root canal original morphology; however, further studies are needed to assure its effectiveness.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.