Analysis of cutting capacity and apical deviation occurrence after preparation of curved canals with ProTaper® Universal instruments, ProTaper Next®, and HyFlex® CM™

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Dr. Tiago André Fontoura de Melo and Letícia Feron analyze instruments after preparation of curved canals with three rotary systems

Abstract

This study aims to analyze the cutting efficiency and apical deviation after preparation of curved canals with three rotary systems.

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For this analysis, 45 simulated curved canals with 35° of curvature were divided according to the endodontic instrument tested in three experimental groups: ProTaper® Universal, ProTaper Next®, and HyFlex® CM™. Each set of instruments was used in five simulated canals by a single operator. At each use, the instruments were cleaned and sterilized. For the cutting efficiency analysis, the canals were weighed on an analytical balance before and after instrumentation. The difference in weight was used to measure cutting efficiency. As for the deviation analysis before and after instrumentation, the canals were filled with ink and photographed on a platform. The images were superimposed in Adobe Photoshop® software; with the ruler tool, the measurement was performed at the 1 mm and 3 mm from working length (WL) deviations. The data was processed by one-way ANOVA followed by post hoc Tukey test with 5% significance level. ProTaper Universal and ProTaper Next instruments showed a higher cutting capacity than HyFlex CM,  as the first and second use showed the highest average cut than the fifth use. At 1 mm from the WL, there was no difference between the instruments. Just at 3 mm from the WL, the HyFlex CM instruments promoted a greater deviation.

Introduction

In recent years, the NiTi alloy from the endodontic instruments has undergone heat treatment by manufacturers in order to improve their properties (Gambarini, et al., 2011). This heat treatment provides a better arrangement of the crystalline structure, which leads to a greater flexibility of the material, as well as changes in the phase percentages (different grain structure) of the alloy, which leads to improved resistance or plastic behavior (Al-Sudani, 2014).

Two existing innovative systems on the market have undergone changes in their alloys. The HyFlex CM rotary files (Coltene/Whaledent, Allstetten, Switzerland; Cuyahoga Falls, Ohio) are made from a new type of NiTi wire — controlled memory (CM), a wire that has been subjected to proprietary thermomechanical processing. It is manufactured by a special thermomechanical process, making the files extremely flexible but without the shape memory of conventional NiTi files (Zhao, et al., 2013). These instruments display a lower percentage in weight of nickel (52 Ni %wt) compared with the great majority of commercially available NiTi rotary instruments (54.5-57 Ni %wt) (Zinelis, et al., 2010). For this reason, in addition to its specific manufacturing process, HyFlex CM files do not rebound to their original shape as do conventional NiTi instruments, which, combined with their greater flexibility, may lead to a reduced risk of ledging, transportation, and perforation (Ninan, Berzins, 2013). The file system ProTaper Next (Dentsply Sirona) is also subjected to thermomechanical processing, resulting in the M-Wire NiTi that increases the flexibility of the material as reported (Alapati, et al., 2009).

The qualities of these materials with respect to the resistance to cyclic fatigue (Braga, et al., 2014; Capar, et al., 2014c), extrusion of debris (Capar, et al., 2014a; Koçak, et al., 2015), and crack formation in dentin (Capar, et al., 2014b) have been proven in numerous studies. However, very little is known with respect to the cutting capacity and effectiveness of the preparation.Therefore, this study aims to analyze the cutting ability and the occurrence of apical deviation after preparation of curved canals with ProTaper Next and HyFlex CM instruments in relation to a system already established in endodontics, the ProTaper Universal.

Materials and methods

Obtaining simulated canals and rotary systems
Forty-five blocks of simulated canals with 35º curvature, 16 mm in length, and apex diameter of 0.15 mm were used.

Endodontic instruments analyzed were ProTaper Universal, ProTaper Next, and HyFlex CM.

Preparation of the simulated canals
During the completion of the preparations, the simulated canals were fixed on a small vise (Vonder®, Curitiba, Paraná, Brazil) in order to facilitate the instrumentation. In addition, the canals were laterally wrapped in foil (Wyda Industry In-packings Ltda., Sorocaba, São Paulo, Brazil), with only the idea of the tilt direction of curvature positioned in a standardized way and always facing the right of the operator.

Before and during the canals’ preparation with the three rotary systems for each change of the endodontic instrument, the canals were irrigated with distilled water (Iodontosul – Dental South LTDA Industrial, Porto Alegre, Rio Grande do Sul, Brazil) to remove the resin scrapings and then anionic detergent Tergensol (Inodon, Porto Alegre, Rio Grande do Sul, Brazil) used for lubrication.

The preparation of the canals was performed by a single operator trained in the use of all systems. Each set of instruments was used to prepare five simulated canals.

For each of the three rotary systems, at each instrument change, an endodontic instrument type K No. 15 (Dentsply Maillefer, Ballaigues, VD, Switzerland) was manually used to WL in order to promote the removal of resin scrapings from the apical region of the canal. The WL was standardized across all simulated canals to a 15 mm in length.

The actuation of the endodontic instruments for all three mechanized systems was carried out by an electric motor X-Smart™ (Dentsply Maillefer, Ballaigues, Switzerland) with the speed and torque recommended by the manufacturer.

The technique sequence in the ProTaper Universal system follows:

  • shaping file No. 1 (3 mm below the WL)
  • shaping file X (cervical preparation)
  • shaping files Nos. 1 and 2
  • finishing files Nos. 1, 2, and 3 (in WL).

For the ProTaper Next system, the instruments used were X1, X2, and X3 (in WL).

For the HyFlex CM system, the following instrumentation sequence was used: step 1 (.08/25 for orifice opener) followed by step 2 (in WL) for apical enlargement with .04/20. For step 3 (in WL), a .04/25 file was used to continue apical enlargement; step 4 used .06/20 (in WL) for middle part shaping, and finalizing was done with step 5 (in WL) using .04/30  file for apical enlargement.

Analysis of apical deviation
clinical_demelo_fig1For the deviation analysis, before and after the completion of instrumentation of the canals, the simulated canals were placed on a platform and photographed by a digital camera D3200 (Nikon Corporation, Nagoya-shi, Tokyo, Japan) always following the same position and the same focal length.To improve the contrast in the photographic display, ink was injected (Corfix, Porto Alegre, Rio Grande do Sul, Brazil) within the simulated canals with the help of a disposable syringe.

After the photographic stage, the images were manipulated in Adobe Photoshop version 6.0. To transform the image in millimeters, a proportion was used associating the original size of the simulated canals with the image size on the computer screen, keeping the images’ sharpness and not reducing image pixels.

With the same program, the images were subjected to contrast adjustment. Each postoperative image was transformed into a 50% layer of transparency and super-imposed  the preoperative image. Thus, we observed by transparency, two images, one atop the other.

Next, we used the tool ruler to find the exact locations of the image where the deviations were measured; the predetermined analysis was 1 mm from WL and at a point located in the middle of the curvature area (3 mm from WL).

The measure of the deviation was performed with the help of the rule tool on the two points to be analyzed. The distance was measured from the opposite side wall of the curvature area of the canal before instrumentation (A) and measured from the same point for the canal position after instrumentation (B) (Figure 1).

Analysis of the cutting efficiency
Regarding the verification of cutting efficiency, each simulated canal was duly numbered and weighed on a high-precision analytical balance (Ohaus Adventurer™, Parsippany, New Jersey). This weighing was done before and after the completion of the canal preparation.

The preparation was finished, the simulated canal was irrigated, and the solution was aspirated. Immediately after, absorbent paper points No. 30 were used (Dentsply Maillefer Instruments SA, Ballaigues,
Switzerland) in order to completely dry the canal. Next, the final weight of the simulated canal was weighted.

Previously, the rotating instruments for instrumentation of the canals passed through a cleaning and sterilization process.

All endodontic instruments were cleaned before their first use, or after being removed from their original packaging, and at the end of each use.

This cleaning procedure was done in two stages, starting with the ultrasonification of the instruments in an ultrasonic tank model 151-7 (Baumer, Joinville, Santa Catarina, Brazil) containing enzymatic detergent Descrost (Multionic-Industry and Co-Mercian Products Chemical Ltd., Taubate, São Paulo, Brazil), the proportion of 10 ml to a liter of water in the ultrasonic frequency of 40 KHz, and a time of 20 minutes according to the manufacturer’s recommendations.

Next, the brushing of the instruments was performed with a soft toothbrush (Oral B, São Paulo, Brazil) humidified with a standard liquid soap (Clonex Products and Cleaning Systems Ltda., Porto Alegre, Rio Grande do Sul, Brazil) under running water until visible residues were no longer detected on the active part of the instruments.

Subsequently, the endodontic instruments were placed in a metal tray (Metallurgical Fava Indústria e Comércio Ltda., Franco da Rocha, São Paulo, Brazil) and allowed to dry randomly.

After completely dried, the instruments were individually placed in envelopes for sterilization. Sterilization took place in a Vitale 12 autoclave in which the instruments have been sterilized by exposure to moist heat to 01 atm and 127ºC for a total sterilization time of 20 minutes.

Cleaning protocol was always performed by the same operator, with a total of six cleaning and sterilization procedures for each of the instruments during the experimental part of this study.

Statistical analysis
Analysis of cutting efficiency and apical deviation between the rotary systems tested was performed by one-way variance analysis (one-way ANOVA), complemented by the post hoc Tukey test. Significance level was 5% (P ≤ 0.05). Statistical analysis was performed using SPSS 22.0 software (SPSS Inc., Chicago, Illinois).

Results

For the cutting efficiency analysis, it was possible to verify that there was no interaction between group and use (Table 1). As for the main effects of group and use (Table 2), both were significant; therefore, regardless of use, systems ProTaper Universal and ProTaper Next had significantly higher average than the HyFlex CM. Regardless of the group, the first and second use had significantly higher averages than the fifth use.

Regarding the presence of apical deviation at 3 mm from WL, no interaction was detected between group and use (Table 3). As for the main effects of group and use, only group was significant; therefore, regardless of use, the HyFlex CM system showed a significantly higher average than the instruments ProTaper Universal and ProTaper Next (Table 4).

In the presence of apical deviation at 1 mm from WL, no interaction between group and use was found (Table 5). As for the main effects of group and use, both were not significant; therefore, the apical deviation at 1 mm from WL showed no statistical difference between the mean values of the three systems (Table 6).

clinical_demelo_table1-2

clinical_demelo_table3-4

clinical_demelo_table506Discussion

According to Rapisarda, et al. (1999), the cutting efficiency and flexibility are important mechanical properties to maintain because they affect the ability of the practitioner to  safely shape the root canal system. Several studies (Capar, et al., 2014d; Deepak, et al., 2015; Elemam, et al., 2015; Uzunoglu, Turker, 2015) compared rotary instruments  for the endodontic canal preparation.

Simulated canals used in the study had a standard diameter and were compatible with size 15. Thus, the recommendation of the manufacturers that a glide path of at least size 15 should be established prior to rotary instrumentation was taken into consideration during this study as well as Elnaghy and Elsaka (2014) and Bürklein, et al. (2015).

The simulated canals were prepared having the final instrument of the apical preparation diameter of approximately .30 in all of the three systems tested, as well as the study of Read, et al. (2015). According to Schäfer and Dammaschke (2006), in clinical conditions, performing slightly longer preparation could result in accidents within the canal and undesirable weakening of the tooth structure. On the other hand, preparations with smaller diameter instruments can leave tissue debris and contaminated material within the canal, thus perpetuating the process of endodontic infections.

To verify cutting efficiency, the method used was initial weight versus final weight of the simulated canal in an analytical balance, due to its highly reliable means, safety, and easy applicability, according to Haikel, et al. (1996).

By analyzing the results, it was verified that the cutting efficiency of ProTaper Universal and ProTaper Next was higher than the HyFlex CM. The cutting ability of root canal instruments involves a complex interrelationship of different parameters such as the cross-sectional design, which seems to be a decisive parameter (Schäfer, Oitzinger, 2008), chip-removal capacity of the instrument, the helical and rake angle, metallurgical properties, and also surface treatment of the instrument (Lam, et al., 1999).

In the study conducted by Saber, et al. (2015) a smaller cutting capacity of the HyFlex CM instrument was also observed when compared to ProTaper Next. This can be attributed to the conicity difference between the instruments on these two systems (4% and 7% HyFlex CM and ProTaper Next).

According to Shabalovckaya and Anderegg (1995), the sterilization process by autoclaving causes changes in the concentrations of nickel and titanium and a decrease on the cutting properties of endodontic instruments. In the present study, we observed a decrease in cutting efficiency on the fifth use of the all three rotary systems tested when compared to the first and second use. Rapisarda, et al. (1999) found a decrease of 20% in the cutting efficiency of the instruments when subjected to seven cycles of sterilizations and 50% when subjected to 14 cycles. When instruments are subjected to a large number of sterilization cycles, they have large amounts of titanium oxide on its surface and therefore show a decrease in cutting efficiency.

However, a study from Seago, et al. (2015) observed no effect in reducing the cutting efficiency of the HyFlex CM instruments after 10 consecutive sterilization processes, as well as Borin, et al. (2008) with ProTaper Universal instruments and Becker, et al. (2009) when using ProTaper Universal and K3 systems for the preparation of five simulated canals.

Regarding the second purpose of the study —  the analysis of apical deviation after the completion of the canal preparation —  an image-overlapping methodology was employed, using before and after images of the instrumented simulated canal. In studies such as Limongi, et al. (2004) and Javaheri and Javaheri (2007) the same methodology for this type of analysis was also used.

The definition of the analyzed area in the apical third of the canal, 1 mm from WL, is due to the fact that is the place where deviations normally occur. And the middle of the curvature (3 mm from the WL) is explained by the higher tensile and compression load that the instrument suffers inside a curved root canal.

Although we observed a greater deviation at 3 mm from WL using HyFlex CM instruments, the original path of the curvature on the simulated canal at the 1 mm from the WL remained the same.

Studies such as Kumar, et al. (2013), Zhao, et al. (2013), Thompson, et al. (2014), and Bürklein, et al. (2014) also achieved the same results with the use of HyFlex CM instruments. Saber, et al. (2015) observed that both the HyFlex CM system and ProTaper Next respected dental anatomy during instrumentation of the root canal.

In this study, ProTaper Universal and ProTaper Next instruments also respected the simulated canal anatomy. This finding was also observed by Gagliardi, et al. (2015), although Zhao, et al. (2014) and Silva, et al. (2016) have obtained in their studies a greater tendency to deviations with ProTaper Universal instruments than with ProTaper Next.

Conclusions

According to the results, the following observations can be made.

  • The cutting capacity of ProTaper Universal and ProTaper Next instruments was higher than the HyFlex CM.
  • The cutting efficiency of the three rotary systems was lower for the fifth use of instruments than it was for the first and second use.
  • Only the HyFlex CM instruments showed some deviation at 3 mm from the WL when compared to the ProTaper Universal and ProTaper Next systems.
  • None of the three rotary systems tested showed significant deviation at 1 mm from WL.

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Tiago André Fontoura de Melo, PhD in Endodontics, is a teacher in the Clinical Department, Dental School, College of Serra Gaúcha (FSG), Caxias do Sul/RS, Brazil.

Letícia Feron is a graduate student in the Clinical Department, Dental School, College of Serra Gaúcha (FSG), Caxias do Sul/RS, Brazil.

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