Comparative Evaluation of Fracture Resistance of Endodontically Treated Bicuspids Instrumented With Hand Files, TruNatomy, ProTaper Next, ProTaper Gold, and WaveOne - An In vitro Study.

Aim: The study aims to compare and evaluate the fracture resistance of endodontically treated bicuspids instrumented with Hand Files, TruNatomy (TRN), ProTaper Next (PTN), ProTaper Gold (PTG), and WaveOne (WO). Materials and
Methods: In total, 45 extracted single-rooted human premolar teeth were selected and sectioned at or below the cementoenamel junction to obtain roots 15 mm long. The samples were divided into four experimental groups and one control group (n = 9): instrumentation with nickel-titanium (NiTi) Hand Files (control group), instrumentation with TRN files, instrumentation with PTN files, instrumentation with PTG files, and instrumentation with WO files. The samples were obturated by lateral compaction technique using gutta-percha points and AH Plus sealer. A vertical load was applied to the tooth using the universal testing machine (Tecsol-TSI-BDS-2Kn-Sr No. 170710) for the fracture resistance test. Statistical analysis was performed.
Results: The study reported that teeth instrumented with NiTi hand files exhibited the highest fracture resistance when compared to all the rotary and reciprocating file systems. Among rotary and reciprocating instruments, root prepared with TRN files showed the most significant resistance to fracture compared with PTN, PTG, and WO files.
Conclusion: Within the limitations of this study, it can be concluded that minimally invasive instrumentation of the system preserves more pericervical dentin, which may increase the resistance to fracture. Results of this study and other studies support the use of minimally tapered instruments to improve the fracture resistance of endodontically treated teeth. Copyright:

INTRODUCTION

The primary goal of endodontic therapy is to eliminate the infected dental tissue and disinfect the entire root canal using various instruments and materials. Root canal preparation with rotary endodontic files removes more root dentin and may result in weakening of dentin integrity, leading to a reduction in the fracture resistance of treated tooth.[1]

Hence, this study was done to ascertain the fracture resistance of bicuspids instrumented by recently introduced TruNatomy (TRN) files in comparison with hand nickel–titanium (NiTi) file, PTG, PTN, and reciprocating WaveOne (WO) files.

MATERIALS AND METHODS SAMPLE SELECTION AND PREPARATION

This study was submitted to and approved by the Ethics Committee of the Yenepoya University, University road, Deralakatte, Mangalore, Karnataka, India.

Forty-five intact single-rooted human premolars extracted for orthodontic and periodontal purposes were selected. Moreover, the coronal portion of all teeth was removed using a diamond-coated disc under water-cooling, leaving roots approximately 15 mm in length. #10 K-file (MANI, JAPAN) were introduced with ethylenediaminetetraacetic acid (EDTA) gel into the root canal to determine the working length and establish a glide path. Forty-five teeth were then be divided into five groups (one control group and four experimental groups) of nine samples each. Cleaning and shaping of the root canal system in five different groups were done as follows.

Group 1: Step-back technique using K-file (control group)

Nine premolars were shaped with ISO 2% NiTi K-files by manual instrumentation. Apical enlargement was progressively done from ISO #10 to #25 using the following sequence 10, 15, 20, and 25, followed by step-back preparation till #40.

Group 2: Crown-down technique using TruNatomy files

Nine premolars were instrumented with 4% TRN files system (Dentsply Sirona, Maillefer, Ballaigues, Switzerland) using crown-down technique. ISO #10 K-file was used to determine the working length and patency. Canal orifice was shaped using TRN Orifice Modifier and a glide path was achieved using #17 TRN Glider till the working length. Subsequently, #20 TRN small files and #26 TRN Prime shaping files were used to complete the preparation till the working length.

Group 3: Crown-down technique using ProTaper Next

Nine premolars were instrumented by PTN files (X1 and X2) using a gentle in-and-out motion with endomotor. The root canal orifices were flared using Sx file from the universal ProTaper set (Dentsply, Tulsa Dental, Tulsa, OK), followed by X1 and X2 PTN files with outward brushing motion till working length.

Group 4: Crown-down technique using ProTaper Gold

Nine premolar teeth were prepared with PTG files (Sx–F2) with a preprogrammed X Smart Plus (Dentsply, Maillefer) at a torque and rpm as recommended by the manufacturer. The coronal third was preflared using Sx followed by sequential files till working length in the following sequence S1 (#17.06), S2 (#20.06), F1 (#20.07), and F2 (#25.08).

Group 5: Crown-down technique using the WaveOne system

Nine premolar teeth were prepared with WO primary files (25/0.08, Dentsply Maillefer, Ballaigues, Switzerland) using X Smart Plus Endomotor (Dentsply, Maillefer). The root canal orifices were flared using Sx as in Group 4. WO instrumentation with pecking motion was done after coronal flaring. The instrumentation was continued in the above manner till the file reached working length.

IRRIGATION AND OBTURATION

In all groups, irrigation was done using 5.25% sodium hypochlorite and EDTA gel was used as a lubricant for the files. After completion of the cleaning and shaping procedure, a final flush was done using 5 ml 17% EDTA for 1 min followed by a final rinse with distilled water. For all groups, obturation by lateral compaction technique was done using gutta-percha of the corresponding file system and AH Plus resin sealer.

EVALUATION OF FRACTURE RESISTANCE

The root surface area of all the samples was covered with two layers of adhesive tape and the apical root end of each specimen (approximately 4 mm in length) was embedded in a block of self-cured acrylic resin with a long axis perpendicular to the base of the acrylic block and exposing approximately 9 mm of the coronal portion of each root. The adhesive tape which was applied before implanting them in acrylic block was removed from the root surface. The obtained space was filled with light body additional silicone impression material.

The specimens were placed in the universal testing machine equipped with a controlled load and loaded through a steel conical tip (tip diameter = 0.5 mm, tapered at 60°) at their center of the canal orifice parallel to the long axis of the tooth. The continuous compressive strength force was applied at a crosshead speed of 1 mm/min. All the samples were loaded until fracture and the maximum breaking loads were recorded in Newtons (N) by a computer connected to the loading machine and each fractured specimen was then examined to evaluate the fracture pattern.[Table 1]

Table 1

Descriptive data

Group	Weight	MD	BL	MD*BL	Fracture load
H	0.4477±0.006	5.5288±0.011	7.5144±0.096	41.541±0.541	509.9133±1.033
TN	0.4511±0.007	5.525556±0.018	7.571111±0.075	41.82889±0.441	457.7244±4.484
PTN	0.4518±0.005	5.5255±0.011	7.5111±0.095	41.527±0.512	389.9456±4.43
PTG	0.4522±0.008	5.2866±0.108	7.4055±0.151	39.14±0.765	346.3533±4.157
WO	0.45±0.008	5.428889±0.093	7.468889±0.039	40.54667±0.897	267.7111±6.421

PTG: ProTaper Gold, PTN: ProTaper Next, WO: WaveOne, TN: TruNatomy, MD: Mesio-Distal, BL: Bucco-Lingual

Statistical analysis

Data were analyzed using the Statistical Package for the Social Sciences (SPSS) 22.0 (SPSS Inc., Chicago, IL, USA) and the level of significance was set at P < 0.05. Descriptive statistics were performed to assess the mean and standard deviation of the respective groups. The normality of the data was assessed using the Shapiro–Wilkinson test. Inferential statistics to find out the difference between and within the groups were done using Student's t-test and one-way analysis of variance (ANOVA) and Tukey's post hoc test.

RESULTS

One-way ANOVA test revealed a statistically significant difference between the groups regarding fracture resistance (P < 0.05). Pairwise analysis using Tukey's honestly significant difference post hoc test reported statistical significance between all the pair groups (P < 0.05).

The study reported that teeth instrumented with NiTi Hand Files exhibited the highest fracture resistance when compared to all the rotary and reciprocating file systems.[Figure 1]

Figure 1

Graphical comparison of mean fracture load values between groups under loading

Among rotary and reciprocating instruments, root prepared with TRN files showed the greatest resistance to fracture compared with ProTaper and WO files.

There is a significant difference that existed when ProTaper files were used in a rotation motion. However, significantly less force was needed to fracture the roots when WO files were used (P < 0.05) [Table 2].

Table 2

Descriptive data of fracture resistance

Group	n	Mean±SD	Minimum	Maximum
H file	9	509.91±1.03	507.73	510.97
TN	9	457.72±4.48	448.84	462.56
PTN	9	389.94±4.43	382.1	394.5
PTG	9	346.35±4.15	340.06	351.82
WO	9	267.71±6.42	259.7	275.8

SD: Standard deviation, PTG: ProTaper Gold, PTN: ProTaper Next, WO: WaveOne, TN: TruNatomy

DISCUSSION

Root canal preparation with NiTi instruments causes dentinal cracks in dentine although less as compared to stainless steel instruments.[2]

Bier et al. also reported that technological advancement in rotary NiTi instruments has improved instrument designs as well as root canal shaping. Root canal shaping procedures and rotary instrumentation with NiTi instruments can exponentially induce more crack formation compared to Hand Files.[2]

Rotary files with greater taper are reported to cause increased stress on root canal walls[3] and could be a potential factor in dentinal crack formation.[2]

Microcrack propagation leads to root fracture clinically. This predisposes to bacterial proliferation in crack lines resulting in secondary infections.[4]

The purpose of this study was to evaluate and compare the fracture resistance of teeth instrumented with NiTi files of different taper and Hand Files. The standardization of the sample is an important aspect in in vitro fracture resistance studies using natural teeth. As teeth with narrow diameters are more susceptible to a fracture.[5] Finite-element analysis can be used in the analysis of stress distributions in shaping procedure and the results were expressed as the circumferential tensile stresses were concentrated on the buccal and lingual canal walls.[6] Carvalho et al. suggested that teeth may be weaker buccolingually than mesiodistally. Thus, proving the presence of microcrack formation in buccolingual direction with lower ultimate dentin tensile strength.[47]

The newly developed TRN instruments are manufactured from heat-treated NiTi alloy with parallelogram cross-section design.[8] This file system uses a 0.8 mm NiTi wire instead of 1.2 mm NiTi wire, which offers maximum preservation of pericervical dentin[8] and tooth integrity due to instrument geometry, regressive tapers, and the slim design.[9]

PTN files as a new generation of ProTaper Universal system haves variable taper design, offset the mass of rotation, and manufactured by heat treatment using M-wire NiTi alloy to improve flexibility and increases cyclic fatigue resistance.[10] The unusual design of PTN rotary files helps in decreasing the screwing effect by minimizing dentin engagement, thus inducing less stress on the root dentinal walls. Hence, these files are associated with less microcracks. PTN produces less dentinal cracks at the apical section than the ProTaper Universal and WO systems due to the rectangular cross-section design.[1112]

Three WO single-file reciprocating system is currently available in 21 mm, 25 mm, and 31 mm length. This single-reciprocating instrument caused significantly more microcracks in the apical third dentin than rotary full sequence files, which may weaken the teeth and lower the fracture resistance. Reciprocating motion generates higher torsional forces and increases debris transportation toward the apical third.[1314]

The concentration of sodium hypochlorite used in the study was 5.25%. In a clinical scenario, the higher concentration of sodium hypochlorite is preferred over lower concentration. To enhance the antimicrobial and tissue dissolving effects of the former.[15]

Taking into account the limitations of this in vitro experiment, a silicone layer was used to simulate the periodontal ligament to avoid external reinforcement, which allowed the root to move within a limited space.[16] However, the complexity of the periodontal ligament is challenging to replicate, and these changes can manipulate the force distributions.[17]

Okitsu et al.[18] concluded that simulated periodontal ligament and loading point are less likely to influence the vertical root fracture compared to the size of the master apical file. Hence, preservation of dentin is a considerable factor to avoid vertical root fracture.

Many studies on fracture resistance compared roots instrumented with different rotary systems under in vitro experimental conditions.[19]

The former study established that increasing the root canal taper resulted in a lower fracture resistance in shaping procedures.[20] At the same time, minimally prepared teeth may act as a hindrance for disinfection,[21] which is in accordance with the previous studies done by Brunson et al.[22] who found that 4% tapered root canals seem to be less disinfected than 8% tapered root canals with the same apical diameter of preparation.[22] However, 4% tapered root canal preparation did not statistically improve the fracture resistance compared to 6% tapered preparation. On the other hand, coronal dentinal tooth structure might have affected the results.[23]

In the present study, decoronated teeth have been used to exclude the influence of access cavity preparation and the coronal maintenance of dentinal tooth structure in fracture behavior of the tested samples.

The results of this study suggest that mandibular premolars prepared with NiTi Hand Files showed higher fracture resistance. However, teeth prepared with WO file systems are more susceptible to fracture than those prepared with TRN, PTN, and PTG file systems. In rotary and reciprocating groups, the TRN group displayed higher resistance to fracture. This is mainly due to the regressive taper, which allows the instrument to maintain the 0.8 mm maximum flute diameter compared to the 1.1 mm diameter of other conventional rotary instruments.[8]

The results of this study are supported by Pawar et al. who found that samples instrumented by PTN exhibited a better fracture resistance when compared to single-reciprocating WO file systems when filled with gutta-percha and epoxy-based sealer.[14] The present study may explain the idea of a minimally tapered root canal preparation that has previously been described in the literature.[24] Minimal taper results in the preservation of the pericervical dentin and increases the fracture resistance of a tooth.[2425]

Various studies have been carried out and have reported that the load required to fracture the teeth instrumented with Hand Files (taper 2%) is more compared to rotary files.[26]

The prevalently used technique for obturating an endodontically treated tooth is the lateral compaction technique. However, the literature suggested that it has a likelihood of Vertical root fracture (VRF) with this technique.[27] On the contrary, many studies have been reported that the load generated by lateral compaction is less. Hence, it is uncertain as to whether different root canal-filling techniques could result in VRFs of endodontically treated teeth.[28]

To the best of our knowledge, the fracture resistance exhibited by the teeth instrumented using TRN have not been reported in the literature. Hence, the study was designed and carried out to compare the fracture resistance exhibited by mandibular premolar teeth after instrumentation with Hand Files, TRN, PTN, PTG, or WO. Further studies are required to assess the short-and long-term impacts of instrumentation on the formation of cracks and vertical root fractures.

CONCLUSIONS

Within the limitations of this study, it can be concluded that fracture resistance of the teeth prepared by TRN file was higher than those in the teeth prepared by PTN file, PTG, and WO. Compared to Group I, the use of the TRN file system caused significant reduction in fracture resistance. The results of this study and of other studies support the use of minimally tapered instruments to improve the fracture resistance of endodontically treated teeth.

Ethics

Yenepoya Ethics Committee 2 Yenepoya (Deemed to be University) DCGI reference number: YEC2/186.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.