| Abstract|| |
Background: Under axial loading, there is stress concentration around the cervical areas especially in the premolars because of their location in the arch. Aim: To evaluate and compare flexural fracture resistance of mandibular premolars after instrumentation with ProTaper®Universal, ProTaper Next®, OneShape®, and WaveOne® endodontic file systems. Methods: Seventy-five mandibular premolar teeth with single straight canals were divided into five different groups (n = 15): Group A: Control, Group B: ProTaper Universal (PTU), Group C: ProTaper Next (PTN), Group D: OneShape, Group E: WaveOne. The teeth were instrumented as per the manufacturer's instructions. After obturation and core placement, the teeth were placed in a customized jig mounted on a universal testing machine where force was applied at 45° on the buccal cusp and the force required to fracture the teeth was noted. Results: Amongst all the instrumented groups, WaveOne showed the highest resistance to fracture (1065.56 ± 175.05) and the control group was 1104.13 ± 188.42. All groups showed a significant difference in the fracture load values with the control group. However, there was a statistically significant (P < 0.05) difference seen with WaveOne vs PTU and WaveOne vs OneShape. Conclusions: Root canals instrumented with reciprocating motion have better flexural fracture resistance than continuous rotary motion. Mandibular premolars when instrumented with ProTaper Universal and OneShape endodontic files showed similar fracture resistance, proving that dentin removal does not depend on the number of files used.
Keywords: Flexural load, fracture resistance, instrumentation, ProTaper NEXT, root canal shaping, WaveOne
|How to cite this article:|
Aidasani GL, Mulay S, Borkar A. Comparative evaluation of flexural fracture resistance of mandibular premolars after instrumentation with four different endodontic file systems: An In Vitro study. Indian J Dent Res 2020;31:701-5
|How to cite this URL:|
Aidasani GL, Mulay S, Borkar A. Comparative evaluation of flexural fracture resistance of mandibular premolars after instrumentation with four different endodontic file systems: An In Vitro study. Indian J Dent Res [serial online] 2020 [cited 2021 Jan 21];31:701-5. Available from: https://www.ijdr.in/text.asp?2020/31/5/701/306439
| Introduction|| |
Successful root canal therapy depends mainly on the removal of micro-organisms through chemo-mechanical debridement of the root canal system. Extremely variable root canal anatomy influences canal preparation due to the lack of visualization of the root structure from radiographs.
The most challenging difficulties in chemo-mechanical preparation of the root canal system are identification, accessing and enlargement of the canals without any procedural errors and maintaining and establishing adequate working length during cleaning and shaping procedures. Stainless steel files have shown to create aberrations, possibly due to the inherent stiffness of stainless steel, which is aggravated by instrument design and root canal shape. The nickel–titanium instruments offer many advantages over conventional files. The evolution of designs and the implementation of new endodontic instruments emphasizes different aspects of the endodontic workflow. To reduce the risk of instrument fracture and minimize torsional loads, most rotary techniques require a crown-down approach.
With the frequent advances in dentistry now variable tapered file systems are designed. Shaping the root canal using greater tapered endodontic instruments are presumably to eliminate more of the contaminated dentin at coronal third and make adequate way for the irrigating solutions and antibacterial agents to eliminate bacterial load. However, preparation procedures tend to damage the root dentin, which may result in fractures or craze lines. The amount of remaining sound tooth structure is directly proportional to the strength of endodontically treated teeth. As a result, under axial and compressive loading in endodontically treated teeth, there is stress concentration around the cervical areas especially in the premolars because of their location in the arch.
The most susceptible roots to fracture are those with a narrow mesiodistal diameter compared with the buccolingual dimension as in mandibular premolars. Removal of excessive dentin, exertion of more stress on canal walls, and increased crack propagation are resulted because of the mechanical instrumentation with greater taper instruments, which could inevitably prompt vertical root cracks and may even require tooth extraction. Various endodontic multiple and single file systems are available, hence the main purpose of this study was to compare the flexural fracture resistance of mandibular premolars after instrumentation with various endodontic file systems.
| Materials and Methods|| |
Seventy-five freshly extracted mandibular first premolars [Figure 1] were selected and radiographs were taken to confirm a single canal. Ethical clearance was obtained from the institutional ethics committee. The teeth were stored in saline after cleaning with ultrasonic scalers. The roots were covered with a single layer of aluminum foil [Figure 2] which was inserted in acrylic resin set in a metal mold. They were then removed from the acrylic resin and after removing aluminum foil from the root surface, a light body silicon-based material was used to fill the space created by the foil to simulate periodontal ligament [Figure 3]. The roots were replaced in the acrylic resin material. The access cavity was prepared using a round diamond point followed by a non-end cutting tapered fissure diamond point. Patency was checked by inserting #10 K-file in the root canal.
Teeth were then divided into 5 different groups:
Group A- Intact teeth (Control) (n = 15)
Group B- ProTaper Universal endodontic files (n = 15)
Group C- ProTaper Next endodontic files (n = 15)
Group D- OneShape endodontic file (n = 15)
Group E- WaveOne endodontic file (n = 15).
The root canal shaping procedures were performed according to the manufacturer's instructions for each endodontic file system.
Group B: The sequence used for instrumentation was as follows: SX, S1 (17/.02), S2 (20/.04), F1 (20/.07), F2 (25/.08). For each ProTaper Universal file, the individual rotational speed (300 rpm) and the torque limit programed in the file library of the motor was used. The first three shaping files were used with a brushing motion away from the root concavities before light resistance is encountered. And the last two finishing files were used until the working length was reached.
Group C: ProTaper Next files were used in the sequence ProTaper Universal SX and then ProTaper Next X1 (17/.04), X2 (25/.06) at a rotational speed of 300 rpm and 200 g/cm torque. Each file was used in a brushing motion similar to that used with ProTaper Universal.
Group D: OneShape file #25/.06 was used in a slow in and out pecking motion with speed and torque set 350 rpm and 4 Ncm, respectively.
Group E: The primary file(#25/.08) in the WaveOne file system was used in a reciprocating handpiece [Figure 4] at speed not more than 350-400 rpm. The file was used in a slow in and out pecking motion. The flutes were cleaned after every 3 pecks. The instrument was worked till apical portion of roots were adequately cleaned using standardized amount of 17% EDTA (RC-Help) as lubricant, intermittent irrigation was done with 3% NaOCl.
Apical gauging was done and master cone (25/.06) showing tug back was selected. RC sealer (AH Plus) was dispensed and mixed as per manufacturer's instructions and sealer was carried to the canals using lentulo spiral. The master cone was placed. A finger spreader of size 25 was penetrated 2 mm shorter than the working length. Accessory cones were then placed if required and excess material was seared off at the root canal orifice and plugger was used for vertical condensation. The composite core build-up was done using self-etching and bonding protocol was done immediately post obturation. The samples were stored at room temperature for 24 hrs.
Load application on study model
Samples embedded in acrylic blocks were placed in a custom made jig and load was applied at an angulation of 45° at a crosshead speed of 3 mm/min, to the tip of the buccal cusp of the specimen teeth until fracture [Figure 5].
|Figure 5: Load application on the study model placed in a customized jig|
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The data were analyzed with Statistical Package for Social Sciences (SPSS) for Windows, version 25.0 (IBM Corp., Armonk, N.Y., USA). Confidence intervals were set at 95% and values of P < 0.05 were interpreted as statistically significant. The results were expressed as the force required to fracture roots in each study group. One way analysis of variance (ANOVA) was used to compare the mean force values recorded for the study groups. Tukey's Post Hoc test was computed to analyze between-group differences for force values.
| Results|| |
Amongst all the instrumented groups, WaveOne showed the highest resistance to fracture, followed by PTN (948.14 ± 120.90) and PTU and OneShape was almost similar. The fracture resistance was highest (1065.56 ± 175.05) for the control group [Table 1]. All groups showed a significant difference in the fracture load values with the control group. However, there was a statistically significant (P < 0.05) difference seen with WaveOne vs PTU and WaveOne vs OneShape [Table 2].
|Table 1: Comparison of Fracture Load (n) for different endodontic file systems|
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|Table 2: Pairwise comparison of Fracture Load (n) for different Endodontic file systems|
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| Discussion|| |
Since greater taper instruments have revolutionized endodontic techniques today, their advantages with respect to more predictable clinical outcome are unarguable. However, greater taper instruments are prone to remove more root canal dentin than conventional root canal instrumentation. Thus, weakening the treated tooth and reducing the fracture resistance. Previous studies suggest that dentinal cracks induced by NiTi greater taper instruments were unavoidable. Various comparative experiments have been conducted in the past to evaluate the propagation of dentinal cracks formed after instrumentation using different greater taper file systems and these cracks are inversely proportional to fracture resistance of the treated tooth, i.e., more the crack propagation lesser the fracture resistance. Gher et al. and Cohel et al. have reported a direct relationship between the incidence of vertical root fracture and endodontically treated teeth. Because of the idea of its confounded analysis, root cracks regularly go undiscovered for more than 10 years by and large post-treatment.
Mandibular premolar teeth were selected due to the increased prevalence of vertical root fractures as reported by Tamse and Lustig. It was additionally detailed that the occlusal load on mandibular premolars while biting is three times higher contrasted with other teeth. This hazard is higher, particularly in mandibular premolars in which the mesiodistal (MD) width is much smaller than the buccolingual. Heymann et al. announced that mandibular teeth experience more prominent tooth flexure due to the lingual inclination of clinical crowns. Furthermore, smaller cross-sectional measurements in the coronal third of premolars undergo greater cuspal flexure as compared to anterior teeth., This study evaluated and compared flexural fracture resistance of mandibular premolars on instrumentation with ProTaper Universal, ProTaper Next, OneShape and WaveOne endodontic file systems.
Overall study samples were divided into 5 different groups, Group A (Control) where unprepared teeth were evaluated for fracture resistance, Group B (PTU), Group C (PTN), Group D (OS), Group E (WO). In overall study groups Control (Group A), i.e., intact teeth showed better fracture resistance followed by WaveOne (Group E), ProTaper Next (Group C), ProTaper Universal (Group B), and OneShape (Group D), respectively. The justification for these may be attributed to the design configuration of various endodontic instruments. ProTaper Universal progressively tapered design replicates the Schilderian Envelope of Motion technique and serves to significantly improve flexibility, cutting efficiency, and safety. Another feature of the ProTaper instruments relates to their convex, triangular cross-section, which enhances the cutting action while decreasing the rotational friction between the blade of the file and dentin. ProTaper files have a changing helical angle and pitch over their cutting blades. The ProTaper files have a non-cutting, modified guiding tip. ProTaper Universal has increasing taper 3.5% to 19%, causing higher removal of dentine. Progressively tapered design along with triangular convex cross-sectional design could have led to aggressive cutting; whereas study groups instrumented with ProTaper Universal showed the least amount of fracture resistance. The ProTaper F2 has a 0.08 taper in the apical portion, removing more apical dentine and hence might have caused more dentinal defects.
ProTaper Next has an off-centered rectangular design results in lowering of the screw effect, dangerous taper lock, and torque on any of the given file by reducing the contact between the file and the dentin. PTN files produce a distinctive asymmetrical rotary motion and, at any given cross-section, the file contacts the wall at particularly 2 points.,
One Shape instrument's unique design helps it to guide down the glide path by 3 cutting edges. Its flexibility assures perfect respect to the original canal path and curvature. Variation in cross-sections offers an optimal cutting action in 3 zones of the canal.
WaveOne endodontic instrument is a single file used in reciprocating working motion, which consists of an unequal clockwise and counterclockwise motion. The greater angle of the counterclockwise rotation ensures the apical advancement of the file while the clockwise motion disengages the file. The reciprocating action acts to reduce the problem of taper lock by continually reversing the direction of rotation and minimizes torsional and flexural stresses on the instrument.
WaveOne induced a lesser number of dentinal cracks compared to ProTaper Universal they stated that this might be due to M-wire technology that imparts more flexibility to WaveOne instruments and that might contribute to lesser dentinal cracks. Also, WaveOne primary files have a noncutting modified tip and a unique cross-sectional design throughout the length of their active portions (a modified convex triangular cross-section at the apical end and a convex triangular cross-section at the coronal third). This is in agreement with the study done by Kansal et al. 2014, comparing reciprocating and rotary motions of endodontic file systems on the effect of dentinal defect formation. They found that the incidence of cracks is less with instruments that are working in reciprocating motion when compared to continuous rotary motion. The reciprocating motion of the instruments that produced lesser stress on dentinal walls during instrumentation.
The results of this study are in disagreement with a study conducted by Burklein et al. (2013), where they evaluated and compared the incidence of dentinal defects on instrumentation using reciprocating files Reciproc and WaveOne with rotary Mtwo and ProTaper instruments. They reported that the root canal instrumentation with both rotary and reciprocating instruments resulted in dentinal cracks and that reciprocating motion resulted in more dentinal cracks than full sequence at the apical level of the canals. However, Burklein et al. did not standardize groups in terms of taper, and it may be possible that the periodontal ligament simulation was also not done. According to a study conducted by Liu et al. (2013), where different endodontic file systems were compared for the incidence of root microcracks, it was reported that files used in reciprocating motion caused fewer dentinal defects than the continuous rotation motion. This might be attributed to work in a reciprocating movement that is similar to the balanced force technique and caused cracks in 5% of teeth only. The reciprocating movement tends to minimize torsional and flexural stresses and also reduces canal transportation. Furthermore, the reciprocating motion shows a higher resistance to cyclic fatigue.
In overall study groups, Control (Group A), i.e. uninstrumented roots showed better fracture resistance followed by WaveOne (Group E). This suggests that reciprocating motion causes less dentinal damage as compared to rotary instrumentation.
The storage time was variable for the study samples, as they were stored in a media. It is practically not possible to get all freshly extracted teeth at the time of study. Also, the diameter of the root canals was not standardized, although only single-rooted premolar teeth were included in the study.
| Conclusions|| |
Root canals instrumented with reciprocating motion have better flexural fracture resistance than continuous rotary motion. Mandibular premolars when instrumented with ProTaper Universal and OneShape endodontic files showed similar fracture resistance, proving that dentin removal does not depend on the number of files used. Not withstanding the elements, which are associated with vertical root fractures, professionals need to be as preservationist as conceivable when performing root canal treatment to constrain the risk due to reduced pericervical dentin.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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Dr. Gaurav L Aidasani
Department of Conservative Dentistry and Endodontics, Dr. D. Y. Patil Dental College and Hospital, Sant Tukaram Nagar, Pimpri, Pune, Maharashtra - 411 018
Source of Support: None, Conflict of Interest: None
[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]
[Table 1], [Table 2]