Basic Research—Technology

Dentinal Damage and Fracture Resistance of Oval Roots Prepared with Single-file Systems Using Different Kinematics Hend Mahmoud Abou El Nasr, PhD, and Karim Galal Abd El Kader, PhD Abstract Introduction: Vertical root fracture is a common finding in endodontically treated teeth, notably oval roots. The aim of the present study was to determine the effect of instrumentation kinematics and the material of instrument construction of single-file systems on dentin walls and fracture resistance of oval roots. Methods: Sixty-five roots with oval canals were allocated into a control group (n = 5) and 3 experimental groups of 20 roots each. Group WO was instrumented with the WaveOne primary file (Dentsply Maillefer, Baillagues, Switzerland), group PT-Rec was prepared with F2 ProTaper files (Dentsply Maillefer, Baillagues, Switzerland) used in a reciprocating motion, and group PT-Rot was prepared with F2 ProTaper files used in a rotation motion. For crack evaluation, half of the samples (n = 30) were embedded in acrylic resin, and the blocks were sectioned at 3, 6, and 9 mm from the apex. The sections were examined under a stereomicroscope and scored for crack presence. The other half of the specimens (n = 30) were obturated using lateral condensation of gutta-percha and AdSeal sealer (Meta Biomed Co, Ltd, Chungbuk, Korea). The specimens were then subjected to a load of 1 mm/min to determine the force required to fracture the roots. Results: WaveOne instruments induced the least amount of cracks and exhibited greatest resistance to fracture compared with ProTaper F2 files whether used in reciprocating or rotating motions. Conclusions: The alloy from which the material is manufactured is a more important factor determining the dentin damaging potential of single-file instruments than the motion of instrumentation. (J Endod 2013;-:1–3)

Key Words Crack, M-Wire, oval canals, ProTaper, reciprocation, root fracture, WaveOne

From the Department of Endodontics, Faculty of Oral and Dental Medicine, Cairo University, Cairo, Egypt. Address requests for reprints to Dr Hend Mahmoud Abou El Nasr, Department of Endodontics, Faculty of Oral and Dental Medicine, Cairo University, 11 EL-Saraya St. Manial, Cairo, Egypt 11553. E-mail address: [email protected]. edu.eg 0099-2399/$ - see front matter Copyright ª 2013 American Association of Endodontists. http://dx.doi.org/10.1016/j.joen.2013.09.020

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V

ertical root fracture (VRF) is a frustrating event because the fracture is usually diagnosed years after all endodontic and prosthetic procedures have been completed, and the affected tooth or root has an unfavorable prognosis (1). The most susceptible roots to fracture are those with a narrow mesiodistal diameter compared with the buccolingual dimension as in maxillary premolars, mesial roots of mandibular molars, and mandibular incisors (2). Most commonly, VRFs occur in endodontically treated teeth. Hence, some endodontic procedures, such as preparation techniques that overenlarge the canal, and aggressive nickel-titanium (NiTi) instruments with large tapers have been identified as etiologic factors (3). During preparation, the contact between instrument and canal walls creates momentary stress concentrations in dentin. Those may leave dentinal defects in which VRF can initiate (4–7). It has been suggested that more dentinal defects may be produced by a higher number of rotations (5) and that instrument design variations alter the forces on a root during instrumentation and increase those defects (6–8). The use of a single file for shaping root canals simplifies instrumentation and reduces the contact with canal dentin (9, 10). ProTaper (Dentsply Maillefer, Baillagues, Switzerland) has been investigated when used as a single file. However, it was tested in a reciprocating motion (9). Consequently, Dentsply Maillefer launched the WaveOne instrument, a single file working in a reciprocating motion. It has the additional advantage of being manufactured with M-Wire, which is a more flexible variant of the NiTi alloy (11). Therefore, the aim of the present study was to determine the effect of instrumentation kinematics and the alloy of instrument construction of single-file systems on dentin walls and the fracture resistance of oval roots.

Materials and Methods Teeth Selection Teeth with mature roots were selected from a collection of recently extracted maxillary premolars with single canals. Those were immediately stored in saline after extraction. The identification of the oval canal was radiographically done by comparing the proximal and clinical views for each root. Sixty-five canals with a buccolingual dimension that was more than twice the mesiodistal dimension at 5 mm from the apex were included in the study. They were all decapitated, and root length was standardized to 14 mm. The roots were examined with a surgical operating microscope (G6; Global Surgical Corp, St Louis, MO) to rule out cracks, fractures, or craze lines. Classification of the Samples Sixty-five roots were allocated to 3 experimental groups and a control group according to the type of instrument and motion of instrumentation as follows: 1. Control (n = 5): Not instrumented 2. Group WO (n = 20): Prepared using the WaveOne single-file technique 3. Group PT-Rec (n = 20): Prepared with the single-file technique using F2 of the ProTaper system in a reciprocating motion 4. Group PT-Rot (n = 20): Prepared with the single-file technique using F2 of the ProTaper system in a rotation motion The samples within each experimental group were further equally divided into 2 subgroups. After root canal instrumentation, 10 specimens from each group were left without obturation and were evaluated for crack presence. The remaining 10 samples were obturated using lateral condensation of gutta-percha and AdSeal root canal sealer

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Basic Research—Technology (Meta Biomed Co, Ltd, Chungbuk, Korea), and they were subjected to the fracture resistance test.

Mechanical Preparation of the Samples The canals were negotiated with a size 15 K-Flexo (Dentsply Maillefer) file followed by instrumentation according to manufacturer instructions for each system as follows: 1. Group WO: Preparation using WaveOne (n = 20): the primary file (#25/.08) in the WaveOne system was used in a slow in-and-out pecking motion. The flutes were cleaned after 3 pecks. Each instrument was worked along the oval root canal at least 3 times in a circumferential motion. 2. Group PT-Rec: Preparation using F2 in a reciprocating motion (n = 20): the NiTi F2 (#25/.08) ProTaper file was carried to the canal in a clockwise counterclockwise motion. The instrument was then pulled out of the canal, cleaned with gauze, and reinserted and employed in the same manner. This step was repeated until the F2 reached the working length (9). 3. Groups WO and PT-Rec: The files were mounted in a 6:1 reducing handpiece, and the X-smart Plus motor (Dentsply Maillefer) was set at the WaveOne program. 4. Group PT-Rot: Preparation using F2 in a rotation motion (n = 20): the F2 ProTaper instrument was used in a torqueand speed-controlled endodontic motor (Endo-Mate DT; NSK Nakanishi, Inc, Kanuma, Japan) at a speed of 300 rpm. The file was moved around the canal at least 3 times until the file was loose.

root canal space all the way to the outer surface of the root. A cracked or defected root was determined when a crack was found in at least 1 section.

Fracture Resistance The obturated 30 samples were mounted in acrylic resin exposing 8 mm of the coronal part. The specimens were secured in the lower plate of the universal testing machine (Model LRX-Plus; Lloyd Instruments, Fareham, UK). A steel spherical tip with a diameter of 3.6 mm was attached to the upper plate and lowered to contact the circumference of the orifice of each specimen. A vertical loading force was applied at a crosshead speed of 1 mm/min until fracture occurred. The force was recorded in newtons using computer software (Nexygen-MT, Lloyd Instruments). Statistical Analysis The results were expressed as the number and percentage of cracked roots in each group. The chi-square test was used to compare between the groups. For the fracture resistance test, the 1-way analysis of variance (ANOVA) test was used to compare the mean force values recorded for the 3 groups, whereas the Tukey post hoc test was used for pair-wise comparisons between the groups when the analysis of variance test was significant. The significance level was set at P # .05. Statistical analysis was performed with IBM SPSS Statistics Version 20 for Windows (Armonk, NY).

Results

During instrumentation, all the roots were covered with moist gauze, and 2 mL 2.5% NaOCl was used to irrigate the canal after each instrument. The final rinse was performed using 2.5% NaOCl followed by distilled water. For each group, each file was used to prepare 5 canals.

Crack Incidence Figure 1 summarizes the crack incidence in each group. No cracks were found in the control group. ProTaper instruments, whether in rotation or reciprocation motion, produced significantly more cracks than WaveOne files.

Lateral Condensation In 10 roots of each of WO, PT-Rec, and PT-Rot, the master cone, which showed tug back at the working length, was selected and carried the sealer into the canal. A finger spreader size 25 was penetrated 2 mm shorter than the working length. Accessory cones were then placed, and excess material was seared off at the orifice. The samples were stored at 37 C and 100% humidity for 24 hours.

Fracture Resistance The mean forces at fracture and standard deviations for each group are given in Table 1. No significant difference existed when ProTaper files were used in reciprocation or a rotation motion. However, significantly more force was needed to fracture the roots when WaveOne files were used (P # .05).

Crack Evaluation The unobturated 10 roots from each of the 3 experimental groups and the control group samples were embedded in acrylic resin blocks. The blocks were sectioned horizontally at 3, 6, and 9 mm from the apex with a low-speed saw under water cooling. Slices were viewed with a stereomicroscope (Leica S8 APO; Leica Microsystems, Czech Republic) at a magnification of 16. The presence of cracks and their location were noted for all sections, and pictures were taken with a digital camera. The photomicrographs were blindly scored according to Yoldas et al (12) as follows: 1. Not cracked: Root dentin devoid of any lines or cracks where both the external surface of the root and the internal root canal wall did not present any evident defects. 2. Cracked: All lines observed on the section that extended either from the outer root surface into the dentin or from the root canal lumen to the dentin. This also included teeth with a complete crack, which was defined as a line extending from the inner 2

Abou El Nasr and Abd El Kader

Discussion VRF is not an uncommon event (2, 13), and its incidence is even increasing (2). It has been assumed that it is not an instant phenomenon but rather a result of crack propagation (14). Thus, the main aim of the present investigation was to study the effect of 2 single-file systems with

Figure 1. The number and percentage of roots presenting a crack.

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Basic Research—Technology TABLE 1. The Mean  Standard Deviation Values (N), Results of Analysis of Variance, and Tukey Test for Comparison between Force Values in the 3 Groups Instrument Force (N)

WO

PT-Rec

PT-Rot

P value

592.2  119.6a 459.4  105b 422.1  50b .001*

Different superscripts are statistically significantly different. *Significant at P # .05.

2 different kinematics on the initiation of dentinal cracks in oval canals. Teeth with oval canals were selected because they are the most susceptible to root fractures, and this root configuration is relatively common (15). The idea behind using a single file for instrumentation is to reduce the number of instruments inside the canal. Moreover, reciprocating motion is claimed to be safer for dentin than conventional rotation (10); therefore, we selected the WaveOne file. The size F2 ProTaper tip was used singly to be compared with the WaveOne primary file because they share the same size, taper, and almost the same geometric design, and it was used in a reciprocating motion as recommended by Yared (9). Microscopic observation showed that the samples in the control group were devoid of cracks, thereby proving that the sectioning method was a safe way for cutting the roots. Statistical analysis revealed that WaveOne produced significantly less amounts of cracks than the F2 ProTaper single file. This might be because of the nature of the manufacturing material of both instruments. Stiffer file designs generate higher stress concentration, which raises the risk of dentinal defects that may lead to root cracking (6). This stiffness is related to the cross-section, size, taper, method of manufacturing, and the material from which the instrument is manufactured (16). M-Wire is a more flexible type of conventional NiTi from which ProTaper instruments are produced. Therefore, not only the material itself induces less stresses on the root canal walls, but also less pressure is exerted on the instrument during instrumentation (17). On the other hand, F2 was used singly in a continuous rotation to find out whether the motion of the instrument within the root canal could be considered another factor determining crack initiation. The amount of cracks did not differ from that initiated with the ProTaper in reciprocation and was significantly greater than those produced by WaveOne. To the best of our knowledge, the use of a single ProTaper file in a rotating motion was not previously reported. Nonetheless, the results were in line with previous studies comparing single-file techniques with the full sequence of ProTaper (5, 12, 18) or comparing single-file reciprocating instruments (Reciproc, VDW GmbH, Munich Germany) with single-file rotating systems (OneShape, Micro-Mega, Besancon France) (19). Crack initiation was considered a stress concentrator and one of the secondary factors predisposing the tooth to fracture after a period of time (20). Thus, the second objective of this investigation was related to the clinical relevance of crack incidence. Therefore, the fracture resistance of roots prepared with the 3 techniques was tested. A single load to fracture was applied vertically to the roots because it has been reported that this method transmits the force uniformly (21). The results showed that

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the force required to fracture the roots was significantly greater in teeth prepared with WaveOne than in those prepared with ProTaper whether in a reciprocating or rotating motion. These findings confirm the direct relationship between the presence of dentinal defects and the root resistance to fracture. Within the limits of the present study, it was concluded that the alloy from which the instrument is manufactured was a more important factor in determining the damaging potential of single-file instruments than the motion of instrumentation.

Acknowledgments The authors deny any conflicts of interest related to this study.

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Dentinal damage and fracture resistance of oval roots prepared with single-file systems using different kinematics.

Vertical root fracture is a common finding in endodontically treated teeth, notably oval roots. The aim of the present study was to determine the effe...
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