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 Table of Contents  
ORIGINAL ARTICLE
Year : 2019  |  Volume : 16  |  Issue : 2  |  Page : 68-72

Effect of heat treatment on the centering ability and dentin removal of a nickel–titanium single file rotary system


Department of Endodontics, Faculty of Dentistry, Tanta University, Tanta, Egypt

Date of Submission31-Dec-2018
Date of Acceptance25-Feb-2019
Date of Web Publication23-Sep-2019

Correspondence Address:
Walaa M Ghoneim
Department of Endodontics, Faculty of Dentistry, Tanta University, Tanta
Egypt
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/tdj.tdj_47_18

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  Abstract 


Aim
The aim of this study was to investigate the effect of heat treatment on the centering ability and amount of dentin removal of OneShape single file rotary system (OS) by cone beam computed tomography.
Materials and methods
Twenty human premolars with single root canals were allocated into two equal groups (n = 10) according to the used rotary system: group 1: OS system, group 2: One Curve which is heat-treated OS single file rotary system. Preinstrumentation and postinstrumentation images were obtained at 3, 6, and 9 mm from the apex and assessed to determine canal centering ability. Amount of removed dentin was estimated by difference between preinstrumentation and postinstrumentation root canal volumes. Data were analyzed using t-test and significance level was set at P value less than or equal to 0.05.
Results
The results showed that One Curve file has higher mean centering ratio and more dentin removal than OS file with no statistically significant difference between them (P > 0.05).
Conclusion
It was concluded that heat treatment did not have significant effect as both files could maintain the original canal path without significant differences in centering ability and amount of removed dentin.

Keywords: centering ability, cone beam computed tomography, dentin removal, One Curve, OneShape


How to cite this article:
Ghoneim WM. Effect of heat treatment on the centering ability and dentin removal of a nickel–titanium single file rotary system. Tanta Dent J 2019;16:68-72

How to cite this URL:
Ghoneim WM. Effect of heat treatment on the centering ability and dentin removal of a nickel–titanium single file rotary system. Tanta Dent J [serial online] 2019 [cited 2019 Nov 17];16:68-72. Available from: http://www.tmj.eg.net/text.asp?2019/16/2/68/267568




  Introduction Top


One of the most important principles of shaping the root canal system is maintaining the original canal anatomy by uniformly preparing all surfaces of the canal to prevent procedural errors such as zipping, ledging, perforations, and apical transportation [1]. However, prevention of these undesirable consequences is a great challenge, especially in severely curved canals [2].

Centering ability is an essential feature of instruments to obtain uniform preparation. It is influenced by instruments characteristics; as alloy [3], cross-section [4], tapering, tip design [5], manufacturing method [6], type of movement [7], and rotation angle [8] in addition to root canal anatomy such as the presence of curvatures [9].

Conservation of radicular dentin, especially in areas of risk such as developmental depressions, concavities and grooves, is mandatory as excessive dentin removal in a single direction within the canal causes canal transportation that may lead to ledge formation, strip perforation and vertical root fractures [10].

Although different nickel–titanium (NiTi) systems have been developed [11], single file systems use was recommended to reduce instrument fatigue and possible cross-contamination [12]. OneShape system (OS), composed of a 55-NiTi alloy, is a single instrument that features continuous rotation for performing a complete root canal preparation. The instrument has size 25 initial diameter and 0.06 mm taper and presents variable cross-sections across the length of its active portion [13].

Many manufacturers have been seeking ways to alter the presently available root canal instrument designs, with a focus on altering the surface of the alloy or altering the alloy microstructure with postmachining or post-twisting heat treatment [14]. One Curve single file system (OC) is heat-treated OS NiTi file, made of C. Wire which is a technique designed, developed and applied by Micro-Mega Company. It is claimed by the manufacturer to be hyperflexible, cyclic fatigue resistant, and without sucking effect [15].

Cone beam computed tomography (CBCT) has been suggested for assessing the tomography of canal walls as it is a nondestructive technique which provides three-dimensional reproduction of the tooth and allows better preoperative and postoperative evaluation of morphological changes in the root canal trajectory [16] for better assessment of root curvatures and canal transportation [1].

Although OS file centering ability is well studied and compared with different rotary and reciprocating systems, no published data is found comparing its centering ability and amount of dentin removal in comparison to OC file. Therefore, this study was designed to evaluate centering ability and amount of removed dentin of OS and OC files in curved root canals to evaluate the effect of heat treatment.


  Materials and Methods Top


Sample selection and preparation

Twenty freshly extracted, fully developed human premolars with single root canals with similar range of canal curvature (21°–39°) according to Schneider's method [17] extracted for orthodontic or periodontal reasons were collected. Approval of this research was obtained from Research Ethics Committee, Faculty of Dentistry, Tanta University. The purpose of this study was explained to the patients and informed consents were obtained to use their extracted teeth on the research. Teeth were thoroughly cleaned, polished, rinsed under running water and stored in sterile saline solution at 4°C until use [18].

All teeth were decoronated using water cooled low speed diamond disc (Dica; Dendia, USA) leaving 13 ± 1 mm long roots. Canals diameters were standardized by selecting roots fitting initial apical file #15 K-type (Dentsuply/Maillefer, Ballaigues, Switzerland) and working length (WL) of each canal was determined by measuring the length of K-type file size #10 at the apical foramen minus 1 mm.

Roots were coded and randomly divided into two equal experimental groups (n = 10) according to instrumentation system and fixed in one acrylic block with a small ball of gutta-percha fixed in one corner of the block as a fixed landmark helping in repositioning during further scanning of the specimens using CBCT.

Preinstrumentation cone beam computed tomography scanning

Preinstrumentation scanning was achieved through adjusting the position of acrylic block by the aid of four laser beams of CBCT machine (Scanora 3D Soredex; Orion Corporation, Helsinki, Finland).

Root canal preparation

In group 1; after glide path creation using #15 K-file, root canals were prepared using OS single rotary file (#25/0.06) (Micro-Mega, Besancon, France) using X-Smart Plus endodontic electromotor (Dentsuply/Maillefer) at a constant speed of 400 rpm and 2.5 N × cm torque following the manufacturer's instructions. OS instrument was coated with Glyde File Prep lubricant agent (Dentsuply/Maillefer) and taken down to two-third of the WL using an in-and-out passive movement while performing an upward circumferential filing movement, then the file was withdrawn and cleaned. Canal was then irrigated and checked for patency.

Then, OS file was inserted 3 mm from WL in a passive movement. This was continued until OS instrument reached the full WL without pressure in one or more passages depending on the complexity of the canal anatomy.

In group 2; glide path was created using #15 K-file and canals were prepared by OC single rotary file (Micro-Mega) using X-Smart Plus endodontic electromotor at a constant speed of 300 rpm and 2.5 N × cm torque in a crown down manner until reaching full WL following manufacturer's instructions.

All root canals were irrigated with 10 ml of 2.5% sodium hypochlorite (Clorox Co., 10th of Ramadan City, Egypt) solution throughout instrumentation using a plastic disposable syringe with a 30-G closed-end needle (NaviTip; Ultradent Products Inc., South Jordan, Utah, USA) positioned 2 mm short of the WL.

Postinstrumentation cone beam computed tomography scanning

After instrumentation, the specimens were scanned using the same standers as preinstrumentation scanning. Preinstrumentation image was fused to the postinstrumentation using a software (Ondemand 3D; Cybermed Inc., Seoul, South Korea) allowing the best possible accuracy.

From fused image, preinstrumentation and postinstrumentation images can be obtained to record the measurement of preinstrumentation and postinstrumentation on the same plane direction and cut to ensure standardization. Cut lines at 3, 6, and 9 mm from the apical end of each root in each group, representing mid-apical, mid-middle and mid-coronal root thirds respectively, were determined on axial view of the fused CBCT image.

Evaluation of centering ability

Centering ability was calculated for each section using centering ratio [19]:



where Y 1 is the shortest distance from the mesial aspect of the root to the periphery of the uninstrumented canal. Y′1 is the shortest distance from the mesial aspect of the root to the periphery of the prepared canal. Y 2 is the shortest distance from the distal aspect of the root to the periphery of the uninstrumented canal. Y′2 is the shortest distance from the distal aspect of the root to the periphery of the prepared canal.

According to this formula, the ratio equal to 1 indicates a perfect centering ability; the closer the result is to 0, the worse is the ability of the instrument to remain centered 213. If these numbers were unequal, the numerator for this formula was the smallest of the two numbers (Y 1 − Y′1) or (Y 2 − Y′2) [20].

Evaluation of amount of removed dentin

The preinstrumented and postinstrumented root canal volume of each root was measured with 3D software (Cybermed Inc.) and volume of dentin removed during instrumentation was determined for each canal by subtracting the preinstrumented canal volume from the instrumented one. The volume of each root canal was determined in cubic millimeters.

Statistical analysis

Mean ± SD values of centering ratio and amount of dentin removal of the two tested root canal preparation systems were calculated and statistically analyzed by t-test using SPSS statistical software (version 20; SPSS Inc., Chicago, Illinois, USA); P values less than or equal to 0.05 were considered to be statistically significant.


  Results Top


Centering ability

Mean ± SD values of centering ratio of the two tested root canal preparation systems were represented and compared at different root canal levels as shown in [Table 1]. Centering ratio mean values were higher in group 2 using OC file system than group 1 using OS file without significant difference between both systems at all root canal levels (P > 0.05).
Table 1: Mean±SD and statistical analysis using t-test for comparison between centering ratio of both groups

Click here to view


Amount of dentin removal

Mean ± SD values of amount of removed dentin for each group was calculated and it was found that OC system recorded higher amount of dentin removal (4.07 ± 0.80) than OS system (3.27 ± 0.38). Comparison between both systems using t-test revealed nonsignificant difference (P = 0.17).


  Discussion Top


Endodontic instruments should remain centered in the root canal throughout the preparation [21] otherwise, canal transportation may occurs and leads to inappropriate dentine removal, with a high risk of straightening the original canal curvature and forming ledges in the dentine wall [22],[23]. Therefore, centering ability is an important feature to be tested in instruments.

Although no consensus on the amount of dentin that should be removed during instrumentation, overinstrumentation of the root canal could result in excessive thinning of the root [7]. Therefore, canal volume was measured preinstrumentation and postinstrumentation in this study to analyze the effect of canal instrumentation on amount of dentine removal [24],[25].

CBCT image technology was used in this study to evaluate centering ability and the canals volume before and after root canal preparation as it provides a precise, reproducible, three-dimensional assessment of alterations in dentine thickness and root canal volume without destructive sectioning of the specimens or loss of the root material during sectioning [26].

Both instruments maintained the original canal curvature well. These results might be explained by similar design of both single files systems used in this study as both files have unique asymmetrical cutting profile, which improves its snake-like movement into the canal and therefore may cause less transportation and preserves the original canal shape [27],[28]. In addition to safety tip of size 25, continuous taper of 0.06 and variable pitch which reduces instrument screwing effects [27],[29].

The results of this study were supported by Rashid and Saleh [30], who reported that all tested single file systems (F360, OS, Reciproc, and WO) maintained root canal curvature well without significant differences and were safe to use. In addition, Santa-Rosa et al. [31] concluded that WO and OS single file systems were able to shape curved root canals producing minor changes in the canal curvature. Moreover, Saleh et al. [32] showed that OS maintained the anatomy of the canals better than WO and Reciproc systems.

Regarding the amount of dentin removal, the unique design of OS, OC instruments which incorporates a variety of different cross-sections along the active length of the file offers an improved cutting action in the root canal. Although the difference between both OS and OC systems was statistically nonsignificant, higher mean centering ratio and more amount of dentin removal recorded in OC group may be because of its heat treatment [33], C-wire technology which increased instrument flexibility and decreased screwing effects [15].


  Conclusion Top


  1. None of the tested instruments remained perfectly centralized within the root canals.
  2. Both tested single file systems (OS and OC) could maintain the original canal path without significant differences between them in centering ability and amount of removed dentin.
  3. Heat treatment in OC file improved its centering ability and amount of dentin removal than OS file but without significant effect.


Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
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