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ORIGINAL ARTICLE
Year : 2016  |  Volume : 5  |  Issue : 3  |  Page : 192-199

An in vitro study to determine fracture resistance of tooth roots after different instrumentation techniques


1 Department of Conservative Dentistry and Endodontics, Malla Reddy Institute of Dental Sciences, Hyderabad, Telangana, India
2 Department of Pedodontia, Carier Dental College, Lucknow, Uttar Pradesh, India
3 Department of Prosthodontia, Al Badar Dental College, Kalaburagi, Karnataka, India
4 Department of Conservative Dentistry and Endodontics, Tagore Dental College, Chennai, Tamil Nadu, India
5 Department of Conservative Dentistry and Endodontics, Sathyabama University Dental College, Chennai, Tamil Nadu, India
6 Department of Oral and Maxillofacial Surgery, Shyamala Reddy Dental College, Bengaluru, Karnataka, India

Date of Web Publication10-Oct-2016

Correspondence Address:
Marri Shilpa Reddy
Department of Conservative Dentistry and Endodontics, Malla Reddy Institute of Dental Sciences, Suraram, Hyderabad, Telangana
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/2277-8632.191839

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  Abstract 

Aim: The aim was to study the influence of rotary and hand instruments techniques on the susceptibility of the root to fracture.
Materials and Methods: Forty extracted human maxillary permanent first and second molars without any defects were selected as sample. Teeth were randomly divided into four groups of ten teeth in each group. Palatal canals were instrumented depending on the instrumentation techniques used in the groups. Group 1: 0.02 taper K-files and Gates Glidden drills (Mani Inc., Japan); group 2: Hand ProTaper (Dentsply-Maillefer, France); group 3: Rotary ProTaper (Dentsply-Maillefer, France); and group 4: Endowave (J. Morita, Japan). After instrumentation, palatal roots were resected and apical 7 mm sectionally obturated. Obturated roots were embedded in putty impression material and mounted on Universal testing machine (Lloyd instruments, UK). Vertical load was applied by means of a spreader inserted into the canal and load at fracture was recorded for each tooth. Fractured roots were embedded in self-cure resin, sectioned horizontally, and viewed under operating microscope to determine the direction of fracture line: Maximum and minimum remaining dentin thickness was measured at coronal, middle, and apical thirds. Statistical analysis of data was accomplished by one-way ANOVA and post-hoc tests with Tukey honest significant difference.
Results: Mean fracture loads was 20.6 ± 3.8 kg (group 1), 9.5 ± 2.4 kg (group 2), 8.4 ± 1.4 kg (group 3), and 13.4 ± 3.1 kg (group 4). There was statistical significance in the values between group 1 and groups 2, 3, and 4 (P < 0.05).
Conclusion: Preparation of canals with a conventional hand instrumentation technique using 0.02 taper K-files showed highest fracture resistance with least amount of dentin removed at all levels followed by Endowave, ProTaper Hand, and Rotary files.

Keywords: Fracture resistance, instrumentation, remaining dentin thickness


How to cite this article:
Reddy MS, Reddy P V, Reddy T J, Balagopal S, Amalavathy K, Reddy G J, Harini T, Reddy S S. An in vitro study to determine fracture resistance of tooth roots after different instrumentation techniques. J NTR Univ Health Sci 2016;5:192-9

How to cite this URL:
Reddy MS, Reddy P V, Reddy T J, Balagopal S, Amalavathy K, Reddy G J, Harini T, Reddy S S. An in vitro study to determine fracture resistance of tooth roots after different instrumentation techniques. J NTR Univ Health Sci [serial online] 2016 [cited 2019 Dec 6];5:192-9. Available from: http://www.jdrntruhs.org/text.asp?2016/5/3/192/191839


  Introduction Top


Vertical root fracture (VRF) is a major clinical failure during or after root canal therapy. Due to its poor prognosis it almost inevitably results in extraction of affected tooth in case of single rooted teeth or resection of the affected root in multirooted teeth. VRF are reported to have been caused by trauma or iatrogenic due to various endodontic [1] and operative procedures.[2] It has been observed clinically that VRF occurs commonly in endodontically treated teeth,[3],[4],[5] and as a result endodontic procedures have been thought as a frequent cause of VRF. The most common cause is thought to be the use of excessive force during lateral condensation [6],[7] and or postplacement.[8],[9]

Lertchirakarn et al. studied forces developed during lateral condensation and concluded that lateral condensation alone should not be a direct cause for VRF. This has led to further investigation into factors that predispose to root fracture. Factors such as dentin thickness, radius of canal curvature, canal size and shape, external morphology have been considered as potentially influencing fracture susceptibility.[10],[11] Finite element analysis (FEA) has demonstrated that canal curvature seems to be more important than external root morphology and reduced dentin thickness increases the magnitude but not the direction of tensile stress.[10],[11] As the dentin thickness decreases, fracture susceptibility increases. A low radius of canal curvature can act as stress raiser area, which makes the root more susceptible to fracture. External root morphology has also been shown with FEA to be a strong determinant of fracture direction.[11]

Canal preparation during root canal therapy involves dentin removal and it may directly compromise the fracture resistance of the roots; it is therefore, considered as a potential cause of VRF. Technological advancements have led to rapid evolution in canal preparation techniques in recent years. The introduction of rotary nickel-titanium (NiTi) instruments for canal preparation has changed canal shape, size, and taper as compared to hand instrumentation. Numerous studies have shown that they exhibit superior performance over hand instrumentation by causing less straightening, apical canal transportation, and perforations.[12],[13],[14] Whereas, canal shape after preparation with hand files can be quite irregular.[15] The presence of structural defects, cracks or canal irregularities also play a major role in determining fracture resistance of the roots because an applied stress may be exponentially amplified at the tip of these defects. With rotary NiTi canal preparation, canal shapes are more likely to be circular or more round and smoother.

The aim of this study was to determine whether the canal preparation techniques with hand and rotary instruments would influence the susceptibility of the root to fracture, and also to determine the location and pattern of fracture lines when fracture occurs from stress within the root canal walls.


  Materials and Methods Top


Forty extracted human maxillary first and second permanent molars were stored in saline until they were tested. Teeth were cleaned and examined under magnifying glass for immature root apices, cracks on root surface, fused roots, gross caries involving the root and short, thin or curved roots. Teeth with these characteristics were discarded.

In the selected teeth, crown of each tooth was sectioned 2 mm coronal to the cementoenamel junction with a diamond disc at slow speed to facilitate straight line access for instrumentation. The working length (WL) was determined with #8 K-file. Apical patency was maintained by passively inserting #10 K-file until it appeared from apical foramen. Palatal canals of all the teeth were prepared by hand instrumentation, until #15 K-file bound at the WL. Then the teeth were divided into four groups of ten teeth each depending on the instrumentation technique used.

Group 1 - Instrumentation with 0.02 tapered stainless steel K-files (hybrid technique)[16] (Mani Inc., Japan).

Coronal flaring of palatal canals was done using Gates Glidden burs (4, 5, and 6) followed by hand instrumentation with 0.02 tapered stainless steel K-files. Instrumentation was done to master apical file (MAF) #30. Then the canals were prepared by step back technique in 1 mm increments for three additional file sizes. Recapitulation was carried out with MAF to WL after instrumentation with each step back file size.

Group 2 - Instrumentation with Hand ProTaper files [17] (Dentsply-Maillefer, France).

Palatal canals were instrumented with Hand ProTaper files according to manufacturer's guidelines. After establishing glide path with #15 K-file canals were instrumented in crown down technique with Hand ProTaper files. S1 followed by S2 file were introduced into the canal and dentin was engaged by gently rotating the handle clockwise and disengaging the file by rotating counterclockwise 45-90 degrees. Dentin was cut by rotating the handle clockwise while simultaneously withdrawing the file. Then coronal shaping was accomplished with SX file. This was followed by instrumentation with S1 and S2 file to WL. Apical preparation was done with finishing files F1, F2, and F3. Canals were instrumented to MAF F3.

Group 3 - Instrumentation with rotary NiTi ProTaper [17] (Dentsply-Maillefer, France).

Palatal canals were instrumented with rotary NiTi ProTaper files according to the manufacturer's guidelines. After establishing a glide path with #15 K-file, canals were instrumented in crown down pressureless technique with rotary ProTaper files. S1 followed by S2 file was introduced into the canal and was used in a brushing motion against canal wall. Then coronal shaping was accomplished with SX file. This was followed by instrumentation with S1 and S2 file to WL. Apical preparation was done with finishing files F1, F2, and F3. Canals were instrumented to MAF F3.

Group 4 - Instrumentation with rotary NiTi Endowave (J. Morita, Japan).

Palatal canals were instrumented with rotary NiTi Endowave files according to manufacturer's guidelines. After establishing a glide path with #15 K-file, canals were instrumented in crown down pressureless technique with rotary Endowave files. Initial coronal flaring was accomplished with size 35, 8% taper instrument. This was followed by crown down preparation with size 30, 6% taper and size 25, 6% taper instrument. Apical preparation was done with increasing sizes 20, 6% taper; 25, 6% taper and 30, 6% taper instrument. Canals were instrumented to MAF size 30, 6% taper.

Glyde (Dentsply-Maillefer, U.S.A) was used as lubricant during instrumentation. Canals were copiously irrigated with saline after using each file. After instrumentation, teeth were stored in saline to prevent from dehydration.

Palatal root for all the groups was resected and standardized to 9 mm length. Canals were obturated as means of distributing spreader load during fracture testing (Lam et al. 2005). Apical 7 mm was obturated with AH plus sealer (Dentsply-Maillefer, Switzerland) and gutta-percha points (Dentsply-Maillefer Switzerland) by single cone technique. And to facilitate setting of sealer, roots were placed in a closed container at 37°C with 100% humidity for 24 h before testing.

Fracture load measurement

Each root was embedded in a putty impression material (Zhermack, Italy). Roots embedded in putty were placed in a custom made metal jig mounted vertically, such that apex of the root was sitting on hard metal surface. The putty was allowed to set for 30 min before testing. Roots were kept wet via damp cotton to prevent dehydration. Universal testing machine (Lloyd instruments, UK) running at a cross head speed of 1 mm/min was used to fracture the roots. Mounted roots were placed on universal testing machine and a hand spreader tip attached to testing machine was inserted into the root canal until spreader tip was in contact with gutta-percha. And force was applied gradually within the canal until fracture was detected. The load at fracture was recorded in kilograms force.

Fractured roots were embedded in self-cure resin and then sectioned horizontally using diamond disc under slow speed hand piece. Three sections were obtained for each root (apical, middle, and coronal) of 3 mm thickness. Each section was captured to a digital image using an operating microscope (Brilliant Operating Microscope, India) under ×33 magnification. And the direction of fracture line (bucco-lingual, mesio-distal or compound) was determined and the remaining dentin thickness (RDT)/cementum thickness was measured.

Remaining dentin thickness measurement

Remaining dentin thickness was measured for each section using an image analyzer. RDT was measured as the distance from canal wall to the outer surface of the root. Each section was measured at the thickest and thinnest area between the canal wall and the external root surface for RDT.

The data were subjected to one-way ANOVA with P < 0.01 (99% significance). Post-hoc tests with Tukey honest significant difference, were calculated at P < 0.05 level, for multiple comparisons between the groups.


  Results Top


Group 1 showed highest fracture resistance followed by group 4 and then group 2 and 3.

Group 2 and 3 showed least fracture resistance than the other groups.

Highest fracture resistance was observed in canals instrumented with 0.02 tapered K-files (20.6 ± 3.8 kg), followed by canals instrumented with Endowave (13.4 ± 3.1 kg), and least fracture resistance was seen in canals instrumented with rotary ProTaper (8.4 ± 1.4 kg) followed by hand ProTaper (9.5 ± 2.4 kg) [Table 1]A and [Table 1]B.
Table 1A: Load At Fracture (Kg Force)

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Table 1B: Statistical Analysis Of Load At Fracture

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Group 1 showed greater RDT at all levels compared to other groups with least remaining dentin in groups 2 and 3 [Table 2]A,[Table 2]B,[Table 2]C.
Table 2A: Statistical Analysis Of Rdt At Coronal Third

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Table 2B: Statistical Analysis Of Rdt At Middle Third

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Table 2C: Statistical Analysis Of Rdt At Apical Third

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There was statistical significance in the values between group 1 and groups 2, 3, and 4 (P < 0.05).

The values between group 2 and group 3 were not statistically significant (P > 0.05).

In this study highest fracture resistance was observed in canals instrumented with K-files (group 1), followed by canals instrumented with Endowave (group 4), and least fracture resistance was seen in canals instrumented with hand ProTaper (group2), and rotary ProTaper (group 3). Most of the fracture lines observed were in bucco-lingual direction followed by proximal and compound fractures.


  Discussion Top


There increasing acceptance of rotary instrumentation as a technique for cleaning and shaping of the root canal space. Due to this it is important to examine the effect of specific tapers imparted by rotary instrumentation of the root canal as it relates to VRF. Zandbiglari et al. also found that greater tapered instruments removed more root dentin and as a result these teeth were susceptible to fracture than those with hand instruments.[18] This was one of the reasons for analyzing the RDT in the present study.

The present study was an in vitro study done using extracted natural teeth and was designed to compare the residual fracture resistance of a root after conventional hand preparation of the root canals using Gates Glidden drills and 0.02 taper K-files with a popularly used rotary instrument ProTaper, their counter-part hand ProTaper and rotary instrument Endowave. The use of rotary ProTaper and hand ProTaper in this study has given an opportunity to compare a similar instrument design with an engine driven mode and on hand use. The study included Endowave so that it provided an opportunity to compare a standard tapered instrument with a nonstandard tapered “ProTaper” instrument. The gold standard of comparison in the study was with that of time tested conventional step back technique with the ISO standard 0.02 tapered K-files and Gates Glidden drills.

The techniques used for preparing the canals were as per manufacturers' instructions and are standard, accepted canal shaping methods. The experimental techniques for investigating root fracture have generally involved generation of force within the canal space by means of a spreader inserted into an obturated canal.[19] Canals were obturated up to apical 7 mm so that uniform force is delivered along the apical 7 mm of canal walls during experiment. The fracture load application in this study were constructed as per this principle and described in a previous study by Lam et al.[19] Similar designs were used in many other tooth root fracture studies.[6],[19],[20],[21],[22]

The present study showed that weakest roots were left by ProTaper instrumentation. The rotary ProTaper prepared roots fractured on a load of about 8.42 kg and the hand ProTaper prepared roots fractured on a load of about 9.59 kg [Table 1]a. Similar studies analyzing the load required to fracture a root after preparation with a hand ProTaper and rotary ProTaper instruments were not found to be documented. Veltri et al.[23] had reported that the canal shaping ability of ProTaper files were similar to Greater Taper “GT Rotary” files, except that the working time was shorter for ProTaper files. In a study done by Yoshimine et al.[24] it was observed that ProTaper instrumentation caused greater widening of canals with increased tendency to ledge or zip formation compared to RaCe or K3.

Zandbiglari et al.[18] observed that after canal preparation the roots lost some amount of strength. They observed that with hand K-file instrumentation the loss of strength was 25%, with flex master 22% and with GT rotary system it was 43.7%. In the present study, conventional hand instrumentation with 0.02 tapered K-files weakened the roots least when compared with other test groups. This is in concurrence with earlier studies by Zandbiglari et al.,[18] Wilcox et al.,[22] McCann et al.,[25] and Katz and Tamse [26] who also showed that engine driven rotary instrumentations as well as sonic and ultrasonic instrumentations tend to weaken the roots more. According to few studies, it is also of the opinion that the canals prepared with rotary leave a smooth wall whereas hand instrumented canals are irregular in their internal surface.[12],[19] The smooth walls result in uniform stress distribution along the wall, compared to irregularities in the wall which produce stress concentration, and propagation through the weak aberrations.

The present study also analyzed the maximum and minimum remaining thickness of dentin in the root at apical, middle and coronal third after canal preparation. It was observed that hand instrumentation with 0.02 tapered K-files removed least amount of dentin at all levels followed by Endowave, hand ProTaper, and rotary ProTaper. Previous studies [25],[27],[28] also showed that canals prepared with conventional hand instrumentation techniques using 0.02 tapered instruments left more RDT, which provided strength to the roots than various rotary NiTi instruments. The VRF resistance values obtained in this study was also in the same order. It is interesting to note that there was no statistical significance in the RDT on using hand or rotary ProTaper as well as in their vertical fracture resistance.

The fracture on loading was seen to occur mostly in bucco-lingual direction and occasionally in mesio-distal direction. This was in conformation with the low RDT. Similar direction of occurrence of fracture was observed in previous studies.[6],[19],[29] It is also presumed that increased tendency for bucco-lingual stress concentration; result from palatal pressure exerted by the instrument in the root that naturally has a buccal curvature.

This study reveals that the resistance of a root to vertical fracture depends on the RDT than the method of canal preparation. The irregularities produced by the hand instrumentation did not weaken the tooth compared to the smooth canal walls of rotary instrumentations. This may be that the irregularities produced by hand instrumentation are compensated by RDT. Raphael Pilo et al.[27] demonstrated that RDT depended on the tooth anatomy and preparation technique. This study has however made no attempt to study the obturation techniques and their role in weakening the root. Further this study was done in the palatal root of maxillary molars which have almost tapered conical roots. It is also observed that only about 5% of the roots have this configuration in the human dentition.[15] It has also been observed that external root morphology, radius of canal curvature influence the fracture susceptibility and pattern of fracture of root [10],[30] and curved canals are weakened more during preparation and become susceptible to stress.


  Conclusion Top


Within the limitations of this study, results of the present study reveals that fracture resistance of root depends on the RDT after biomechanical preparation and not on the method of instrumentation.

Preparation of canals with a conventional hand instrumentation technique using 0.02 taper K-files showed highest fracture resistance compared to rotary instrumentation technique. This is due to least amount of dentin removed by 0.02 taper hand instrumentation technique followed by rotary Endowave, ProTaper Hand and rotary.

 
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    Tables

  [Table 1], [Table 2], [Table 3], [Table 4], [Table 5]



 

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