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The remaining dentin thickness investigation of the attempt to remove broken instrument from mesiobuccal… Yang, Qian; Cheung, Gary S; Shen, Ya; Huang, Dingming; Zhou, Xuedong; Gao, Yuan Jul 28, 2015

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RESEARCH ARTICLE Open AccessThe remaining dentin thickness investigationof the attempt to remove broken instrument frommesiobuccal canals of maxillary first molars withvirtual simulation techniqueQian Yang1, Gary Shun-Pan Cheung2, Ya Shen3, Dingming Huang1, Xuedong Zhou1 and Yuan Gao1*AbstractBackground: To investigate differences in the estimated minimum remaining dentin thickness (RDT) betweenperiapical radiographs using the paralleling and parallax technique, after simulated removal of broken instrumentfrom the mesiobuccal (MB) canal of maxillary first molar in virtual simulation model. The 3D measurement wastaken as the standard for comparison.Methods: Thirty-six maxillary first molars were scanned by micro-CT and reconstructed as 3-dimensional (3D)model. A virtual fragment of an instrument was created within the MB canal in software. Removal of the brokeninstrument was simulated in both the 3D and 2D dataset. Then, the models of all specimens were submitted to 2Dand 3D measurements for the lowest (RDT) value in each. Differences in the values between the paralleling andparallax radiographic technique and the 3D-RDT value were analyzed with two-way Analysis of Variance. TheIntra-class Correlation Coefficient (ICC) was used to assess consistency of the RDT measurements between the twoperiapical radiographic and techniques and 3D analysis.Results: There was significant difference between RDT value obtained from the paralleling technique and 3D-RDT.There were no differences between RDT obtained from parallax (angled) technique and 3D-RDT. The ICC of RDTvalues between paralleling technique and 3D measurement were lower than 0.75. ICC between angled radiographsand 3D technique was close to 0.75. The optimal horizontal angle for the parallax technique was about 21°.Conclusions: The virtual simulation technique can provide valuable insight into the benefit/risk analysis beforeremoval of a broken instrument. Parallel radiographs overestimate the actual remain dentin thickness inmesiobuccal canals of maxillary first molars, whereas the parallel technique would give a closer estimate to theactual thickness at a projection angle of about 21°.Keywords: Broken instrument, Virtual simulation, Periapical radiography, Remaining dentin thicknessBackgroundRoot canal preparation is an essential stage of rootcanal treatment aiming to clean and shape the canalsthoroughly. The introduction of rotary nickel-titanium(NiTi) endodontic instruments has improved the effi-cacy of the process compared with manual stainlesssteel files [1], as well as enhanced the success rate oftreatment [2]. There is a concern about the separationof instrument [3], which has been reported to occurmost often in the mesiobuccal canal of maxillary molarsand mesial canal of mandibular molars, due to theircanal curvature and complex anatomy [4]. The pres-ence of a broken fragment would hinder the thoroughlycleaning and shaping of the root canal system, and mayaffect the long-term prognosis of treatment [5].In considering the removal of broken instruments,the clinician needs to evaluate the risk and consider the* Correspondence: gaoyuan@scu.edu.cn1Department of Operative Dentistry & Endodontics, State Key Laboratory ofOral Diseases, West China College & Hospital of Stomatology, SichuanUniversity, 14#, 3rd section of RenMin South Road, Chengdu 610041, ChinaFull list of author information is available at the end of the article© 2015 Yang et al. This is an Open Access article distributed under the terms of the Creative Commons Attribution License(http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium,provided the original work is properly credited. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.Yang et al. BMC Oral Health  (2015) 15:87 DOI 10.1186/s12903-015-0075-xpossible complications. Excessive loss of dentin can in-crease the risk of lateral perforation or root fracture[6]. The remaining dentin thickness (RDT) is probablythe most important factor affecting the decision of re-moving the fragment instrument, as that contributes tothe resistance against root fracture [7, 8]. Typically, theRDT is estimated on periapical radiographs. Accordingto Lim and Stock [8], 200 to 300 μm of dentin thick-ness should be present after preparation, to withstandthe compaction forces during obturation to preventperforation or fracture. If RDT falls below a certainvalue, it would be risky to attempt removal of the frag-ment. Instead, one may then attempt to bypass thebroken instrument, or to clean/shape and fill the rootcanal up to the fragment [9]. Earlier studies usuallysectioned the tooth to measure the canal wall thicknessin cross section [9–11]. Such method is destructive,and the samples cannot be used for further studies oras their own control. Furthermore, it is not easy tocompare the results with other reports, because of thevariability of root canal anatomy. Recently, micro-computed tomography (micro-CT) and the techniqueof virtual simulation provide promising applications inendodontic research [12, 13]. Micro-CT is as a non-destructive method that has been used to investigatethe three-dimensional (3D) morphologic features ofroots and root canals. Tomographic images are digitallyreconstructed in 3 dimensions [14]. Simulated 2-dimensional (2D) radiographs can be generated, basedon micro-CT data by a direct ray casting technique insoftware, without taking a real radiograph [15–17].Thus, one can measure and calculate the dentin thick-ness from 3D micro-CT data and the 2D simulatedradiographs.Although radiographs are widely used in clinical end-odontics, they are not accurate for determining theactual root anatomy, because of distortion and presenceof overlapping structures. In addition, film-based radio-graph has the limitation of being two-dimensionalprojection of a three-dimensional object [18]. For in-stance, the zygomatic process typically overlaps theroots of maxillary first molar. So, some details aboutthe root anatomy can be misinterpreted or lost, whichhinder the visualization of the root canal anatomy andany concavities that may be present in the proximalroot surface. This may compromise clinical judgment,especially when the decision to remove broken instru-ment is concerned. There are few reports on the evalu-ation and calculation of dentin thickness before theremoval of broken instrument in maxillary first molarsby radiographic means. The purpose of this study wasto evaluate the remaining dentin thickness measure-ments based on paralleling and parallax (angle) radio-graphic image, versus 3D tomography, after the virtualremoval of broken instruments from the mesiobuccalcanal of maxillary first molars.MethodsThirty-six maxillary first molars were selected from acollection of extracted human teeth from a Chinesepopulation sample based on mature apices without vis-ible apical resorption. After understanding and writtenconsent was obtained from patients, the extracted teethwere collected by the West China Hospital of Stomatol-ogy for teaching and research. The present study wasapproved by the Ethics Committee of the West China Hos-pital of Stomatology, and the molars were selected from theteeth bank of the hospital. These teeth were ultrasonicallycleaned and stored in thymol solution until use. The teethwere scanned by using a micro-CT system (microCT-50,Scanco Medical, Bassersdorf, Switzerland) with an isotropicvoxel size of 30 μm. All scanned data were processed on anHP 6600 W workstation [Hewlett Packard, Palo Alto, CA]running Windows 7.The MeVisLab package (www.mevislab.de/index)(MeVis Medical Solution, Bremen, Germany) was used,which provided a visual data-flow program environmenton a graphic user interface [19], to build a virtualsimulation platform for the mesiobuccal (MB) canal of allspecimens. The steps of the workflow were similar withthose described in another study [19], and included thefollowing steps: (i) Build a 3D dataset from the scannedmaxillary molar image; (ii) a 3 mm-long apical segment ofa size 25, taper 0.06 endodontic instrument was assumedto have fractured in the MB canal and situated at 3 or5 mm below the orifice; this was created virtually in the3D reconstructed model (Fig. 1b and c); (iii) the toothmodel was rotated at various angles using the “DRR mod-ule” to “isolate” the mesiobuccal root by rotating the toothmodel so that it was not overlapped by the palatal root;and (iv) simulated x-ray images, either paralleling orangled (parallax), were generated to represent radio-graphic images obtained clinically with the techniques,respectively (Fig. 2a-d).Virtual simulation of the removal of the broken fragmentThe clinical procedures were simulated in the Mevislabpackage as follows: First, the tooth model created as instep (i) and (ii) above. Then, a modified Gates Gliddenburs #4 was used to prepare, a “staging platform” up tothe coronal aspect of the fractured piece; a scaled anddimensionally correct 3D image of the instrument wasinserted into the model in software (see Fig. 1d). Afterthat, ultrasonic tips, (CPR number 7, Obtura-Spartan,Fenton, MO, USA)were used to trephine the dentinaround the fragment for 1.5 mm along the fragment(Fig. 1d) to allow the broken instrument to “jump out”of the canal or to retrieve it by using a micro-tubeYang et al. BMC Oral Health  (2015) 15:87 Page 2 of 8instrument removal system (Fig. 1). The most conserva-tive space requirement was assumed in this simulatedprocess: the diameter of the coronal end of the brokeninstrument (Db) was 0.43 mm for the 0.06 tapered fileand the minimum diameter (Dc) of the CPR ultrasonicat 0.4 mm. Therefore, theoretically, the diameter of thetrough created by the ultrasonic tip (D = Db + 2Dc) was1.23 mm. A cylindrical space of this diameter was posi-tioned around the broken instrument uing the “SoTrans-formerDragger module” of MeVisLab (Fig. 1e and f).The 2D simulation steps of fragment removal wereperformed in ImageJ software (http://imagej.nih.gov/ij/).First, simulated radiodgraphs were generated with adirect ray casting technique from the 3D dataset. Then, arectangle (4.5 mm × 1.23 mm&6.5 mm × 1.23 mm) thatcorresponded to the space for straight-line access was setin the resultant paralleling and parallax x-ray images. Asimilar trepan space (1.23 mm diameter) was created byaround the fragment (Fig. 2).Measurement of remaining canal wall thicknessModel dataset of each tooth after the simulation proced-ure was submitted to 3D measurement in Mevislab. Theremaining dentin thickness measurements were madefrom the root canal wall to the external root surfacealong the root using the “3D SurfaceDistance module” ofthe software. These distances were stored in the nodesfor color-coding and analysis. A 3D marker was placedon the surface to allow visualization of the dentin thick-ness there (Fig. 3). A 3D-RDT value was obtained foreach tooth.The 2D canal wall thickness was estimated on boththe paralleling (Pa-RDT) and parallax radiograph in theImageJ software. The RDT value was taken as the mini-mum distance from the side of the rectangle to the ex-ternal root surface (Fig. 2).Statistical analysisThe RDT values were submitted to two-way analysis ofvariance. Then, the 3Dunnett t test was used to identifythe differences in RDT between radiographic and actual3D thickness. Intra-class correlation coefficient (ICC)was used to assess consistency between the radiographicand actual thicknesses. The level of significance was setat p < 0.05. All analyses were performed a statisticalpackage (SPSS 21.0, SPSS Inc., Chicago, IL).ResultsThis virtual simulation platform can provide a safeenvironment for planning the removal of a broken in-strument interactively. The often-proposed approachwas followed, i.e. by creating a staging platform andFig. 1 a Morphological reconstruction of one maxillary first molar; b & c size 25/.06 NiTi instrument with 3 mm apical segment assumed to befractured in the mesiobuccal canal with 3 mm and 5 mm away from the canal orifice; d using modified Gates Glidden burs to create a stagingplatform and CPR ultrasonic tip to trephine dentin a 1.5 mm distance apically from the coronal part of the fragment around the fragment; e access tothe fragment at 3 mm; f access to the fragment at 5 mmYang et al. BMC Oral Health  (2015) 15:87 Page 3 of 8then troughing around the fragment. The space createdin such process was simulated in both the 2D and 3Ddatasets. RDT measurements were obtained from dif-ferent radiographic projections and from the 3D ana-lysis; the mean and standard deviations were deportedin Fig. 4.For the group with fragment 3 mm below orifice, theminimum RDT value obtained from paralleling radio-graphic technique (1058 ± 216 μm])was significantlygreater than that by the parallax (angled) technique(An-RDT) (606 ± 155 μm), as well as the 3D-RDT(581 ± 159 μm) (p < 0.05). For the 5 mm-deep group,the An-RDT (389 ± 126 μm) was only slightly greaterthan 3D-RDT (368 ± 159 μm). The Pa-RDT was (895 ±220 μm), which value was significantly greater than theformer two (p < 0.05). Considering the effect offragment location, the minimum RDT of the 3 mm-deep group was generally greater than that with frag-ments situated deeper (5 mm below the orifice) in thecanal. There were no differences between parallax an-gled radiograph (An-RDT) and 3D-RDT value for bothlocations (3 mm versus 5 mm below the orifice) of thefragment. The ICC values of remaining dentin thick-ness measurements between the paralleling techniqueand the 3D analysis were 0.479 and 0.574 two for thefragment locations, respectively. Noted that both valueswere lower than 0.75. The ICC between parallax-RDTand 3D analysis were 0.721 and 0.667 for the two loca-tions, which values were close to 0.75.The average rotation angle from the paralleling tech-nique to obtain a parallax radiograph with unimpededimage of the mesiobuccal root was 21.06 ± 4.34°.Fig. 2 Simulated X-ray image by parallel and parallax technique when broken instrument below the orifice 3 mm (a, c) and 5 mm (b, d) andmeasurement by ImageJ softwareYang et al. BMC Oral Health  (2015) 15:87 Page 4 of 8DiscussionIn a recent survey conducted in the UK, 85.1 % of gen-eral dental practitioners and 94.8 % of endodontists haveexperienced fracture of endodontic instruments [20].Instrument fracture often occurs in narrow and curvedcanals, such as the mesiobuccal canal of maxillary mo-lars [21, 22]. Removing a fractured instrument from theroot canal is a demanding task. Sufficient enlargement ofthe root canal coronal to the fragment is essential forsuccessful retrieval. Usually a staging platform coronalto the fragment is prepared to allow straight-line accessand direct sight of the fragment under the operatingmicroscope. This is followed by the application of ultra-sonic tips. If the direct application of ultrasonic energydoes not loosen the fragment sufficiently to remove it,then there is a need to grab and retrieved the fragmentwith some variant of micro-tube [23].Gao et al. [19] reported that the application frame-work, based on the freeware MeVisLab, enables the 3Dreconstruction and measurements of root canal andteeth scanned by micro-CT. The virtual simulation plat-form can provide a safe environment for planning forFig. 3 3D color-coded image of residual dentin thickness distribution around the narrow parallel space in root dentin after created a staging platformwhen the instrument placed in mesiobuccal canal below the orifice with 3 mm (a) and 5 mm (b) depthFig. 4 The means and standard deviations for RDT by different methods. 3D-RDT(=3D remain dentin thickness), Pa-RDT(= remain dentin thicknessobtained from parallel technique), An-RDT(= remain dentin thickness obtained from angulated technique), green and blue color were instrumentbroken 3 mm and 5 mm below the orifice (group 3 mm and group 5 mm)Yang et al. BMC Oral Health  (2015) 15:87 Page 5 of 8the removal of fractured instruments. Virtual digital ra-diographs can be generated from the reconstructedmicro-CT data. This permits an assessment of remainingdentin wall thickness, as estimated by plain radiographs,with the measurement from 3D analysis serving as thestandard for comparison. The software platform in 3 di-mensions has facilitated the realistic simulation andevaluation of any changes in dentin thickness that oc-curs in the roots, if the clinical procedure were to beperformed. This platform also allows the comparison ofdentin wall thickness obtained from radiographs takendifferent angles. The technique described in our presentstudy allows each root to serve as its own control andovercomes the problem of sample variation. The virtualsimulation platform provides useful and intuitive infor-mation in education and research, with potential to ex-tend to the clinical situation.During the removal of any broken instrument, dentinreduction must be done carefully to avoid root perfor-ation. Hence, treatment planning should include a riskassessment. The clinician has to evaluate the options ofeither attempting to remove the fragment, bypassing it,or leaving the broken fragment inside the root canal.The decision is often based on information about theroot canal wall thickness, especially when root fractureor perforation is to be avoided. The risk of the endodon-tically treated teeth to fracture increases proportionallyto the amount of dentin removed [7]. A direct relation-ship exists between remain dentin thickness and thestrength of the root [24–26]. Thus, preservation ofsound dentin is very important during removal ofbroken instrument. In previous studies, teeth were sec-tioned at one or several selected levels of the root withmeasurements done in 2D in cross sections [11, 27]. Un-avoidably, some parts of the root were destroyed duringsectioning and could not be assessed. In the presentstudy, all levels of the root were examined in a virtualplatform that also permitted the quantification of the ra-dicular wall thickness if an attempt was made for thebroken instrument. The images may be color-coded foreasy visualization of the result after these manipulationsdrilling and troughing were carried out in the tooth.One may argue that cone beam computed tomography(CBCT) is an accurate and noninvasive technique thatmay be applied in the clinical situation. However, thecost and radiation dose to the patient must be consid-ered. Periapical radiograph is likely to remain as themost important tool in clinical practice, which is a com-promise when dentin thickness information is con-cerned. Raiden et al. [18] and Souza et al. [28] evaluatedthe post preparation in premolars using paralleling(bucco-lingual) radiographs, and concluded that periapi-cal radiographs after overestimate the actual root canalwall thickness. Our present study supported the findingthat paralleling radiographic technique would overesti-mate the actual RDT. On the other hand, the parallaxtechnique seems give a closer or more accurate estima-tion of the actual RDT. As the root may display differentappearance in varied projection angle, the projectedshape and curvature of the mesiobuccal root could influ-ence the measurement on a periapical radiograph. Whenthe beam crosses the tooth at a certain angle (as in aparalleling technique), the tooth appears blurred in theradiograph. Thus, by the angulating the beam, the shapeand concavity of the mesiobuccal root may be bettervisualized. This is reflected in the results that angled film(parallax technique) produced thickness measurementthat is close to, but still slightly greater than the actual3D-RDT. It might be related to the presence of concav-ities on the distal (or furcal) surface of the mesiobuccalroot of maxillary first molars that were not visible radio-graphically and thus concealed the true distance betweenthe outer root surface and the root canal wall. Simplyput, plain radiographs provide an over-optimistic estima-tion of the dentin root canal wall thickness on the furcalaspect of the mesiobuccal root. Using a parallax techniquewould help to reduce the discrepancy in the thickness esti-mation for risk assessment.For the actual RDT, the coefficient of variation was0.034 and 0.049 in the two fragment-location groups (3and 5 mm). When this coefficient was small, that ICCvalue would not be high [29]. The ICC values of RDTmeasurement between parallax radiograph and 3D ana-lysis were close to 0.75 in the present study, suggestingthat the parallax technique may provide a better predic-tion of the true thickness. The thicknesses estimatedfrom these two methods were closer to each other, andwere significantly different from that obtained from theparalleling radiographs. Thus, an angled radiographshould be taken when an attempt to remove the brokeninstrument from the MB canal of maxillary molar iscontemplated.Changing the angulation of the radiation source mayhelp in determining the presence of root or strip perfor-ation [30], additional roots, the localization of periradi-cular pathosis, and other anatomic structures. Theparallax radiographs can avoid the problem of overlap-ping structures to some extent. For instance, the bestangle would show the MB root clearly, separate from thedistobuccal and palatal root. In the present study, thishorizontal offset angle was about 21°. This may be aguide to the radiologist or clinicians when faced with abroken instrument in such a situation. Morphologically,the anatomy of the MB root of maxillary first molar wascomplex with a high incidence of MB2 canals, isth-muses, accessory canal, apical delta and loop [31]. Rootcanal curvatures are most pronounced in the MB canal,in which most cases of instrument fracture occur. In theYang et al. BMC Oral Health  (2015) 15:87 Page 6 of 8coronal part, the furcal [i.e. distal] wall of the MB root israther thin and, often, is much thinner than the mesialwall at similar level [32]. Realizing that intraoral radio-graphs will overestimate the RDT would be helpful forclinicians to make decisions during clinical procedures;the parallax technique is more accurate than parallelingtechnique in this regard.ConclusionsIn conclusion, based on virtual simulation platform, theminimal remaining dentin thickness after attempt to re-move a fracture instrument was affected by the projec-tion angle, the position of the fractured instrument.There was a high risk of perforation in the middle thirdof the mesiobuccal canal in the maxillary first molar.Although the results from virtual simulation modelscannot always completely extrapolate to the in vivo/pa-tient situation, they can provide valuable insight into thebenefit/risk analysis before removal of a separated in-strument. To evaluate the RDT during remove brokeninstrument in maxillary first molars, parallel radiographsoverestimate actual remain dentin thickness and angu-lated technique were significantly more accurate thanparallel technique when the angle was 21°. It providesreference information for endodontists and radiologists.AbbreviationsRDT: Remain dentin thickness; MB: Mesiobuccal; 3D: 3-dimensional; ICC:Intra-class correlation coefficient; NiTi: Nickel-titanium; Micro-CT:Micro-computed tomography; Pa-RDT: Remain dentin thickness obtainedfrom parallel technique; An-RDT: Remain dentin thickness obtained fromangulated technique; CBCT: Cone beam computed tomography.Competing interestsThe authors declare that they have no competing interests.Authors’ contributionsQY carried out the studies, performed the statistical analysis and drafted themanuscript. YG conceived of the study, DM and XD participated in its designand coordination. GC and YS provided clinical guidance and helped to draftthe manuscript. All authors read and approved the final manuscript.AcknowledgmentsThis study was supported by the National Natural Science Foundation ofChina (No. 81200781 and No.11272226). The authors deny any conflicts ofinterest related to this study.Author details1Department of Operative Dentistry & Endodontics, State Key Laboratory ofOral Diseases, West China College & Hospital of Stomatology, SichuanUniversity, 14#, 3rd section of RenMin South Road, Chengdu 610041, China.2Area of Endodontics, Comprehensive Dental Care, Faculty of Dentistry,University of Hong Kong, Hong Kong, China. 3Division of Endodontics,Department of Oral Biological & Medical Sciences, Faculty of Dentistry,University of British Columbia, Vancouver, Canada.Received: 28 March 2015 Accepted: 22 July 2015References1. Schafer E, Schulzbongert U, Tulus G. Comparison of hand stainless steel andnickel-titanium rotary instrumentation: a clinical study. 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