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 Table of Contents  
REVIEW ARTICLE
Year : 2013  |  Volume : 3  |  Issue : 2  |  Page : 57-64

Radiographic assessment of endodontic working length


1 Division of Endodontics, King Abdulaziz University, Jeddah, Saudi Arabia
2 Department of Oral Rehabilitation, Sir John Walsh Research Institute, University of Otago, Dunedin, NewZealand

Date of Web Publication13-Sep-2013

Correspondence Address:
Nicholas P Chandler
School of Dentistry, University of Otago, P.O. Box 647, Dunedin 9054
NewZealand
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/1658-5984.118145

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  Abstract 

The use of radiographs for working length determination is usual practice in endodontics. Exposing radiographs following the principles of the paralleling technique allows more accurate length determination compared to the bisecting-angle method. However, it has been reported that up to 28.5% of cases can have the file tip extending beyond the confines of the root canals despite an acceptable radiographic appearance. The accuracy of radiographic working length determination could be affected by the location of the apical foramen, tooth type, canal curvature and superimposition of surrounding structures. Variations among observers by virtue of training and experience may also influence the accuracy of the procedure. The interpretation of radiographs could be affected by film speed and viewing conditions, with the superiority of digital imaging over conventional radiography for working length determination remaining debatable. The combination of several methods is recommended for acquiring the most accurate working length.

Keywords: Electronic apex locator, endodontics, radiography, root canal treatment, working length


How to cite this article:
Alothmani OS, Friedlander LT, Chandler NP. Radiographic assessment of endodontic working length. Saudi Endod J 2013;3:57-64

How to cite this URL:
Alothmani OS, Friedlander LT, Chandler NP. Radiographic assessment of endodontic working length. Saudi Endod J [serial online] 2013 [cited 2018 Oct 23];3:57-64. Available from: http://www.saudiendodj.com/text.asp?2013/3/2/57/118145


  Introduction Top


The search strategy for this review involved a combined manual and electronic investigation of references published in English. The former was run over the Medline via Ovid interface to identify articles published from 1948 to November 2012 using the search terms: Apical foramen (AF), apical constriction (AC), cemento-dentinal junction, root apex, anatomic apex, radiographic apex, major diameter, minor diameter and outcome of root canal treatment. The latter search involved a cross-check of the electronic results with review articles and textbook chapters to identify possible relevant publications.

There are two radiographic exposure techniques used for conventional and digital intra-oral radiography; the bisecting-angle technique and paralleling technique.

The vertical angulation during the bisecting-angle technique is determined by directing the X-ray beam perpendicular to an imaginary line bisecting the angle formed between the film and the long axis of the tooth. This method usually results in some linear distortion (elongation or shortening) depending on the accuracy of vertical angulation. It is also accompanied by dimensional distortion, due to the angular relation formed between the film and the tooth. Objects positioned away from the film will be shortened, while those located nearer the film will be less affected. Overall, this technique depends on operator experience and lacks consistency. [1]

The second exposure method is the paralleling technique where the film is placed parallel to the tooth and the X-ray beam is directed perpendicular to the film. This method minimizes dimensional distortion, with parallelism dictated by local anatomy. It is often helpful to increase the film-tooth distance in order to achieve parallelism, although this might not be necessary in the mandibular molar region. To minimize the loss of sharpness associated with the long film-tooth distance, a long cone must be used to increase the distance between the X-ray source and film. [1],[2]


  Length Measurements with Both Techniques Top


The mean radiographic tooth length of anterior teeth was found to be longer than the actual tooth length when the paralleling technique was used. The radiographic length was magnified by 5.4%. [3] Another study reported that, when the paralleling technique was used, 85% of the radiographic lengths of anterior teeth were longer than the actual length, 14% were shorter and only 1% of the lengths were exactly the same. The radiographic tooth length ranged from 1.3 mm short of actual length to 2.1 mm longer. [4] However, when radiographic length magnification associated with the paralleling technique was taken into consideration, the mean radiographic length was shorter than the mean actual length by 0.14 (±0.05) mm. [5] Tooth length determination with the paralleling technique was more accurate than with the bisecting-angle method. Nevertheless, both resulted in elongation of all roots, except for the mesio-buccal and disto-buccal roots of maxillary molars which were shortened when the bisecting-angle principle was used. [6] Magnification is inherent to the central projection principle used for intra-oral radiography. [4],[5]

The radiographic working length (WL) produced by both techniques has been compared. Chunn et al., [7] reported that the paralleling and bisecting-angle techniques gave comparable WLs and the slightly better performance of the former would be clinically irrelevant. The paralleling technique depicted the distance between an intra-canal marker and the root tip more consistently and with fewer errors. Varying the vertical angulation influenced the radiographic position of the intra-canal marker, especially with the bisecting-angle technique. [8],[9] The likelihood of over-instrumentation and overfilling with the bisecting-angle technique is high, as it depicts a shorter than actual file tip position, resulting in longer adjustments than required. Even when the file appears beyond the apex of the tooth, length subtraction would be shorter than required. [10] An in vitro comparison of conventional and digital images obtained using the paralleling technique showed that digital images resulted in over-estimation of the actual file length by 2.81-7.58%, while the same value for conventional images was 1.13%. [11]

The paralleling technique with a beam-aiming device resulted in a significantly lower frequency of incorrect vertical angulations, cone cuts and incorrect film placements during WL determination when compared with the bisecting-angle technique. Nevertheless, the beam-aiming device required experience for correct use. [12] Images taken with a beam-aiming device were more diagnostic than those obtained with a hemostat or the patient's finger supporting the film. [6],[13] Even when radiographs were taken with the bisecting-angle method the use of a film holder gave better standardization, less variation in tooth length measurements on follow-up and improved intra-examiner consistency. [14]


  Accuracy of the Radiographic WL Top


The most commonly employed radiographic WL determination method is that of Ingle. [15],[16] Ingle's method depends on estimating the distance between the file tip and the radiographic apex and adjusting it until the file tip is 0.5-1 mm short of the radiographic apex. [17] One in vivo study compared the accuracy of several radiographic methods for WL determination including those of Best, Bergman, Bramante and Ingle when the bisecting-angle technique was used. Best's method depends on securing a 10 mm steel pin to the labial surface of the tooth and parallel to its long axis before obtaining the radiograph. The radiograph is related to a gauge allowing tooth length determination. Bergman's method is based on calculating the real tooth length based on the radiographic appearance of a 25 mm probe that has an acrylic stop which allows it to penetrate 10 mm in the canal. Bramante introduced a technique that utilized stainless steel probes of different gauges and lengths bent at one end to form a right angle. A radiograph is obtained with the probe in place, then tooth length could be calculated (similar to Bergman's method) or the distance between the probe end and the radiographic apex used to determine the adjustment needed (similar to Ingle's method). The results of the study showed that the best length determination was obtained using Ingle's technique. [18]

Several studies examined the accuracy of the radiographic WL and related the file tip position to the AF. It was found that placing the file exactly at the radiographic apex resulted in a mean radiographic WL beyond the AF by 0.25 (±0.19) mm, with 97% of the measurements being within ±0.5 mm of the AF. [19] Setting the WL 1 mm short of the radiographic apex resulted in 60% of the measurements being within the ±0.5 mm range from the AF, although only 16% were at the AF. The mean radiographic WL was coronal to the AF by 0.1 (±2.11) mm. [20] Radiographic WL resulted in measurements beyond the AF in 19-28.5% of cases, regardless of an acceptable radiographic appearance. [7],[21],[22],[23] All these studies involved the paralleling technique.

Other researchers have related the radiographic file tip position to the AC. These studies showed that adjusting the WL 1 mm short of radiographic apex precisely detected the AC in 15-18% of the teeth. [24],[25] In 68% of anterior and premolar teeth, setting the WL 1 mm short of the radiographic apex positioned the file tip beyond the AC, with the file tip exactly at the AC in the remaining 32%. [26] Another study reported a higher accuracy of the WL when set to 1 mm short of the radiographic apex. The AC was precisely detected in 43% of the teeth. This high rate was attributed to the ideal projection geometry obtainable using extracted teeth. [27]

Hassanien et al., [28] studied 10 mandibular premolars and related the radiographic position of the file tip to the position of the cemento-dentinal junction, AF and AC in vivo. The WL was adjusted to be 0.5-mm short of the radiographic apex, the files were cemented in place and the teeth extracted. The mean radiographic WL was coronal to all three landmarks by 0.26 (±0.03) mm, 0.56 (±0.03) mm, and 0.64 (±0.08) mm, respectively. [28]

The position of the file tip in relation to the radiographic apex was consistent regardless of the images being exposed bucco-lingually or mesio-distally. [23],[29] File tip position on radiographs has been reported to be shorter than its actual position. [7],[8],[18],[23],[29],[30],[31],[32],[33] Only one study reported the opposite. [34]


  Factors Influencing the Accuracy of Radiographic WL Top


Location of the AF

File position in radiographic WL determination is related to the radiographic apex. [17] The AF is not always situated over the apex and is frequently found on the buccal or lingual sides of the root tip. Several studies have reported that radiographs could result in erroneous WL determination as a result of this anatomical feature. [7],[8],[30],[35],[36],[37],[38],[39],[40],[41],[42]

Tooth type and canal curvature

Radiographic WL determination might be more accurate for anterior teeth because the degree of AF deviation in anterior teeth is usually less than posterior teeth. [32] Indeed, the frequency of WL leading to instrumentation beyond the AF in premolars and molars was 51% and 22%, respectively, while this never occurred in anterior teeth. [21] In contrast, another study found the radiographic method to be more accurate in premolars compared to anterior teeth and molars. [43]

It has been reported that as the curvature of the apical part of the root increases, the chances for erroneous radiographic WL also increased. [8],[31] Specifically, when the angle of curvature of palatal roots of the maxillary first and second molars increased above 25°, the discrepancy between actual tooth length, radiographic tooth length and file length increased to more than 0.5 mm. [30] Conversely, the estimation of WL for the mesial canals of mandibular molars did not significantly differ from their actual length, regardless of the degree of canal curvature. [11]

Palatal and mesiobuccal roots of maxillary molars were associated with the highest incidence of inaccurate radiographic WL compared with other roots in vitro[21] and in vivo. [18],[44] The in vivo radiographic WL measurements of the distobuccal and palatal roots of maxillary first and second molars were within ±0.5 mm from the AF in 88.5% of the teeth. Longer measurements were seen in 9.6% while shorter measurements were seen in 1.9% of the sample. The accuracy of the method in distobuccal roots was higher than the palatal roots. [45]

Radiographic WL in maxillary molars may be complicated by the superimposition of the zygomatic arch when the bisecting-angle technique is used. [46] This helps explain why the highest discrepancy in radiographic WL assessment among different observers was seen during evaluations of the palatal root of maxillary molars. [47] Newer digital radiography systems might overcome this problem by image manipulation. [48]

Observer variability and clinical experience

Cox and coworkers [49] compared the agreement of nine observers; three endodontists, three oral radiologists, and three general practitioners with the adjustment needed for best radiographic WL. Assessors were asked to identify the distance needed to place the file tip 0.5-mm short of radiographic apex or short of where the AF was perceived. Excellent agreement of the nine assessors, defined as ±0.5-mm variation from the actual adjustment, was achieved for only 68% of the images. [49] Although the subjects were given identical instructions, their implementation varied. This may be because of differences in their clinical experience, which were not investigated. Recently, this factor was investigated and it was reported that the agreement of endodontists, restorative dentists, general practitioners, endodontic postgraduates and undergraduate students was high, and that the difference in clinical experience did not influence their interpretation. [50] Another study found that the accuracy of estimating the adjustment needed for radiographic WL was directly related to and significantly influenced by clinical experience. [51]

Film speed and viewing conditions

E-speed film was found to be as good as D-speed for radiographic WL determination when a size 15 file was used. [52] Although the accuracy of WL determination with D-, E- and F-speed films was similar, D-speed film received a higher subjective rating than other types regarding the clarity of a size 10 hand file. [47] Another study found that the use of E-speed film was significantly better than F-speed and equal to D-speed when size 10 and 15 files were used for WL determination. Again, D-speed film received better subjective scoring than the others. [53]

The position of size 8 and 10 files was statistically more accurately identified on D-speed film than E-speed film in both in vitro and in vivo conditions. [54],[55]

Optimum radiographic interpretation requires the radiograph to be mounted in a light-masking frame, extraneous light from the viewer be blocked and the amount of reflected light from the film reduced by dimming room lights. [56] Magnification is also recommended. [57],[58] Viewing box illumination intensity was found to be insignificant. [59] The use of a viewing box and masking allowed more accurate file length determination than when no masking was done. Further, viewing the images on a light box with masking and magnification provided higher accuracy than when no masking/magnification was used. There was no difference in the estimation of file length when films were viewed on a light box and masked with or without magnification. [50] This implies that blocking extraneous light might be more important than using magnification during file length estimation.

Digital radiography for WL determination

The first intraoral digital radiography system introduced was RadioVisio Graphy (RVG; Trophy Radiologie, Vincennes, France). [60] Since then, continuous development and improvements have been achieved and new digital systems have been introduced. [61] Digital images can be acquired directly using solid-state receptors such as charge-coupled devices (CCD) and complementary metal oxide semi-conductors (CMOS). Semi-direct capture is achieved via photo-stimulable phosphor (PSP). The indirect capture is acquired by flatbed scanner or digital camera. [61]

RVG, a CCD-based digital system, has been compared with conventional films for WL determination. RVG was as good as E-speed film for the determination of file length adjustment when a size 10 file was used. [62] Another study found that E-speed films were statistically better for WL determination when file size 15 was used, although the difference might not be clinically relevant. [63] D-speed and E-speed films were statistically better than RVG images for the detection of size 10 and 15 files, although image manipulation improved the performance of RVG to comparable levels. [54] Nevertheless, in vivo testing of the same three modalities revealed that D-speed films were significantly better than original and modified RVG images. Original RVG images were significantly worse than E-speed films, while no difference was found between the modified digital images and E-speed films. [55] However, this study did not use identical projection geometry to obtain the radiographs. When this was ensured, there was no difference between the WL of mandibular molars obtained from RVG images and D-speed films. In fact, there was no significant difference between radiographic length and actual length. [64]

The error margin of radiographic WL determination using the regular digital images of two CMOS-based systems, the RVG 6000 system (Kodak Dental Systems, Atlanta, USA) and Schick CDR system (Schick Technologies, NY, USA) was lower than the conventional, indirectly digitized F-speed film. Contrast enhancement of the digital images provided better visibility and improved the interpretation of the position of files of size 10 and 15. [48]

E-speed films were better than the CCD-based Sens-A-Ray (Regam Medical Systems AB, Sundsvall, Sweden) images for WL determination, especially when the digital images were viewed on the monitor. [63] Another study found that estimation of WL on Sens-A-Ray and Vixa (Gendex Dental Systems, Hatfield, USA) images was better than or equal to E-speed films. [65]

Another digital modality is the PSP marketed as Digora (Soredex Orion Corporation, Helsinki, Finland). This system was recommended for WL measurements, even at low radiation dosage. [66] Length adjustments obtained from Digora images were consistent with those obtained from E-speed films. Digital image manipulation allowed better delineation of the fine tips of files compared to conventional images. [67] In contrast, tip clarity of a size 6 file was significantly worse on Digora images than for E-speed films although manipulated images were evaluated. [68] Digora and RVG were inferior to D-speed film for WL determination although their accuracy improved when larger files were used. [69] File sizes 15 and 20 were more accurately visualized on digital images than sizes 8 and 10. [70],[71],[72] The accuracy of file length determination using the PSP-based sensors improved when the images were acquired with high spatial resolution and high contrast together with an appropriate endodontic filter. [73]

The image quality of a PSP-based system (Vistascan; Dürr Dental GmbH, Bissingen, Germany) was superior to a CCD-based system (Sidexis; Sirona Dental Systems GmbH, Bensheim, Germany). The latter required lower radiation but it was associated with more retakes. [74] Images obtained by three digital systems were assessed by six observers to determine WL. The highest error was with the PSP-based system (DenOptix, Gendex Dental Systems) followed by the CCD-based system (Visualix II, Gendex Dental Systems). The CMOS-based system (Schick CDR) was the best among the three systems. [75] Another study concluded that CMOS and CCD (CygnusRay MPS, Cygnus Technologies, Scottsdale, USA) images were better than the PSP (Digora) images to determine file length. [76]

It is important to note that all these studies compared the performance of different systems and attributed variations to differences in image clarity. A factor overlooked is that evaluation of image clarity also depends on observer perception and experience, and most evaluated earlier digital models. Newer brands overcame many of their limitations. [61]

Image manipulation is the strength of digital radiography; [48],[77] however, enhancement did not always result in better interpretation when compared to conventional films. [54],[55],[62],[68],[78] Thus, image enhancement should be task specific. [61] Although digital images are instantly available, their manipulation takes time and requires experience. [63]

From the above it can be concluded that digital radiography was found to be comparable to conventional radiography for WL determination especially when larger files were used. Image manipulation and enhancement is mandatory for optimum interpretation. Digital radiographs are exposed at a lower radiation dose, reducing patient exposure to ionizing radiation. Nair and Nair [61] recommended the use of CCD or CMOS systems for endodontic purposes.

Recommended WL determination technique

Recent guidelines recommend a combination of electronic and radiographic methods for WL determination. [79] This is in line with several studies which have recommended that an electronic apex locator (EAL) reading should be radiographically confirmed to reduce the chances of erroneous WL determination. [21],[27],[80],[81],[82],[83],[84],[85],[86],[87],[88],[89],[90],[91] This recommendation is further supported by the complexity and variability of the apical part of the root canal, which makes it obligatory to utilize multiple WL determination techniques in the same canal. [40],[41],[92]

The higher accuracy of the combined method has been demonstrated. In 51% of premolars, radiographic WL determination resulted in long measurements although they were radiographically acceptable. When the EAL was used, this frequency dropped to 21%. The combination of the two methods resulted in a further reduction of long measurements to only 14%. [93] EAL measurements were ±0.5 mm from the AC in 84% of the teeth. The combined method raised the accuracy to 96%. [81]

Conflicts in the length measurements given by the two techniques have also been reported. The mean length given by the EAL was beyond the radiographic apex by 0.66 mm and measurements ranged from 2.13-mm short of radiographic apex to 3.14 mm beyond it. [80] Another study reported that the mean length measured by the EAL was 1.08 (±0.73) mm short of the radiographic apex with recordings ranging from 4.1 mm short of the radiographic apex to 2 mm beyond it. [83] If the radiographic image of the indicated EAL length appeared to be well short of the radiographic apex, the EAL reading should be chosen as this should be closer to the vicinity of the AC. [80],[94] However, it is not always mandatory to follow the electronic length. [91] The quality guidelines of the ESE state that if the radiographic length is short by more than 3 mm of the desired position, the length should be adjusted and confirmed by another radiograph. [79] It might also be helpful to use tactile sensation in such situations. [84]

Another advantage of the combined method is the reduction in the number of radiographs needed for WL determination, meaning clinical time and radiation hazards are reduced. [20],[88],[95],[96] This was especially true for maxillary molars. [97] In addition, EALs offer the advantage of measuring the canal to the level of its terminus instead of its radiographic apex. [98] Meanwhile, the radiographic method has the advantages of being able to inspect root anatomy and document it in patient records. [45],[95]

It was recently reported that canal lengths measured on existing cone-beam computed tomography (CBCT) images were highly correlated with electronic lengths. [99],[100] More studies are required before implementing CBCT as a method for WL determination.


  Conclusions Top


The paralleling technique is the recommended exposure technique for radiographic WL determination. The accuracy of WL determination is affected by several factors, so a combination of methods is recommended for determining the best working length.

 
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