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In vivo measurement accuracy in vital and necrotic canals with the endex apex locator
0099-2399/93/1911-0545/$03.00/0 JOURNAL OF ENDOOONTICS Copyright © 1993 by The American Association of Endodontists
Printed in U.S.A. VOL. 19, NO. 11, NOVEMBER1993
In Vivo Measurement Accuracy in Vital and Necrotic Canals with the Endex Apex Locator David Lee Mayeda, BS, DDS, James H. S. Simon, AB, DDS, David F. Aimar, DDS, and Kurtis Finley, DDS
Currently apex Iocators are being used to determine working length. This study was undertaken to see what is actually being measured and if the pulp status, i.e. vital or necrotic, makes a difference in the determination. In this in vivo study, 33 teeth, both vital and necrotic, were measured by the Endex apex Iocator and then radiographed. After the length determination, the file was cemented to place, the tooth extracted, and then shaved back until the file and the apex were exposed. The position of the file was measured in relation to the apical foramen. Results indicate that all measurements were within a narrow range ( - 0 . 8 6 mm to 0.50 mm). There was no statistical difference in measurements between vital and necrotic canals.
Stein and Corcoran in 1991 (3) and 1990 (4) found that EALs seemed to consistently measure within a narrow band (SD = 0.76 mm) near the apical constriction (where the mean value was 0.24 mm coronal to the cementodentinal junction). The electronic device Berman and Fleischman (5), tested, measured consistently within a small range near the apical foramen (-0.48 mm from the apical foramen with a SD of 0.44 ram). This study was done on mature vital teeth that were destined for orthodontic extraction. Numerous other authors (6-8) have investigated and evaluated commercially available EALs. Although some of these studies identified vital and necrotic teeth, no study specifically examined the accuracy of the EALs in vital versus necrotic cases. In the necrotic cases where there is inflammatory root resorption, the apical constriction may be altered and even nonexistent. In teeth with periapical lesions, no periodontal ligament may remain to respond to the "apex locator." The purpose of the present study was to evaluate, in vivo, the measurement accuracy in vital and necrotic canals by means of the Endex (Osada Electric Co., Los Angeles, CA) apex locator.
Several in vivo studies have been conducted on various electronic apex locators (EALs) to determine their accuracy and consistency. These studies examined the relationship of the measurement file to the apical constriction (minor diameter), the foramen, radiographic apex, and/or anatomical apex. They measured their consistency without regard to vital or necrotic conditions. Chunn et al. (1) in a 1981 study found the Forameter to be inaccurate 65% of the time and radiographs to be inaccurate 40% of the time. These authors defined accuracy as any measurement 0.5 mm to 1.0 mm short of the foramen opening (major diameter). Any measurement beyond these parameters was considered inaccurate. In another study Fouad et al. in 1990 found the five EALs they investigated in vivo to vary in accuracy from 55 to 75% within +_ 0.5 mm of the apical foramen. After the measurements were taken with the various EALs, the file or probe was withdrawn from the tooth and the length was measured _+ 0.25 mm. After extraction, a #10 file was placed and advanced to a point "just" inside the apical foramen and this was determined to be the true length. The measurements derived from the EALs were compared with this true length and any measurement within __. 0.5 mm was deemed to be acceptable. In their study they did not label the true length as the measurement to the major diameter (foramen), although just inside the apical foramen and the illustrations included in the study would indicate this to be the case.
MATERIALS AND METHODS Thirty-three teeth destined for extraction in 19 patients were selected. All were single-rooted maxillary or mandibular teeth with mature apices. Radiographs were taken to determine whether or not a periapical radiolucency and/or resorption could be detected. The teeth were tested using an Analytic Technology pulp tester, a carbon dioxide ice pencil, percussion, and palpation. Seventeen teeth were determined to be vital and 16 were necrotic. The patient was anesthetized and the tooth was isolated using a rubber dam. The majority of the coronal portion of the tooth was removed using a 557 carbide bur or a bullet-shaped diamond in the high-speed handpiece. Once access was gained, the canal orifice was widened using Gates Glidden drills to facilitate placement of the file. The canals were irrigated using 5.25% sodium hypochlorite and, as recommended by the manufacturer, were not dried prior to insertion of the measurement file. A file was then inserted within 2 to 3 mm of the radiographic apex. The Endex probe and lip clip were then placed and the instrument turned on and reset. The size of the measurement file was between a #15 and a #35, dependifig on the size of the canal initially.
545
546
Journal of Endodontics
Mayeda et al.
TABLE 1. Observed distances in millimeters from file tip to major diameter of foramen
Vital Teeth
Necrotic Teeth
-0.35 -0.57 +0.28 0.0 -0.36 0.07 -0.71 0.0 +0.50 0.0 -0.36 0.0 +0.28 +0.21 0.0 0.0 0.0
-0.43 -0.43 -0.07 +0.14 -0.57 +0.07 -0.50 0.0 +0.29 +0.21 0.0 +0.43 0.0 -0.86 0.0 0.0
The Endex apex locator (Osada Electric Co.) is considered a "third generation" type of EAL and measures the m a x i m u m impedance difference (9) between the lip clip and the file probe. The impedance difference is supposed to be the greatest at the apical constriction. The instrument allows measurements in a canal filled with some form of electrolyte which can include purulent exudate, blood, a n d / o r sodium hypochlorite. In the length determination process, an inverted cone (#33) bur was used to establish undercuts within the coronal 3 m m of the access prior to placement o f the files. The files were then placed within 2 to 3 m m of the apex. As the file was advanced apically, the analog dial of the Endex would sweep from the left to the right. On the dial face there is a place marked "apex" preceded by a region o f green. Once the Endex indicated the apex, another radiograph was taken. Self-curing Silar composite (3M, St. Paul, MN) was injected into the remaining canal space. Once the composite set-up, the tooth was extracted and stored in formalin.
The samples were collected and the roots were encased in clear orthodontic acrylic resin to aid in handling. The acrylic and the root were then cut, first using a melite bur to remove the acrylic, then sequential softflex discs to trim the root. An initial penetration o f the root to the file was made approximately 8 to 10 m m from the apex. The plane of the cut was made in the same plane as the greatest curvature of the root. Once the file could be visualized, the additional root bulk was carefully removed while tracking the file as it progressed apically. This was done until the terminus o f the file and the root was revealed. The exposed files and root apices were examined under a dissecting microscope with a calibrated ocular grid. The distance from the end of the file to the foramen (major diameter) was measured and recorded. Two evaluators examined the lengths and there was no disagreement in the measurements. RESULTS All measurements were made from the major diameter because it was reproducible and consistent. F o r the vital cases a maximum and m i n i m u m o f - 0 . 7 1 m m (coronal to the major diameter) and +0.50 m m (extended beyond the major diameter) were noted. The mean was - 0 . 0 5 7 m m with a SD of 0.32 mm. For the necrotic cases the m a x i m u m and minimum were - 0 . 8 6 m m to +0.43 m m with a mean o f - 0 . 1 1 m m and a SD of 0.35 mm. (Table 1 and Fig. 4). To determine whether there was any significant difference between the means of the vital and necrotic cases a pooled variance estimate t test was performed. This revealed a t score = 0.4151 with 31 df where p = 0.6822. By these statistical data it is apparent that there is n o significant difference between the means of the vital and the necrotic cases. DISCUSSION The Endex electronic apex locator measured, within a narrow band (-0.71 m m to +0.50 m m in vital cases and - 0 . 8 6 mm to +0.43 m m for necrotic cases), what it perceived
FIG 1. A, X-ray of three vital teeth (7, 8 and 9) with measurement probes in place. B, Ground section of tooth 9 showing the measurement probe past the apical foramen. C, Ground section of tooth 8 showing the file at the apical foramen.
Vol. 19, No. 11, November 1993
Apex Locator Accuracy
547
FIG 2. A, X-ray showing two vital teeth (7 and 8). B, Ground section with the measurement file considerable short of the apical foramen in tooth 7.
FtG 3. A, X-ray of necrotic canal with measurement file appearing at the radiographic apex (note large periapical lesion). B, Ground section with the measurement file at the apical foramen.
to be the apex (Figs. 1 to 3). The means (-0.057 m m for the vital teeth and - 0 . I07 m m for the nonvitals) would seem to indicate this apex is actually just short of the major diameter of the apical foramen. Kuttler (l 0) states that the average apical constriction (minor diameter) is 0.524 to 0.659 m m coronal to the apical foramen (major diameter). It is at this apical constriction that the cementodentinal junction may exist. Ideally, the root canal filing should terminate at this junction (1 l) and an effective EAL should measure to this junction. Grove (1 l) states that pulpal tissue changes to periodontal tissue in this region. Sunada (12) in his original work in the development
of EALs states it is this periodontal tissue that the electronic device is measuring. However, in necrotic cases with inflammatory resorption at the foramen, there is no periodontal tissue to measure. It is unclear exactly what this (third generation type) instrument is measuring. The maximum impedance value is supposedly at the apical constriction (minor diameter); however, it is here that pulpal tissue meets periodontal tissue, in the vital case. In the necrotic case, there is no periodontal tissue to measure and the apical constriction may very well be missing also. Therefore, one would expect the accuracy and/or the consistency of the EALs in necrotic cases to be suspect. Much to the surprise of the investigators
548
Mayeda et al.
Journal of Endodontics
Frequency 8 7
6 5 4 3 2 1 0 -0.76 to - 0.60
-0.60 to 0.00
0.00
0.00 to *0.60
mm From Apical Foramen m
vital
U
neorotl¢
FIG 4. Frequency of the measurement in relation to the apical foramen for both the vital and necrotic specimens.
this was not the case. The Endex readings were very consistent within a narrow band in both vital and necrotic cases. Although the mean of the readings were beyond the apical constriction (minor diameter) (Fig. 4), the investigators felt this was more a function of the location of the apex on the analog dial face of the instrument. If the measurements were taken at some point before the apex, i.e. in the green area, the measurement would most likely have approximated the apical constriction more accurately. Thus, each operator should correlate his own radiographic and clinical findings with the analog dial readings on the instrument to determine exactly where on the dial the operator wants to call his apex. From this investigation, the Endex consistently located a point that was closer to the major diameter than the apical constriction (minor diameter). As mentioned earlier, the root canal should be obturated to a point as close to the apical foramen as possible and still be within sound tooth structure. If one were to take the apex measurement from the Endex apex locator, according to this study, most of the measurements would actually be beyond the apical constriction (minor diameter). Since positive measurements are long (one third) and 0.0 measurements (one-third) for practical purposes are long, about two thirds of the measurements are actually long, i.e. past the minor diameter. Thus, it is recommended that each operator determine, with the use of radiographs, exactly where the apex is and not necessarily take what is indicated on the dial face of the Endex. Although the Endex gave all readings within a narrow range plus/minus from the major diameter, it is not known from the individual reading whether or not it is 0.5 m m past the foramen, right at the foramen or 0.5 m m short of the foramen, i.e. at the minor diameter.
Dr. Mayecla is a senior resident in endodontics, VAMC Long Beach, CA. Dr. Simon is chief, Enclodontic Section and director, Endodontic Residency Program, VAMC Long Beach, CA and is professor of endodontics, Loma Linda University, Loma Linda, CA. Dr. Aimar is a former resident in endodontics and is currently in private practice in Walnut Creek, CA. Dr. Finley is a former resident in endodontics and is currently in private practice in Palo Alto, CA. Address requests for reprints to Dr. James Simon, VAMC, 5901 E. Seventh St., Long Beach, CA.
References 1. Chunn CB, Zardiackas LD, Menke RA. In vivo root canal length determination using the forameter. J Endodon 1981 ;7:515-20. 2. Fouad AF, Krell KV, McKendry DJ, Koorbusch GF, Olson RA. A clinical evaluation of five electronic root canal length measuring instruments. J Endodon 1990; 16:446-9. 3. Stein T J, Corcoran JF. Nonionizing method of locating the apical constriction (minor foramen) in root canals. Oral Surg Oral Meal Oral Pathol 1991 ;71:96-9. 4. Stein TJ, Corcoran JF, Zillich RM. The influence of the major and minor foramen diameters on apical electronic probe measurements. J Endodon 1990;16:520-2. 5. Berman LH, Fleischman SB. Evaluation of the accuracy of the neosonoD electronic apex Iocator. J Endodon 1984;4:164-7. 6. Blank LW, Tenca JI, Pelleu GB. Reliability of electronic measurig devices in endodontic therapy. J Endodon 1975;4:141-5. 7. O'Neill LJ. A clinical evaluation of electronic root canal measurements. Oral Surg Oral Med Oral Patho11974;3:469-73. 8. McDonald NJ, Hovland EJ. An evaluation of the apex locater endocater. J Endodon 1990;1:5-8. 9. McDonald NJ. The electronic determination of working length. Dent Clin North Am 1992;2:304. 10. Kuttler Y. Microscopic investigation of root apexes. J Am Dent Assoc 1955;50:544-52. 11. Grove CJ. The value of the dentinocemental junction in pulp canal surgery. J Dent Res 1931 ;11:466-8. 12. Sunada I. New method for measuring the length of the root canal. J Dent Res 1962;2:375-87.
Printed in U.S.A. VOL. 19, NO. 11, NOVEMBER1993
In Vivo Measurement Accuracy in Vital and Necrotic Canals with the Endex Apex Locator David Lee Mayeda, BS, DDS, James H. S. Simon, AB, DDS, David F. Aimar, DDS, and Kurtis Finley, DDS
Currently apex Iocators are being used to determine working length. This study was undertaken to see what is actually being measured and if the pulp status, i.e. vital or necrotic, makes a difference in the determination. In this in vivo study, 33 teeth, both vital and necrotic, were measured by the Endex apex Iocator and then radiographed. After the length determination, the file was cemented to place, the tooth extracted, and then shaved back until the file and the apex were exposed. The position of the file was measured in relation to the apical foramen. Results indicate that all measurements were within a narrow range ( - 0 . 8 6 mm to 0.50 mm). There was no statistical difference in measurements between vital and necrotic canals.
Stein and Corcoran in 1991 (3) and 1990 (4) found that EALs seemed to consistently measure within a narrow band (SD = 0.76 mm) near the apical constriction (where the mean value was 0.24 mm coronal to the cementodentinal junction). The electronic device Berman and Fleischman (5), tested, measured consistently within a small range near the apical foramen (-0.48 mm from the apical foramen with a SD of 0.44 ram). This study was done on mature vital teeth that were destined for orthodontic extraction. Numerous other authors (6-8) have investigated and evaluated commercially available EALs. Although some of these studies identified vital and necrotic teeth, no study specifically examined the accuracy of the EALs in vital versus necrotic cases. In the necrotic cases where there is inflammatory root resorption, the apical constriction may be altered and even nonexistent. In teeth with periapical lesions, no periodontal ligament may remain to respond to the "apex locator." The purpose of the present study was to evaluate, in vivo, the measurement accuracy in vital and necrotic canals by means of the Endex (Osada Electric Co., Los Angeles, CA) apex locator.
Several in vivo studies have been conducted on various electronic apex locators (EALs) to determine their accuracy and consistency. These studies examined the relationship of the measurement file to the apical constriction (minor diameter), the foramen, radiographic apex, and/or anatomical apex. They measured their consistency without regard to vital or necrotic conditions. Chunn et al. (1) in a 1981 study found the Forameter to be inaccurate 65% of the time and radiographs to be inaccurate 40% of the time. These authors defined accuracy as any measurement 0.5 mm to 1.0 mm short of the foramen opening (major diameter). Any measurement beyond these parameters was considered inaccurate. In another study Fouad et al. in 1990 found the five EALs they investigated in vivo to vary in accuracy from 55 to 75% within +_ 0.5 mm of the apical foramen. After the measurements were taken with the various EALs, the file or probe was withdrawn from the tooth and the length was measured _+ 0.25 mm. After extraction, a #10 file was placed and advanced to a point "just" inside the apical foramen and this was determined to be the true length. The measurements derived from the EALs were compared with this true length and any measurement within __. 0.5 mm was deemed to be acceptable. In their study they did not label the true length as the measurement to the major diameter (foramen), although just inside the apical foramen and the illustrations included in the study would indicate this to be the case.
MATERIALS AND METHODS Thirty-three teeth destined for extraction in 19 patients were selected. All were single-rooted maxillary or mandibular teeth with mature apices. Radiographs were taken to determine whether or not a periapical radiolucency and/or resorption could be detected. The teeth were tested using an Analytic Technology pulp tester, a carbon dioxide ice pencil, percussion, and palpation. Seventeen teeth were determined to be vital and 16 were necrotic. The patient was anesthetized and the tooth was isolated using a rubber dam. The majority of the coronal portion of the tooth was removed using a 557 carbide bur or a bullet-shaped diamond in the high-speed handpiece. Once access was gained, the canal orifice was widened using Gates Glidden drills to facilitate placement of the file. The canals were irrigated using 5.25% sodium hypochlorite and, as recommended by the manufacturer, were not dried prior to insertion of the measurement file. A file was then inserted within 2 to 3 mm of the radiographic apex. The Endex probe and lip clip were then placed and the instrument turned on and reset. The size of the measurement file was between a #15 and a #35, dependifig on the size of the canal initially.
545
546
Journal of Endodontics
Mayeda et al.
TABLE 1. Observed distances in millimeters from file tip to major diameter of foramen
Vital Teeth
Necrotic Teeth
-0.35 -0.57 +0.28 0.0 -0.36 0.07 -0.71 0.0 +0.50 0.0 -0.36 0.0 +0.28 +0.21 0.0 0.0 0.0
-0.43 -0.43 -0.07 +0.14 -0.57 +0.07 -0.50 0.0 +0.29 +0.21 0.0 +0.43 0.0 -0.86 0.0 0.0
The Endex apex locator (Osada Electric Co.) is considered a "third generation" type of EAL and measures the m a x i m u m impedance difference (9) between the lip clip and the file probe. The impedance difference is supposed to be the greatest at the apical constriction. The instrument allows measurements in a canal filled with some form of electrolyte which can include purulent exudate, blood, a n d / o r sodium hypochlorite. In the length determination process, an inverted cone (#33) bur was used to establish undercuts within the coronal 3 m m of the access prior to placement o f the files. The files were then placed within 2 to 3 m m of the apex. As the file was advanced apically, the analog dial of the Endex would sweep from the left to the right. On the dial face there is a place marked "apex" preceded by a region o f green. Once the Endex indicated the apex, another radiograph was taken. Self-curing Silar composite (3M, St. Paul, MN) was injected into the remaining canal space. Once the composite set-up, the tooth was extracted and stored in formalin.
The samples were collected and the roots were encased in clear orthodontic acrylic resin to aid in handling. The acrylic and the root were then cut, first using a melite bur to remove the acrylic, then sequential softflex discs to trim the root. An initial penetration o f the root to the file was made approximately 8 to 10 m m from the apex. The plane of the cut was made in the same plane as the greatest curvature of the root. Once the file could be visualized, the additional root bulk was carefully removed while tracking the file as it progressed apically. This was done until the terminus o f the file and the root was revealed. The exposed files and root apices were examined under a dissecting microscope with a calibrated ocular grid. The distance from the end of the file to the foramen (major diameter) was measured and recorded. Two evaluators examined the lengths and there was no disagreement in the measurements. RESULTS All measurements were made from the major diameter because it was reproducible and consistent. F o r the vital cases a maximum and m i n i m u m o f - 0 . 7 1 m m (coronal to the major diameter) and +0.50 m m (extended beyond the major diameter) were noted. The mean was - 0 . 0 5 7 m m with a SD of 0.32 mm. For the necrotic cases the m a x i m u m and minimum were - 0 . 8 6 m m to +0.43 m m with a mean o f - 0 . 1 1 m m and a SD of 0.35 mm. (Table 1 and Fig. 4). To determine whether there was any significant difference between the means of the vital and necrotic cases a pooled variance estimate t test was performed. This revealed a t score = 0.4151 with 31 df where p = 0.6822. By these statistical data it is apparent that there is n o significant difference between the means of the vital and the necrotic cases. DISCUSSION The Endex electronic apex locator measured, within a narrow band (-0.71 m m to +0.50 m m in vital cases and - 0 . 8 6 mm to +0.43 m m for necrotic cases), what it perceived
FIG 1. A, X-ray of three vital teeth (7, 8 and 9) with measurement probes in place. B, Ground section of tooth 9 showing the measurement probe past the apical foramen. C, Ground section of tooth 8 showing the file at the apical foramen.
Vol. 19, No. 11, November 1993
Apex Locator Accuracy
547
FIG 2. A, X-ray showing two vital teeth (7 and 8). B, Ground section with the measurement file considerable short of the apical foramen in tooth 7.
FtG 3. A, X-ray of necrotic canal with measurement file appearing at the radiographic apex (note large periapical lesion). B, Ground section with the measurement file at the apical foramen.
to be the apex (Figs. 1 to 3). The means (-0.057 m m for the vital teeth and - 0 . I07 m m for the nonvitals) would seem to indicate this apex is actually just short of the major diameter of the apical foramen. Kuttler (l 0) states that the average apical constriction (minor diameter) is 0.524 to 0.659 m m coronal to the apical foramen (major diameter). It is at this apical constriction that the cementodentinal junction may exist. Ideally, the root canal filing should terminate at this junction (1 l) and an effective EAL should measure to this junction. Grove (1 l) states that pulpal tissue changes to periodontal tissue in this region. Sunada (12) in his original work in the development
of EALs states it is this periodontal tissue that the electronic device is measuring. However, in necrotic cases with inflammatory resorption at the foramen, there is no periodontal tissue to measure. It is unclear exactly what this (third generation type) instrument is measuring. The maximum impedance value is supposedly at the apical constriction (minor diameter); however, it is here that pulpal tissue meets periodontal tissue, in the vital case. In the necrotic case, there is no periodontal tissue to measure and the apical constriction may very well be missing also. Therefore, one would expect the accuracy and/or the consistency of the EALs in necrotic cases to be suspect. Much to the surprise of the investigators
548
Mayeda et al.
Journal of Endodontics
Frequency 8 7
6 5 4 3 2 1 0 -0.76 to - 0.60
-0.60 to 0.00
0.00
0.00 to *0.60
mm From Apical Foramen m
vital
U
neorotl¢
FIG 4. Frequency of the measurement in relation to the apical foramen for both the vital and necrotic specimens.
this was not the case. The Endex readings were very consistent within a narrow band in both vital and necrotic cases. Although the mean of the readings were beyond the apical constriction (minor diameter) (Fig. 4), the investigators felt this was more a function of the location of the apex on the analog dial face of the instrument. If the measurements were taken at some point before the apex, i.e. in the green area, the measurement would most likely have approximated the apical constriction more accurately. Thus, each operator should correlate his own radiographic and clinical findings with the analog dial readings on the instrument to determine exactly where on the dial the operator wants to call his apex. From this investigation, the Endex consistently located a point that was closer to the major diameter than the apical constriction (minor diameter). As mentioned earlier, the root canal should be obturated to a point as close to the apical foramen as possible and still be within sound tooth structure. If one were to take the apex measurement from the Endex apex locator, according to this study, most of the measurements would actually be beyond the apical constriction (minor diameter). Since positive measurements are long (one third) and 0.0 measurements (one-third) for practical purposes are long, about two thirds of the measurements are actually long, i.e. past the minor diameter. Thus, it is recommended that each operator determine, with the use of radiographs, exactly where the apex is and not necessarily take what is indicated on the dial face of the Endex. Although the Endex gave all readings within a narrow range plus/minus from the major diameter, it is not known from the individual reading whether or not it is 0.5 m m past the foramen, right at the foramen or 0.5 m m short of the foramen, i.e. at the minor diameter.
Dr. Mayecla is a senior resident in endodontics, VAMC Long Beach, CA. Dr. Simon is chief, Enclodontic Section and director, Endodontic Residency Program, VAMC Long Beach, CA and is professor of endodontics, Loma Linda University, Loma Linda, CA. Dr. Aimar is a former resident in endodontics and is currently in private practice in Walnut Creek, CA. Dr. Finley is a former resident in endodontics and is currently in private practice in Palo Alto, CA. Address requests for reprints to Dr. James Simon, VAMC, 5901 E. Seventh St., Long Beach, CA.
References 1. Chunn CB, Zardiackas LD, Menke RA. In vivo root canal length determination using the forameter. J Endodon 1981 ;7:515-20. 2. Fouad AF, Krell KV, McKendry DJ, Koorbusch GF, Olson RA. A clinical evaluation of five electronic root canal length measuring instruments. J Endodon 1990; 16:446-9. 3. Stein T J, Corcoran JF. Nonionizing method of locating the apical constriction (minor foramen) in root canals. Oral Surg Oral Meal Oral Pathol 1991 ;71:96-9. 4. Stein TJ, Corcoran JF, Zillich RM. The influence of the major and minor foramen diameters on apical electronic probe measurements. J Endodon 1990;16:520-2. 5. Berman LH, Fleischman SB. Evaluation of the accuracy of the neosonoD electronic apex Iocator. J Endodon 1984;4:164-7. 6. Blank LW, Tenca JI, Pelleu GB. Reliability of electronic measurig devices in endodontic therapy. J Endodon 1975;4:141-5. 7. O'Neill LJ. A clinical evaluation of electronic root canal measurements. Oral Surg Oral Med Oral Patho11974;3:469-73. 8. McDonald NJ, Hovland EJ. An evaluation of the apex locater endocater. J Endodon 1990;1:5-8. 9. McDonald NJ. The electronic determination of working length. Dent Clin North Am 1992;2:304. 10. Kuttler Y. Microscopic investigation of root apexes. J Am Dent Assoc 1955;50:544-52. 11. Grove CJ. The value of the dentinocemental junction in pulp canal surgery. J Dent Res 1931 ;11:466-8. 12. Sunada I. New method for measuring the length of the root canal. J Dent Res 1962;2:375-87.