THE EFFECT OF THE SIZE OF THE APICAL

FORAMEN AND CORONAL FLARING ON THE

ACCURACY OF THREE ELECTRONIC TOOTH

LENGTH MEASURING DEVICES.

(AN INVITRO STUDY)

Hossam M. Tewfik. Assoc. Prof. Of Endodontics. Ain shams University. Ahmed A. Hashem. Lecturer of Endodontics. Ain Shams University.

Abstract

The purpose of the present study was to evaluate the effect of the size of

the apical foramen and the coronal flaring of the root canals on the

accuracy of three apex locators. The actual lengths of forty mesio-buccal

canals were measured under surgical microscope. Canals were then

divided into two equal groups each representing size of the apical

foramen (equivalent to #15 and #35 K-files). Each group was subdivided

into two equal subgroups according to the degree of canal coronal flaring.

The electronic tooth measurements were done using three different apex

locators (Tri auto ZX, Neo Sono and AFA apex finder). The results

showed no difference in accuracy among the three tested devices. Canals

with open apices decreased the accuracy of the electronic measurements

as compared to canals with closed apices. On the other hand, coronal

flaring of the root canals increased the accuracy of the three tested

electronic devices.

Introduction

Cleaning and shaping of the root canal system cannot be initiated

without accurate determination of tooth length. Several methods have

been used for proper establishment of working length. Perhaps the Ingle’s

radiographic technique (1) received the greatest popularity being the most

widely used method for length determination. Limitations and

shortcomings of the radiographic technique led to the introduction of the

electronic devices for tooth length measurement. Electronic devices for

tooth length measurement were first introduced by Sunada(2) and since

then, different devices based on several electrical terms were presented

and tested.

O’Neill et al(3) tested the accuracy of the Sono explorer apex

locator invivo as compared to the actual tooth length after extraction. The

results showed that both measurements were equal in 83% of the times

while in the other 17% the electronic measurement was shorter than the

actual length. This high accuracy in the electronic measurement was not

attained in the study by Seidberg et al(4) where the Sono Explorer was

accurate in only 48% of the times. The authors thus suggested that this

device is not a reliable tool for locating the working length.

Fouad et al(5) evaluated five brands of electronic tooth length

measuring devices in invivo conditions and compared them to the

radiographic estimates of canal length. The accuracy of the electronic

devices ranged from 55-75% with no statistical difference between

different brands.

Keller et al(6) clinically compared the Endocater apex locator with

the traditional radiographic length determination performed by an

experienced endodontist. Results showed that the evaluator was 95.8%

accurate in positioning the file within +/- 1 mm from the cemento-

dentinal junction. On the other hand, the Endocater was only 67.7%

accurate. Close results were recorded by Wu et al(7) when testing the Sono

Explorer Type III. The actual root canal lengths were measured after

tooth extraction and compared to the electronic measurement. The rate of

agreement of the two measurements was 77.5% within a range of +/-

0.5mm.

Again, Hembrough et al(8) evaluating the Sono explorer Mark III

presented similar data where they found the device to be accurate in

73.1% as compared to 88.5% achieved by the radiographic method.

Shabahang et al(9) evaluated the newly introduced Root ZX apex locator

in locating the apical foramen in unprepared root canals of vital teeth and

found it to be efficient in locating the apical foramen within the range of

+/- 0.5 mm with an accuracy of 96.2%.

Different root canal conditions have direct effect on the accuracy

of different root canal electronic measurement devices. These conditions

include the type of tissue inside the canal (vital or non vital), type of

fluids inside the canal, size of apical foramen and degree of canal

enlargement.

Fouad and Krell(10)

compared five different electronic devices

under different root canal conditions. The tested variables were the

presence of different fluids, presence of conductive gutta percha and the

size of the root canal. The results showed that no difference was seen

among different devices while most fluids caused short measurements

with the greatest effect with blood and sodium hypochlorite. Further

more, canal enlargement increased the accuracy of all electronic devices.

Stein et al(11)

studied whether width of the apical foramen

influenced the accuracy of the Neosono-D apex locator and found that the

increase in width of the foramen altered the accuracy of the device.

The efficiency of different brands of apex locators invivo in both

vital and necrotic cases were tested by Mayeda et al(12)

and showed no

statistical difference in measurements between vital and necrotic cases.

Similar results were reported by Dunlap et al(13)

where the measurements

recorded in vital and necrotic cases by the Root ZX apex locator were

82.3% accurate to within 0.5 mm from apical constriction.

The effect of root canal instrumentation in recapitulated and non-

recapitulated canals on the accuracy of apex locators was examined by

Rivera and Seraji(14)

. They found that recapitulation of non-patent root

canals was necessary to obtain accurate length determination when using

apex locators.

Fouad et al(15)

tested four apex locators under different root canal

fluids with the variation of apical foramen size. The results showed that

no significant difference was noted among devices in dry canals

regardless of the foramen size. Only the Endex apex locator was superior

to the other devices in the moist canals. Similarly Frank and

Torabinejad(16)

reported an accuracy of 89.64% when Endex apex locator

was used in moist root canals.

In 1994, Kobayashi and Suda(17)

developed a new method for

electronic canal measurement to overcome the effect of presence of

electrolytes in the canal. The method depend on measuring two

impedances of the canal using current sources with two different

frequencies. The accuracy in electronic tooth measurement devices was

shown not to be affected by the condition of the root canal either dry or

wet regardless the type of device used(18)

. Recently, Jenkins et al(19)

and

Kaufman et al(20)

in two different studies evaluated the efficiency of Root

ZX with different root canal irrigants and showed high reliability of the

device.

Stabholz et al(21)

showed in a clinical study that the ability to

determine the apical constriction by tactile sensation was significantly

increased when the root canals were preflared. Should this alter the

accuracy of electronic devices for tooth measurement ? A question which

was addressed by several studies.

Nguyen et al(22)

evaluated the root ZX as a reliable method for

tooth length measurement. Furthermore, they tested the accuracy in

measurement when using small or large files in enlarged canals. Results

showed that the file size is not a contributing factor in the accuracy of

measurement.

Moor et al(23)

compared four apex locators in unflared dry, flared

dry or flared wet root canals and showed that early coronal flaring did

not ensure better or more precise electronic tooth length measurement.

However, opposite to the later study, Ibarrola et al(24)

showed that root

canal preflaring increased the efficacy of the Root ZX apex locator

suggesting that the preflaring allow the working length files to reach the

apical foramen more consistently thus improve their accuracy.

The effect of some of the previously reviewed canal conditions on

the accuracy of apex locators needs further testing. Thus, it was the

purpose of the present study to evaluate the effect of the size of the apical

foramen and coronal flaring of the root canal on the accuracy of three

apex locators.

Materials and methods

1- Selection and classification of samples

Forty human freshly extracted mandibular molars were selected. Teeth

were intact with no carious lesions or restorations. Absence of internal

calcifications was verified radiographically. Any molar with external

resorption, fracture or extensive curvature was discarded.

Samples were classified into two equal groups according to the size of

the apical foramen. Samples of group I had an apical foramen which

permitted size 15 K-file to project 1-2 mm beyond the apical foramen

while samples of group II had larger apical foramen which permitted the

passage of size 35 K-file 1-2 mm beyond the apical foramen.

Each group was subdivided into two equal subgroups where samples

of subgroup A (groups I and II) had their lengths measured without any

manipulation or enlargement of their root canals. Lengths of samples of

subgroup B (groups I and II) were measured after enlargement of the

coronal 2/3 of their root canals.

2- Preparation of samples

All molars were scaled and curetted to remove external debris and

calculus deposits. The occlusal surface of all samples was slightly

grounded to produce a flat surface for better adjustment of the rubber

stopper during length measurement. An endodontic coronal access

preparation was done for all samples using carbide round burs and

tapered diamond stones mounted on high speed contra angle with water

as a coolant. Pulp chambers were thoroughly irrigated with sodium

hypochlorite 2.6%.

All length measurements for all molars were done for the mesio-

buccal canal only. The forty mesio-buccal canals were checked for root

canal patency using # 15 K-files. Using a tapered diamond stone with

water coolant, one millimeter root-end resection was done for the mesial

root of samples of group II. This horizontal cut resulted in larger apical

foramen which permitted size 35 K-file to pass beyond the apical

foramen. All canals were irrigated with sodium hypochlorite 2.6%.

Samples of subgroup A (groups I and II) were stored with no further

manipulation until length measurements. Samples of subgroup B (groups

I and II) were subjected to flaring of the coronal 2/3 of the canal. Flaring

was done using gates glidden drills # 4, 3, 2 mounted on slow speed

contra angle in a crown-down fashion. Root canals were thoroughly

irrigated and patency was rechecked.

3- Actual tooth length Measurement:

All forty samples were subjected to tooth length measurement by

introducing a # 15 K file (group I ) or # 35 Kfile (group II) until it

appeared just projecting from the apical foramen and flushing with the

root end. This position was verified under the surgical microscope at X8

magnification. The rubber stopper on each file was adjusted on the

flattened cusp, file was withdrawn from the canal and length was

measured to the nearest 0.25mm.

4- Invitro model for electronic tooth measurement:

The in vitro model used was that introduced by Aurelio et al(25)

and

modified by Nahmias et al(26)

. The model consists of 12x72 mm

polystyrene culture tube containing agar in phosphate buffered saline.

The crown of each tooth was tightly fitted in the tube cap such that the

roots would be totally emersed in the medium. A ¾ inch nail extending

from the medium through the sides of the tube at a 3 cm distance from the

base of the tube acted as a negative electrode.

5- Electronic tooth length measurements:

Three different electronic tooth length measuring devices (Fig. 1) were

evaluated which were: the Tri auto ZX (J Morita, Japan), Neo Sono

(Satellic, France) and the Apex Finder (AFA)(Analytic Technology,

USA). The manufacturer instructions for operation of each device were

accurately followed. The file holder for each device was mounted with K

file either #15 (group I) or #35 (group II) while the lip clip for each

device was hanged on the metal nail at the base of the culture tube. Each

file was slowly introduced inside the root canal until the apical foramen

was reached as indicated by each device (Fig. 2). The instrument stopper

was adjusted on the flattened cusp, file was withdrawn from the canal,

measurements were verified three times and recorded to the nearest 0.25

mm.

6- Data collection

The mean difference between the electronic measurement and the

actual tooth length measurement was used for data evaluation. A positive

value indicated long measurement while negative value indicated short

measurement. Measurements that fell within +/- 0.5 mm were determined

as acceptable. Data were subjected to multiple range analysis of variance

(ANOVA) and student t-test at p<0.05.

Results

The difference in length measurement by the three tested devices

among the two groups and subgroups as compared to the actual tooth

length is presented in tables 1-4.

The total number of measurements (120 measurements)

representing the readings of the forty samples measured by each of the

three electronic devices is presented in table 5. This overall data showed

that the electronic measurement was exactly the same as the actual tooth

length (zero value) in 44 measurements (36.6%). A difference of +/- 0.5

mm or less in the electronic measurement as compared to the actual tooth

length was seen in 62 measurements (51.6 %). A difference more than +/-

0.5 mm was recorded in only 14 measurements (11.6%). Considering the

data that fell within +/- 0.5 mm to be clinically acceptable showed that

the overall accuracy of the electronic measurements regardless of the

device used was 88.2%.

As regard the accuracy of the three tested devices, no significant

difference was recorded among the data recorded at p<0.05. However, the

readings of the Tri auto ZX and the Neo Sono were identical and showed

higher degree of accuracy reaching 90% whereas, the AFA recorded

lower degree of accuracy (85%).

The degree of accuracy of electronic length measurement among

the two groups with both subgroups is presented in table 6. The highest

degree of accuracy was attained when the root canals were flared before

measurement. This was seen in samples with closed apices (group I

subgroup B) as well as in samples with open apices (group II subgroup

B). This high degree of accuracy reached 100% when adding the

measurements with zero value and the measurements that fell within the

+/-0.5 mm. This was followed by the samples that had closed apices and

their coronal 2/3 was not flared (group I subgroup A). These samples had

their electronic measurement exact as the actual measurement in 33.3 %

of the times. While in 60 % of the times the electronic measurement was

considered clinically acceptable. The lowest degree of accuracy was seen

in samples which had open apices and were not coronaly flared (group II

subgroup A). These samples had the highest percent of unacceptable

measurements (40%). While the acceptable measurements were 59.9%.

The data recorded for such subgroup (Group II subgroup A) was

statistically significant than the other subgroups at p<0.05.

The effect of the size of the apical foramen on the accuracy of the

electronic length measurement is presented in table 7. Adding the zero

value to clinically acceptable values (+/- 0.5 mm) showed that the degree

of accuracy of the three tested devices reached 96.65% when the apical

foramen was not enlarged. On the other hand, canals with large apical

foramina decreased the accuracy of electronic measurement reaching a

value of 79.95%, however, this decrease was not statistically significant.

The effect of coronal flaring on the accuracy of electronic length

measurement is presented in table 8. Again, adding the zero value to the

+/- 0.5 mm values showed that coronal flaring resulted in a 100%

accuracy of the three tested devices. On the other hand, unflared canals

showed significant decrease in the accuracy of electronic length

measurement reaching 76.6%.

Table 1: The difference in length measurements of samples with closed apices and

no coronal flaring (Group I Subgroup A)

Sample Tri ZX Neo Sono AFA

1 0 0 - 0.25

2 - 0.5 - 0.5 - 0.25

3 - 0.5 - 0.5 - 1.0

4 0 0 0

5 - 0.25 - 0.25 + 0.5

6 0 - 0.5 0

7 - 0.5 0 + 0.5

8 - 0.25 0 - 0.25

9 -0.5 - 0.25 - 1.0

10 0 - 0.5 - 0.25

Mean - 0.25 - 0.25 - 0.2

Table 2: The difference in length measurements of samples with closed apices and

coronal flaring (Group I Subgroup B)

Sample Tri ZX Neo Sono AFA

1 + 0.5 + 0.5 0

2 + 0.5 0 0

3 - 0.5 0 - 0.5

4 0 - 0.5 - 0.5

5 - 0.5 - 0.5 0

6 0 - 0.5 0

7 - 0.5 0 0

8 - 0.5 0 - 0.5

9 + 0.5 - 0.5 0

10 + 0.5 + 0.5 - 0.5

Mean 0 - 0.1 - 0.2

Table 3: The difference in length measurements of samples with open apices and

no coronal flaring (Group II Subgroup A)

Sample Tri ZX Neo Sono AFA

1 + 1.0 + 1.0 + 1.0

2 0 0 + 0.25

3 - 1.0 - 1.0 0

4 0 0 + 0.75

5 + 0.25 + 0.25 + 0.25

6 0 + 0.25 0

7 + 0.25 0 + 0.25

8 0 0 + 0.25

9 + 1.0 + 1.0 + 1.0

10 - 1.0 - 1.0 + 0.75

Mean + 0.05 + 0.05 + 0.45

Table 4: The difference in length measurements of samples with open apices and

coronal flaring (Group II Subgroup B)

Sample Tri ZX Neo Sono AFA

1 - 0.5 0 - 0.5

2 0 + 0.25 0

3 0 0 + 0.5

4 0 - 0.5 - 0.25

5 + 0.25 + 0.5 + 0.5

6 0 - 0.5 0

7 - 0.5 0 + 0.5

8 + 0.25 0 - 0.25

9 0 + 0.5 - 0.5

10 0 + 0.25 + 0.5

Mean - 0.05 + 0.05 + 0.05

Table 5: The total number of measurements for all samples with the representative

percentages in between brackets.

Tri ZX

(n=40)

Neo Sono

(n=40)

AFA

(n=40)

Total

(n=120)

0 16 (40%) 16 (40%) 12 (30%) 44 (36.6%)

+/- 0.5 mm 20 (50%) 20 (50%) 22 (55%) 62 (51.6%)

> 0.5 mm 4 (10%) 4 (10%) 6 (15%) 14 (11.6%)

Table 6: Comparison among groups with representative percentages in between

brackets.

Group Group I /A

(n=30)

Group I /B

(n=30)

Group II /A

(n=30)

Group II /B

(n=30)

0 10 (33.3%) 12 (40%) 10 (33.3%) 12 (40%)

+/- 0.5 mm 18 (60%) 18 (60%) 8 (26.6%) 18 (60%)

> 0.5 mm 2 (6.6%) 0 12 (40%) 0

Table 7: Effect of the size of apical foramen on the accuracy of measurement

represented as percentages.

Closed apex Open apex

0 36.65% 36.65%

+/- 0.5 mm 60% 43.3%

> 0.5 mm 3.3% 20%

Table 8: Effect of the coronal flaring on the accuracy of measurement represented

as percentages.

Unflared canals Flared canals

0 33.3% 40%

+/- 0.5 mm 43.3% 60%

> 0.5 mm 23.3% 0%

Discussion

The accuracy of electronic apex locators have been

evaluated by many studies with conflicting results ranging from 48% up

to 96.2%. In the present study the overall data showed an accuracy of

88.2% with no difference between the three tested devices. This was not

much different than most of the recent studies where the new generations

of apex locators proved to be equally accurate(10,15,18,26).

The purpose of the present investigation was to evaluate the effect

of certain canal conditions on the accuracy of three different electronic

devices for tooth length measurements. Since the introduction of the first

generation of apex locators by Sunada(2), it have been always recognized

that wet contaminants in the root canal affects the accuracy of measuring

devices. Therefore, all recently introduced newer generations of apex

locators were designed to operate very accurately in different root canal

fluids. The three tested devices in the present study are recommended to

be used in wet environment so sodium hypochlorite was selected as the

root canal irrigant in all measurements to mimic clinical conditions.

Evaluating the accuracy of electronic devices invivo presents some

difficulties in testing certain conditions. Therefore, the use of an invitro

model is considered a needed alternative. Different invitro models for

examining the accuracy of electronic devices for tooth length

measurement have been suggested and tested(7,17,18,27)

. In the present

study, the agar model was used which was developed by Aurelio et al(25)

,

modified by Nahmias et al(26)

and tested for its validity by several

studies(10,15,26)

. This model provides the advantage of being easy to

assemble, highly accurate and its data are reproducible.

All electronic measurements recorded in the present investigation

were compared to the actual tooth length and categorized into three

levels: Zero value (representing the actual tooth length), +/- 0.5 mm and

more than +/- 0.5 mm. Measurements falling within the range of zero to

+/- 0.5 mm were considered clinically acceptable. This assumption was

followed by most of the studies testing these electronic devices based on

the fact that deviation of the apical foramen from the exact tooth apex is

considered a normality(5,8,9,28)

.

Two variables were evaluated in the present study the first was the

effect of the size of the apical foramen on the accuracy of the electronic

measurement. Wide apical foramen can be seen in certain stages of apical

development, presence of periapical lesions or in cases of iatrogenic over

instrumentation. Should this affect the accuracy of apex locators?

Huang(29)

noted that the larger the size of the apical foramen, the less

accurate was the measuring device. Other studies(11,30,31)

also showed that

the electronic devices are inaccurate in presence of wide or immature

apical foramen. Wu et al(7) reported a direct relation ship between the

size of the apical foramen and the accuracy of electronic apex locators.

The results of the present study were not much different than those

previously reviewed studies where a decrease in the degree of accuracy of

the electronic measurements was noted among teeth with open apices as

compared to those with closed apices (table 7). However, under the

conditions of the present investigation this difference was not significant.

Perhaps the only disagreement was reported by Fouad et al(15)

where they

showed in their study superiority of the measurements done by the Endex

apex locator when the apical foramen was widened.

The second variable included in the present study was to evaluate

the effect of coronal preflaring on the accuracy of the electronic length

measurement. Our results showed that coronal flaring did significantly

increased the accuracy of the tested devices reaching 100 % when adding

the zero values to the +/- 0.5 mm values. This was similar to the study by

Ibarrola et al(24)

who found that preflaring increase the accuracy of apex

locators. It is suggested that coronal flaring aids in removing any

interference caused by cervical dentin which provides unobstructed

access to the apical foramen(21)

. Opposite to these results, Moor et al(23)

suggested that early coronal flaring did not ensure better root length

measurement.

Although the invitro model utilized in the present study proved to

be efficient, and enabled us to examine variables that are not practical to

be tested clinically yet, the obtained results should not be directly

extrapolated to clinical situation. Future invivo investigations including

such tested variables are strongly recommended.

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Fig. 1: The three tested electronic devices for tooth length measurement.

Fig 2: The invitro model during tooth length measurement