Determination of Working Length

Determination of Working Length
The determination of an accurate working length is one of the most critical steps of Endodontic
therapy. The cleaning, shaping and obturation of the root canal system cannot be accomplished
accurately unless the working length is determined precisely.
Exploration for the canal orifice
Before the canals can be entered, their orifices must be found. In older patients, finding a canal
orifice may be more difficult to find. The endodontic explorer is used to find the canal orifice
and exploring the walls of the pulp chamber. When the orifices are found, a small size instrument
as file size 10-15 is used to ensure canal patency.
The pulp should be removed by a barbed broach which should enter to the canal without bending
nor engagement with the canal walls.
 Working length: - It is the distance from a coronal reference point to the point of which canal
preparation and obturation should terminate which is called the apical stop.
1- Apical constriction:- It is the apical portion of the
root canal having the narrowest diameter. This
position may vary but is usually 0.5 to 1.0 mm short
of the center of the apical foramen.
2- Apical foramen: - It is the main apical opening of
the root canal. The foramen is generally not in the
center and may even be situated on the lateral side of
the root.
3- Radiographic apex: - It is the end of the root
determined radiographically.
4- Anatomic apex: - It is the end of the root
determined anatomically,
5- Cementodentinal junction:- It is the area where
the dentin and cementum are joined, the point of
which the cemental surface terminates at or near the
apex of the tooth.
Methods of determining working length:-

1-Radiographic methods:-
The following items are essential to perform this procedure:-
1) Undistorted preoperative radiographs showing the total length and surrounding structures.
2) Adequate coronal access to all canals.
3) Endodontic millimeter ruler.
4) Knowledge of the average length of all teeth.
5) A definite reference point.
Reference point:- It is the site on the occlusal or incisal surface from which measurement of the
working length is made. This point is used throughout canal preparation and obturation. This
should be a stable area that will not change during the course of treatment.
To establish the length of the tooth, a stainless steel reamer or file with an instrument stop on the
shaft is needed. The exploring instrument size must be small enough to negotiate the total length
of the canal but large enough not to be loose in the canal.
Method:-
1- Measure the tooth on the preoperative radiograph (initial measurement).
2- Place the file inside the canal to a length 1 mm less than the length from the preoperative
radiograph
3- Adjust the rubber stopper on the reference point.
4- Take the x-ray by the bisecting angle or parallel techniques.
5- On the radiograph, measure the difference between the end of the instrument and the end of
the root and adjust the working length accordingly.
6- From this adjusted length of tooth, subtract a 1.0 mm to calculate subjectively the position of
the apical constriction.
7- If there is external root resorption there may be destruction of the apical anatomy therefore the
length of the root canal should be shortened 2 mm.
8- Set the endodontic ruler at this new corrected length which will be the lenbth used during root
canal treatment.
Significance of working length

The working length determines the length of the tooth having the pulpal tissue. Endodontic
treatment should be confined to this area.
Failure to accurately determine & maintain working length leads to:

A- Length too long can lead to:
1. Perforation through apical constriction.
2. Overfilling or over instrumentation.
3. Increased incidence of post operative pain.
4. Prolonged healing period.
5. Lower success rate.
B- Short working length can lead to:

1. Incomplete cleaning.
2. Underfilling.
3. Persistent discomfort.
4. Incomplete apical seal.
5. Lower success rate.
2-Electronic determination of working length:-

The electronic principle is relatively simple and is based on electrical resistance to determine
canal length.
How do apex locators work?
The apical constriction has a specific electronic characteristic which is a resistance of 650 ohms.
This finding was used to develop the first generation electronic apex locators.
Based on Ohm’s law (V = R x I), these devices are generators that deliver a direct current of a
known voltage (V), and include an ammeter that measures the intensity (I) of the current after its
passage through the file and being recaptured by the labial hook. An electronic component
calculates the ratio V/I and deducts the resistance at the level of the canal where the instrument is
located. When the resistance is 650 ohms, the screen displays a ‘0’ and the clinician then
estimates that the tip of his instrument is at the apical constriction.
The principle is based on the electrical resistance of different tissues. When the circuit is
complete, resistance decreases and current begins to flow.
Old types were affected by the presence of saliva, blood inside the canal while recent types are
not affected by them and work efficiently in their presence.
Uses of apex locators:

1-They are useful in conditions where apical portion of canal system is obstructed by:
a-impacted teeth, b-zygomatic arch, c-excessive bone density, d-tori, e-overlapping roots,
f-shallow palatal vault.
In such cases, they can provide information
which radiographs cannot.
2-They are useful in patient who cannot
tolerate X-ray film placement because of
gag reflex.
3-In case of pregnant patients, to reduce the
radiation exposure, they can be valuable
tool.
4-They can also be used in children who
may not tolerate taking radiographs,
disabled patients and patients who are
heavily sedated.
5-They are helpful in root canal treatment
of teeth with incomplete root formation,
requiring apexification and to determine
working length in primary tooth.
Contraindications:- The use of apex locators are contraindicated for patients who have cardiac
pacemakers. Electrical stimulation to the pacemaker patient can interfere with pacemaker
function.
They are simple to use, but several precautions must be taken to ensure accuracy and
reproducibility.
􀂄 Remove any metallic parts from the crown of the
tooth (amalgam, crown, etc) that could affect the flow
of the current.
􀂄 Initially, estimate the root length of the tooth by
using the preoperative x-ray.
􀂄 Prepare the access cavity, locate the canal openings,
and clear the content from the pulp chamber.
􀂄 Place the lip hook under the rubber dam, making
sure the mucous membrane is damp.
􀂄 Clip the second electrode of the locator on the hand
instrument. Start the device.
􀂄 Place the file in the canal and slowly advance toward
the apex, using alternating 1/4 turns (watch winding
motion).
􀂄 Stop advancing when the locator displays ‘0’.
􀂄 Check and confirm the measurement three times.
􀂄 Confirm the working length during the procedure
using digital or conventional radiography.
Generations of Apex locators

First Generation
The first generation apex locators use resistance
method for measuring the WL. The first apex locator
was developed in 1969. When compared with
radiographs, these devices were found to be
unreliable. Patients also experienced pain due to
high current machine. Presence of strong
electrolytes such as endodontic irrigants,
haemorrhage, pus or pulp tissue led to inaccurate
results.
Second Generation
Impedance was utilized instead of resistance. In 1971, an apex locator was developed
which needed to be calibrated at the periodontal pocket of each tooth. Inaccurate readings
were gained when electrolytes were present in the canals and even when the canals were
dry.
Third Generation
Multiple frequencies were used to measure the WL in the canals. The apex locators can
measure WL of the canal even with the presence of electrolytes and have 90-100%
accuracy.
Fourth Generation
Similar to the third generation units, the apex locators use two separate frequencies 400 Hz
and 8 kHz. The limitations of these apex locators are that they need to be performed in
partially dried canals and in heavy exudates or blood where it becomes inapplicable.
Fifth Generation
Fifth generation apex locators work on the basis of the comparisons of the data taken from
the electrical characteristics of the canal and additional mathematical processing. These
devices perform extremely well in the presence of blood and exudate but in dry canals they
experience considerable problems. Hence, additional liquids inserted in the canals become
necessary.
Sixth Generation
Sixth generation adaptive type apex locator has been developed which combines the
advantages of the fifth and fourth generation appliances. Measurement using the adaptive
apex locator eliminates the necessity of drying or moistening of the canal, while also
achieving high degree of measurement precision in the presence of blood, sodium
hypochlorite or while manipulating dry canals.

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Determination of Working Length

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Determination of Working Length

Methods of determining working length:-

Significance of working length

Failure to accurately determine & maintain working length leads to:

B- Short working length can lead to:

2-Electronic determination of working length:-

Uses of apex locators:

Generations of Apex locators

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