Working length determination in RCT

Introduction:

Determination of an accurate working length(WL)is one of the most critical steps of endodontic therapy.the cleaning, shaping and obturation of root canal system cannot be acomplished accurately unless WL is determined precisely.

According to endodontic glossary :

W.L is defined as” the distance from a coronal reference point to a point at which canal preparation obturation should terminate.”

Working Length Determination

The objective of working length determination is to establish the length (distance from the apex) at which canal preparation and subsequent obturation are to be terminated.

Importance of accurate working length determination:

· Confine the instrumentation to the canal system (within dentin).

· Create and maintain an apical stop or seat at the minor constriction.

· Prevent under- instrumentation that could leave tissue and debris in the apical segment.

· Prevent over- instrumentation which could cause patient discomfort, damage periapical tissue, or potentially cause an infection or cyst development from the placement of irritating materials beyond the apex.

Reference point : is that site on occlusal or the incisal surface from which measurements are made .

A reference point is chosen which is stable and easily visualized during preparation.

Anatomic apex: is tip or end of root determined morphologically .

Radiographic apex: is tip or end of root determined radiographically.

Apical foramen: is main apical opening of root canal which may be located away from anatomic or radiographic apex.

Apical constriction: is apical portion of root canal having narrowest diameter .

It is usually 0.5-1mm short of apical foramen.

The Cementdentinal junction:is the region where cementum and dentine are united ,the point at which cemental surface terminate at or near the apex.

Different methods of WL estimation:

Average root length from anatomic studies

Radiographic

Mathematics method

Tactile sensation

Bleeding on paper point

Apical periodontal sensitivity

Electronic apex locator

Radiographic method

1. Measure the estimated WL from preoperative periapical radiograph

2. Adjust stopper of instrument to this estimated WL and place it in the canal up to the adjusted stopper

3. Take the radiograph

4. On the radiograph measure the difference between the tip of the instrument and the root apex add or subtract this length to the estimated WL to get the new WL.

5. correct WL is finally calculated by subtracting 1 mm from this new length .

Modification in the length subtraction:

1. No resorption -subtract 1mm

2. Periapical bone lesion - subtract 1.5 mm

3. Periapical bone +root apex resorption

-subtract 2 mm

Mathematic method:

it is based on simple mathematical formulation to calculate the WL.

In this an instrument is inserted into the canal ,stopper is fixed to the reference point and radiograph is taken.

The formula to calculate actual length of the tooth ia as follow:

Actual length of tooth apperent length of tooth in x-ray

-------------------------------- --- = ------------------------------------------------

Actual length of instrument apperent length of instrument

in radiograph

So,

actual length of instrument X

apperent length of tooth in radiograph

Actual length of tooth= ------------------------------------------------------------

apperent length of instrument in radiograph

disadvantages:

Wrong readings can occur because of :

A. Variation in angles of radiograph

B. Curved root

C. S-shaped ,double curvature roots.

Other methods of WL determination including:

tactile sensation

Apical periodontal sensitivity

Paper point measurement

These methods don’t always provide the accurate readings for example :

In case of narrow canals instrument may feel increased resistance as file approaches apical 2-3mm and in case tooth with immature apex instrument can go beyond apex.

So these methods should not used alone should be used as supplement to radiographs apex locators.

Electronic method:

The electronic method calculates the working length by comparing the electrical resistance of the periodontal membrane with that of the gingival surrounding the tooth, both of which should be similar. The processors of these both of these machines basically work on four mechanisms.

§ Log readings

§ Subtract method

§ Taking averages

§ Taking ratios

These mechanisms are more important as compared to the number of frequencies used to collect data, as these signify essentially how the device interprets the data. All apex locators function by using human body to complete a circuit. The apex locators are also classified based on the principle they work on.

Suzuki conducted research in 1942 and this was used by Sunanda

The apex locators are base on three principles:

§ Resistance method

§ Impedance method

§ Frequency ratio method

Resistance Method(First Generation Apex Locators)

The apex locator has a built in resistant value of 6.5 kilo Ohms. The apex locator is attached to the patients lip on one side and the other side is attached to the file. The file is then advanced into the canal until it touches the periodontal tissue at the apex which then completes the circuit.

Impedance Method(Second Generation Apex Locators)

Operates on the principle that there is electrical impedance across the walls of the root canal due to the presence of the transparent dentin. The tooth exhibits an increasing electrical impedance across the walls of the root canal, which is greater apically than coronally. At the DCJ, the level of impedance drops dramatically. The unit detects the sudden change and indicates it on the analogue meter. To overcome the problem of a wet environment, insulated are utilized.

Frequency Ratio Method(Third Generation Apex Locators)

Operates very similarly to the Impedance-type because it measures the impedance of the tooth at two different frequencies. In the coronal portion of the canal, the impedance difference between the frequencies is constant. As the file is advanced apically, the difference in the impedance value begins to differ greatly with maximum difference at the apical area.

EAP are used for determining WL as an adjunct to radiography.

They are basically used to locate the apical constrictions or cementodentinal junction or apical foramen and not the radiographic apex.

Uses of apex locator:

1. Provide high degree of accuracy

2. Useful in conditions where apical portion is obstructed.

3. Useful in patient with gag reflex

4. Pregnant patient

5. Root perforation ,resorption ,root fracture .

6. Pulp vitality

7. RCT of teeth with incomplete root formation.

Advantages of EAL:

Accurate

Objective measurement

Easy and fast

Reduction of exposure to radiation

Perforation can be detected

Can measure pulp space exactly to constriction

Can detect resorption and root fracture

Basic conditions for accuracy of EAL:

Canal should be free from debris

Canal should be relatively dry

No cervical leakage

Proper contact of file with canal walls and periapex

No blockages or calcification in canal

Contraindication of EAL:

In patient who have cardiac pace maker,electrical stimulation to such patient can interfere with pace maker function

In teeth with periapical radiolucency and necrotic pulp associated with root resorption the use of apex locator is not much beneficial.

Summery:

The most important to understand when determining WL is morphology of apical one third of the canal.

The consideration should given to adopt the parameter 0.5-0.0mm(from apical constriction)as most ideal terminating point in canal.

We should use many of these techniques as possible during the course of treatment.