بسم الله الرحمن الرحيم

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Ahmed LabibAhmed Labib

Working length: is the length to which the root

canal preparation and obturation will terminate.

So the working length of a tooth: is the length between an external reference point on the crown of the tooth, and the cemento-dentinal junction of the root.

CDJ: (the apical foramen) the anatomical apex is 0.5 mm to 1 mm shorter than the radiographic apex.

The optimum length is at the apical constriction.

Biological rationale for working length:

-Establishing a good working length of the tooth is the most important step in cleaning and shaping of the root canal.

-It greatly facilitates accurate instrumentation and filling of the root canal.

Access Cavity

Tooth length

RC Prep

Obtur.

Over instrumentation

Over instrumentation results in perforation of the apical foramen with the following results:

1-The periapical tissue might become traumatized by the instrumentation → to post treatment inflammation, pain, and swelling.

2-Necrotic material might be forced into the periapical region with subsequent acute inflammation.

3-The irrigation solutions and intracanal medicaments may leak out through the apical foramen with subsequent irritation of the periapical area.

4-The apical foramen might be enlarged by the perforating instrument so that the subsequent filling material may be extruded from the foramen and irritate the periapical tissue.

N.B.: Instrumentation to the radiographic apex is considered over instrumentation.

Under instrumentation

Instrumentation shorter than the cemento-dentinal junction may result in::

2-Any bacteria or necrotic material left in the canal beyond this point of shelfing

might result in case failure.

3-Even if there is no pulp tissue beyond the ledge (when ledge is formed after

complete cleaning) one can’t fill this area, with subsequent microleakage, which may

lead to case failure.

1-Shelfing the canal, (a ledge in a root canal formed during instrumentation ) which will catch the instrument.

Methods of Working

Length Determination

Methods of Working

Length Determination

Methods of working length determination:

1-Radiographic.

2-Electronic apex locator.

3-Tactile sensation.

4-Paper point.

5-Apical Periodontal sensitivity.

1.Radiographic method:

a) Average length of the tooth.

b) Preoperative radiograph.

c)  Tactile sensation.

A properly angulated, developed and fixed X-ray film with an instrument inside the root canal, is still the most accurate method for length determination , which can be determined by:

Radiographic Apex Location:

The following items are essential to perform this procedure:

1. Good, undistorted, preoperative radiographs showing the total length and all roots of the involved tooth.

2. An endodontic millimeter ruler.

3. Knowledge of the average length of all teeth.

4. Adequate coronal access to all canals.

5. A definite, repeatable plane of reference to an anatomic landmark on the tooth, a fact that should be noted on the patient’s record.

Preoperative radiograph should be examined to reveal the following:

1-The number, size, shape, curvature and angulations of the root(s).

2-Presence of any periapical pathosis and degree of bone and root resorption if present.

3-Presence of vertical or horizontal roots fracture.

4-Root obstructions by pulp stone or obliteration of the root by secondary dentin.

5-Old root canal treatment if present.

6-Estimation of the tooth working length.

.

-The estimated length is the length to which the initial file will be inserted into the canal, and then confirmed by another radiograph.

-If the estimated length is 21 mm. the stopper must be adjusted on the initial file shaft to be 21 mm away from the file tip, then the file is inserted into the canal and a confirmatory radiograph is taken.

Radiographic estimation of working length:

- Measure the tooth length on the preoperative radiograph from cusp tip or incisal edge to the radiographic apex to get an estimation of the actual working length of the tooth.

- Subtract 1 mm from this length to get the position of the anatomical apex or the cemento-dentinal junction.

1-No Bone or Root resorption (normal case):

the difference is 1mm. from the apex.

2-Bone resorption but No Root resorption:

the difference is 1.5 mm. from the apex.

3-Bone and root resorption: the difference is

2 mm. from the apex.

Important considerations for estimated tooth length:

1-Tactile perception: “sense of feel” or “tactile sense”

Feeling the constriction of the cemento-dentinal junction with the measuring instrument can assist in estimating the working length of the tooth.

2-Average length of tooth:

The knowledge of different average teeth lengths can be of great value in estimating working length.

Selection of the external reference point:

1-In anterior teeth incisal edge.

2-In posterior teeth cusp tip for each canal.

-In case of more than one-reference point for canals of posteroir teeth, this must be registered in the patient chart.

3-Reference point should be easily checked during instrumentation.

4-It should be selected in sound tooth structure to avoid breakage of the restoration or undermined enamel between visits.)

5-Avoid inclined planes as a reference point.

6-The file stopper must be rested in a straight position on the reference point to prevent length discrepancies during instrumentation and filling of the root canal.

Placing the stopper

Initial radiograph with file in the canal:

Three possibilities may be faced at this step:

1-The file tip is just on the cemento-dentinal junction (1mm shorter than the radiograph apex), so this is our working length to which all instrumentation and filling procedure will be confined.

2-The file tip is at the radiographic apex or longer than it, so we subtract the length of the file beyond the

radiographic apex + 1mm. (The distance between the radiographic apex and the cemento-dentinal junction).

And another confirmatory X-ray must be taken to get sure that we got the accurate working length of the tooth.

Over estimated length

3-The file tip is shorter than the radiographic apex by more than one mm. In this case we add the length difference between the file tip and the radiographic apex and then subtract one mm. to get the accurate working length. Again, another Confirmatory X-ray must be taken to be sure that we reach the accurate working length.

Under estimated length

Confirming working length of tooth:1. Reset the stopper to the new estimated length.

2. Reinsert the file into the canal as done before to its new estimated length.

3. Take a new radiograph to confirm the length.

4. Examine the new radiograph to determine if the correct working length of tooth has been reached (just to the cemento-dentinal junction). If this length has been reached that is it, and this length should be recorded in the patient chart.

5. If the working length of tooth has not yet been reached, additional adjustment of the length must be done, until working length of tooth is reached and confirmed.

Summary

Disadvantages of the radiographic method :1. Health hazards.

2. The film reveals only two dimensional picture for a three dimensional object (Tooth)

• So the third dimension of the tooth structure does not appear in the radiographic film.

•This may lead to superimposition of root canals over each other in the radiograph, which may lead to inaccurate working length determination.

•Hence there was a need to develop a radiographic technique that can overcome this disadvantage which is called the Buccal Objective Rule (Changing the Horizontal Angulation ) or ( Tube-Shift ).

Stand. Standard

Mesial shift

Distal shift

Tube-Shift Localization (Clark)

SLOB RuleSame Lingual Opposite Buccal

The SLOB rule is used to identify the buccal or lingual location of objects (impacted teeth, root canals, etc.) in relation to a reference object (usually a tooth). If the image of an object moves mesially

when the tube head is moved mesially (Same

direction), the object is located on the Lingual. If the image of the object moves distally when the tube

head moves mesially (Opposite direction), the object

is located on the Buccal.

INGLE’s Rule ( M -► B -► D , M -► L -► M )

The buccal (yellow) and lingual

(red) objects are superimposed

on each other because the beam is

directed perpendicular to both of

them and they are in the same relative position mesiodistally.

mesial

distal

mesialdistal

Horizontal movement

B

When the tube head is moved mesially ( the beam is directed

distally ). The buccal object (yellow) moves distally (Opposite to tube head

movement) and the lingual object (red) moves mesially

(Same direction as tube head) in relation to the second molar.

mesial

distal

mesial distal

Horizontal movement

Mesial Shift

P

B

When the tube head is moved distally ( the beam is directed

mesially). The buccal object (yellow) moves mesially

(Opposite to tube head

movement) and the lingual object (red) moves distally

( Same direction as tube head) in relation to the second molar.

mesial distal

mesial

distal

Horizontal movement

Distal Shift

P

Are devices developed to:

1-facilitate the determination of the working length.

2- Avoid the hazards of exposure to multiple X-ray doses.

2- Electronic Method:( Electronic apex locators )

The principle idea of electronic apex locator is based on electrical resistance when the file (which is attached to the device) come closer to the apex; the device will recognize the impulses in the periapical area; this will indicate the accurate length.

Classification and Accuracy of Apex Locators.

This classification is based on:1. the type of current flow. 2.the opposition to the current flow. 3.the number of frequencies involved.

First-Generation Apex Locator: ( resistance apex locators)

• It measure opposition to the flow of direct current or “resistance.”

• When the tip of the reamer reaches the apex in the canal,the resistance value is 6.5 kilo-ohms (current 40 mA). • It had some problems. • Today, most first-generation apex location devices are off the market.

2.The Digipex has a visual LED digital indicator and an audible indicator.

3.The Digipex II is a combination apex locator and pulp vitality tester.

4.The Exact-A-Pex has an LED bar graph display and an audio indicator.

5.The Foramatron IV has a flashing LED light and a digital LED display.

6.The Apex Finder has a visual digital LED indicator.

7.The Endo Analyzer is a combined apex locator and pulp tester.

Second-Generation Apex Locators:

(impedance apex locators) They measure opposition to the flow of alternating current or

impedance. 1.Sono-Explorer, one of the earliest of the second generation apex

locators.

IMPROVEMENTS OF SONO-EXPLORER:

Formatron IV

Disadvantage of second generation apexlocators is that: 1.The root canal has to be free of Electroconductive

materials to obtain accurate readings.

2.The presence of tissue and electro-conductive irrigants changes the electrical characteristics and leads to inaccurate, (usually shorter measurements).

3.This created a Question : Should canals be cleaned and dried to measure working length, or should working length be measured to clean and dry the canals?

Third-Generation Apex Locators. They depend on the “Frequency of current flow”.

• These devices have been called “frequencydependent”

• More advanced and more accurate.

• work in wet canals (in the presence of electrolyte). eg: Saline or NaOCL.

Endex (aka APIT), the original third-generationapex locator. It measures the impedance between two currents and works in a “Wet” canal with sodium hypochlorite.

The Apex Finder A.F.A. (All Fluids Allowed) Third generation apex locator. It functions best with an electrolyte present and displays, on an LCD panel, the distance of the file tip from the apex in 0.1 mm increments.

Root ZX third-generation apex locatorThe Root ZX microprocessor calculates the ratio of two impedances and displays a file’s approach to the apex on a liquid crystal display (LCD). It functions in both a “dry” or “wet” canals .

Tri-auto ZX :has 3 automatic safety mechanisms 1.Auto start – stop.2.Auto torque reverse.3.Auto apical reverse.•Combination of Apex Locator & Endodontic Handpiecewith a built-in Root apex locator.

3-Tactile Sensation method:• If the coronal portion of the canal is not constricted; an experienced clinician may detect an increase in resistance as the file approaches the apical 2 to 3 mm.•This detection is by “Tactile sense”.

• Constriction and Course change apply pressure to the file which give the sensation to the operator that the working length is reached.

Two facts make tactile sensation possible:

• Canals commonly constrict before the end of the root.

• Canals frequently change it’s course in the last 2-3 mm.

It is a supplementary method.

4- Paper Point Method:• Can be used In a root canal with an immature (wide open) apex.• After profound anesthesia has been achieved.• Gently pass the blunt end of a paper point into the canal.

• The moisture or blood on the portion of the paper point

that passes beyond the apex may be an estimation of

working length or the junction between the root apex

and the bone.

• In cases in which the apical constriction has been lost owing to resorption or perforation,( and in which there is no free bleeding or suppuration into the canal), the moisture or blood on the paper point is an estimate of the amount the preparation is overextended.

• This paper point method is a supplementary one.

5-Apical Periodontal sensitivity method:

• Any method of working length determination, based on the patient’s response to pain, does not meet the ideal method of determining working length. Working length determination should be painless.

• If an instrument is advanced in the canal toward inflamed tissue, the hydrostatic pressure developed inside the canal may cause moderate to severe, instantaneous pain. At the onset of the pain, the instrument tip may still be several millimeters short of the apical constriction.

• When the canal contents are totally necrotic, however, the passage of an instrument past the apical constriction may evoke a mild or possibly a flare-up reaction.

•

• Vital pulp tissue with nerves and vessels may remain in the most apical part of the main canal even in the presence of a large periapicallesion.

• This suggests that a painful response may be obtained inside the canal even though the canal contents are “necrotic” and there is a periapical lesion.

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