Slide 4 - Working Length

7/29/2019 Slide 4 - Working Length

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Working Length Determination,

Endodontic Radiology

Dr Nawaf Al-Hazaimeh


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WORKING LENGTH DETERMINATION

The working length is defined as the distance

from a predetermined coronal reference point

(usually the incisal edge in anterior teeth and

a cusp tip in posterior teeth) to the point that

the cleaning and shaping, and obturation

should terminate.

The reference point must be stable so fracturedoes not occur between visits.


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Optimal length 1-2mm short of the apex.

Apical foramen

Anatomical apex

Apical constriction


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A, The apical foramina(small arrows) do notcorrespond to the trueanatomic apex (largearrows).

B, In most situationsthe apical terminus orseat of the preparationwill vary from the apicalforamen andradiographic apex


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Apical Constriction

The presence of an apical constriction is unpredictable.Frequently there is no apical constriction. It has beenproposed that the cementodentinal junction forms theapical constriction; however, this concept is incorrect.In fact, the junction is difficult to determine clinicallywith accuracy, and the intracanal extent of cementumis variable. If an apical constriction is present, it is not

visible on a radiograph and usually is not detectablewith tactile sense using a file, even by the most skilledpractitioner.


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Reference point

Before access an estimated working length is

calculated by measuring the total length of

the tooth on the diagnostic parallel radiographor digital image.


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2.0 millimeters are subtracted to account for the foramen

distance (1.0 mm) and radiographic image distortion

/magnification (1.0 mm). This provides a safety factor so

instruments are not placed beyond the apex.

After access preparation, a small file is used to explore the

canal and establish patency to the estimated working length.

The largest file to bind is then inserted to this estimated

length


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Millimeter markings on the file shaft or rubber

stops on the instrument shaft are used for

length control. A sterile millimeter ruler or

measuring device can be used to adjust the

stops on the file


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To obtain an accurate measurement, the minimum size

should be a No. 20. With files smaller than No. 20, it is difficult

to interpret the location of the file tip on the working length

film or digital image.

It is imperative that the rubber dam be left in place during

working length determination to ensure an aseptic

environment and to protect the patient from swallowing or

aspirating instruments.


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With the modified paralleling technique, the film is positioned

by using a film holder parallel to the long axis of the tooth.

The cone is then positioned so the central beam will strike the

film at a 90-degree angle

Other clinical factors should be considered in establishing the

corrected working length. These include tactile sensation, the

patients response, and hemorrhage.


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Corrected working length

WL distance from the apex is

Determined when

radiographically there is

No bone or root resorption

(A)

1 mm from apex

Bone but no root resorption(B)

1.5 mm from apex

Bone and root resorption (C

)

2 mm from apex


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When the correction is

greater than 3.0 mm, it

is advisable to make a

second working lengthradiograph with the file

placed at the adjusted

length.


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An apex locator is very helpfulin patients with structures orobjects that obstructvisualization of the apex,patients that have a gag reflex

and cannot tolerate films, andpatients with medicalproblems that prohibit theholding of a film or sensor.

The use of apex locators andelectric pulp testers in patients

with cardiac pacemakers hasbeen questioned.


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IMPORTANCE OF RADIOGRAPHY IN ENDODONTICS

Radiographs perform essential functions in threeareas. However, they have limitations that requirespecial approaches.

A single radiograph is but a 2-dimensional shadowof a 3-dimensional object. For maximuminformation, the third dimension must be visualizedand interpreted.

The three general areas of application arediagnosis, treatment, and recall; each requires its

own special approach.


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Diagnosis

Root and pulp anatomy

Identifying Pathosis

Characterizing normal structures.

Treatment

Determinig working length

Moving superimposed structure Locating canals

Evaluating obturation

Recall

Identifying new pathosis

Evaluating Healing


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A, Parallel preoperative

radiograph. B, The

mesial working length

film is made correctly.The apices and file tips

are clearly visible.

Note the mesiolingual

canal (arrow).


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Diagnosis

The facial projection of this

premolar gives some limited

information about

pulp/root morphology. Fast

break (small arrow) usuallyindicates canal bifurcation.

B, The same premolar from

the proximal view. The

presence of two definitivecanals, each in its own root

bulge, is confirmed.


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when two objects and the film are in a

fixed position and the radiation source (cone) ismoved, images of both objects move in theopposite direction.

The facial (buccal) object shifts farthest away; thelingual object shifts less. The resulting radiographshows a lingual object that moved relatively inthe same direction as the cone and a buccal

object that moved in the opposite direction. This principle is the origin of the acronym SLOB

(same lingual, opposite buccal)


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The SLOB rule


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The film is positioned parallel to the plane of

the arch. The cone has the central ray (arrow)

directed toward the film at right angles.

This is the basic cone-film relationship used

for horizontal or vertical angulations. B, There

is a clear outline of the first molar but limitedinformation about superimposed structures

(canals that lie in the buccolingual plane). The

arrowpoints to a periodontal ligament space

adjacent to a superimposed root bulge, not to

a second canal

A, The horizontal angulation of the cone

is 20-degrees mesial from the parallel,

right-angle position (mesial projection).

B, The resultant radiograph

demonstrates the morphologic features

of the root or canal in the third

dimension. For example, two canals are

now visible in the distal root of the first

molar


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SLOB Rule

Central (x-ray) beam passing directly through a root containing two

canals will superimpose the canals on the film.

When the cone is shifted to the mesial or distal aspect, the lingual

object will move in the same direction as the cone; the buccal

object will move in the opposite direction (SLOB rule).


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Determination of Working Length

A, Mesial projection gives limited information about morphologic features and relationship of

four canals. B, Correct distalprojection for mandibular molars opens up roots. Mesial canals are easily visualized for their

entire length. The distal canal is a single wide canal

because instruments are close and parallel


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Facial

Maxillary anterior teeth rarely have more than asingle root and a single canal, thus only a facial

(straight-on) projection is required. This is alsotrue for maxillary molars unless a secondmesiobuccal (mesiolingual) canal is detected andnegotiated during access.

The straight facial projection provides maximumresolution and clarity (which is difficult at bestwith maxillary molars).


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Mesial

The mesial projection is indicated for maxillary

and mandibular premolars and for mandibular

canine teeth. A mesial projection is used for

maxillary molars with a mesiolingual canal.


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Distal

The distal projection is used for mandibular

incisors and mandibular molars. The distal is

preferred to the mesial projection for

mandibular molars because of the relative

position of the canals. Generally, the distal

angle more effectively opens up the

mesial root.


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Separation of the mesiobuccal and mesiolingual canals

achieved by varying the horizontal angle. With maxillary

molars maximum

separation occurs with a mesial cone angulation

because of the mesial location of the mesiolingual canal

in relation to the mesiobuccal canal.

Separation of the mesiobuccal and the mesiolingual canals

achieved by varying the horizontal angle. With mandibular

molars,

maximum separation occurs with a distal orientation

because of the mesial location of the mesiobuccal canal in

relation to the mesiolingual canal.

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Slide 4 - Working Length