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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