DA118, Chapter 18
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Bisecting is based on the simple geometric rule
of isometry, which states two triangles are
equal if they have two equal angles and share a
common side
The film must be placed along the lingual
surface of the tooth
The point where the film contacts the tooth,
and the plane of the film form an angle
The dental radiographer must visualize a plane
that bisects, or divides in half, that angle that is
formed by the tooth and film
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The dental radiographer must then direct the
central beam perpendicular to that imaginary
bisector.
When the central beam is perpendicular to that
bisector, two imaginary congruent triangles are
formed
When these principles are followed strictly, the
tooth image is accurate.
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Film holders are recommended so the patient does
not have to hold the film themselves
This reduces patient exposure, and the possibility
that the film might shift around when being held
in place
Some film holding devices used for bisecting angle
technique are:
Rinn BAI instrument: work like XCP, except it is designed
for the film to be placed closer to tooth surface
 Stabe
 Snap-A-Ray
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Used as an alternative when a film holding device
is not possible
The patient’s finger or thumb is used to stabilize
the film (placed behind the film and teeth)
The patient’s hand is usually in the path of the xray beam, resulting in unnecessary exposure
The patient may bend the film, causing distortion
The patient may not hold the film with enough
pressure, causing it to slip
Without the use of a beam alignment device, cone
cut occurs more readily
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When dealing with a patient who has difficult
or unusual anatomy
When the patient has a disabling condition that
may prevent them from closing on a bite block
When dealing with an uncooperative patient,
such a small child
Used with certain endodontic films
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Film Placement:
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Film position: placed against lingual surface of
tooth
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similar to paralleling BUT use all size 2 films, and
only three views on maxillary anteriors
Anterior teeth: against incisal edge
Posterior teeth: against occlusal surface
Vertical Angulation: “yes-yes”
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Central ray directed perpendicular to imaginary
bisector that divides angle formed by film and long
axis of tooth
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If we aim the central ray at the long axis of the
tooth, the result will be longer teeth than
normal, or elongated
If we aim the central ray at the film, the result
will be shorter teeth than normal, or
foreshortening
In order to get an accurate image, that angle
needs to be bisected, or divided in two equal
parts. That imaginary line is what the central
ray is aimed perpendicular to
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The horizontal angle, or the plane going side to
side, is the same as paralleling.
When the horizontal angulation is correct, the
central x-ray beam will be directed through the
contact areas, otherwise, overlap will occur
The vertical angle refers to the plane that is
positioned either up or down
The correct vertical angulation is critical to an
accurate image record
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Foreshortened Images: are a result of excessive
vertical angulation. This also occurs if the
central ray is directed perpendicular to the film
and not the bisector
Elongated Images: are a result of insufficient
vertical angulation. This also occurs if the
central ray is directed perpendicular to the long
axis of the tooth, and not the bisector
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Horizontal Angulation: “no-no”
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Central ray directed through contact areas between
teeth
Correct horizontal angulation will create “open
contacts”, allowing doctor to see proximal surfaces
of the individual teeth
Incorrect horizontal angulation will create
overlapping which obscures detail in the image
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Number of films and groups of teeth are the
same as paralleling
Horizontal angle is the same as paralleling
Film should extend 1/8 – ¼ inch beyond the
incisal or occlusal edge
Vertical angle is determined by the imaginary
bisector