1. 0
Bisecting Angle Technique
Occlusal Technique
The following slides describe the bisecting
angle technique and occlusal technique
for taking intraoral films.
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2. Patient Preparation
Prior to starting to take films, the patient must
be positioned properly. Seat the patient and ask
them to remove their glasses and any removable
appliances. Adjust the headrest to support the
head while taking films. Raise or lower the chair
to a comfortable height for the operator. Place
the lead apron and thyroid collar on the patient.
You are now ready to begin taking films.
It is a good idea to inform the patient about the
number of films you will be taking so they know
what to expect.
4. The Bisecting Angle Technique is an alternative
to the paralleling technique for taking periapical
films. The paralleling technique is recommended
for routine periapical radiography, but there are
some instances when it is very difficult due to
patient anatomy or lack of cooperation. In these
situations, the bisecting angle technique may be
used. The film can be held in the mouth with the
thumb or index finger or a bisecting instrument
may be used. During this discussion, finger
retention will be stressed; patient acceptance of
the bisecting instrument is not much better than
for the paralleling instrument.
5. In the Bisecting Angle Technique, the x-ray beam
is directed perpendicular to an imaginary line
which bisects (divides in half) the angle formed
by the long axis of the tooth and the long axis of
the film (see diagram below).
Long axis of tooth Bisecting line
X-ray beam
Long axis of film
6. Bisecting Angle Technique
(Advantages)
When comparing the two periapical techniques, the
advantages of the bisecting angle technique are:
1. More comfortable: because the film is placed in
the mouth at an angle to the long axis of the teeth,
the film doesn’t impinge on the tissues as much.
2. A film holder, although available, is not needed.
Patients can hold the film in position using a finger.
3. No anatomical restrictions: the film can be
angled to accommodate different anatomical
situations using this technique
7. Anatomical Variations
Anatomical situations which might require using
the bisecting angle technique are:
• a shallow palate
• a large palatal torus
• a shallow or tender floor of the mouth
• a short lingual frenum (tongue-tie)
8. Bisecting Angle Technique
(Disadvantages)
When comparing the two periapical techniques, the
disadvantages of the bisecting angle technique are:
1. More distortion: because the film and teeth are
at an angle to each other (not parallel) the
images will be distorted (see next slide).
2. Harder to position x-ray beam: as mentioned
previously, because a film holder is often not used
it is difficult to visualize where the x-ray beam
should be directed.
3. Film less stable: using finger retention, the film
has more chance of moving during placement
9. Distortion
In the bisecting technique, the long axis of the tooth is
not parallel with the long axis of the film. This results in a
distortion of the image produced using this technique. In
the left radiograph below, the buccal roots appear much
shorter than the palatal root, even though in the actual
tooth the lengths are not that much different. In the other
radiograph taken with the paralleling technique, the
lengths are projected in their proper relationship
(minimal distortion).
bisecting paralleling
10. Head Position
When using a bisecting instrument, head position is not
critical. However, when using finger retention, head
position is important. When radiographing the maxillary
arch, the head should be positioned so that the maxillary
arch is parallel to the floor. For mandibular films, the
head is tipped back slightly so that the mandible is
parallel to the floor when the mouth is open (The mouth
is always open when using finger retention). Make sure
head is supported by headrest.
Maxilla Mandible
headrest (head tipped back)
11. Head Position
When viewed from the front of the patient, the
Midsagittal Plane (which divides the head into
right and left halves) is perpendicular to the floor.
MSP
floor
12. Bisecting Angle Technique
Film Selection for Adults
The # 2 size film is routinely used for all periapical
films using the bisecting angle technique. The long
axis of the film is vertical for anterior films and
horizontal for posterior films.
#2 #2
anterior posterior
13. Bisecting Angle Technique
Film Selection for Children
For children with small mouths, the # 0 size film is
used for both anterior and posterior periapical
films. However, if the child’s mouth is large
enough to reasonably accommodate the larger
size films, and the child is cooperative, they
should be used.
#0 #0
anterior posterior
14. Anterior Periapical
The # 2 (or # 0) size film is positioned vertically with the
all-white side of the film facing the teeth. The identifying
dot is placed at the incisal edge of the teeth. The thumb or
finger is applied to the back (colored) side of the film at
approximately the junction of the tooth with the gingiva;
this provides good support for the film and avoids film
bending. The film should extend ¼” beyond the incisal
edges of the teeth.
15. Posterior Periapical
The # 2 (or # 0) size film is positioned horizontally with
the all-white side of the film facing the teeth. The
identifying dot is placed at the occlusal surface of the
teeth. The finger is applied to the back (colored) side of
the film at approximately the junction of the tooth with
the gingiva; this provides good support for the film and
avoids film bending. The film should extend ¼” beyond
the occlusal surface of the teeth.
16. Bisecting Instrument
The Bisecting Angle Instrument is shown below.
Notice that the biteblock support, against which the
film will be aligned, is not parallel with the ring; it is
slightly angled to accommodate the bisecting
technique. This slight tilt of the film does little to
make film placement more comfortable for the patient
over the paralleling technique; that is why finger
placement is recommended if the bisecting technique
is indicated.
17. Snap-A-Ray
Another instrument that may be used for posterior
periapical films is the Snap-A-Ray shown below. The
alligator jaws hold the film tightly and, since there is no
support behind the film, the film can flex as the patient
closes. This makes it more comfortable for the patient.
18. Finger Retention
When using finger placement, always use the
hand opposite to the side of the mouth being
radiographed. (e.g., use the left index finger
when taking the right maxillary premolar film).
Use either thumb for the max. incisor film, the
thumb or index finger (opposite hand) for the
maxillary canines, and the index finger for all
mandibular films and for the maxillary
posterior films (opposite hand). Help the
patient by positioning their thumb or finger
where you want them to apply pressure.
19. Bisecting Angle Film Placement
The film placements below are appropriate for
both maxillary and mandibular arches.
20. Vertical Angulation
Using finger retention of the film, there is no external
guide to help you align the x-ray beam, as there is
when using the paralleling instrument. You have to
“imagine” where the bisecting line is and align the
beam perpendicular to this line. This makes the
technique much more difficult, but with practice it can
be a beneficial adjunct to your radiographic
technique.
When using this technique, keep in mind that all teeth
incline slightly toward the middle of the head; they
are not straight up-and-down. This will influence your
visualization of the long axis of the tooth and the
angle it forms with the film.
21. Vertical Angulation
The x-ray beam is directed perpendicular to the
bisecting line shown below. You can see the film long
axis, but you have to “visualize” the inclination of the
long axis of the tooth. Once you determine the angle,
imagine the bisecting line and direct the x-ray beam
at a 90-degree angle (perpendicular) to this line. This
is the vertical angulation.
Bisecting line
Long axis of tooth
X-ray beam
Long axis of film
22. Vertical Angulation 0
In the diagram below, the tooth is imagined to be
more upright than it really is. As the tooth is rotated
into its correct inclination (click to rotate), the angle
changes and the bisecting line (green dotted line) is
less steep, requiring an increased vertical angulation
(green arrow). Because most people imagine the
tooth to be more upright than it really is, it is
recommended that 5 degrees be added to the vertical
angulation you have chosen.
23. Horizontal Angulation
The horizontal angulation is adjusted so that a line
connecting the front and back edge of the PID (yellow
line below) is parallel with a line connecting the buccal
surfaces of the premolars and molars (green line below).
The x-rays will then be perpendicular to the film.
correct incorrect
24. Centering the Beam
For the anterior periapicals it is easy to see the sides of
the film and makes it easy to center the beam on the
film side-to-side. You then need to make sure the PID
extends ¼” below the visible (incisal) edge of the film
(maxillary arch) or above the visible edge (mandible).
In the posterior region, the front edge of the PID should
be ¼” anterior to the front edge of the film and the PID
should extend ¼” beyond the visible (occlusal) edge of
the film (above or below, depending on which arch is
being radiographed). These steps will help to insure
that the film is completely covered by the x-ray beam,
avoiding cone-cuts.
¼”
25. Maxillary Incisors
The film is held in place using the thumb of either hand.
The x-ray beam is directed perpendicular to the bisecting
line vertically and the horizontal angulation aligns the x-
ray beam perpendicular to the film. The x-ray beam is
centered on the film. The film shows both central
incisors and most of the lateral incisors.
26. Maxillary Canine
The film is held in place using the thumb or index finger
of the opposite hand. (Right hand for maxillary left
canine pictured below). The x-ray beam is directed
perpendicular to the bisecting line vertically and the
horizontal angulation should open the contact between
the canine and first premolar (see next slide). The x-ray
beam is centered on the film. The film shows tooth # 11.
27. Canine Horizontal Angulation
If you direct the beam perpendicular to the canine,
there will normally be overlap between the canine and
first premolar. In order to open this contact, the
horizontal angulation must be rotated posteriorly. Try
to imagine the mesial surface of the first premolar and
align the beam parallel with this surface. (see diagram
below right).
Incorrect Correct
28. Maxillary Canine 0
In many patients, especially ones with narrow maxillary
arch widths, it is difficult to align the film ideally
because the top edge of the film contacts the palate on
the opposite side and doesn’t allow enough film to
register the apex of the canine. By rotating the film into
a diagonal placement, this won’t be a problem.
Film can’t be placed
far enough into the diagonal placement
mouth (narrow arch)
29. Maxillary Premolar
Using the index finger of the opposite hand,
position the film properly and align the beam
vertically and horizontally. Center the x-ray beam
on the film.
30. Maxillary Molar
Using the index finger of the opposite hand,
position the film properly and align the beam
vertically and horizontally. Center the x-ray beam
on the film.
31. Sometimes it is difficult to
get the film far enough
back to cover the third
molar region due to
gagging or anatomy, and
all of the third molar will
not be seen on the film
(see diagram at left). By
rotating the tubehead so
that the beam is directed
more anteriorly (diagram
at right), the third molar is
projected on to the film,
giving us the needed
information. Note,
however, the increase in
overlap that results.
32. Mandibular Incisors
Using the index finger of either hand, position the
film properly and align the PID as discussed earlier.
All four incisors appear on the film.
33. Mandibular Canine
Using the index finger of the opposite hand,
position the film properly and align the beam
vertically and horizontally. Center the x-ray beam
on the film. # 22 is shown on the film below.
34. Mandibular Premolar
Using the index finger of the opposite hand,
position the film properly and align the beam
vertically and horizontally. Center the x-ray beam
on the film.
35. Mandibular Molar
Using the index finger of the opposite hand,
position the film properly and align the beam
vertically and horizontally. Center the x-ray beam
on the film. This film clearly shows all of the third
molar roots (# 17).
36. Adult full-mouth series, BisectingTechnique0
Using all # 2 size film, an adult full-mouth series of
films consists of 14 periapical films; 6 anterior (from
canine to canine, 3 maxillary and 3 mandibular) and 8
posterior (premolar and molar films in each quadrant).
All # 2 films
R L
37. Anterior First
When taking films on a patient, you should always
start with the anterior films. If you are doing a full
series, start with the maxillary canine film and
then finish all the anterior films, both maxillary
and mandibular. Then complete the posterior
films, starting with the premolar, then molar, in
each quadrant. When doing only a few films on a
patient, start with the most anterior film and work
your way back in the mouth. This sequence of
taking films allows the patient to get used to the
procedure with a minimum of discomfort and
helps to avoid stimulation of the gag reflex.
38. Bisecting Angle Technique
Errors
The following slides identify some of the most
common errors seen when using the bisecting
angle technique.
39. Elongation
If you have too little vertical angulation, as in the
diagram below, the image will be elongated or
stretched out on the film. The angle the x-ray beam
forms with the bisecting line is less than 90°. The red
lines on the film represent the actual length of tooth #
9; the black arrow points to the end of the “image” of
the tooth.
long axis of tooth
bisecting line
x-ray beam
film
bisecting line
40. Foreshortening
If you have too much vertical angulation, as in the
diagram below, the image will be foreshortened or
reduced in length. The angle the x-ray beam forms
with the bisecting line is greater than 90°. The red lines
on the film represent the actual length of tooth # 9; the
black arrow points to the end of the “image” of the
tooth.
long axis of tooth
bisecting line
film
41. When using the bisecting angle technique with finger retention,
the incisal edge /occlusal surface will always be in contact with
the film. This part of the tooth will always appear at the same
spot on the film no matter what the angulation is. However, the
apex of the teeth, being farther away from the film, will be
imaged at different positions depending on the vertical
angulation. The arrows in the diagram below identify where the
apex of the tooth will be at different angulations; e. g., at >90°
the apex will be imaged lower on the film, shortening the overall
image. Remember, a 90° angle between the x-ray beam and the
bisecting line is the ideal alignment.
>90º = foreshortening
<90º = elongation image lengths
42. Improper Film Placement
As with the paralleling technique, improper film
placement is one of the most common errors
seen in the bisecting angle technique. In the
molar film below, the film was placed too far
forward, cutting off the distal root of the second
molar and failing to image the third molar region.
AP
Mandibular molar periapical
43. Film Placement 0
With finger retention, it may be hard to keep the
film from rotating around the end of the finger as
it presses the film against the teeth. This may
result in a tipped film as seen below. Notice the
tip of the second molar is not visible, resulting in
the need for a retake. (The teeth are also
elongated; is this too little or too much vertical
angulation?)
Too little (not
enough) vertical
angulation
44. Film Placement 0
It is important to place the film so that ¼” of film
extends beyond the incisal edge (anterior) or
occlusal surface (posterior). However, if too
much film extends beyond, the roots of the
teeth will usually not appear on the film, as seen
below.
45. Film Placement 0
When placing the film using finger retention, it is
important to make sure that finger pressure is applied
where the film is supported by tooth structure, ideally
at the junction of the crown of the tooth with the
gingiva. If the film is not supported, film bending will
result. In the canine film below, the canine root
“bends” off of the film. What other error is seen on
this film?
Film not centered
on canine
Canine periapical
46. Reversed film 0
If the colored portion of the film faces the teeth
being radiographed, the lead foil in the film packet
will be between the teeth and the film. This results
in the pattern stamped on the lead foil appearing
on the film (see right side of film below). The film
will also be lighter than the other films taken at the
same time. What other situations could result in a
film that is too light?
Underexposure or
processing error
(e.g., developer
solution too cold)
47. Cone-cutting
If the x-ray tubehead is not positioned properly, the x-ray
beam may not cover the entire film. This is known as
conecutting, which results in a clear (white) area on the
film where the silver halide crystals were not exposed to
x-rays (see film below). In the diagram below left, the
dotted circle represents where the x-ray beam should
have been positioned; the solid circle shows the actual
position of the x-ray beam (too posterior).
48. Overlap (incorrect horizontal angulation)
Overlap is the superimposition of part of one tooth
with part of the adjacent tooth (dotted circles below
left). The red arrow represents the direction of the x-
ray beam; the x-ray beam should be perpendicular to
the dotted line below. (See discussion of horizontal
angulation on earlier slide).
49. Film “Softening”
If you try to make the film more comfortable for the
patient by “softening” the corners or edges, the
emulsion of the film will be affected, resulting in
black lines (see film below). With finger retention,
film placement is usually not very uncomfortable;
therefore, film softening is not needed.
50. Double exposure
When taking films, you should always place each film in
a container or paper bag immediately after it is exposed.
Exposed films should never be placed in the same area
where unexposed films are located. If you inadvertently
pick up an exposed film and use it for another exposure,
the result is a double exposure. Two different areas of
the mouth are superimposed, making the images
worthless. This is the worst error because two films have
to be retaken.
The film at left shows
images of mandibular
posterior teeth , both
upright and inverted. The
film was used for both the
premolar and molar films
on the same side.
51. Patient Movement 0
If the patient moves slightly during the exposure
of the radiograph, the image will be blurred as in
the film below. Always advise the patient to
remain still for the very short time it takes to
complete the exposure. What other error is
evident on this film?
Less than 1/4” of film
was extending above the
occlusal surface on this
premolar periapical film,
cutting off the top part of
the crowns of the teeth.
52. Thyroid collar
With finger retention of films in the mandibular arch, the
tubehead may be positioned so that the x-ray beam
passes through part of the thyroid collar (see photo
below). This lead in the thyroid collar prevents x-rays
from passing through, resulting in an unexposed, clear
area on the film as seen below right.
PI D
53. Incorrect Exposure Factors
The standard exposure settings on your x-ray machine
will be acceptable for the majority of your patients.
However, if you are taking radiographs on a child you
would need to decrease the settings. If your patient is
very large, you would need to increase the settings.
Underexposure results when the exposure factors are
set too low for the patient size. Overexposure results
when the exposure factors are set too high.
underexposure correct exposure overexposure
55. Occlusal Film
The occlusal film is used to:
• identify the extent of lesions in a buccolingual
direction
• identify the buccolingual location of impacted
teeth or other abnormalities
• show the location of developing teeth in
children, using # 2 size film
• image patients with trismus that have limited
mouth opening
56. Occlusal Technique
Head Position
Maxillary film: the maxillary arch is parallel to the
floor; the midsagittal plane is perpendicular to
the floor.
Mandibular film: the head is tipped back so that
the mandibular arch is as close to perpendicular
to the floor as possible.
57. Occlusal Technique
Film position
The film is placed so that the all-white side of
the film (# 4 for adults, # 2 for children) faces the
arch being radiographed. The film is usually
placed with the long axis side-to-side, but this is
not critical. The film is large enough to normally
cover the entire arch, but make sure it covers
the area of interest. Position the film as far back
in the mouth as possible and the patient gently
bites on it to keep it in place.
58. Occlusal Technique
X-ray Beam Position
There are three types of occlusal films (to be
discussed on the following slides):
“Normal” Maxillary
“True” Maxillary
Mandibular
For all three of these, the x-ray beam is centered on
the area of interest. Because of the curved beam,
the corners of the film that sticks out of the mouth
are often not exposed, resulting in slight conecuts.
This is not an error, since these areas contain no
needed information.
59. Normal Maxillary Occlusal
The Normal Maxillary Occlusal film is the most
common occlusal film taken in the maxillary arch.
The vertical angulation is set at 65 degrees.
Because of this angle, structures located toward
the back of the mouth may be projected off the
back edge of the film and not be imaged.
65 degrees
60. True Maxillary Occlusal
The True Maxillary Occlusal film is not often used
because of the much higher exposure time needed to
properly expose the film. (Because the vertical
angulation is 90 degrees, the x-ray beam passes
through the very dense frontal bone; this is the reason
for the increased exposure). Structures located farther
back in the mouth are more likely to be imaged on this
film.
90 degrees
61. Mandibular Occlusal
With the head tipped back as much as possible,
the x-ray beam is directed at a 90 degree angle to
the film. Bony expansions of the mandible as well
as abnormalities or pathology in the floor of the
mouth can be imaged with this film.
90 degrees
62. Occlusal Technique
Exposure Settings
The exposure times for the “normal” maxillary and
mandibular occlusal films are the same as for a
periapical or bitewing film of comparable film
speed. For the “true” maxillary occlusal film, the
exposure time is four times as long, allowing
enough x-rays to pass through the frontal bone
and properly expose the film.
65. Modified Bisecting Occlusal
If a patient has difficulty opening the mouth due to
trismus, an occlusal film can be used to provide a
reasonable image of the teeth. The film is centered on
the side of interest with the long axis front to back. The
beam is aligned using the Bisecting Angle technique.
The images will be greatly distorted, but may provide
the necessary information.
66. 0
This concludes the section on Bisecting
Angle and Occlusal Techniques.
Additional self-study modules are available
at: http://dent.osu.edu/radiology/resources.htm
If you have any questions, you may e-mail
me at jaynes.1@osu.edu.
Robert M. Jaynes, DDS, MS
Director, Radiology Group
College of Dentistry
Ohio State University
