Intraoral Radiographic Techniques (The Bisecting Angle Technique)-Chapter 3

Basic Principles
The bisecting-the-angle or bisecting angle technique is based on the principle of aiming the central ray of the x-ray beam at right angles to an imaginary line which bisects the angle formed by the longitudinal axis of the tooth and the plane of the film packet.  While it is not necessary to go into a long dissertation on plane geometry to understand this concept, a quick review will help make the technique more clear.  To bisect is to divide a line or angle into two equal portions.  A bisector is a plane or line that divides a line or angle into two equal portions.  Figure 37 shows an equilateral triangle, with legs AB=BC=CA, and the angles ABC=60 degrees, CAB=60 degrees and BCA=60 degrees.
 

Figure 37
 
We see in Figure 37 the following:
  1. The dotted line BD bisects the triangle, dividing it exactly in half.  Thus, two equal triangles are formed from the original. Legs AB and BC were unchanged and thus are still equal.
  2. The original line CA was divided in half by D, and thus the lines AD and CD are equal.
  3. We know that the angle at point B was 60 degrees, and since it was bisected (divided equally), it now is 30 degrees at the intersections of AD and BD.
  4. We also know that bisecting the angle did not affect the angle at the old point A which was 60 degrees, and still is.
  5. The angle at the bisecting point DC must be 90 degrees because the sum of all the angles in any triangle is 180 degrees, and thus 180-(60+30)=90.
  6. Cyzynski’s Rule of Isometry states that two triangles are equal when they share one complete side, and have two equal angles.  We can see that triangles ADB and BDC share the common side BD.
  7. We know further that the angles ADB and BDC are equal because D was defined as a bisector of the old angle ABC.
  8. Lastly, we know that the angles CAB and BCA were unchanged by bisecting and are still equal.  Therefore, under Cyzynski’s Theorem, we can prove the triangles ABD and CBD are equal.
In dental radiography, the theorem is applied in the following manner.  The film is positioned resting on the palate or on the floor of the mouth as close to the lingual tooth surfaces as possible.  The plane of the film and the long (vertical) axis of the teeth to be radiographed form an angle with the apex at the point where the film packet contacts the teeth.  The apex in Figure 38 is located at the point labeled B.
In Figure 38, the long axis through the tooth forms one leg of a triangle (AB), the plane of the film packet another leg, (BC), both of which intersect at the apex, point B.  A line representing the central x-ray beam will form the third leg of the triangle, AC.  If an imaginary line bisected this axis-packet-ray triangle, the bisector, DB, would form the common side of two equal triangles as defined by Cyzynski’s Theorem.
 
Figure 38
Since the sides formed by the tooth’s long axis and the film packet are equal, the image cast onto the radiographic film would be the same length as the tooth or teeth casting that image.  This linear equality is the basis for diagnostic quality bisecting angle radiographs.

The Bisecting Angle Technique
Anatomical Considerations
The bisecting angle technique is of value when the paralleling technique cannot be utilized.  This may include patients with small mouths and those with low palatal vaults.  Because of the increased exposure to radiation in this technique, it should only be employed as necessary.
Beam Angulation
The bisecting technique calls for varying beam angulations, depending on the region to be examined.
Horizontal angulation:  The horizontal angulation of the tube head should be adjusted for each projection to position the central ray through the contacts in the region to be examined.  This angulation will usually be at right angles to the buccal surfaces of the teeth to be radiographed.
Vertical angulation:  In practice, the operator should position the central ray of the x-ray beam so that it is perpendicular to the imaginary line bisecting the angle formed between the tooth long axis and the film.  This principle works well with flat, two-dimensional structures, but teeth that have depth or are multirooted will produce distorted images.  If the vertical angulation is excessive the image will appear foreshortened.  Insufficient vertical angulation produces an elongated image.
The optimum angle will vary from patient to patient, but the chart below serves as a general guideline for beam angulation.
Projection
Maxilla
Mandible
Incisors
+40 degrees
-15 degrees
Canine
+45 degrees
-20 degrees
Premolar
+30 degrees
-10 degrees
Molar
+20 degrees
2-5 degrees

Film Holding Devices
Supporting the film pack with the patient’s forefinger is not recommended.  This method has several drawbacks.  In addition to exposing the patient’s digit to additional radiation, the patient may exert excessive force, thus bending the film and distorting the radiograph.  The film may slip without the operator’s knowledge, and produce a radiograph outside the proper image field.  Therefore, intraoral support is best accomplished using instruments that restrain the film and help align the beam properly.


Quiz
  1. On what principle is the bisecting angle technique based?
  2. How did this principle originate?

Answers
  1. The bisecting angle technique is based on the principle of aiming the central ray of the x-ray beam at right angles to an imaginary plane bisecting the angle formed by the longitudinal axis of the tooth and the plane of the film packet.
  2. The principle originated from Cyzynski’s Rule of Isometry (Cyzynski’s Theorem) which states that two triangles are equal when they share one complete side and have two equal angles.

Bisecting Angle Methodology
 
Patient Positioning
Maxillary region:  For bisecting angle radiographs of the maxilla, the patient should be positioned so that the maxillary occlusal plane is parallel to the floor and the sagittal plane of the patient’s head is perpendicular to the floor.
Mandibular region:  For bisecting angle radiographs of the mandible, the patient should be positioned so that the mandibular occlusal plane is parallel to the floor and the sagittal plane of the patient’s head is perpendicular to the floor.
Before any radiographs are exposed, the patient must be protected with a lead apron and thyroid collar.  The apron must be properly placed to avoid interference with the radiographic exposure.
 
Full Mouth Exposure
Procedure for the Maxillary Central/Lateral Incisors
  1. Assemble the anterior film holder and insert the film packet vertically on the biteblock.  Use a #1 film.
  2. Center the film on the central/lateral incisors as close as possible to the lingual surfaces of the teeth with approximately a one-eighth inch border of the film extending below the incisal edge of the centrals (Figure 39).  Position the biteblock on the incisal edges of the teeth to be radiographed (Figure 40).
  3. A cotton roll may be inserted between the mandibular teeth and the biteblock for patient comfort.  Ask the patient to slowly, but firmly, bite onto the block to maintain film position.
  4. Align the central ray perpendicular to the bisector vertically and at the desired interproximal contact to be viewed.  Horizontally, the central ray should bisect the central/lateral (Figure 41).  For maxillary exposures the tube head will be pointed down for positive (+) angulation.
  5. Follow the film and equipment manufacturer’s recommendation concerning exposure factors.  Make the exposure.

Figure 39 

Figure 40 

Figure 41

Procedure for the Maxillary Canines
  1. Assemble the anterior film holder and insert the film packet vertically on the biteblock.  Use a #1 film.
  2. Center the film on the canine as close as possible to the lingual surfaces of the teeth with approximately a one-eighth inch border of the film extending below the incisal edge of the centrals (Figure 42).  Position the biteblock on the incisal edges of the teeth to be radiographed (Figure 43).
  3. A cotton roll may be inserted between the mandibular teeth and the biteblock for patient comfort.  Ask the patient to slowly, but firmly, bite onto the block to maintain film position.
  4. Align the central ray perpendicular to the bisector vertically and at the desired interproximal contact to be viewed.  Horizontally, the central ray should bisect the canine (Figure 44).  For maxillary exposures the tube head will be pointed down for positive (+) angulation.
  5. Follow the film and equipment manufacturer’s recommendation concerning exposure factors.  Make the exposure.


Figure 42

Figure 43

Figure 44

Procedure for the Maxillary Premolars
  1. Assemble the posterior film holder and insert the film packet horizontally on the biteblock.  Use a #2 film.
  2. Center the film on the premolars as close as possible to the lingual surfaces of the teeth (Figure 45).  Position the film in the palate so that the entire tooth length will appear on the film with approximately a one-eighth inch border below the cuspal ridge.  Align the anterior border of the film packet with the canine so that the image captured on the anterior edge of the film will be the distal third of the canine.  Position the biteblock on the occlusal surface of the teeth being radiographed (Figure 46).
  3. A cotton roll may be inserted between the mandibular teeth and the biteblock for patient comfort. Ask the patient to slowly, but firmly, bite onto the block to maintain film position.  (Watch the occlusal border of the film packet; it tends to slip down anteriorly.)
  4. Align the central ray perpendicular to the bisector vertically and at the desired interproximal contact to be viewed. Horizontally, the central ray should pass between the contact of the first and second premolar (Figure 47).  For maxillary exposures the tube head will be pointed down for positive (+) angulation.
  5. Follow the film and equipment manufacturer’s recommendation concerning exposure factors.  Make the exposure.

 

Figure 45

Figure 46

Figure 47

Procedure for the Maxillary Molars
  1. Assemble the posterior film holder and insert the film packet horizontally on the biteblock.  Use a #2 film.
  2. Center the film on the molars as close as possible to the lingual surfaces of the teeth (Figure 48).  Position the film in the palate so that the entire tooth length will appear on the film with approximately a one-eighth inch border below the cuspal ridge.  Align the anterior border of the film packet with the second premolar so that the image captured on the anterior edge of the film is the distal third of the second premolar.  Position the biteblock on the occlusal surface of the teeth being radiographed (Figure 49).
  3. A cotton roll may be inserted between the mandibular teeth and the biteblock for patient comfort. Ask the patient to slowly, but firmly, bite onto the block to maintain film position.
  4. Align the central ray perpendicular to the bisector vertically and at the desired interproximal contact to be viewed. Horizontally, the central ray should pass between the contact of the first and second molar (Figure 50).  For maxillary exposures the tube head will be pointed down for positive (+) angulation.
  5. Follow the film and equipment manufacturer’s recommendation concerning exposure factors.  Make the exposure.

 

Figure 48

Figure 49

Figure 50

Procedure for the Mandibular Central/Lateral Incisors
  1. Assemble the anterior film holder and insert the film packet vertically on the biteblock.  Use a #1 film.
  2. Center the film on the central/lateral incisors as close as possible to the lingual surfaces of the teeth with approximately a one-eighth inch border of the film extending above the incisal edge of the centrals.  Position the biteblock on the incisal edges of the teeth to be radiographed (Figure 51).
  3. A cotton roll may be inserted between the maxillary incisors and the biteblock for patient comfort.  Ask the patient to slowly, but firmly, bite onto the block to maintain film position.  The film should be straightened as the patient closes and the floor of the mouth relaxes.
  4. Align the central ray perpendicular to the bisector vertically and at the desired interproximal contact to be viewed.  Horizontally, the central ray should pass between the central/lateral incisors (Figure 52).  For mandibular exposures the tube head will be pointed up for negative (-) angulation.
  5. Follow the film and equipment manufacturer’s recommendation concerning exposure factors.  Make the exposure.

 

Figure 51
 
Figure 52

Procedure for the Mandibular Canines
  1. Assemble the anterior film holder and insert the film packet vertically on the biteblock. Use a #1 film.
  2. Center the film on the canine as close as possible to the lingual surfaces of the teeth with approximately a one-eighth inch border of the film extending above the incisal edge of the canine. Position the biteblock on the incisal edges of the teeth to be radiographed (Figure 53).
  3. A cotton roll may be inserted between the maxillary teeth and the biteblock for patient comfort. Ask the patient to slowly, but firmly, bite onto the block to maintain film position. The film should be straightened as the patient closes and the floor of the mouth relaxes.
  4. Align the central ray perpendicular to the bisector vertically and at the desired interproximal contact to be viewed. Horizontally, the central ray should bisect the canine (Figure 54). For mandibular exposures the tube head will be pointed up for negative (-) angulation.
  5. Follow the film and equipment manufacturer’s recommendation concerning exposure factors. Make the exposure.

 

Figure 53
 
Figure 54

Procedure for the Mandibular Premolars
  1. Assemble the posterior film holder and insert the film packet horizontally on the biteblock. Use a #2 film.
  2. Center the film on the premolars as close as possible to the lingual surfaces of the teeth. Align the anterior border of the film packet with the canine so that the image captured on the anterior edge of the film will be the distal third of the canine. Position the biteblock on the occlusal surface of the teeth to be radiographed (Figure 55).
  3. A cotton roll may be inserted between the maxillary premolars and the biteblock for patient comfort. Ask the patient to slowly, but firmly, bite onto the block to maintain film position. The film should be straightened as the patient closes and the floor of the mouth relaxes.
  4. Align the central ray perpendicular to the bisector vertically and at the desired interproximal contact to be viewed. Horizontally, the central ray should pass between the first and second premolars (Figure 56). For mandibular exposures the tube head will be pointed up for negative (-) angulation.
  5. Follow the film and equipment manufacturer’s recommendation concerning exposure factors. Make the exposure.

 

Figure 55

Figure 56

Procedure for the Mandibular Molars
  1. Assemble the posterior film holder and insert the film packet horizontally on the biteblock.  Use a #2 film.
  2. Center the film on the molars as close as possible to the lingual surfaces of the teeth.  Align the anterior border of the film packet with the second premolar so that the image captured on the anterior edge of the film will be the distal third of the second premolar.  Position the biteblock on the occlusal surface of the teeth to be radiographed (Figure 57).
  3. A cotton roll may be inserted between the maxillary molars and the biteblock for patient comfort.  Ask the patient to slowly, but firmly, bite onto the block to maintain film position.  The film should be straightened as the patient closes and the floor of the mouth relaxes.
  4. Align the central ray perpendicular to the bisector vertically and at the desired interproximal contact to be viewed.  Horizontally, the central ray should pass between the contact of the first and second molar (Figure 58). For mandibular exposures the tube head will be pointed up for negative (-) angulation. 
  5. Follow the film and equipment manufacturer’s recommendation concerning exposure factors.  Make the exposure.

 

Figure 57   

Figure 58
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