1. The Bisecting Technique
DA118, Chapter 18

2. Principles of Bisecting:
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

3. Bisect angle concept

4. Principles of Bisecting, con’t
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.

5. Film Holding Devices:
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

6. Film holding devices

7. Finger Holding Method:
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 x-ray 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

8. Finger-holding Method: Mx Premolars

9. Finger-holding Method Md Incisors

10. Why do we use the Bisecting Technique?
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

11. Rules for Bisecting Technique
Film Placement:
similar to paralleling BUT use all size 2 films, and only three views on maxillary anteriors
Film position: placed against lingual surface of tooth
Anterior teeth: against incisal edge
Posterior teeth: against occlusal surface
Vertical Angulation: “yes-yes”
Central ray directed perpendicular to imaginary bisector that divides angle formed by film and long axis of tooth

12. Rules on Bisecting Film Placement:
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

13. Angulation:
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

14. Vertical Angulation:
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

15. Vertical Angulation Too Steep (excessive)

16. Foreshortening

17. Vertical Angulation Too Flat (insufficient)

18. Elongation

19. Placement

20. Rules (continued) Horizontal Angulation: “no-no”
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

21. Correct Horizontal Angulation

22. Incorrect Horizontal Angulation

23. Review Bisecting:
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