THE BISECTING TECHNIQUE

DA118, Chapter 18

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

Bisect angle concept

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.

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

Film holding devices

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

Finger-holding Method:Mx Premolars

Finger-holding MethodMd Incisors

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

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

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

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

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

Vertical Angulation Too Steep (excessive)

Foreshortening

Vertical Angulation Too Flat (insufficient)

Elongation

Placement

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

Correct Horizontal Angulation

Incorrect Horizontal Angulation

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