Comparison of the paralleling and bisected angle techniques
The advantages and disadvantages of the two techniques can be summarized as follows:
Advantages of the paralleling technique
• Geometrically accurate images are produced with little magnification.
• The shadow of the zygomatic buttress appears above the apices of the molar teeth.
• The periodontal bone levels are well represented.
• The periapical tissues are accurately shown with minimal foreshortening or elongation.
• The crowns of the teeth are well shown enabling the detection of approximal caries.
• The horizontal and vertical angulations of the X-ray tubehead are automatically determined by the positioning devices if placed correctly.
• The X-ray beam is aimed accurately at the centre of the film — all areas of the film are irradiated and there is no coning off or cone cutting.
• Reproducible radiographs are possible at different visits and with different operators.
• The relative positions of the film packet, teeth and X-ray beam are always maintained, irrespective of the position of the patient’s head. This is useful for some patients with disabilities.
Disadvantages of the paralleling technique
• Positioning of the film packet can be very uncomfortable for the patient, particularly for posterior teeth, often causing gagging.
• Positioning the holders within the mouth can be difficult for inexperienced operators.
• The anatomy of the mouth sometimes makes the technique impossible, e.g. a shallow, flat palate.
• The apices of the teeth can sometimes appear very near the edge of the film.
• Positioning the holders in the lower third molar regions can be very difficult.
• The technique cannot be performed satisfactorily using a short focal spot to skin distance (i.e. a short spacer cone) because of the resultant magnification.
• The holders need to be autoclavable or disposable.
Advantages of the bisected angle technique
• Positioning of the film packet is reasonably comfortable for the patient in all areas of the mouth.
• Positioning is relatively simple and quick.
• If all angulations are assessed correctly, the image of the tooth will be the same length as the tooth itself and should be adequate (but not ideal) for most diagnostic purposes.
Disadvantages of the bisected angle technique
• The many variables involved in the technique often result in the image being badly distorted.
• Incorrect vertical angulation will result in foreshortening or elongation of the image.
• The periodontal bone levels are poorly shown.
• The shadow of the zygomatic buttress frequently overlies the roots of the upper molars.
• The horizontal and vertical angles have to be assessed for every patient and considerable skill is required.
• It is not possible to obtain reproducible views.
• Coning off or cone cutting may result if the central ray is not aimed at the centre of the film, particularly if using rectangular collimation.
• Incorrect horizontal angulation will result in overlapping of the crowns and roots.
• The crowns of the teeth are often distorted, thus preventing the detection of approximal caries.
• The buccal roots of the maxillary premolars and molars are foreshortened.