Chapter 13 Oblique lateral radiography
INTRODUCTION
Oblique lateral radiographs are extraoral views of the jaws that can be taken using a dental X-ray set (see Fig. 13.1). Before the development of dental panoramic equipment they were the routine extraoral radiographs used both in hospitals and in general practice. In recent years, their popularity has waned, but the limitations of panoramic radiographs (see Ch. 17) have ensured that oblique lateral radiographs still have an important role.
TERMINOLOGY
Lateral radiographs of the head and jaws are divided into:
The differentiating adjectives true and oblique are used to indicate the relationship of the image receptor, patient and X-ray beam, as shown in Figure 13.2.
True lateral positioning
The image receptor and the sagittal plane of the patient’s head are parallel and the X-ray beam is perpendicular to both of them. This is the positioning for the true lateral skull radiograph taken in a cephalostat unit described in Chapter 15.
Oblique lateral positioning
The image receptor and the sagittal plane of the patient’s head are not parallel. The X-ray beam is aimed perpendicular to the image receptor but is oblique to the sagittal plane of the patient. A variety of different oblique lateral projections is possible with different head and X-ray beam positions.
MAIN INDICATIONS
The main clinical indications for oblique lateral radiographs include:
• Assessment of the presence and/or position of unerupted teeth
• Detection of fractures of the mandible
• Evaluation of lesions or conditions affecting the jaws including cysts, tumours, giant cell lesions, and other bone lesions
• As an alternative when intraoral views are unobtainable because of severe gagging or if the patient is unable to open the mouth or is unconscious (see Ch. 9, Fig. 9.1)
• As specific views of the salivary glands or temporomandibular joints.
EQUIPMENT REQUIRED
This includes (see Fig. 13.3):
• An extraoral cassette containing film and intensifying screens or a digital phosphor plate (usually 13 × 18 cm)
• A lead shield to cover half the cassette when taking bimolar views.
Fig. 13.3 Equipment used for oblique lateral radiography. (i) An 13 × 18 cm cassette A and lead shield B. (ii) An example of an angle board showing the cassette A, lead shield B and the plastic earpieces P for patient positioning.
Specially constructed angle boards as shown in Figure 13.3B can be used to facilitate positioning, but are not considered necessary by the author.
BASIC TECHNIQUE PRINCIPLES
As stated, a wide range of different oblique lateral projections of the jaws are possible. However, all the variations rely on the same basic principles regarding the position of:
Cassette position
The cassette is held by the patient against the side of the face overlying the area of the jaws under investigation. The exact position of the cassette is determined by the area of interest.
Patient’s head position
The patient is normally seated upright in the dental chair and is then instructed to:
1. Rotate the head to the side of interest. This is done to bring the contra-lateral ramus forwards, avoiding its superimposition and to increase the space available between the neck and shoulder in which to position the X-ray set.
2. Raise the chin. This is done to increase the triangular space between the back of the ramus and the cervical spine (the so-called radiographic keyhole, see Fig. 13.4) through which the X-ray beam will pass.
Fig. 13.4 A The view through the radiographic keyhole (arrowed) showing the right mandibular and maxillary posterior teeth. Note the anterior teeth are obscured by the left ramus of the mandible. B The view from underneath the left body of the mandible showing the right mandible and right posterior mandibular teeth. Note the right maxillary teeth are obscured by the left body of the mandible.
X-ray tubehead position
The X-ray tubehead is positioned on the opposite side of the patient’s head to the cassette. There are two basic positions, depending on the area of the jaws under investigation:
• Behind the ramus aiming through the radiographic keyhole. The X-ray tubehead is positioned along the line of the occlusal plane, just below the ear, behind the ramus aiming through the radiographic keyhole at the particular maxillary and mandibular teeth under investigation. The view from this position is illustrated in Figure 13.4A. As shown, the X-ray beam will not pass directly between the contact areas of the posterior teeth. This may result in some overlapping of the crowns.
• Beneath the lower border of the mandible. The X-ray tubehead is positioned beneath the lower border of the contra-lateral body of the mandible, directly opposite the particular mandibular teeth under investigation, aiming slightly upwards. The view from this position is illustrated in Figure 13.4B. As shown, the X-ray beam will now pass between the contact areas of the teeth. However, there will still be some distortion of the image in the vertical plane owing to the upward angulation of the X-ray beam. In addition, the shadow of the body of the mandible will be superimposed over the maxillary teeth.
Once these principles are understood, the technique becomes straightforward and can be modified readily for different anatomical regions and clinical situations.
POSITIONING EXAMPLES FOR VARIOUS OBLIQUE LATERAL RADIOGRAPHS
Examples of the required positioning for different oblique laterals and the resultant radiographs are shown in Figures 13.5-13.8. Illustrations show the positioning for both adults and children.
Fig. 13.5 A Cassette and X-ray tubehead positions for the RIGHT mandibular and maxillary molars on an adult. B Diagram of the positioning from above showing the cassette overlying the molar teeth and the X-ray beam passing between the cervical spine and mandibular ramus. C A typical resultant radiograph. The shadow of the superimposed left ramus, overlying the premolars, has been drawn in to emphasize its position. Compare with Figure 13.4A.
Note the radiograph is mounted and viewed as if the observer is looking at the patient’s right side.
Fig. 13.6 A Positioning of a child, cassette and X-ray tubehead for the RIGHT deciduous maxillary and mandibular molars. B A typical resultant radiograph. The shadow of the superimposed left ramus has been drawn in.
Fig. 13.7 A Cassette and X-ray tubehead positions for the RIGHT mandibular and maxillary canines. Note the displacement of the nose needed to achieve the desired position for the cassette. B Diagram of the positioning from above, showing the cassette overlying the canine teeth and the X-ray tubehead aimed through the radiographic keyhole. C A typical resultant radiograph of a patient in the mixed dentition. The shadow of the superimposed left ramus has been drawn in—it now overlies the lateral incisors. Again note the orientation of the radiograph and how it is viewed.
Fig. 13.8 A Cassette and X-ray tubehead position for the RIGHT mandibular molars. Note the upward angulation of the X-ray tubehead and its position beneath the left body of the mandible. B Diagram of the positioning from above. Note the position of the X-ray tubehead and compare with Figure 13.5B. C A typical resultant radiograph showing the right mandibular molars. The superimposed shadow of the left mandibular body has been drawn in overlying the maxillary molars. Compare with Figure 13.4B.
Important points to note
• For stability, a small child is usually rotated through 90° in the chair, so the shoulder is supported and the cassette and head can be rested on the headrest.
• The area under investigation determines the position of the cassette and the X-ray tubehead.
• An X-ray request for an oblique lateral must specify the exact region of the jaws required.
BIMOLAR TECHNIQUE
As mentioned earlier, bimolar is the term used for the radiographic projection showing oblique lateral views of the right and left sides of the jaws on the different halves of the same radiograph as shown in Figure 13.9.
Fig. 13.9 A (i) Position of a child, cassette and X-ray tubehead for the LEFT side of the jaws and (ii) positioning for the RIGHT side. Note the lead shield covering the half of the cassette not being used. B An example of a child’s bimolar.
The technique can be summarized as follows:
1. The patient is positioned with one side of the face in the middle of one half of the cassette, with the nose towards the midline. The precise positioning depends on which teeth or area of the jaws are being examined (like any other oblique lateral).
2. The other half of the cassette is covered by a lead shield to prevent exposure of this side of the image receptor.
3. The X-ray tubehead is positioned to show the desired area, and the exposure is made.
4. The lead shield is then placed over the other side of the cassette to protect the part of the film already exposed.
5. The patient is then positioned in a similar manner with the cassette held on the other side of the face.
6. The X-ray tubehead is re-positioned and a second exposure made.