beam computerized tomography
It is not the purpose of this chapter to present a complete manual on dental radiography, but rather to highlight concisely those techniques and methods that are useful in the clinical setting, as they pertain to impacted teeth.
The methods offered have two main aims [1, 2]. The first relates to the furnishing of qualitative information regarding normal and abnormal conditions that may be associated with unerupted teeth. Thus, we will discuss and compare the different ways of radiologically displaying and recognizing pathological entities, including supernumerary teeth, enlarged eruption follicles, odontomes and root resorption. The second aim is to describe the various radiological techniques that the clinician may find helpful in accurately pinpointing the position of a clinically invisible, unerupted tooth in the three planes of space. The relative merits of these techniques are discussed and indications for their use are suggested in relation to the different groups of teeth concerned.
Planar radiography
Periapical radiographs
The first, simplest and most informative radiograph is the periapical view. This view is oriented to pass through the minimum of surrounding tissue, in order to give accuracy and quality of resolution. There are two techniques to review: the paralleling technique and the bisecting technique. In the paralleling technique, the receptor, which is a sensor or phosphor storage plate (PSP), is placed as close to the tooth as possible, but parallel to its long axis, with the X‐ray beam directed perpendicular to it. In areas where the parallel technique is impossible due to poor access, the bisecting angle technique is used, in which the receptor is placed as close to the tooth as possible, but not parallel to its long axis. The X‐ray beam is aimed perpendicular to an imaginary plane, which bisects the angle between the long axis of an erupted tooth and the plane of the receptor, thus ensuring a minimum of distortion. The periapical radiograph is designed to view the tooth itself from the angle of best advantage, unrelated to its position in space.
From the periapical view, it will be immediately obvious if there is an impacted tooth and if its stage of development is similar to that of its erupted antimere, with at least two‐thirds of its root length. The presence and size of a follicle will be obvious and crown or root resorption, root pattern and integrity will be possible to ascertain. The presence and description of hard tissue obstruction will be evident, allowing the observer to distinguish connate, incisiform and barrel‐shaped supernumeraries, as well as odontomes of the complex or compound composite type. Similarly, this view will show soft tissue lesions, such as cysts. The great clarity that the view offers is superior to other views and should always be used as the initial radiograph of a suspected impacted tooth in a radiographic examination. The periapical view is two‐dimensional, and thus can give no information in the bucco‐lingual plane. Overlapping structures cannot be differentiated on a single radiograph as to which is lingual and which buccal.
For this radiograph to give the most advantageous view of the teeth in the maxillary arch and in the mandibular anterior segment, the central ray of the periapical view must be oblique and vary between 20° and 55° to the occlusal plane [3] (depending on the region to be X‐rayed), while attempting to be as true to the paralleling technique as possible. Given this oblique direction, any attempt to estimate the height of an impacted tooth or its bucco‐lingual location, without additional information, must fail.
When performing periapical radiography on the posterior teeth in the mandibular arch, however, the most advantageous direction has the central ray very close to the horizontal and, as such, also offers a true lateral view of these teeth. Thus, except for the bucco‐lingual situation details, the observer will see the most precise detail of the tooth and its surrounding tissues; it will also be possible to accurately assess its height in the jaw.
Occlusal radiographs
Mandibular arch
Occlusal radiographs in the mandibular arch are properly executed by tipping the patient’s head backwards and pointing the X‐ray tube at right angles to a receptor held between the teeth in the occlusal plane (Figure 4.1). The head will need to be tipped back to permit the positioning of the X‐ray tube under the chin. In the lower canine/premolar region, the occlusal view is a ‘true’ occlusal view and should depict all the posterior standing teeth in cross‐section, thereby providing bucco‐lingual positional information on the tooth and any associated structures in a plane at right angles to that seen on the periapical radiograph. Due to the thickness of bone traversed, detail is much poorer, unless there is expansion owing to a large cyst or a bucco‐lingually displaced tooth.
Fig. 4.1 The angle of the central ray in a true occlusal view of the lower jaw depends on the area of interest.
Source: Reproduced from previous edition. Adrian Becker, The Orthodontic Treatment of Impacted Teeth, 2nd ed., 2007 with the kind permission of Informa Healthcare – Books.
In order to produce a true occlusal view in the anterior region of the mandibular arch (Figure 4.1), the head will need to be tipped back further and the tube pointed at the symphysis menti, at an angle of 110° to the occlusal plane, in line with the long axes of the incisor teeth. To achieve the same for the molar teeth, the 90° angle to the occlusal plane will need to be augmented by a 15° medial tilt of the tube, to compensate for the characteristic slight lingual tipping of these teeth [3]. This means that, ideally, the radiograph should be performed individually for each side, in order to capture each molar in its long axis and its true occlusal view.
Maxillary arch
Maxillary anterior occlusal
In the maxillary arch, the nose and forehead interfere with the positioning of the X‐ray tube close to the area to be viewed. The best that can be achieved by positioning the tube close to the face is an oblique, anterior, maxillary occlusal view of the teeth, which is perhaps better described as a high or steeply angled periapical view (Figure 4.2). This view will shorten the apparent length of the roots and will be a far cry from the cross‐sectional view that is so easy to achieve in the mandibular arch. The central ray passes through cancellous bone rather than the compact bone that is found in the mandible, so detail is usually good, although not as clear as with the periapical view.
Fig. 4.2 A diagram showing incisor inclination, receptor position and central X‐ray beam, differentiating the periapical view, the anterior (oblique) occlusal view and the true vertex occlusal view.
Source: Reproduced from previous edition. Adrian Becker, The Orthodontic Treatment of Impacted Teeth, 2nd ed., 2007 with the kind permission of Informa Healthcare – Books.
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