4.8, left canine).
This principle was used in an investigation of this phenomenon, which revealed that when the mesio‐distal width of the crown of an unerupted canine (as it appears and is measured directly on the panoramic radiograph) is 1.15 times larger (i.e. 15% greater) than that of the adjacent central incisor (the canine‐to‐incisor index), then the canine will be palatally displaced [10, 11]. This was found to be reliable in 100% of cases in which the canine was seen on the radiograph to be superimposed on the coronal or middle portions of the root of the adjacent incisor.
As can be seen in this illustration (Figure 4.8), by direct measurement of the crown of the patient’s right canine, the mesio‐distal width of the crown appears considerably more than 15% larger than that of the right central incisor, while the left incisor is approximately the same width as the left central incisor. Each of them is superimposed on the middle portion of the root of its immediate neighbour, validating the conclusion that the right canine is palatal and the left buccal.
Earlier studies that had attempted to diagnose canine position on a panoramic radiograph using the principle of differential enlargement revealed only an 80–89% degree of reliability of diagnosis [12, 13]. This was due to the inclusion of cases where the image of the canine was superimposed on the apical portion of the root of the incisor. The anatomy of the anterior portion of the maxilla is responsible for this aberration. Erupted permanent incisor teeth do not stand vertically upright, but their roots tip palatally at a significant angle to the vertical (Figure 4.9). This means that the root apices are considerably more distant from the panoramic machine receptor than are the crowns. If a canine is located high up on the labial side of the root apices (in the labial alveolar depression in the incisor region inferior to the nose), the tooth may still be considerably more distant from the receptor than are the crowns of the incisors. Thus, the image of the canine crown will be enlarged to a greater extent than that of the incisor crowns and will appear disproportionately large on the radiograph.
Fig. 4.7 The lateral tube shift method using a panoramic radiograph and a lateral cephalogram. (a) In the panoramic view, the X‐ray cone projects this image in the premolar area when it is behind the ear of the opposite side and therefore provides an oblique lateral view. This gives the misleading impression that the unerupted right second premolar (arrow) is rotated. (b) The lateral cephalometric view of the same patient shows only a very mild mesial displacement of the second premolar (arrow), with a minimal rotation of its palatal cusp in a mesial direction. Since this projection is a true lateral view, this is the true mesio‐distal position of the tooth. The imaging of the second premolar for a panoramic view is made when the X‐ray cone is behind the ear on the other side. This means that the apparent mesial displacement and apparent rotation of the tooth, seen here, confirm that it is also palatally displaced. Its size relative to the first premolar also confirms its palatal displacement. A tooth placed palatally vis‐à‐vis the focal trough appears wider than the same tooth in a buccal position.
Fig. 4.8 The enlarged premaxillary segment of a panoramic radiograph showing two unerupted maxillary canines. Note the inequality of image size of the two canines.
Fig. 4.9 On the dry skull, the roots of the maxillary incisor teeth can be seen to tip palatally at a significant angle to the vertical, creating a depression in the bone (arrow) at the level of their apices. A canine impacted labially in this depression will be more distant from the panoramic receptor than the incisor crowns and will therefore cast a much enlarged image on the radiograph. The use of the panoramic view for positional diagnosis at this relative height would therefore be incorrect.
Accordingly, the 1.15 canine‐to‐incisor index formula excludes all canines whose superimposition on the incisor root is high in the apical area. If the method is restricted to those cases in which the canine traverses the root of the incisor inferior to its apical third, then its use in determining the bucco‐lingual positioning of the crown of an impacted tooth is valid, without the need to resort to other views.
It is very important to clarify that panoramic radiography is extremely sensitive to deviations in patient positioning. The patient is positioned with the jaws placed exactly in the middle (in the bucco‐lingual aspect) of the focal trough. Any deviation from the middle of the focal trough will cause distortion, especially in the horizontal magnification. In such cases the apparent mesio‐distal dimension of the teeth is unreliable.
Radiographic views at right angles
Radiographic views may be taken at right angles to one another in various ways but, for the method to be of value, it must be possible to determine the exact orientation in space of both the receptor and the central ray [1, 2]. The observer must be in a position to deduce these from observation of other structures on the radiograph whose locations are known. Thus, if one begins with a periapical view, it becomes necessary to provide another view that is at 90° to it, in order to satisfy the minimum geometric conditions. However, having done this, it must be possible to mentally reconstruct the exact orientation of this second view at a later date, by looking at the radiograph alone and without necessarily having prior knowledge of exactly how the tube and receptor were placed. This is obviously very confusing and completely impractical.
Standardization
Standardization of views is required within the confines of strict adherence to the planes of space. Any 2D view, carried out in meticulous accordance with the standards, becomes simple for the observer to appreciate and when information from the other views at right angles to it is merged with it, the composite 3D picture is easy to reconstruct mentally. A true lateral view (Figure 4.10a) will give exact information regarding both the antero‐posterior and vertical location of an object, relative to other structures that may be seen both on that radiograph and clinically. It will provide bucco‐lingual (transverse) plane information for the incisors, but not for the posterior teeth. A true occlusal view will offer positional information in both the antero‐posterior and the transverse planes, but not in the vertical plane. Thirdly, there is the true antero‐posterior view (Figure 4.10b), which defines the height (vertical plane) of a tooth as well as providing bucco‐lingual information for the area of the premolars and molars, but not the incisor area. By combining the information provided by any two of these three radiographs, three‐dimensional localization may be determined.
