Orthodontic Treatment of Impacted Teeth. Adrian Becker

Fig. 4.6 The vertical tube shift method using a panoramic radiograph and periapical views. (a) The panoramic radiograph shows the left canine very high and above the root apices of the incisors (arrow). The right canine superimposes on the apical third of the adjacent incisor. (b) The periapical views show the left and right canines overlapping one‐third and two‐thirds of the incisor roots, respectively. Both canines are labial.

If all the teeth are in the same approximate semi‐circular line of the arch, then their mesio‐distal relationships will be fairly accurately represented on the radiograph. However, a palatally displaced canine or premolar tooth is imaged when the X‐ray cone is at a point, in its arc of circle, just behind the ear on the opposite side. Viewed from this position, the palatally placed tooth will be ‘thrown’ mesial to its true mesio‐distal position and will be shown superimposed more mesially on other structures than would be evident from its appearance on a lateral cephalogram [9]. Accordingly, a panoramic radiograph (an oblique lateral view) and a lateral cephalogram (a true lateral view) may be coordinated to determine the bucco‐lingual location in the canine or premolar regions, in a similar manner to the use of two periapical views in Clark’s parallax method [5]. Obviously, this is dependent on the individual teeth being clearly discernible on the cephalogram, bearing in mind that sometimes unavoidable superimposition in the anterior region may invalidate the method (Figure 4.7).

      The panoramic X‐ray machine is normally adjusted so that its circling of the jaws maintains a fixed distance from the focal spot to the dental arch (for a patient with the ideal jaw shape). The perimeter of the arch falls within the centre of the focal trough that the machine produces. Teeth that are palatal to the line of the dental arch are displayed as enlarged horizontally and those that are buccally displaced are shown reduced horizontally.

The mesio‐distal width of a maxillary permanent canine is approximately 90% of the width of the maxillary central incisors. With a normally located canine, the distance between it and the receptor may be slightly larger than that of the central incisor, due to the form of the arch in that area. Thus, in these cases it is common to see, on the panoramic radiograph, similar mesio‐distal widths of these two teeth. A buccally displaced canine, on the other hand, will generally reflect the true width difference between the two teeth, because its distance from the receptor is similar to that of the central incisor (Figure 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.