Orthodontic Treatment of Impacted Teeth. Adrian Becker

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.

Fig. 4.10 (a) The true lateral cephalometric radiograph shows both canines superimposed at a higher level than the other teeth. Their axial inclination on the antero‐posterior plane is favourable, with the crowns and apices apparently normally located. (b) The postero‐anterior cephalometric radiograph shows the two canines similarly angulated, with their apices in the line of the arch and their crowns close to the midline. From these two radiographs, we may conclude that the apices are ideally placed and that the long axes of the teeth have a downward, mesial and palatal inclination. (c) The panoramic view of the same patient. The appearance of canines close to the midline is very similar to that seen on the posterior‐anterior cephalometric radiograph.

Translating these principles into radiographic practice demands repeatability of patient positioning standards for each radiograph. This might present some difficulties when these radiographs are not consistently executed by the same person, since they have to deal with different and many times challenging jaw structures. However, the problems are not insurmountable and, insofar as the radiographs provide the clinician with accurate positional visualization of the unerupted tooth, doing so may be entirely worthwhile.

      For most orthodontic cases, a lateral cephalometric radiograph (a cephalogram) is an essential initial step, whose primary purpose is the routine measurement of angles and planes. On the other hand, this radiograph potentially contains much useful information regarding the location and angulation of unerupted teeth. The radiograph represents a true lateral view of the skull and, for the present purposes, of the jaws and the anterior maxilla in particular (Figure 4.10a). Although there are many superimposed structures in this area, the outline of a canine may be clearly seen. The direction of the long axis of the tooth in the anterior–posterior and vertical planes may be clearly defined, together with the mesio‐distal position of both crown and apex.

With regard to the mandibular posterior area, we have pointed out that the routine periapical radiograph produces a true lateral view, with the X‐ray tube pointing at right angles across the body of the mandible and in the horizontal plane. The height and mesio‐distal position of a buried tooth may then be accurately defined. The occlusal radiograph of this area is directed perpendicular to the occlusal plane and adds the bucco‐lingual dimension, thereby completing the 3D picture. Accordingly, these two views will provide a good assessment of the position of unerupted teeth in this area (Figure 4.11).

      If a cephalometric radiograph is not available, the same view of the anterior maxilla may be obtained on a small, occlusal‐sized receptor. The receptor is held vertically against the cheek and parallel to the sagittal plane of the skull. The X‐ray tube is directed horizontally above and parallel to the occlusal plane from the opposite side of the face and at right angles to the receptor. The result is called the tangential view and has the advantage of simplicity. This view is particularly useful in monitoring progress in the resolution of impacted incisors during active treatment.

      At the age that most patients first present with an impacted central incisor, around 8–10 years, the permanent canine teeth are unerupted and are located both well forward and high in the anterior maxilla. Thus, on the lateral cephalometric or tangential view, right and left canines will be impossible to distinguish from one another. The roots of the incisors, at the same height as the canines, as well as the superimposed images of the more inferiorly placed crowns of the erupted incisors and deciduous canines, will all be impossible to differentiate from one another and from any supernumerary teeth that may also be present. For this reason, the lateral view may be of limited value in cases where there is obstructive impaction, with minimal displacement. When gross displacement is present, however, the outlines of the altered axial inclination and height of the tooth are usually possible to delineate, despite the considerable superimposition of other teeth.

Nowhere is this view a greater asset than when a dilacerate central incisor is present, since, because of its relative height, it separates out this malformed tooth superiorly from the root apices of the other teeth and from the permanent canines (Figure 4.12). Furthermore, the morphology may be seen to best advantage from this aspect, which allows definitive and accurate diagnosis to be made of the condition, together with its precise relation to surrounding structures. The lateral cephalogram/tangential view should be considered an essential requirement in the radiographic recording of a dilacerated central incisor.

Fig. 4.11 The true lateral and true occlusal views, taken together, provide all the information needed for a good positional assessment of crown and root in the three planes of space. (a) The periapical view (a true lateral in this case) of an impacted mandibular right second premolar shows the tooth to be tipped 60° distally from the vertical, with its incomplete apex at the correct height and mesio‐distal