id="ulink_5963d403-7099-5d4b-ac25-5f4d9c66af03">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 location. (b) The true occlusal view shows the crown of the tooth to be lingual to the molar and the apex to be in the bucco‐lingual line of the arch. The long axis of the tooth, proceeding from its ideally sited apex, can be described as rising at a 30° angle in a distal and lingual direction, to overlap the molar roots on the lingual side.
Fig. 4.12 A dilacerated central incisor (arrow) seen in a lateral cephalometric radiograph.
For maxillary canines, the lateral view is extremely useful. It should be remembered that most impacted maxillary canines are diagnosed in the full permanent dentition, when all the other teeth have erupted. This permits clear radiographic imaging of the canine when it is sited at a higher level than the other teeth.
A postero‐anterior cephalometric radiograph is used less routinely in orthodontics, but it does offer the clinician the opportunity to view the maxilla in a different plane, the true postero‐anterior view (Figure 4.10b), which is at right angles to the lateral cephalogram. The overlap of structures of the base of the skull and the maxilla renders detail of individual teeth less clear, but a good postero‐anterior radiograph will show the height of both the crown and the root of a significantly displaced tooth, as with the lateral radiograph. This view also shows whether the root apex of an ectopic posterior tooth is in the line of the arch and how far the crown is deflected in the palatal direction. The bucco‐lingual tilt of the long axis of the tooth will be plainly visible (Figure 4.10b). However, the view is less practical in the mandible, where the left and right sides of its V‐shaped body converge, as they proceed forward towards the anterior midline and are thus oblique to the central ray. There is usually excessive overlap, more radio‐opaque bone and difficulty in discerning even markedly bucco‐lingually displaced teeth. For structures close to the midline, the panoramic view offers a very similar representation (Figure 4.10c) and a much clearer picture. Since this view is a rotational tomograph, it eliminates all structures that are either lingually or buccally outside the narrow focal trough at which it is aimed.
An occlusal projection of the anterior maxilla (Figure 4.2) offers the possibility of viewing in the third plane of space, at right angles to each of the two earlier
