A true (vertex) occlusal 2D view of the anterior maxilla [4] needs a very long exposure time and it is for this reason that the method has never been popular. It is therefore almost with a collective sigh of relief among professionals that the method has been totally superseded by the introduction of volumetric cone beam computerized tomography (CBCT) scanning. The CBCT imaging modality, which can give much more information with little or no increase in radiation dosage, is discussed towards the end of this chapter.
Extra‐oral radiographs
The panoramic view, while not showing detail to the same degree as a periapical radiograph, has the advantage of simply and quickly offering a good scan of teeth and jaws, from the temporo‐mandibular (TM) joint on one side to the TM joint on the other. It is probably true to say that today orthodontists are in general agreement that this radiograph gives the most qualitative information to act as a starting point from which to proceed to other forms of radiography, in line with the demands of the particular situation in any given case.
True and oblique extra‐oral views (Figure 4.3a–c) and the variously angulated oblique occlusal radiographs all provide information that may be used to complement the periapical radiograph, particularly when tooth displacement is severe. However, the use of any oblique radiograph, be it a single periapical, an occlusal or a lateral jaw radiograph, for the accurate localization of a buried tooth may frequently be misleading. This being so, two incipient dangers exist. First, a surgical procedure may be misdirected and a flap opened on the wrong side of the alveolar process. Second, misinterpretation of the tooth’s position may lead the operator to consider there to be a very favourable prognosis for biomechanical resolution when in fact the tooth may be in a completely intractable position. In such circumstances, therefore, the choice of treatment will be inappropriate.
In view of these and other shortcomings, these cases are now diagnosed and treatment planned using CBCT imaging and the only extra‐oral radiographs still in use, to complement panoramic radiographs, are the lateral and PA cephalometric projections.
Three‐dimensional diagnosis of tooth position
As dentists, we are used to seeing periapical radiographs of individual teeth and, provided that the teeth concerned are erupted and in the line of the arch, these radiographs have many advantages. However, in this view the X‐ray tube is not directed in either the true horizontal, true vertical or true lateral plane. Aside from radiography of the mandibular posterior teeth, the tube is always tipped at an angle to one or more of these planes. For an erupted tooth this is unimportant, since the third dimension is supplied by direct vision within the mouth. However, while it gives a good 2D representation of the tooth, this view has limited value when visualization of an unerupted tooth is required in the three planes of space.
Fig. 4.3 (a) The periapical view shows an impacted left maxillary central incisor, due to an inverted, unerupted, supernumerary tooth. The deciduous tooth is over‐retained. Accurate diagnosis of the height of the impacted tooth in the alveolus is not possible to determine from this view. (b) The anterior maxilla seen on a lateral cephalometric radiograph shows the high impacted central incisor (arrow) and its bucco‐lingual location, facing the labial vestibular sulcus. (c) The parallel intra‐oral photographic view at surgical exposure. The radiograph has been laterally inverted to simplify comparison.
Courtesy of Dr D. Harary.
Parallax method
By following the principles involved in binocular vision, two periapical views of the same object, taken from slightly different angles, can provide depth to the flat, 2D picture presented by each of the radiographs individually (Figure 4.4) [5–7]. This is of considerable help in distinguishing the buccal or lingual displacement of the canine, which is low down and fairly close to the line of the arch, and is performed in the following manner (Figure 4.5):
1 A periapical‐sized receptor is placed in the mouth, preferably using the bisecting angle technique, placing it against the palatal aspect of the area where the tooth would normally be situated. The X‐ray tube is directed at right angles (ortho‐radial) to the tangent to the line of the arch at this point, as for any periapical view, and at the appropriate angle to the horizontal plane for the tooth in question (50° for the central incisor, etc.).
2 A second receptor is placed in the mouth in the identical position but, on this occasion, the X‐ray tube is shifted (rotated) mesially or distally round the arch, although held at the same angle to the horizontal plane and directed at the mesially or distally adjacent tooth. To achieve this, the tube should describe an arc of between 30° and 45° of a circle whose centre is somewhere in the middle of the palate.
There should be no problem identifying which of the two radiographs is the ortho‐radial view and which was taken from the distally deviated aspect, simply by comparing the relative distortion of the erupted teeth on the two radiographs. However, by radiographically ‘labelling’ the deviated receptor with the placement of a paper clip in one corner or by using a different receptor size for the deviated view, such as an occlusal‐sized receptor, this distinction will be simplified.
Let us assume that a right unerupted canine is palatally placed (Figure 4.5), so that this tooth will be close to the middle of the picture obtained in both radiographs. In the first picture (direction B), where the tube is directed over the designated canine area of the ridge, the lateral incisor root will be on the right of the picture. If the canine is also mesially displaced, there will be some overlap of the canine crown and the lateral incisor root. On the second picture, taken from the front (direction A), the right lateral incisor root and the crown of the palatal canine will be in the middle of the picture, superimposed on one another, to a much greater degree.
Fig. 4.4 The left periapical view, oriented for the central incisors, shows the crown of the canine superimposed on the distal half of the central incisor root. The middle radiograph, rotated 30° to the left, shows the canine overlapping only the lateral incisor root. By rotating the central beam a further 30°, superimposition of the canine over the lateral incisor root has been eliminated. The canine is palatally displaced.
Fig. 4.5 A diagrammatic representation of the parallax method. If the observer’s eye peers along the axis of the X‐ray beam in each case, the image on the radiograph will be easy to reconstruct.
