Читать книгу Orthodontic Treatment of Impacted Teeth - Adrian Becker - Страница 75

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 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 radiographs, and recording the position of the displaced incisor or canine without overlap. However, for maximum effect it is important to project the X‐ray beam through the long axis of the maxillary teeth, as we have just described.

Any two of these three views (the lateral cephalogram or tangential view, the postero‐anterior cephalogram view and the true occlusal view) will provide complete information regarding every aspect of the height, bucco‐lingual and mesio‐distal location of the crown and the root, and the degree of tilt of the long axis of the impacted tooth and its relation to neighbouring teeth. The postero‐anterior cephalogram and the occlusal views, however, are not always as clear as is desirable, and they may need to be repeated or discarded. In a case of bilateral canine impaction, the lateral cephalometric or the tangential view may create confusion, since one canine will be superimposed on the other and distinguishing them may be a problem, whereas other views will usually facilitate differentiation. Two identically oriented and superimposed canines (Figure 4.10) will obviously not need to be differentiated.

It is essential to be aware that many of the radiographs that we take for other reasons should always be scrutinized for useful information regarding positions of unerupted teeth. This is particularly so regarding cephalograms, whose principal aim is to measure and compare angles and distances on tracings of specific landmarks on the radiograph. Sadly, these potentially valuable views are often filed away without sufficient thought.

From these projections, it is very easy to build up a 3D picture of the exact position and angulation of the impacted tooth and to define the type of movement that will be necessary to bring the tooth into alignment. When a composite mental reconstruction is made of the position of the unerupted tooth in space, the design of the appliance needed to resolve the impaction is simplified and fewer surprises are likely to be encountered. It is, however, a sine qua non in all these cases to examine a periapical view of the tooth in order to eliminate the possibility of local pathology, which is much more likely to be missed on the extra‐oral views.

The foregoing description has covered the various methods available for envisioning the anatomical form and 3D location of unerupted teeth, using plain film radiography. We have seen that, in order to achieve an adequate picture of the relationships between the crowns of these teeth and the surrounding anatomical structures, including adjacent teeth, information gleaned from several different types of view needs to be put together to make up the complex picture. Nevertheless, even when enough information is available, mistakes and misdiagnoses are sometimes made by experienced orthodontists [6], occasionally with serious repercussions for the patient.

It would be appropriate now to consider the question of whether the same degree of precision in root apex location and 3D root orientation is essential for both the orthodontist and the surgeon. From the point of view of the oral surgeon, diagnosis of the position of the crown, buccal or palatal to the line of the arch, is generally all that is needed, regardless of whether the tooth is to be exposed for orthodontic alignment or extracted. The position of the root apex and the orientation of the long axis of the tooth are irrelevant for surgical exposure. If the tooth is to be extracted in one piece, careful dissection of the tissue surrounding the crown and dislodging it with an elevator or extraction forceps will deliver the tooth, together with its root. Even if the root portion is sectioned and scheduled for extraction, after removal of the crown portion, its general orientation and apex position can be determined by the anatomy and general orientation of the crown. For these reasons and until CBCT became first choice, many surgeons have had to rely solely on the tube shift parallax method of positional diagnosis for exposure or extraction of impacted teeth, and have done so with a degree of exaggerated confidence. However, today the majority will overwhelmingly prefer cross‐sectional slices reconstructed from a CBCT series.

From the point of view of the orthodontist, however, while the position of the crown is important, the position of the apex and the orientation of the long axis of the tooth are crucial. When the root apex of the impacted tooth is displaced, re‐siting it in its correct position is fraught with technical difficulty and can only be done once the crown has been brought into its place in the arch and ligated into the main archwire. As we shall discuss in Chapter 7, root movement is the most damaging movement to the root itself and to the supporting tissues. It has been shown to be one of the factors that most undermine the periodontal prognosis of the treated result [14–16]. Orthodontists cannot be expected to be confident in their ability to bring a tooth into full alignment if an accurate 3D diagnosis of its position is not available. In the event, a surgical exposure may be attempted from the wrong side of the alveolus and unnecessary damage will ensue. Alternatively, the impacted tooth may be drawn in the wrong direction and will be brought into contact with the root of an adjacent tooth, which may lead to resorption of that root or to the blocking of further progress. The reader is referred to Chapter 18 for an illustrated description of failures and how most of these were due to positional misdiagnosis.

Unfortunately, the above‐mentioned plane radiograph methods exhibit many shortcomings. This is particularly true in relation to the bucco‐lingual plane. Undoubtedly, the most difficult aspect to define is the relative proximity of the impacted tooth to the root of an adjacent tooth on which its image is superimposed in the bucco‐lingual plane, as seen on a periapical, anterior occlusal, panoramic or cephalometric radiographs. Whether there is a small distance between them or whether the crown of the impacted tooth lies in a resorption crater on the palatal or labial aspect of the root of an incisor may be impossible to determine using plane 2D radiography. As a result, an undiagnosed and severely resorbed tooth, with a poor long‐term prognosis, may be mistakenly included as an integral but ‘weak link’ in the final scheme of the dentition in a projected treatment plan [17].

The relative accuracy of positional diagnosis using planar radiography is, therefore, inadequate in many instances. While this is so, there can be no question that a good number of cases continue to be successfully treated, despite a lack of adequate imaging documentation that would be needed to make even an approximate positional diagnosis. In some cases, no serious attempt at definitive diagnosis of the position of the impacted tooth is made until the unsuspecting and potentially unfortunate patient is on the operating table.