NEWSLETTERS

Positional diagnosis of impacted teeth. Part 1: Pairing planar radiographs

Published: May 2016

Bulletin #55, May 2016

Positional diagnosis of impacted teeth. Part 1: Pairing planar radiographs

Accurate positional diagnosis of impacted teeth is essential if we are to create the optimum conditions for their resolution. Many a palatal flap has been opened up by a surgeon who was unsuccessfully hunting for a canine which turned out to be on the labial side!

Fig. 1.

In my practice, patients with all but the simplest impacted teeth will be referred to an imaging center for a cone beam CT. This is because information regarding this tooth is lacking compared with information available for an erupted tooth. I need to have an all-round 3D view of the tooth in all its aspects, in its relationship to adjacent teeth and anatomical structures, whether it has caused resorption to adjacent roots, whether it has abnormal crown or root form and whether a reason for its impaction may be seen. It will also show me the direction in which the tooth must be moved to circumvent obstacles in the way or how the adjacent teeth need to be moved to open an eruption path for it. For me, the all-round best 3D view available is a well composed video clip created by a competent CT technician (Fig. 1), although a comprehensive series of single screen shots may often suffice.

In addition, cross-sectional, axial, coronal and various longitudinal slices at 1-1.5mm intervals will offer the possibility of providing confirmatory evidence of root resorption of adjacent teeth. These slices will also give the orthodontist the unrivalled opportunity to recognize specific hard tissue pathological entities, such as ankylosis, invasive cervical root resorption (ICRR) and pre-eruptive intracoronal resorption (PEIR), which may be the sole reason that the impacted tooth has not erupted. Planar radiographs alone do not have the capability to provide this level of information, but they are significantly more sparing on ionizing radiation than CBCT – factors that need to be weighed carefully, especially with young children, when choosing between the two modalities.

At a first examination the likelihood is that the parent and patient will have come into the office, after shopping around in search of you or your nearest competitor, to provide 11 year old Janie with “the great smile”. In my office, I count the teeth as part of my full clinical examination. I report to and often surprise the unsuspecting parent that a tooth is unerupted and that a radiographic examination is required to check this and to see if other abnormalities exist. The mood of the discussion will have moved from the lighter to the more serious, now that surgery will have been mentioned for the first time. With all the advantages that CBCT has to offer, we now need to ask ourselves whether we should immediately scan the patient in a cone beam unit and prepare all the available data.

My approach in these situations is initially to request a panoramic radiograph, supplemented with one or two periapical views of the suspect area. If the permanent dentition is largely complete, except for the area of the impaction and the timing is suitable for the commencement of orthodontic treatment, I will also include a cephalogram and intra-oral and extra-oral clinical photographs, together with dental impressions. At a subsequent visit I will explain the need for a CBCT evaluation to the parent, because the population in my country is largely well educated with a good measure of sophistication, access to the internet and a grasp of the benefits and drawbacks of ionizing radiation. I then arrange for this to be done. If the impacted tooth is a maxillary central incisor, a phase 1 treatment is likely to be advised and, again, a CBCT evaluation will be ordered.

If the patient is too early for treatment and, assuming that the location of the impacted tooth and its relationship with its neighbors are non-threatening, I will usually place the patient on annual or semi-annual recall for reassessment. Occasional supplementary single radiographs may be later needed to monitor the movements of the impacted tooth, until active treatment is indicated and only then will I perform my CBCT evaluation.

For the purposes of the present discussion in this month’s bulletin, we shall assume that active orthodontic intervention will not be undertaken for a year or more and, if we can agree on the line of argument expressed above, a CBCT will not be considered justified at this stage. As such, I regard it as incumbent on me to gather as much information as is available from planar radiographs, during the long period that the patient is under observation prior to treatment.

My approach is “….let’s maximize the information available from simple inexpensive methods before moving on the costly and often premature CT imaging modality, which is relatively much heavier on radiation".

The frame of reference to be considered in this context includes periapical radiographs, panoramic films, possibly with the addition of an occlusal film. A lateral cephalogram will sometimes be included, particularly if there is a need for a baseline to monitor growth.

A pair of periapical radiographs:

Periapical films appear to be going out of fashion in relation to orthodontics in general and to the diagnosis of tooth impaction in particular, which is a shame because their depiction of individual teeth represents the best qualitative representation that is available among the planar films. The film shows the clearest detail of the tooth in question, particularly in relation to pathologic changes in the sharp outline of the crown and root of the tooth, its follicle, its root canal and its apical region. By itself, however, it is only a two-dimensional view and, as such, it cannot be used to give clues regarding the bucco-lingual location of one tooth vis-à-vis another on which its shadow is superimposed.

jjj.Fig._2a__bjjj.Fig._2c__d

Fig. 2a, b. Diagrammatic representation of the direction of the x-ray cone for periapical radiographs by the “tube-shift” or “parallax” method. Direction A represents an orthoradial view of the canine (13). Its degree of superimposition on the root of the lateral incisor (12) is colored red. Direction B is taken from a horizontally rotated direction of the x-ray cone and shows a changed superimposition of the canine.

Fig. 2c, d. Parallel periapical radiographic views of a typical case.

As early as in 1909, Clark1 published his technique using two periapical views of the same tooth, taken at different angles, to diagnose the relative positions of two teeth which superimpose upon one another. Thus, if we are faced with a maxillary canine superimposed on the lateral incisor, the x-ray cone would be positioned directly facing the lateral incisor, at an angle of 50 degrees up from the horizontal plane, to produce an orthoradial periapical view of the canine crown and the superimposed lateral incisor root. For the second periapical view, the intraoral sensor/film is positioned in the identical place. The 50 degree angle of the cone of the x-ray machine to the horizontal remains unaltered, but the cone is moved 15 degrees distally and horizontally around the arch, pointing at the same 2 teeth from this eccentric angle (Fig. 2a-d).

The relationship between the two teeth will have changed on the second film compared with that of the first. Thus, if the canine is on the palatal side of the incisor root, the crown of the canine will have “moved” distally, to overlap the incisor root to a lesser degree or to be “separated” from it, depending on the degree of mesio-distal displacement of the canine. This method is known as the “lateral tube-shift” or “lateral parallax” method.2

A vertical variety of this method may also be used, in which the angle of the x-ray tube remains unchanged in the mesiodistal plane for the second film, but the angle of the tube to the horizontal plane is increased. In this way the overlap of the canine crown will appear higher up on the incisor root for a palatal canine.

This is a very simple exercise, based on the simplest of optical principles, which is cheap in financial cost and in dosage of ionizing radiation. It is also a method that every orthodontist knows and would seem to be too mundane to even be mentioned here. However, for some reason orthodontists find it confusing. To illustrate this, a study was carried out involving 6 trained orthodontists, in which they were each asked to evaluate 39 cases with ectopic canines.3 The orthodontists were required to diagnose canine location using the lateral tube shift technique and then to repeat the assessment using the vertical technique, with a significant time gap between the two. This highly qualified cadre of professionals could only diagnose 83% of the cases correctly with the lateral shift and only 68% in the vertical shift cases!

A single panoramic film:

This film, too, is a two dimensional representation of a three dimensional scenario. However, it can be used to shine some light on the bucco-lingual dimension. When the panoramic view is being recorded, the central ray of the x-ray tube is directed towards the maxillary canine and premolars from behind the patient’s ear of the other side and, from there on to the revolving drum of the sensor/film. The size of the image of the canine on the drum will depend on the distance of the canine from the drum and, thus, a palatal canine will project a larger image than a labial or normally-placed canine, which is much closer to the drum.

jjj.Fig._3

Fig. 3. Anterior portion of the panoramic view of a patient with an unerupted canine, with horizontal marking to delineate the coronal, middle and apical portions of the incisor root. The crown of the canine has an enlarged mesio-distal width (a), compared to that of the adjacent central incisor (c), by direct measurement on the film, indicating a palatal displacement.

In a study of this phenomenon in a large sample of patients, using panoramic films, it was found that an enlargement of the mesiodistal width of the canine of at least 15%, when compared with the mesiodistal width of the ipsilateral central incisor, as measured directly on the film, indicated palatal canine displacement (Fig. 3). However, the method is not valid when the superimposition of the crown of the canine covers the apical third of the root of the incisor.4, 5

A panoramic film paired with a periapical or anterior occlusal film:

jjj.Fig._4

Fig. 4a. The canine is viewed on the cropped anterior portion of a panoramic view and shows the canine to be superimposed almost as far down as the CEJ of the central incisor and about half the root of the lateral incisor.

Fig. 4b. Viewed on an anterior occlusal film, which is taken at a more vertical angle, the degree of overlap is markedly less, indicating palatal displacement.

The panoramic film is produced by the x-ray tube being directed from just below the horizontal plane in a slight 7 degree gradient upwards towards the anterior dentition. Thus the image produced represents a largely true height superimposition of the impacted canine in relation to the incisor root in the vertical plane. On the other hand the periapical or anterior occlusal views, with angles of 50 -60 degrees respectively from above downward, “elevates” the image of a palatal canine in an apical direction, in relation to the incisor root. This creates a vertical tube shift situation, from which the positional diagnosis of the canine may be assessed (Fig. 4a, b).

A panoramic film paired with a cephalogram:

If we attempt to identify the individual teeth on a standardized lateral skull radiograph, we can almost always pick out the incisor crown and root. It is less certain that we can identify the shadow of the canines. A normally placed canine will always appear posterior to the incisor roots, because the canine is situated distally along a curved dental arch. However, if we know from the panoramic or anterior periapical/occlusal film that the unerupted canine is close to the maxillary anatomic midline, seeing the image of the canine located posterior to the incisor roots means that it is palatally impacted.

jjj.Fig._5a__b

Fig. 5a. The unerupted second premolar is seen here on a section of the panoramic view. The tooth appears to be tipped mesially and its lingual cusp rotated mesially. The canine appears to have a normal presentation. It should be remembered that this is a rotated lateral view of the maxilla and, as such, it will exaggerate the mesiodistal displacement of any tooth that is lingually displace from the line of the arch.

Fig. 5b. A cephalogram view of the same area of the same patient film is a true representation of the lateral plane. It shows little or no mesial tip and no rotation of the premolar. The view indicates that the canine is not displaced mesially. The conclusion must be that both teeth are lingually displaced from the line of the arch and the canine is rotated, while the premolar is not.

Frequently overlooked is the positional distortion that occurs particularly regarding individual unerupted maxillary premolars and canines. As pointed out above, the premolar is imaged when the x-ray source is located behind the ear on the opposite side. Additionally, in a young child, the location of the developing maxillary second premolar is slightly lingual to the line of the dental arch. It is therefore “thrown” mesially relative to the general line of the arch of erupted teeth. Furthermore, its palatal cusp will be affected more that its buccal cusp, because of its more palatal location, making it appear rotated. A comparison of the depiction of the unerupted second premolar on panoramic and lateral cephalogram of the same patient can be seen in the above illustrations (Fig. 5a-d).

An antero-posterior and a lateral cephalogram:

jjj.Fig._6a__b

Fig. 6a, b. Lateral and p-a cephalograms repectively of the same patient who exhibits severe displacement of the canine due to the existence of a dentigerous cyst. From the lateral view, one may only note that it is displaced much higher in the maxilla than its antimere. This view also shows either that the tooth lies almost horizontally or that it has no root. The p-a view shows (with difficulty due to the superimposition of bony structures) that the tooth has a root of good length with its apex at the level of the inferior border of the orbit and the long axis proceeding inferiorly and laterally, due to the size of the cyst.

These two films are exactly at right angles to each other and each of them represents the skull in 2 planes of space.6 The antero-posterior view encompasses the lateral and vertical planes, while the lateral view depicts the a-p and vertical planes. This means that, together, they cover all aspects. A third view which is at right angles to each of the above is the vertex occlusal film, which is taken along a line from the vertex of the skull downward and forward along the long axes of the anterior teeth. This takes in the lateral and the antero-posterior planes. However, in view of the very large exposure needed for this view, on a par with that required for a CBCT, the technique has been completely abandoned. The biggest problem using cephalograms is that it is not always easy to identify the individual teeth on these films because of the superimposition of other teeth and of other anatomic bony structures (Fig. 6a, b).

N.B. Figs. 2a-c, 3 & 6 are reproduced from the author's text "Orthodontic Treatment of Impacted Teeth", published by Wiley-Blackwell, Oxford 2012 and are copyright.

References

1. Clark CA. A method of ascertaining the relative position of unerupted teeth by means of film radiographs. Royal Society of Medicine Transactions, 1909;3:87-90

2. Jacobs SG. Radiographic localization of unerupted maxillary anterior teeth using the vertical tube shift technique: The history and application of the method with some case reports. American Journal of Orthodontics and Dentofacial Orthopedics, 1999;116, 415–423.

3. Armstrong C, Johnston C, Burden D, Stevenson M. Localizing ectopic maxillary canines--horizontal or vertical parallax? European Journal of Orthodontics, 2003;25:585-9.

4. Chaushu S, Chaushu G, Becker A. The use of panoramic radiographs to localize maxillary palatal canines. Oral Surgery, Oral Medicine, Oral Pathology, Oral Radiology and Endododontics, 1999; 88:511-516.

5. Chaushu, S., Chaushu, G. and Becker, A. Reliability of a method for the localization of displaced maxillary canines using a single panoramic radiograph. Clinical Orthodontics and Research, 1999;2:194-199.

6. Seward GR. Radiology in general practice IX. Unerupted maxillary canines, central incisors and supernumeraries. British Dental Journal, 1968;115:85-91.