From Volume 4, Issue 1, January 2011 | Pages 14-17
This case illustrates the benefits of CBCT over plain radiography in accurately localizing an unerupted transmigratory permanent mandibular canine and determining if it was aetiologically responsible for resorption of the permanent mandibular incisors.
Canine impactions occur 20 times more frequently in the maxilla in comparison to the mandible.1 Transmigration of a permanent mandibular canine is rarely symptomatic2 and is rarely suspected when the deciduous canine is retained beyond its normal exfoliation time.
When a permanent mandibular canine is significantly ectopic, it may not be identified on intra-oral radiographs, whilst a panoramic radiograph is unlikely to provide accurate information on the bucco-lingual position, proximity to incisor roots, and existence of pathology related to the canine.3 When permanent mandibular canines transmigrate, the distance they travel depends on the length of time before diagnosis. It should be noted that the tip of the canine may be detected to be crossing from the midline to the distal root of the first permanent molar on the opposite side.4 Moreover, they may be associated with a variety of pathologies, including dentigerous cysts and extensive root resorption of adjacent teeth. The destructive nature of transmigratory mandibular canines makes intervention almost always advisable in order either to prevent or limit root resorption. However, this treatment strategy has historically been dependent upon information provided by 2D radiographs. 3D imaging provides an opportunity to re-evaluate whether or not surgical removal of transmigratory mandibular canines is always desirable.
Cone-beam computed tomography (CBCT) has been available for imaging the craniofacial region for around ten years. In comparison with conventional CT, which acquires data as a series of 'slices' using a linear array of detectors, CBCT uses a cone-shaped beam of X-rays and a flat panel (or image intensifier) receptor to capture the data in a single volume. As the data is divided into cube-shaped isotropic voxels (same length in each dimension), the images can be reconstructed in any dimension. Cone-beam CT has very limited use in the evaluation of soft tissues and is ideally suited to imaging the jaws and dental hard tissues.
The dose received by patients undergoing CBCT examination has been measured in a number of studies5,6,7,8 and ranges from 69 to 560 μSv (2007) for a medium field of view (FOV) CBCT scan,5 with 23.9/96.9 μSv (1990/2007) for a 6 cm standard mandible scan.8 In comparison, the use of the panoramic scout for CBCT was found to be marginally higher in dose than its 2D counterpart.7 This compares with an annual radiation dose of approximately 2.5 μSv.9
CBCT equipment is now broadly divided into a group of machines imaging a large volume (such as the I-CAT, Imaging Sciences International) and those imaging a smaller, more limited, volume (such as the Kodak 9000 and Accuitomo, Morita, Japan). The 'Next Generation' I-CAT machine has the facility to customize the vertical height of the area imaged to reduce the area of the patient exposed following an initial scout view. As well as the volume of the region imaged, the patient dose is also affected by the size of the voxels from which the image is produced. The I-CAT machine has the ability to image in voxel sizes of 0.125, 0.2, 0.25, 0.3 and 0.4mm. Larger voxel sizes result in reduced radiation dose but lower resolution images, so for each patient there is a compromise between the amount of radiation associated with the exposure and the resolution of the resultant images.
This case demonstrates the benefit of three-dimensionally evaluating buried teeth using CBCT rather than traditional plain radiography by accurately localizing an unerupted transmigratory permanent mandibular canine and determining whether it is unlikely to continue causing resorption of the permanent mandibular incisors.
A 15-year-old female patient was referred for orthodontic management as a result of the general delay in eruption of her permanent dentition. It was noted that there was a Class 1 malocclusion associated with crowding in both arches on a skeletal Class 1 antero-posterior base. The permanent mandibular right canine had not erupted and could not be palpated buccally. In addition, the permanent left maxillary canine had not erupted and could also not be palpated buccally. A panoramic radiograph demonstrated the permanent mandibular right canine was positioned horizontally below the apices of the incisor teeth and the morphology of the apices of these teeth was indicative of previous resorption having taken place (Figure 1).
This image also revealed the permanent maxillary left canine to be palatally ectopic when using parallax in association with a maxillary anterior occlusal radiograph. This tooth was surgically removed as the prognosis for alignment of this tooth was poor. Owing to the potential for iatrogenic damage, surgical removal was not deemed advisable for the permanent mandibular right canine and this tooth was monitored closely. It was agreed, however, that, should the patient be interested in active orthodontic treatment in the future, or should there be any lower incisor root resorption, surgical removal would be clinically advisable.
A subsequent radiograph determined that there had been no further horizontal movement of the permanent mandibular right canine. However, the morphology of the apical regions of the permanent mandibular incisors appeared to be subtly different from their appearance at initial presentation and it was thought to arise from foreshortening due to a difference in patient position in the radiographic equipment (Figure 2)
Three years after initial referral, the patient was keen to pursue active orthodontic appliance treatment. It was determined that further panoramic radiography would be unlikely to provide any additional information about the precise location of the permanent right mandibular canine or, indeed, the status of the roots of the permanent mandibular incisors. Three-dimensional imaging would, however, provide this information and, in addition, would determine optimal surgical access for surgical removal of the permanent right mandibular canine.
A 4.9 second, 4.8 cm height and 0.3 mm voxel size scan I-Cat Next Generation CBCT scan (Imaging Sciences International, Hatfield, PA, USA) of the mandibular incisors (extending to the lower border of the mandible) was performed. This demonstrated that the long axis of the permanent right mandibular canine was, in fact, parallel with the mandibular plane, with the crown lying in the anterior mandible and crossing the midline (Figure 3). The root was found to be within the body of the mandible, apical and slightly anterior to the apex of the mandibular right first premolar. Close examination of the CBCT slices indicated that the labially positioned crown only had a thin layer of covering bone, which was deficient in some areas. Furthermore, the follicular space was noted to be slightly enlarged, measuring around 4 mm at the widest point. However, whilst the follicular space and crown of the permanent right mandibular canine were in close proximity to the apices of the permanent mandibular incisors, intervening alveolar bone was noted, indicating the permanent mandibular canine had not moved in the direction of the roots of the permanent mandibular incisors with eruptive movement (Figures 4 and 5). Reassuringly, there was no evidence of any marked incisor root resorption.
Following discussion with the patient, it was agreed that there was no need for surgical removal of the transmigratory permanent right mandibular canine, and this tooth continues to be monitored. Orthodontic treatment for the maxillary arch was initiated involving an upper fixed appliance.
CBCT imaging was very useful for the management of this case. The panoramic images that had been recorded previously were not conclusive for determining the precise labio-lingual position of the crown and root of the permanent right mandibular canine, the proximity of this tooth to the roots of the permanent mandibular incisors, and the presence, location and extent of any resorption affecting the permanent mandibular incisors.
The CBCT views demonstrated the permanent right mandibular canine to be lying across the arch, with the root being in approximately the normal anatomical location (mesial to the root of the mandibular right first premolar) and the crown being located in the labial aspect of the parasymphyseal region. This information could not be gleaned from panoramic views without recourse to using either the parallax principle10 or a true occlusal view. Both of these would necessitate an additional radiograph. Nevertheless, superimposition of the crowns and roots of the incisors would have obscured the root of the canine. By limiting the CBCT scan height, the radiation dose was minimized. Although cone beam CT has the advantage of a much lower radiation dosage than conventional medical or multi-slice CT,11 the radiation dose is higher than that for conventional plain radiography. However, limiting the field of view, as in this case, can reduce the radiation dosage from 45 times that, to around twice the radiation dose of a panoramic radiograph.12 This was justified in this case in order to localize the permanent right mandibular canine and the root status of the mandibular incisors precisely. It should be noted that evidence-based guidelines are limited at present13,14 and, whilst these support the acquisition of CBCT images for the localization and consequences of a transmigratory permanent mandibular canine, it is important to ensure that all CBCT exposures are justified in line with IR(ME)R regulations.15
The presence of alveolar bone between the crown and root of the permanent right mandibular crown and the permanent mandibular incisors, coupled with the follicular space being generally narrow, indicated that the eruptive potential of this tooth had reduced considerably since the previous panoramic radiograph had been recorded. As a result, the CBCT image determined that the permanent right mandibular canine was unlikely to cause any further permanent incisor root resorption.
Furthermore, the apical regions of the permanent mandibular incisors had not, in fact, been affected by obvious root resorption. This was an unexpected finding and the indication from the previous panoramic images is explainable on the basis that the permanent mandibular incisors were proclined, with the apices being close to the lingual extent of the focal trough. It is well known that panoramic images can be inconclusive when detecting root aberrations and overt root resorption, however, the presence of the transmigratory permanent right mandibular canine, in this case in association with the radiographically shortened permanent incisor roots, was indicative of a cause and effect relationship, which proved not to be the case. Notwithstanding, there was also superimposition of the cervical spine, which partially obscured the symphyseal region.
The presence of thin (and in places, fenestrated) bone overlying the permanent right mandibular canine crown was useful additional information that would not be provided by further plain radiographs in this case. The patient has been advised that, should there be any pathological changes in the crown or follicle, including cystic change, this would result in swelling being detectable at an early stage. The overlying bone would be breached and consequential hard tissue damage unlikely. Should surgical removal be required in the future, a labial approach would be appropriate and would be less likely to result in damage to the incisor roots.
CBCT is a useful method for localizing unerupted teeth where there is any suspicion of iatrogenic damage taking place. The information may also provide additional information on the possibility of the direction of eruption of such teeth and is of considerable utility in determining the management strategy for cases. In some cases, such as that above, surgical intervention can be avoided.
