Orthodontic Treatment of Impacted Teeth. Adrian Becker

which is then reconstructed into a 3D volume. This procedure is called a primary reconstruction and is carried out by a software algorithm. The algorithm will calculate and create the volume according to the protocol chosen and is depicted in the form of a cylindrical stack of axial slices, reminiscent of thin slices of salami, one on top of the other.

       Processing the scanned information

      When the orthodontist’s CBCT machine is ‘in‐house’, the imaging process is initiated with the software supplied by the machine manufacturer, or with third‐party software. When the patient is referred to an imaging centre, the imaging technician will produce a DICOM (Digital Imaging and Communications in Medicine) set, representing the international standard for image format and file structure for communication, handling, storing, and printing of medical and dental imaging and image‐related information. It will be handed over together with a viewer program supplied by the CBCT unit manufacturer, or with a third‐party software viewer preferred by the imaging centre. Often, however, more digitally savvy orthodontists may decide to use their own third‐party software to achieve the same ends. In this case only, the DICOM set is handed over and the orthodontists employ their own software.

      Many orthodontists will prefer to patronize a professional imaging centre, who may offer a service to perform a work‐up of the case. The technicians at the centre are experts in specifically providing an accurate positional diagnosis of the tooth/teeth in the three planes of space and in relation to the adjacent teeth and other anatomical structures. The work‐up should be adequate to the task of analysing and discovering the existence of pathological entities, leading to the diagnosis of the cause of the impaction, a task for which this modality is second to none.

      Secondary and online reconstructions can alter the patient position by tilting in any desired axis in space and slicing it in any direction and in any chosen slice thickness. The slices are not limited to straight cuts, with the panoramic view being the most popular curved cut. A thick panoramic view looks similar to the traditionally irradiated 2D panoramic view and it has the advantage that its form is without horizontal or vertical magnification and, therefore, without distortion. It is possible to avoid overlapping of individual teeth, especially in the premolar area, and there is no projection of other structures, such as the hyoid bone on the mandible or spinal vertebrae on the anterior region. The only information presented on the images is precisely what exists in the focal trough. The disadvantage of the reconstructed panoramic view is that artifacts found in the focal trough will be seen in the reconstructed panoramic view even if their source is outside the focal trough or even the FOV. If an extra‐large FOV is chosen, 3D orthodontic software has the capacity to produce standardized cephalometric views and the many other views needed for an orthodontic portfolio. A handful of devices have this capability. It is very important to note that all devices that enable this extra‐large FOV will perform this scan using half‐beam scan technology (up to 40% less radiation). The result is a normal‐ to low‐resolution scan, which in most cases will be sufficient for all the reconstructed views needed for the orthodontic portfolio. When zooming in on an impacted tooth and its surroundings, one might find it difficult to go into delicate details, like minor resorption, early‐stage invasive cervical root resorption (ICRR), etc.

       3D module

      A majority of dental CBCT software will have some kind of 3D volume‐rendering module, which is a very valuable tool for the accurate positional diagnosis and treatment planning of impacted teeth. The 3D volume‐rendering module depicts the individual teeth in their exact spatial arrangements and proximity to one another, from root apices to crown tips and viewable from every angle. The capabilities of this module vary between software programs and normally include several viewing modes. A logical examination sequence would start with the 3D rendering module, during which the ROI is identified, before moving on to slicing these areas.

      It is possible to move and rotate the volume, to ‘sculpt’ away areas that interfere or obstruct, to clip in a given axis and to peel away bone. In relation to impacted teeth, the most popular viewing modes in the orthodontic context are the transparent mode and the opaque bony appearance. The 3D module is good for a general, overall survey and will help clarify the crown and root relationships of impacted and supernumerary teeth with adjacent structures. Unfortunately, 3D portrayal cannot be trusted to discern tooth contact or minor resorption, not even using ideal viewing angles. Bone peeling, relevant to the opaque bony viewing mode, is not a tooth segmentation procedure, because this will peel off visual information with a density below the set threshold. When cortical bone areas have a similar density to that of dentine, the software is unable to distinguish between the two and will peel both. When the 3D threshold control is altered, the visible tooth volume changes. Thus, when thicker cortical bone needs to be peeled off, more dentine will be peeled off with it and a smaller tooth volume will result.

      Case 1: Ways of imaging and their effect on tooth size

Figure 4.13 represents six different ways to image the area of the dentition immediately surrounding the maxillary right permanent canine. The clinical aim was to determine if the space between the lateral incisor and first premolar, which was partially filled by the retained deciduous canine, offered sufficient mesio‐distal width to accommodate the permanent canine in its trajectory down to the occlusal level. The 3D opaque bony view needs to be peeled in order to clarify this point. This is presented with progressively more aggressive peeling from parts 4.13(a) to 4.13(d). Part 4.13(a) appears to have peeled off only the soft tissue. However, the alveolar bone covering the labial side of the canine is extremely thin and, because of its low density, will ‘disappear’ along with the soft tissues. Reducing the peeling would cause ‘reappearance’ of the soft tissue and obscure the thin bone covering. Proceeding from part 4.13(a) through to part 4.13(d), actual tooth volume begins to be peeled away. Thus, while there is visible interproximal solid contact in parts 4.13(a) and 4.13(b), there is also the suspicion of minor root resorption at the distal of the incisor. In part 4.13(c) the solid contact transmutes into a lighter contact and in part 4.13(d) into an apparent open contact, as the peeling process depletes the tooth volume. When rendering the volume in the 3D transparent mode, as depicted in part 4.13(f), there is no interproximal contact with the lateral incisor and, therefore, it may be assumed (wrongly) that there is a clear path to accommodate the unerupted canine. Since valid accurate information is an obvious clinical requirement, it is essential to understand how minor errors such as this may creep into the assessment of space by this method.

Fig. 4.13 Bone peeling in 3D. (a–d) Progressive bone peeling and how it may mislead by altering the teeth volume and interproximal contact. (e) Volume rendering in the 3D transparent mode. (f) Longitudinal slice cropped from the multi‐planar reconstruction screen (Figure 4.14) showing the deepest point of interproximal contact (see text).

The multi‐planar reconstruction (MPR) screen presentation in Figure 4.14 is a typical example. In order to define the exact mesio‐distal contact area between the lateral incisor and canine on the right side, the sagittal (Figure 4.14b) and coronal (Figure 4.14c) planes are tilted until the long axis of the incisor is brought exactly vertical. Once the tooth is vertically positioned, it may be rotated on its axis. The yellow line with arrows at both ends is a custom section tool, which has