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 the ability to rotate around an axis marked at its centre. It is placed on the axial (Figure 4.14a) view with its centre at the point at which it meets the tooth axis, on which rotation may be made. The window in Figure 4.14(d) displays the cut produced by the rotation tool. By rotating the tool, the tooth outline may be depicted at any point on the 360° circle. The deepest point of contact is illustrated in the window in Figure 4.14(d) and enlarged separately in Figure 4.13(f).
Fig. 4.14 A view of the multi‐planar reconstruction screen for Case 1, as presented in InVivoDental software (see text).
Peeling the 3D opaque bony mode and exploiting the 3D transparent mode offer many advantages in appreciating the inter‐relations of the teeth and the surrounding structures. Understanding of the process by which they are produced and their reducing effect on tooth volume are factors that often need to be taken into account.
In light of the foregoing discussion, it will not come as a surprise to learn that peeling of bone in the body of the mandible will result in much loss of teeth volume, particularly the thick bone of the buccal side of the mandible. In practice, because the densities of the teeth and the surrounding compact bone are of a similar order, it is largely impossible to peel this thick buccal bone without excessively reducing tooth volume and it becomes necessary to combine clipping and, in some cases, sculpting to achieve the desired results (as demonstrated in Case 2).
Case 2: Peeling, clipping and sculpting
In
