Orthodontic Treatment of Impacted Teeth. Adrian Becker

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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

Photos depict 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. Photo depcits a view of the multi-planar reconstruction screen for Case 1, as presented in InVivoDental software.

      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

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