Читать книгу Orthodontic Treatment of Impacted Teeth - Adrian Becker - Страница 81

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).