Читать книгу Biological Mechanisms of Tooth Movement - Группа авторов - Страница 47

The initial phase is completely determined by the biophysical properties of the PDL (van Driel et al., 2000; Jónsdóttir et al., 2006). In the first few seconds the tooth moves at a rate of approximately 10 μm/s, in the subsequent 20 seconds at a rate of approximately 1 μm/s, and thereafter at 0.1 μm/s or less. It stabilizes within 5 hours. In the initial few seconds the movement is determined by a rapid reallocation of fluid, and the remaining movement indicates the viscoelastic behavior of the ECM of the PDL (van Driel et al., 2000; Jonsdottir et al., 2006, 2012) (Figure 3.4).

As a result, blood vessels in the PDL are occluded, causing hypoxic conditions. This causes local cell death through the loss of cell membrane integrity and an uncontrolled release of organelles and debris into the ECM, with cell‐free areas as a result.

The necrotizing tissues initiate an inflammatory response through the action of inflammatory mediators such as interleukin‐1β (IL‐1β) and PGE2 in the surrounding tissue, which attract leukocytes and nearby phagocytes, such as macrophages and foreign body giant cells. These cells eliminate the dead cells and debris by phagocytosis (Murdoch et al., 2004). Also, the ECM changes by protein denaturation. This means that the secondary and tertiary structures of the collagen type I fibers, for example, are lost but their primary structures remain. Therefore, the proteins can no longer perform their function, and a gelatinous (gel‐like, hyaline) substance is formed. This process in called hyalinization and is mediated by enzymes from the matrix metalloproteinase family (MMP‐1, MMP‐8, MMP‐13). Furthermore, osteoclasts migrate to the area from nearby marrow spaces, after having resorbed the bone immediately adjacent to the necrotic PDL area, in a process known as undermining resorption (Krishnan and Davidovitch, 2006; von Böhl and Kuijpers‐Jagtman, 2009), This process is enabled by the mechanosensory action of the lacuna–canalicular system in the alveolar bone. OTM becomes only possible after all necrotic tissue has been removed. Because it is almost impossible to avoid blood vessel occlusion completely, hyalinization, and the subsequent interruption of tooth movement, is a very common process.

Figure 3.11 Photomicrograph of the trailing side of orthodontically moving premolar of a dog. The bone surface is covered with active osteoblasts indicating rapid bone deposition. H & E staining.

(Source: Jaap Maltha.)

The inflammatory processes related to orthodontic force application are described in detail in Chapter 4 of this book.

At the trailing side of the tooth, the very fast initial movement of the tooth leads to a rapid influx of fluid in the first few seconds. In the subsequent hours, widening of the blood vessels and tensioning of the collagenous fibers are seen. During the hyalinization phase, the situation at the trailing side remains stable.