Читать книгу Biological Mechanisms of Tooth Movement - Группа авторов - Страница 46

In 1962, Burstone suggested that, if the rates of OTM were plotted against time, there would be three phases of OTM: the initial phase, a lag phase, and a post‐lag phase. The initial phase is characterized by a period of very rapid movement, which occurs immediately after application of force to the tooth. This rate is attributed to the displacement of the tooth within the PDL space and bending of the alveolar bone. This phase is followed by a lag period, when no or low rates of tooth displacement occur. This lag results from hyalinization of the PDL in areas of compression. No further tooth movement will occur until cells complete the removal of all necrotic tissues. During the third phase, the rate of movement gradually or suddenly increases. Experiments by Hixon and co‐workers (Hixon et al., 1969, 1970) revealed two phases in OTM: an initial mechanical displacement, and a delayed metabolic response.

Figure 3.8 General time–displacement curve of OTM.

(Source: Jaap Maltha.)

More recently, a new time–displacement model for OTM was proposed (van Leeuwen et al., 1999; Von Böhl, et al., 2004b) (Figure 3.8). These studies, performed on beagle dogs, divided the curve of tooth movement into four phases.

The first phase lasts 24 hours to 2 days and represents the initial movement of the tooth inside its bony socket, causing structural changes in the ECM. In this initial phase, the ECM is compressed in the direction of the tooth movement, leading to a temporal increase in tissue pressure, constriction of blood vessels, and deformation of nerves. In many cases this results in an anoxic situation leading to local tissue necrosis, called hyalinization (Figure 3.9).

At the trailing side of the tooth (formerly incorrectly called the tension side), the periodontal space is widened, which leads to a temporal decrease in tissue pressure, and a widening of the blood vessels (von Böhl and Kuijpers‐Jagtman, 2009).

The initial phase is, in most cases, followed by a second phase, where there is an arrest in tooth movement lasting for approximately 20–30 days. As long as the hyalinized tissue remains, tooth movement is prevented, as direct bone resorption is not possible, because osteoclasts cannot differentiate within the necrotic areas of the PDL. Actual OTM, the third phase, begins with an increasing rate only after the complete removal of the hyalinized tissue.

In the fourth phase, tooth movement takes place at a constant rate, as long as the force is exerted and no obstacles are encountered. This is the linear phase. In this phase, alveolar bone is resorbed at the leading side of the root (formerly incorrectly called the pressure side) (Figure 3.10), and bone deposition is found at the trailing side (Pilon, Kuijpers‐Jagtman, and Maltha, 1996; van Leeuwen et al., 1999) (Figure 3.11).

Figure 3.9 Photomicrograph of the PDL after orthodontic force application for 36 hours on a rat molar. The internal structure of the PDL is almost completely lost due to hyalinization.

(Source: Jaap Maltha.)

Figure 3.10 Photomicrographs of the leading side of orthodontically moving premolar of a dog. A. Herovici staining showing the absence of type I collagen fibers and their replacement by type III collagen. B. ED1 staining, specific for osteoclast cytoplasm. Arrows indicate osteoclasts.

(Source: Japp Maltha.)

In fact, this pattern of OTM concurs with the three phases described by Burstone (van Leeuwen et al., 1999; Von Böhl et al., 2004a, 2004b).

If the force application is discontinued because the tooth is in its desired position, the tooth tends to move in the opposite direction. This process is called relapse and can be prevented by stabilizing the tooth in its position by a retention appliance (van Leeuwen et al., 2003; Littlewood et al., 2017).