Читать книгу Biological Mechanisms of Tooth Movement - Группа авторов - Страница 43

Orthodontic forces are applied in an environment where the mechanical properties depend on the material properties, sizes, and shapes of the constituent entities within the dento‐alveolar units (Viecilli et al., 2008, 2013). The initial effect of the application of an orthodontic force is deformation or strain in the PDL and, to a lesser extent, the alveolar bone. This deformation depends on the biomechanical characteristics of the PDL, which in turn are dependent on the constituents of the extracellular compartment, i.e. fluid, fibers, and ground substance. This complicates mechanical behavior. In most biomechanical literature on the PDL a linear elastic behavior is assumed, which is an oversimplification of the real situation.

Experimental studies have described the time‐dependent behavior of the PDL in the first period after orthodontic force application (van Driel et al., 2000; Jónsdóttir et al., 2006; Bergomi et al., 2010). Two subperiods can be recognized: the first lasts for only a few seconds when a rapid transposition within the socket indicates a redistribution of free‐floating fluid; the second lasts for about 5 hours and shows a decelerating transposition, indicating viscoelastic behavior of the ECM of the PDL, with the collagen fibers leading to fiber‐reinforced properties (Jónsdóttir et al., 2006; Jonsdottir et al., 2012) (Figure 3.4).

Therefore, the most likely model for the biomechanical properties of the normal PDL would be a biphasic poroviscoelastic fiber‐reinforced material (van Driel et al., 2000; Wang et al., 2012; Ortún‐Terrazas et al., 2018; Uhlir et al., 2017; Wu et al., 2019).

However, a serious drawback of all the literature pertaining to the mechanical properties of the PDL is that it is exclusively based on the normal state. The dramatic changes in the structure and composition of the PDL during OTM have never been taken into account.