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CHAPTER 3 Cellular and Molecular Biology of Orthodontic Tooth Movement
Jaap C. Maltha, Vinod Krishnan, and Anne Marie Kuijpers‐Jagtman
Summary
Orthodontic tooth movement is the result of a goal‐oriented application of an external force to a complex biological system. For a proper understanding of the processes underlying this complicated system, knowledge of its constituents is necessary. Therefore, this chapter begins with a description of the morphology and physical characteristics of fiber systems and the ground substance of the extracellular matrix, and a description of the different cell types involved in the synthesis and turnover of this matrix. Also, the biomechanical characteristics of the periodontal ligament are described. Orthodontic tooth movement is a result of mutual interactions between cells and between cells and the extracellular matrix. General systems of cell–cell and cell–matrix interactions are described in detail. Tooth movement and the different phases that can be distinguished in this process are explained. Each phase is characterized by specific cell biological regulatory systems. Particular attention will be given to the linear phase in which a mechanosensory system is responsible for a cascade of events that ultimately leads to bone resorption at the leading side and bone deposition at the trailing side of the moving tooth.
Introduction
Orthodontic tooth movement (OTM) is the result of externally applied forces on a complex biological system that contains the alveolar bone, the periodontal ligament (PDL), the tooth, and the gingiva. Under physiologic conditions, this complex is adapting to ever changing mechanical conditions due to chewing, swallowing, and muscle activities in and around the oral cavity. The application of orthodontic forces leads to a cascade of reactions in the extracellular matrix (ECM) and the cells in the dento‐alveolar complex.
In orthodontics, the fields of biology and mechanics are intertwined, and research keeps unfolding the details of this relationship. The chief questions are about the nature of the biological response of a living organism to applied mechanical forces, and what the features of an optimal force are, for each individual patient. Basic research, at the cellular and molecular levels, has revealed meaningful information about the mechanism of mechanotransduction, and about the signaling systems controlling the interactions between cells during periods of tissue remodeling. This chapter reviews details of the biological responses of paradental tissues and cells to applications of mechanical forces in vitro and in vivo.
Entities important for tooth movement – the players in the game
Extracellular matrix
The PDL, the root cementum, and the alveolar bone consist, like all connective tissues, of cells and ECM of which the principal component is formed by fibers, embedded in a gel‐like ground substance (Kerrigan et al., 2000; Nanci and Bosshardt, 2006).
The most predominant type of fiber in the alveolar bone, the PDL, and the root cementum is collagen type I, which is mainly present as strong extracellular fibers (Figure 3.1A). This protein is synthetized by fibroblasts starting with the intracellular synthesis of triple helices called procollagen, containing two type‐α1 and one type‐α2 “pre‐procollagen” peptide chains. Procollagen is secreted by exocytosis. Outside the cell, procollagen is assembled into collagen fibrils and subsequently, by crosslinking, into collagen fibers. These fibers have a very well organized and complex internal structure that resembles a hawser. This structure results in flexible fibers with a great tensile strength (Kerrigan et al., 2000; Wenger et al., 2007; Dean, 2017).
Figure 3.1 Photomicrographs of the normal PDL in a dog, showing the main orientation of the collagen fibers (A, H & E staining) and the oxytalan fibers (B, Oxone‐Halmi Aldehyde Fuchsin staining).
(Source: Jaap Maltha.)
A second type of fiber in the PDL is the oxytalan fiber (Figure 3.1B). This type of fiber belongs to the elastic fiber family, which consists of elastic, elaunin, and oxytalan fibers. The elastic and elaunin fibers mainly contain elastin and fibrillins, while the oxytalan fibers lack elastin and only contain fibrillin‐1 and fibrillin‐2. These
