ERM functions, EGF and orthodontic movement or Why doesn’t orthodontic movement cause alveolodental ankylosis?

Can orthodontic movement induce alveolodental ankylosis? This question is often asked and the answer involves further questioning: Why don’t the teeth naturally evolve to alveolodental ankylosis if they are separated from the bone by only 0.2 to 0.4 mm (the minimum and maximum thickness of the periodontal ligament)? The periodontal ligament is richly cellularized and vascularized, featuring numerous elastic and reticular collagen fibers, typical of connective tissues (Figs 1, 2 and 3). In between these structures it has a “gel”, namely, the extracellular matrix. Among the fibers, fibroblasts, vessels and nerves of the periodontal ligament there is a network of epithelial cords and islands that continuously release mediators, especially EGF, i.e., Epithelial or Epidermal Growth Factor (Fig 2). Areas on the surface of the bone tissue that contain EGF stimulate bone resorption, hindering the formation of new layers. This epithelium network interposed between bone and tooth in the ligament tissue is known as Epithelial Rests of Malassez (ERM), derived from apoptosis in Hertwig’s Epithelial Root Sheath (HERS). Malassez’ original drawings (Fig 4) depicted these epithelial cords and islands in the same manner as we analyze them microscopically today. It was long believed that ERM comprised latent or quiescent cells devoid of structure and function, often associated with the genesis of cysts and tumors. However, these epithelial periodontal components are active, produce mediators and fulfill key functions in maintaining periodontal health and root integrity even during orthodontic movement. In this paper we will discuss these wonderful structures and their functions to assist us in understanding the relevant responses to the two questions posed above.