The role of the inflammatory process in orthodontic tooth movement

Introduction: It is now well-known that an orthodontic tooth movement will always create areas of pressure and tension within periodontium. The pressure area is characterized by partial and complete interruption of the local blood flow which will inevitably result in death of some cells. As a typical physiological reaction of any vital organism an inflammation process soon emerges. Although practitioners are now aware of inflammation during orthodontic tooth movement (OTM) it is still not always clear what is its purpose and whether it’s beneficial or harmful for orthodontic treatment. Objective of the study: To study the role and features of the inflammatory process during orthodontic dental displacement. Material and methods: The design of the study involves a systematic review of information from articles, textbooks and manuals in English found in electronic sources. Only the original content without translation was used. Web platforms were used as a database: PubMed, ScrienceDirect, Reserch4Life, Oxford Academic. Articles that contained the terms ‚Orthodontic appliances’, ‚Orthodontic techniques’, ‚Biomechanics’ were excluded because this study was not conducted to assess the principles of orthodontic treatment. Results: The OTM is always associated with an aseptic inflammatory process. The areas of pressure and tension in the periodontium affect the local vascular and nerve network. Areas of cell death as well as the alteration of the vessel endothelium with increase of its permeability lead to the emergence of leukocytes, monocytes, macrophages, and platelets from the bloodstream. These cells together with the local periodontal cells release inflammatory factors such as cytokines (IL1β, IL-6, IL-10, TNF-α, TGF-b, M-CSF), chemokines (MCP-1, CCL5), arachidonic acid derivatives (prostaglandins, leukotrienes, nitric oxide), which lead to remodeling of local tissues. IL-1β and TNF-α are released at periodontal pressure areas and act on osteoclasts and their precursors, stimulating their differentiation, increasing their activity and resistance. They also contribute to the maintenance and promotion of the inflammatory process and increase the level of matrix metalloproteinases (MMPs). The absence of TNF-α or its receptor, of PGE2 or leukotrienes result in a significant decrease in the rate of OTM. The most important element responsible for the transfer of orthodontic force into structural changing in periodontal tissues is RANKL/RANK/OPG pathway. It controls bone remodeling/modeling processes by regulating the osteoclastic function. Most pro-inflammatory cytokines exert their osteoclastogenetic effects via one of three pathways: stimulation of RANK synthesis, modification of RANK-induced intercellular interactions, or stimulation of RANKL synthesis. Several original studies have concluded that orthodontic forces lead to an increased expression of the gene encoding RANKL. The absence of RANKL leads to a complete cessation of OTM. The presence of OPG and RANKL has been revealed on both the tension side of the periodontal ligament and the compression side. RANKL/OPG ratio, regardless of the exact amount of each substance, affects the activity of osteoclasts in a particular periodontal area. Orthodontic treatment of patients with inflammatory periodontal diseases in some cases results in a combination of a de novo aseptic inflammation with a preexisting septic inflammation. This leads to accelerated progression of periodontal disease. Conclusions: 1) The inflammatory process during OTM is aseptic in nature and represents a physiological and favorable (expected) response of periodontal tissues. The released inflammatory factors lead to opposite results in different parts of the orthodontically moved teeth: to tissue resorption on the pressure side and to its formation on the tension side. 2) The RANK-RANKL-OPG mechanism represents the key pathway for the transformation of exogenous forces into bone resorption and apposition through the inflammatory process. The rate of osteoclast activity and tooth displacement depends largely on the RANKL/OPG ratio, with the amount of these substances being a less relevant factor. On the side of compression, inflammatory factors change this ratio in favour of RANKL-ligand (RANKL), while on the other side the opposite phenomenon is observed. 3) Patients with an inflammatory disease of periodontium are at risk of worsening the preexisting condition when being treated orthodontically. A thorough periodontological therapy is recommended before starting an orthodontic therapy.