Inflammatory Response in the Periodontal Ligament and Dental Pulp During Orthodontic Tooth Movement

Orthodontic tooth movement is induced by mechanical stimuli and facilitated by remodeling of the periodontal ligament and alveolar bone. A precondition for these remodeling activities, and ultimately for tooth displacement, is the occurrence of an inflammatory process in the periodontium and dental pulp, in response to the mechanical damage caused by orthodontic forces. Recent data suggests that cellular/tissue stress or damage-related products, such as damage-associated molecular pattern molecules, can trigger an aseptic inflammatory response. Vascular and cellular changes were the first events to be recognized and described, and a number of inflammatory mediators of immune and neural origin, such as cytokines, growth factors, and neuropeptides have been demonstrated in the periodontal supporting tissues. Their increased levels during orthodontic tooth movement have led to the assumption that a network of interactions between cells producing these substances (i.e., nerve, immune, and endocrine system cells), regulate the biological responses that occur following the application of orthodontic forces. Peripheral nerve fibers and neurotransmitters are also involved in the inflammatory process and bone remodeling as evidenced by the presence of neurogenic inflammation-related substances such as calcitonin gene regulated peptide and substance P, leading to increased vasodilation, increased microvasculature permeability, production of exudate, and increased proliferation of endothelial cells and fibroblasts. Inflammatory mediators of immunological origin, such as prostaglandins, interleukins (ILs; IL-1, IL-6, IL-17) as well as cytokines of the tumor necrosis factor α superfamily, which includes the RANK/RANKL/osteoprotegerin system, are also described in the periodontal ligament and dental pulp in increased levels after orthodontic force application. Considering the importance of RANK, RANKL, and osteoprotegerin in physiological osteoclast formation, it is reasonable to propose that the RANKL/RANK/osteoprotegerin system plays an important role in orthodontic tooth movement. This chapter reviews current knowledge regarding the role of inflammation in the periodontal tissue reactions in response to orthodontic forces.