**6. Orthodontic movement of teeth and inflammation**

The mechanical stimulus stemming from the orthodontic forces causes aseptic inflammatory reaction within periodontal tissues, which initiates biological processes, which are connected to the reshaping of the bone [16]. Even though in normal conditions the movement of teeth is a sterile process, the early stage of orthodontic tooth movement is observed as a type of tissue injury and it is accompanied by the acute inflammatory response.

Generally speaking, the acute inflammation is an initial stage of defense reaction of the mechanism to the tissue injury (mechanical, physical, chemical, nutritive, biological). It occurs fast and does not last long and it emerges as the result of numerous, complex, and mutually related processes via which certain proteins and cells are transmitted from blood to the damaged tissue and whose final result is the recovery of the tissue. The acute phase of the inflammation is characterized by vascular changes (vasodilatation and increased permeability of blood vessels) and consequently, plasma leakage (exudation) and relocation of leukocytes (extravasation) from blood into the injured tissue.

Immediately upon the application of orthodontic force, the disturbance of the microcirculation of periodontal ligament occurs, which results in the ischemia of local tissue, periodontal vasodilatation, and migration of leukocytes via capillaries of periodontal ligament. The changes are temporary and, by the rule, do not have pathological effects.

Even though inflammatory changes occurred during the orthodontic tooth movement are mostly the consequence of reactive processes in the support tissue, mechanical stimuli may be transmitted also to the tooth pulp and may initiate the inflammatory response within this dental tissue [17].
