Ultrastructural changes in pressure zones of human periodontium incident to orthodontic tooth movement

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Abstract
In order to characterize the ultrastructural alterations of cells, vasculature and fibrils that occur on the pressure side of the periodontal ligament when teeth are subjected to orthodontic forces of a magnitude generally used in clinical practice, eleven premolar teeth were moved buccally by means of a fixed appliance. The experimental periods were 2, 21 and 50 days. Previous light microscopic observations of cellular and vascular disturbances in compressed zones of the periodontal ligament were verified. In addition, ultrastructural details of the process of degeneration and necrosis of cells and vascular elements were obtained. It was demonstrated that cells and blood vessels in the hyalinized zones are damaged to an extent where restitution to normal function is inconceivable. Although collagen fibrils disintegrated by longitudinal splitting into filaments without periodicity, the majority of fibrils retained the cross striations. Apart from differences related to the time factor, there was close agreement between the results of the present study on human material and in recent studies on rats where structural alterations have been systematically observed.