Scanning electron microscopic study of orthodontically induced injuries to the periodontal membrane

Scanning electron microscopy was used to study vascular and cellular changes in the periodontal membrane (PDM) in rats after orthodontic treatment ranging from 3 h to 9 days. The orthodontic forces ranged between 200 mN and 400 mN. Within the first hours of treatment extravasated erythrocytes, eichinocytes and platelets in various stages of degeneration were found in the pressure zones. The connective tissue of the pressure zones developed an amorphous appearance lacking discernible fibers and blood vessels following treatment for 24 h. These areas are known as hyaline zones, an area of local aseptic necorsis. Three days after the application of an orthodontic force macrophage-like cells were abundant close to the hyaline zone. They adhered to the hyaline zone with filopods radiating from pseudopods. Large osteoclast-like cells with several projections towards the bone surface were found around the hyaline zone and in the marrow spaces close to the pressure zones. The results indicated that the tissue reactions leading to remodeling of the PDM in the pressure zones are closely related to vascular injuries and subsequent formation of coagulated blood. Furthermore, macrophages seem to be the cells which play the most important role during degradation of the hyaline zone.