Periodontal ligament changes depend on amount, duration and type of forces. Teeth movement toward periodontal ligament starts from the initial hours by formation of pressure and traction areas. These areas as well as vascular system must be preserved under the forces. Bone loss is resulted from osteoclasts activity. In spongy bone spaces, osteoid is formed. Due to heavy forces, periodontal tissues are pressed, vessels are blocked and periodontal ligament becomes acellular and hyalinised. Also,The osteocytes of undermining bone are damaged in localized areas. In traction areas, fibroblasts proliferation and elongation of periodontal tissues are resulted and osteoid is formed on the areas which are replaced by bundle bone (a part of lamina dura that includes calcified collagen ligaments). Due to heavy forces, collagen in traction area and blood vessels may become ruptured.