Ultrastructural changes of the periodontal fibers and their attachment in rat molar periodontium incident to orthodontic tooth movement

Changes of the periodontal fibers and their attachment to cementum and alveolar bone in pressure zones of rat molars subjected to experimental forces were studied in the transmission electron microscope. In 67 rats, one maxillary first molar was moved buccally with a fixed appliance, using forces of 5–25 g. After experimental periods of 30 min to 28 d the animals were sacrificed and the experimental teeth with surrounding periodontal structures were processed for electron microscopy. After 24 h individual fibrils revealed bending and loss of functional orientation, while dense packing of fibrillar elements in irregular arrangement was a characteristic finding between 3 and 7 d. Within localized areas non‐striated filaments had formed by longitudinal splitting of collagen fibrils. Even in the hyalinized zones of longest duration (7–14 d) less than 5% of the area seen in the electron microscope had degradèd in this manner. Regenerative processes predominated in the 14–28 d specimens. The apposition of cementum and alveolar bone had come to a halt during hyalinization, as indicated by distinct demarcation lines. The matrix of the new cementum and alveolar bone which had been deposited during the period of repair of the tooth attachment appeared more granular than the old hard tissues, due to a lower content of fibrillar elements.