Ultrastructural Cellular Reactions in Pressure Zones of Rat Molar Periodontium Incident to Orthodontic Tooth Movement

Abstract

Changes in the cells of the periodontal ligament of teeth subjected to orthodontic forces have been described. the purpose of the present study was to characterize these changes at the ultrastructural level. In 67 rats, maxillary first molars were moved buccally by means of fixed appliances rxerting forces of 5–25 g. Animals were sacrificed after periods of 30 min to 28 days. the experimental teeth with surrounding periodontal tissues were removed and processed for electron microscopy. Cellular changes not involving the nucleus, such as dilation of the endoplasmic reticulum and moderate swelling of mitochondria occurred within 30 min of force application. After 2 hrs, this cellular swelling was marked. Large vacuoles had formed in the cytoplasm, and the mitochondria frequently revealed considerable enlargement. More advanced degradation of cellular components was observed after experimental periods of 2 hrs or more. Rupture of the cell membrane had frequently occurred leaving isolated nuclei in various stages of decomposition.

It seems that hyalinization involves severe injuries of fibroblasts and other connective tissue cells, resulting in cell death. However, necrosis was limited to circumscribed areas in the periodontal ligament. Dispersed globuli and nuclear fragments were occasionally observed within hyalini Aed zones at 2–7 days. Regenerative processes were prevailing in the 7–28 days specimens.