Orthodontically induced microvascular injuries in the tension zone of the periodontal ligament.

A review of the literature pertaining to the histological and ultrastructural changes related to orthodontic tooth movement reveals that much attention has been devoted to the fibrillar and cellular components of the periodontal ligament (PDL). Although the vascular response to experimental tooth movement has been investigated, specific morphological and morphometric data on changes in the tension zone of the PDL microvascular system are lacking. The present investigation was conducted to study the ultrastructural vascular response in the tension zone of the PDL following a short-term extrusive force. Six 12-week-old male Porton rats were anesthetized and cannulated via the aorta after applying a 1-N extrusive force for 30 min to the right maxillary first molar. The contralateral molar served as a control in all animals. The apical portion of both the control and experimental PDL was prepared for TEM examination. In the experimental PDL, degenerative changes in the endothelial cell wall and incipient nuclear shrinkage were observed in about 30% of the postcapillary venules and 18% of the capillaries, whereas the arterial components were generally spared. Compared to the control PDL, these degenerative changes were significantly marked in the experimental PDL (p less than 0.01). The results of this study suggest that the postcapillary venules are the most vulnerable segment in the PDL microvascular system.