Effect of duration of force application on blood vessels in young and adult rats

Introduction: Age effects on orthodontically induced periodontal vascular reactions have not been studied. The aim of the present study was to test the hypothesis that prolonged tooth movement induces age-related increases in periodontal vascularity. Methods: A standardized orthodontic appliance was placed in 2 groups of 30 rats aged 6 weeks and 9 to12 months. At 1, 2, 4, 8, and 12 weeks, animals were killed. Blood vessels (BV) were identified based on their morphology and by immunohistochemical staining for alpha-smooth muscle actin. At each study region, surface areas (SA) of the periodontal ligament space and each BV were measured; BV mean SA, BV relative SA (the summed BV SA as a percentage of the periodontal ligament SA), and BV numbers were calculated. Results: Pressure and tension regions showed similar vascular changes. Young rats had lower BV relative SA and BV mean SA in the early phase of force application (<4 weeks); this increased in the late phase, reaching the same level as adult rats. In the late phase (4-12 weeks), young rats had increases of both small- and large-sized BV that did not affect the BV mean SA; adult rats had an increase of small-sized BV only; this resulted in decreased BV mean SA. Conclusions: The hypothesis was confirmed that prolonged tooth movement increases periodontal vascularity, which is age related. These results suggest that clinicians should consider age-related difference in tissue reactions during orthodontic tooth movement.