Quantitative Analysis on Effects of Aging on Orthodontic Tooth Movement

Objectives: The purpose of this study was to investigate the effects of aging on the amount of tooth movement and concomitant changes of cell proliferation in the periodontal tissue adjacent to orthodontically treated tooth roots. Methods: Wistar rats (n =32) of four different age groups (10, 30, 50, and 80 weeks of age) were used in the experiment. Maxillary first molars (M1) were retracted mesially with a nickel titanium alloy coil-spring for 2 weeks by a continuous force of 10 cN. Twenty-four age-matched untreated rats were used as controls. Amount of tooth movement was measured the distance between the most mesial point of the M1 and the maxillary incisor at the gingival level. Parahorizontal serial sections of the first molar were made after tooth movement. The changes in periodontal tissue turnover were assessed histochemically and immunohistochemically. The sections were stained for tartrate-resistant acid phosphatase (TRAP) activity or Proliferating Cell Nuclear Antigen (PCNA) activity. Results: Two weeks after the start of tooth movement, the amount of tooth movement was found to decrease with age and was significantly different among the four age groups (P < 0.05). The quantitative analysis demonstrated that, in all experimental groups, the number of PCNA positive cell in periodontal membrane increased significantly compared with that of each age-matched untreated group (P < 0.001). However, the rate of increase decreased in an age-related manner. Conclusions: These results suggest that the age-dependent decrease cell proliferation activity in periodontal tissue, in response to mechanical forces, might decrease the amount of tooth movement.