Duration of orthodontic force affecting initial response of nitric oxide synthase in rat periodontal ligaments

Clinical experience suggests that successful orthodontic tooth movement can be produced with a threshold for force duration at about 6 hours, however, the changes in the periodontal ligaments (PDL) during this period is still unclear. Nitric oxide (NO) is a marker of signal transduction relating to bone remodeling. The aim of this study was to observe the initial response of NO synthase (NOS) when PDL equilibrium would be broken against light continuous orthodontic force. Rat maxillary first molars were moved mesially with 2 gf Titanium-Nickel closed coil springs for 1, 3 and 6 hours. The number of NADPH-diaphorase positive cells in PDL was counted for investigating NOS activity. At the control group, NOS activity in the distal area of the PDL was significantly higher than that of the mesial area (P<0.001). The activity of mesial area increased at 1-hour group (P<0.01), while the activity of distal area dropped down at 3- and 6-hour groups (P 3-hour<0.05, P 6-hour<0.001), compared with the control group. These results suggest that 1-3 hours would be the threshold of force duration for tooth movement with light continuous force.