Abstract
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
