Light pressure against a tooth for less than 1 second causes the periodontal ligament fluid to be expressed and the tooth to move within the periodontal ligament space. Within 3-5 seconds, blood vessels in the periodontal ligament are partially compressed on the pressure side and dilated on the tension side, disturbing periodontal ligament fibers and cells. Tooth movement begins within 2 days as osteoclasts and osteoblasts remodel the bony socket. Heavy pressure against a tooth for less than 1 second has similar initial effects, but causes blood vessel occlusion on the pressure side within minutes and cell death in compressed areas within hours. Tooth movement occurs after 7-14 days as undermining resorption removes the lam

1. Physiologic Response to Sustained
Pressure Against a Tooth

2. Time Event
Light pressure
≺ 1 second PDL fluid incompressible ,
alveolar bone bends ,
piezoelectric signal generated.
PDL fluid expressed,
tooth moves within PDL space.
3-5 seconds Blood vessels within PDL partially compressed on
pressure side,
dilated on tension side,
PDL fibers and cells mechanically disturbed.
Minutes Blood flow altered ,
oxygen tension begins to change ,
prostaglandins and cytokines released.

3. Time Event
Light pressure
Hours Metabolic changes occurring,
chemical messengers affect cellular activity,
enzyme levels change.
4 hours Increased cAMP levels detectable,
cellular differentiation begins within PDL
2 days Tooth movement beginning as osteoclasts / osteoblasts
remodel bony socket
PDL, Periodontal ligament;
cAMP, cyclic adenosine monophosphate.

4. Time Event
Heavy pressure
≺ 1 second PDL fluid incompressible ,
alveolar bone bends ,
piezoelectric signal generated.
PDL fluid expressed,
tooth moves within PDL space.
3-5 seconds Blood vessels within PDL occlude on pressure side
Minutes Blood vessels within PDL occluded on pressure side
Hours Cells death in compressed areas
3-5 days Cells differentiation in adjacent marrow spaces
undermining resorption begins
7-14 days Undermining resorption removes lamina dura adjacent
to compressed PDL , tooth movement occurs

5. Appliance activation.
• The duration of initial lag phase ie the time taken from the
beginning of force application to the onset of tooth
movement usually takes about 2 to 3 days with light force
and 7 to 14 days with heavy force.
• In other words a tooth start moves only after 7 to 14 days.
• Therefore appliance adjustment should be made after every
three to four weeks for optimal tooth movement and to
allow time for cell reactions to occur.