To define the mechanics of orthodontics, we need to discuss Newton’s third law of motion:
"For every action, there is a reaction equal but opposite."
Orthodontic force is not an exception to this rule. If we apply force to a few teeth, there will be a reaction on some other teeth. When it comes to braces and tooth movement we always have two different units:
Active unit: Teeth that need to be moved in a certain direction
Anchorage unit: Teeth that are used as an anchor to move the active unit
To clarify the above statement let’s say Dr. Amin Movahhedian or Dr. Hamed Vaziri needs to close the space between your teeth by using braces. He needs to get help from the back teeth to move the front teeth. By doing this he applies force to both active and anchorage units equally but in opposite directions. The consequence is that both units will move toward each other. But what if we do not want the anchorage unit to move at all or very minimal in comparison to the active unit? Then our option will be to either reinforce the anchorage unit by:
Adding more teeth to the anchorage unit
Using an extra-oral device like headgear
Using an intra-oral device like a Nance
The disadvantage of some of these appliances such as headgear is that they are bulky and most of the time they need patient’s compliance.
Another option is to not even consider other teeth as an anchorage unit and use a TAD instead. TAD is a device that is temporarily placed in the bone to be utilized as an anchorage for tooth movement.