Types of dental movement
Dental movement can be classified in many ways:
a. Uncontrolcd inclination.
b. Controlled inclination,
3. Root displacement.
1. Inclination; Is the movement in which the crown of the teeth suffers greater displacement than the root of the tooth. This can be classified according to the location of the center of resistance:
a. Uncontrolled inclination: This type of inclination happens when the center of rotation is between the center of resistance and the apex of the tooth. This movement can be obtained by any orthodontist, but is usually not wanted. This uncontrolled inclination happens when we have a round arch wire in the slot of the brace and force is applied to the tooth.
Another way to produce uncontrolled inclination is with the use of elastomeric power chains in order to close spaces.
When an elastomeric power chain is placed from a molar to a canine, with a round arch, the outcome is an inclination (moment) of the crowns toward the space; meanwhile the mots will become completely divergent. This happens because the force is applied on the crown, away from the center of resistance.
b. Controlled inclination: This is a wanted movement; it is obtained with the application of a force to move the crown, and the placement of a moment in order to control or maintain the position of the root apex. An example of this inclination is when we want to retract the anterior sector without moving the location of the tooth apex. This is obtained wiLh rectangular arch wires. When we softly insert the wire in the slot of the brace, It counteracts (torque) part of the moment caused by the dental retraction.
2. Translation: Is one of the most difficult movements in orthodontics. It is also known as in-mass or bodily movement. Happens when the crown and the dental apex are horizontally displaced. This is only possible when the line of action of a Force passes through the center of resistance of the tooth. We obtain this type of movement with the use of power arms, which allow the line of action of the force to pass directly through the center of resistance.
As it was previously mentioned, translation is one of the most difficult movements to achieve in orthodontics. This is caused by the anatomical characteristics that surround teeth that make the application of a force through the center of resistance very difficult, so pure translation is hard to obtain. Although in a high percentage of cases, each time we want to move a tooth in a bodily manner, it will not translate, instead it will tend to rotate slightly in the direction of the exerted force, because the force is away from the center of resistance of the tooth, so we will have translation and rotation of the tooth.
3. Root displacement: In this movement a Moment and a force are applied to displace only the root, meanwhile the dental crown is not displaced. This is the best movement to alter the longitudinal axis of the tooth without altering the incisal edge of the tooth. Root displacement is commonly used to torque incisors, to upright canine roots after space closure, upright posterior teeth that arc mesially inclined, etc.
4. Rotation: To make this movement a couple or coplanar forces are required, which can produce a pure rotation around the longitudinal axis of the tooth (seen from the occlusal view).