What is the center of resistance?

The center of resistance is the point a force has to pass through in order to move an object freely in a linear manner. Said in another way, every free body has a point known as mass center, therefore, every time that the line of action of a force passes through de mass center of a free body will suffer a translation movement.

A tooth in the mouth is not a free body because periodontal support tissue does not let it move freely. The center of resistance is equivalent to the mass ccntcr for free bodies. Any force that acts through the center of resistance of a tooth makes it translate in a bodily manner.

Hie center of resistance of a tooth depends on the length and morphology of the root, the number of roots and the level of support of the alveolar bone.

The center of resistance for a single root tooth with a normal level of alveolar bone is situated between the middle and cervical third part of the root, meanwhile in a multiple root tooth the center of resistance is located at one or two millimeters apical from the furcate.

Because braces can only be bonded on the crowns of teeth, there are very few opportunities in which it is possible to apply a force that can act through the center of resistance of the tooth to produce pure dental translation.

What is the Center of Rotation?

The tenter of rotation of a tooth is the arbitrary point located at a distance from the center of resistance on which a tooth is going to turn around if a force is applied.

The center of rotation can be very close, but it will never coincide with the center of resistance. In orthodontics, when the rotational process occurs along the larger axis of the tooth it is called rotation or first order dental movement {in-out movement).

When rotation occurs around the mesiodistal axis it is called angulation or second order dental movement (Tip), if it happens around the buccolingual axis, it is called torque or third order movement.

What is a moment?

A moment is produced when the line of action of a force passes along the center of resistance provoking a tendency to rotate; said in another way, a force applied on a brace that does not act through the center of resistance provokes the rotation of the tooth,(U) A Moment is the result of a force times distance. When a force is applied on braces and it does not pass through the center of resistance of the tooth, a distance is produced between the line of force and the centcr of resistance of the tooth. This distance (in a perpendicular way) is the cause of moment in the tooth, giving dental rotation as a result.1151 Hie measuring unit for a moment is g x mm2 and the graphic representation is a curved arrow that in 2-dimensional diagrams can be drawn clockwise or anticlockwise.

Every force that passes through the center of resistance will not produce a moment; furthermore, the body will translate without any type of rotation; the greater the distance between the center of resistance to the line of force, the greater the moment that will be produced.