Mechanics of tooth movement

- RICHARD J. SMITH-CHARLES J. BURSTONEVOLUME 85, NO 4 , AM. J. ORTHODONTICS , APRIL 1984

MECHANICS OF TOOTH MOVEMENT::

PRESENTED BY: KIRTI AGRAWAL J.R.1 DATE: 14-11-13

Scalars : it do not have direction , it have only magnitude. Example: weight, temperature , distance , mass etc.

Vectors: it have both magnitude and direction. Example : force etc.

Force vector is represented by arrows: Origin of arrow: point of application of force Body of arrow: line of application Arrowhead: direction of force( sense)

RESULTANT AND COMPONENT OF ORTHODONTIC FORCE SYSTEM: RESULTANT FROM FORCE WITH COMMON

POINT OF APPLICATIO N:THE PARALLELOGRAM METHOD: If 2 force have common point of application, than

they are considered as the side of the parallelogram

We complete the parallellogram.

And diagonal is considered as the resultant.

(length: magnitude of resultant F, arrowhead: direction )

The parallelogram method:

RESULTANT WITH DIFFERENT POINT OF APPLICATION: THE LAW OF TRANSMISSIBILITY OF

FORCE: First we make artificial/ constructed point of

application in space, along the line of application of force.

Vectors are moved along the line of application to the constructed point of application ,maintaining their original length.

Then parallelogram is constructed, and resultant F is obtained

RESOLVING THE FORCE INTO COMPONENT: We divide a single force into

components at right angle to each other.

We consider the force vector to be the diagonal of parallelogram,

The component are parallel and perpendicular to occlusal plane, dividing into vertical and the horizontal component.

Force and movement:

Center of gravity: a point at which a free body is perfectly balanced.

Center of resistance : a force with a line of action passing through the center of resistance produce translation.

Example: center of resistance of a single rooted tooth , probably between 1/3 and ½ of root length apical to alveolar crest.

Center of resistance of a multirooted tooth , is probably 1-2 mm apical to the furcation.

Center of resistance depends on : 1) root length 2) alveolar bone height

Types of movements:

Translation Rotation Combination of translation and

rotation

Translation: if line of action of force passes through the center of resistance of a tooth, the tooth will respond to pure bodily movement / translation

MOMENT OF A FORCE:

If line of application of force does not pass through the center of resistance, the force will produce some rotation, the potential for rotation is measured a MOMENT.

M= F × d( perpendicular distance of line of action of force to the center of resistance)

Unit: gram milimeters Symbol: curved arrow. Clockwise moments defined as POSITIVE

and counterclockwise moments negative.

The direction of the moment can be determined by continuing the line of action of force around center of resistance.

COMBINED TRANSLATION AND ROTATION: If the line of action of force does not

pass through the center of resistance.

It will create a moment about center of resistance, and result is a combination of translation and rotation.

Rotation :

Pure rotation means tooth spins about center of resistance.

Type of movement Center of rotation

translation infinity

Pure rotation At center of resistance

Root movement Incisal edge

Controlled tipping Apex

Uncontrolled tipping slightly apical to the center of resistance

Intrusion On a line which is approximately perpendicular to long axis

COUPLE:

A single F cannot produce pure rotation , only a force system can produce pure rotation of tooth is called a COUPLE.

Example : 2 F are the same magnitude, parallel to each other , in the opposite direction but not coincide, form a couple.