Mechanics of tooth_movement-_ortho

MECHANICS OF TOOTH MOVEMENT

Rohan Vadsola

Contents:IntroductionForceStress and StrainCoupleCentre of resistanceMomentCentre of rotation

Introduction:Physics, mathematics and engineering are

three important disciplines that can effectively be applied to the study of orthodontic tooth movement.

A clinician is in part an engineer.Mechanics is defined as that branch of

engineering science that describes the effect of force on a body.

- S. I. Bhalajhi -Gurkeerat

singh

A clear understanding of the theories of mechanics has potential applications in 3 areas:

1. Precise application of force.

2. A better understanding of clinical and histological response to various magnitude of force.

3. Improving the design of orthodontic appliances.

- S. I. Bhalajhi

The response of a tooth to an applied force can be at 3 levels i.e. clinical, cellular and stress-strain level of activity within the investing tissues that is the least understood.

There exist no stress-strain gauges, at present, which can be placed within the periodontium to evalute the stress-strain activity.

Therefore, the importance of the study of mechanics can not be over-emphasized as almost all the assumptions made about the stress-strain activity in the periodontium are based on several mathematical formulations and conclusions.

- S. I. Bbhalajhi

Proper application of biomechanical principles to the orthodontic mechanotherapy helps in increasing the treatment efficiency and also helps reduce the treatment time.

- S. I. Bhalajhi

Force:

Force can be defined as an act upon a body that changes or tends to change the state of rest or of uniform motion of that body.

Being a vector, force has a definite magnitude, a specific direction and a point of application.

In clinical practice it is either a ‘push’ or a ‘pull’.A force is equal to mass times acceleration. Its units are newtons or gram

millimeters/second2.

- S. I. bhalajhi

The point of application of a force is indicated by origin of the arrow, which is at the bracket.

Direction is indicated by the body of the arrow itself and the arrowhead.

Without the head of the arrow, the body alone indicates the line of action.

- Gurkeerat Singh

In clinical orthodontics the unit used is grams.Multiple forces can be combined to form a

resultant force by vector addition.The resultant force is determined by the law of

parallelogram of vectors by connecting the vectors ‘head to tail’ while maintaining the length and direction of the line of action.

Similarly a given force can be resolved into two or more components.

Orthodontic correction is based on application of appropriate force on the teeth.

- S. I. bhalajhi

Stress and strain:Stress is the force applied per unit area while

strain can be defined as the internal distortion per unit area.

Stress and strain are inter-related terms as stress is an external force acting upon a body while strain is the resultant of stress on that body.

Strain can be expressed in the form of a change in either the external dimension or internal energy of the body.

-S. I. bhalajhi

Couple:Couple is a pair of concentrated forces having

equal magnitude and opposite direction with parallel but non-collinear line of action.

In other words a couple is two parallel forces of equal magnitude but opposite in direction.

The magnitude of a couple is calculated by multiplying the force with the distance between them.

The unit used is grams millimeters.

- S. I. bhalajhi

A couple when acting upon a body brings about pure rotation around the centre of resistance.

Moment of the couple is the rotational tendency of the couple.

- S. I. Bhalajhi

Two forces of same magnitude, parallel to each other & acting in opposite direction.

Clinical application of a couple in treatment of rotations.

Engaging a wire in an angulated bracket slot results in a couple

Engaging a rectangular wire into a slot produce a couple

Centre of resistance:Every body or free object behaves as if its

mass is concentrated at a single point on which it can be perfectly balanced.

For physical calculation, this point can be taken as the point where the whole body weight is concentrated and can be termed centre of gravity.

Teeth can not move in a free manner within the jaws.

- S. I. Bhalajhi

They are restricted by the investing tissues around their roots.

In such a situation, a point analogous to the centre of gravity is made use of and is called the centre of resistance.

Centre of resistance of a tooth can be defined as that point on the tooth when a single force is passed through it, would bring about its translation along the line of action of the force.

The centre of resistance of a tooth is constant.

- S. I. bhalajhi

In a single rooted tooth it lies between one third and one half of the root, apical to the alveolar crest while in a multi-rooted tooth the centre of resistance lies between the roots, 1-2 mm apical to the furcation.

The centre of resistance exists for single tooth, units of tooth, the entire dental arch and the jaws.

Factors which can change the position of the centre of resistance are the root length and alveolar bone height.

Longer the root, the centre of resistance will be placed more apically.

-S. I. bhalajhi

If the alveolar crest is higher the centre of resistance will be placed more coronally.

Similarly the morphology and the number of roots also influence the location of the centre of resistance.

- S. I. Bhalajhi

Centre of resistance of multirooted and single rooted teeth.

A-normal alveolar bone

B-Alveolar bone loss results in apical shift of centre of resistance

C-Root resorption and shortening results in more coronal centre of resistance

Moment:Moment can be defined as the measure of

rotational potential of a force with respect to a specific axis.

Orthodontic forces are most commonly applied at the level of the crowns of the teeth.

Therefore, the force are seldom applied through the centre of resistance of the crowns.

Thus these forces not only produce a linear motion, they also produce a rotation.

-S. I. bhalajhi

Moment = magnitude of force x distance (perpendicular distance from the centre of resistance of the body to the line of action of the force).

The unit of measurement of moment is gram millimeters.

The two variables that determine the moment of force are the magnitude of force and the distance from the centre of resistance.

- S. I. bhalajhi

Moment is calculated by multiplying the magnitude of force by the perpendicular distance of the line of action to the centre of resistance.

Centre of rotation:Centre of rotation is a point, about which a

body appears to have rotated, as determined from its initial and final positions.

The centre of rotation is a variable point and changes according to the type of tooth movement.

It can be at any position on or off the tooth.

-S. I. Bhalajhi

In case of controlled crown tipping, the centre of rotation will be at the root apex while in case of a perfect translation it will be at infinity.

- S. I. Bhalajhi

A-At the incisal edge during torquing B-At the root apex during controlled tippingC-Away from the root apex during uncontrolled tippingD-Outside the tooth during intrusion or extrusion

Thank

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