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TOOTH MOVEMENTS & BIOMECHANICS

Prepared by – SK AZIZ IKBALFinal Prof. 2015-2016NORTH BENGAL DENTAL COLLEGE

& HOSPITAL

DEPARTMENT OF ORTHODONTICSUnder the Guidance of -:Dr Prakash Ch RoyDr Rajib ShealDr Abhijit DasDr Amit Shaw

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Orthodontic treatment was possible due to the fact that whenever a

prolonged force is applied on a tooth, bone remodelling occurs around the

tooth resulting in it's movement.

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Biomechanics refers to the science of mechanics in relation to biologic systems.

Where as term mechanics refers to the discipline that describes the effect of forces on bodies.

WHY TO STUDY BIOMECHANICS? Knowledge of the bio mechanical principles and

governing forces is necessary for the control of orthodontic treatment

What is Biomechanics?

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It is the naturally occurring tooth movementsthat take place during and after tooth eruption

1. Tooth eruption2. Migration or drift of teeth3. Changes in tooth position during

mastication

Physiologic Tooth Movement

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Normal force of mastication – 1 to 50 kg It occurs in cycles of 1 second or less duration Teeth exhibit slight movement within the socket

and return to their original position on withdrawal of the force

Whenever the force is sustained for more than 1 second, periodontal fluid is squeezed out & pain is felt as the tooth is displaced into the the periodontal space

Tooth Movement During Mastication

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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.

FORCEIt is defined as an act upon a body that changes or tends to change the state of rest or motion of the body. Force is a vector it has both magnitude and direction. The forces are indicated by straight arrows.

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RESULTANT AND COMPONENT OF ORTHODONTIC FORCE SYSTEM:

1.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 )

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The parallelogram method:

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2.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

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3.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.

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Each body has a point in its mass, which behaves as if the whole mass is concentrated at that single point, which we call CENTER OF MASS in a gravity free environment.

Centre Of Mass

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Centre Of Gravity

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Centre Of Gravity

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Centre Of Gravity

It is the 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

Centre Of Resistance

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The center of resistance is on the long axis of tooth between one third and one half of the root length apical to alveolar crest.

Centre Of Resistance: (Single rooted tooth with healthy periodontium)

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For multirooted tooth, the center of resistance is probably b/w the roots 1-2 mm apical to furcation.

Centre Of Resistance: (Multirooted tooth with healthy periodontium)

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• Factors affecting Centre of Resistance1. Number of roots2. Degree of Alveolar Bone loss3. Degree of Root Resorption

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Point, around which body appears to rotate, determined from initial and final position.

Centre of rotationThe centre of rotation can be at the centre of resistance,apical to it,at the root apex or at infinity

TYPES OF TOOTH MOVEMENT & POSITION OF THE CENTER OF ROTATION Translation – lies at infinityUncontroued tipping – slightly apical to COR  Controlled tipping – Apex of rootRoot movement of torquing - incisal or occlusal edge

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Extrusion or Intrusion- outside the body

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The moment of the force is the tendency for a force to produce rotation. It is determined by multiplying the magnitude of the force by the perpendicular distance of the line of action to the center of resistance

It is measured in gm-mm.

Moment of force

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Moment of force

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If two forces of the couple act on opposite sides of the center of resistance, their effect is additive.

However, if they are on the same side of the center of resistance, their effect is subtractive.

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If two forces of the couple act on opposite sides of the center of resistance, their effect is additive.

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However, if they are on the same side of the center of resistance, their effect is subtractive.

COUPLE - A couple consists of two forces of equal magnitude, with parallel but non colinear lines of action and opposite senses.

The result is a moment with no net force.

The object rotates about it’s centre of resistance regardless of the point of application of the couple.

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MOMENT FORCE RATIOIt is the relationship between the force and the

counterbalancing couple.

‘Moment-to-Force’ ratio= Counterbalancing moment/ Force

Different Types of Orthodontic Tooth Movements

CONTROLLED TIPPING

TIPPING –simplest tooth movement.Controlled tipping- Centre of rotation at its apex.Uncontrolled tipping- Centre of rotation is very close or at centre of resistance.

UNCONTROLLED TIPPING

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BODILY MOVEMENT

TRANSLATION – •All points of tooth move equally and in same direction. •Here line of action of applied force passes through centre of resistance.•To counteract the tendency for tipping, a couple can be applied intentionally to produce a moment of equal magnitude in the opposite direction. when a moment –to –force ratio of 10:1 is applied to the bracket ,the equivalent force system at the center of resistance is a single force with no net moment.•In the pure translation, the center of rotation is considered to be at infinity.

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ROTATION – If only a couple ,and no force is applied to a tooth the tooth will rotate around it’s centre of resistance and the tooth will not translate. because the action of a couple does not depend on it’s point of application,a pure moment always acts at the center of resistance.

ROTATION

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ROOTUPRIGHTING

ROOT UPRIGHTING –When the counter moment applied intentionally at the bracket is more than the moment of force , the root moves in the direction of force but the crown tips in the opposite direction.when the moment-to –force ratio of 13:1 is applied at the bracket,the equivalent force system at the center of resistance is a force to move the tooth plus a small net tendency for the root to tip in the direction of force. The center of rotation ,when the moment to force ratio is 13:1,is at the crown of the tooth and only root movements occur.

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EXTRUSIONINTRUSION

INTRUSION & EXTRUSION- They are bodily displaced of a tooth along the long axis of it. When the displacement is in apical direction, it is intrusive and when towards occlusal direction, it is extrusive.

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M/F 0 to 5 : 1 Uncontrolled tippingM/F 7 : 1 Controlled tipping M/F 8 to10 : 1 Translation M/F >10 : 1 Root movement or Torque

MOMENT TO FORCE RATIO FOR VARIOUS TOOTH MOVEMENTS

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Optimum Forces For Orthodontic Tooth MovementsTYPE OF MOVEMENT FORCE REQUIRED

(gm)*Tipping 35-60 Bodily movement (translation)

70-120

Root uprighting 50-100Rotation 35-60 Extrusion 35-60Intrusion 10-20*values depend in part on the size of the tooth, smaller values appropriate for

incisors, higher values for multirooted tooth

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A moment can be referred as ROTATING,TIPPING,TORQUING When the tooth is embedded within the alveolar bone, a

couple can be applied only on the exposed part of the tooth.Various tooth alignment procedures can be achieved by this couple mechanism. Depending on the plane in which the couple is acting this rotational tendency is called

a.Rotation{first order} b.Tipping{second order} c.Torque{third order}

Couple-clinical point

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Force applied on a tooth

Overcome the moment Created by the force applied to the crown

Counter moment

Crown moves more than root

To maintain the inclination Of the tooth

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To maintain axial inclination

Apply the force close to the center of resistance

Create a 2nd moment In the direction opposite to the first

Practical difficulty

Power arm

Counter moment

Tooth remain upright And move bodily

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COUNTER MOMENT

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Mc/Mf=0 –PURE TIPPING (Crot and Cres same, thus the tooth rotates around the Cres) 0‹Mc/Mf‹1-CONTROLLED TIPPING (Crot displaced away from Cres, and the root and crown move in the same direction) Mc/Mf=1-BODILY MOVEMENT (equal movement of crown and root)

Mc/Mf›1-TORQUE (root apex moves further than crown)

The type of tooth movements depend on the ratio of the counter-balancing moment produced to the net force that is applied to the tooth

[Mf- Moment produced by force applied on tooth, Mc- Counter balancing moment]

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TORQUE

A rectangular wire ina rectangular slot

Generate the momentof a couple necessaryto control rootposition

Torque acting as the counter moment

Bracket system Clinical implication

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Equal and opposite force system. One couple appliance system. Two couple appliance system.

Biomechanical classification of orthodontic appliances

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Simplest orthodontic appliance ,an elastic band stretched between two points of attachment is the best example. This produces force of equal magnitude on either end but opposite direction.

 

Equal and opposite force system.

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One end of the appliance experiences couple and the other end is tied as a point contact.

ONE COUPLE APPLIANCE SYSTEMS.

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The both the ends of the appliance are engaged into attachments{brackets or tubes}.A couple may be generated by the wire at either or both attachment sites. The force systems produced by two couple appliances cannot be measured clinically and so they are referred as statically indeterminate.

Two couple appliance system.

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Variety of combinations of two- bracket systems and their force systems

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Continuous Force (ideal spring) Interrupted Force (removable active plates) Intermittent Force (removable appliances)

Forces Delivered by Appliances

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Continuous Force

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Interrupted Force

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Intermittent Force

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Forces that bring about orthodontic tooth movement are continuous and should have a minimum magnitude (threshold)

Below this threshold limit, the PDL has the ability to stabilize the tooth by active metabolism

The minimum pressure required is 5 to 10 gm/cm2 (current concept)

Force for Orthodontic Tooth Movement

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Contemporary Orthodontics . Orthodontics – The Art and Science . Biomechanical and esthetic considrations NANDA Text book of orthodontics TOM GRABER

REFERENCE

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