Mechanics of tooth movement

Parmanum Jaya

4th year BDS

Introduction To understand how tooth movement occurs in orthodontics,

these terms are important:

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

Orthodontic correction is based on application of appropriate force on teeth by means of ortho appliances.

Force

Stress: force applied per unit area (external force acting upon a body)

Strain: internal distortion per unit area (result of stress on a body)

Couple: pair of concentrated forces with equal magnitude & opposite direction with parallel but non collinear line of action.

Couple brings about pure rotation around the centre of rotation.

Moment: measure of rotational potential of a force wrt a specific axis. (tendency of a force to produce rotation)

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

Types of tooth movement The main aim of orthodontic Rx is to move teeth into more

desirable & favourable positions.

Movement occurs in 3 planes:- sagittal - coronal - transverse

Classification of tooth movement

Classification of tooth movts: Pure translation Pure rotation Generalised rotation

Types of tooth movement Bodily movement Intrusion/extrusion Tipping Torquing Uprighting

Pure translation All points on tooth move an equal distance in the same

direction.

Bodily movement: Movt of an equal distance of crown & root (translation) Intrusion & extrusion:Bodily displacement of a tooth along its long axis apically or occlusally/incisally.

Intrusion/extrusion & centre of rotation

Uprighting:Mesio-distal tipping of roots for a parallel orientation.

Pure rotation Displacement of a tooth with centre of rotation coinciding

with centre of resistance.

2 types transverse long-axis

Transverse rotation Tipping Single force applied to the crown resulting in movement of the crown in same direction of the force & the root in opposite direction.

Torquing:Reverse tipping with lingual movt of root. These are transverse rotation: tooth movt with change in long axis orientation.

Torquing

(A) Controlled tipping: When a tooth tips about a centre of rotation at its apex.Lingual movt of crown with minimal movt of root labially.

(B) Uncontrolled tipping:Movt about a centre of rotation apical to & close to centre of resistance.

Long-axis rotation:Labial or lingual movts of teeth around its long axis.

Generalised rotation

Is a combination of both translation & rotation.

Types of force

o Continuous o Intermittent o Interrupted

Continuous force:Active orthodontic force that decreases little in magnitude between appointment periods. (light wire appliance)

-Highly flexible-Activation done at a low force level-Bring about direct resorption of root socket-Should not be too high to cause occlusion of blood vessels supplying the tooth

Intermittent force:Active orthodontic force that decreases to 0 magnitude or nearly so prior to following appointment. (removable active plates)High stiffness & activation after application of force to teeth

Resorption/tooth movt

period of rest (no force)

Repair of necrosed soft tissue

Interrupted force: Inactive ortho force for intervals of time bet appointments.(force exerted by an extraoral appliance worn at night only)

o Should deliver heavy forceso No force decayo The inactive period of each day should be enough to keep the

periodontal ligament healthy.