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Types of orthodontic tooth
Biomechanics of tooth movement
Types of orthodontic tooth movement
Mechanics is the science which deals with the action of
forces on the form and motion of bodies.
Biomechanics is the study of the action of forces
delivered by the orthodontic appliances on the biologic tissues
such as the teeth, periodontal ligament and the bones
An orthodontic appliance is a system, storing
and delivering forces against the teeth, muscles
or bone and creating a reaction within the
periodontal ligament and alveolar bone that
causes movements of the teeth or alters bone
morphology or growth.
Force is an energy or strength brought to bear on an
object causing a change in shape or motion.
Force is a vector matter.
It may be represented by its vectors as :
- Direction - line of action.
- Point of application.
Center of gravity (CG) or center of mass (CM).
Every object or free body has one point where all of
the weight is concentrated at this point.
The object would be perfectly balanced, if
supported, on this point as if the rest of the object does
This point is known as the center of gravity (CG) or
center of mass (CM).
Center of resistance. (C-ris)
Unlike free body, movement of a tooth is constrained
by the periodontal attachments to the roots, and the alveolar
Therefore in orthodontic biomechanics, a point
analogous to the center of gravity or center of mass is used.
This point is termed center of resistance. (C-ris)
Center of mass
Center of gravity
Basically, there are three types of movement.
- Translation. (Bodily movement)
- Pure rotation.
- Combined rotation and translation.
Translation or bodily movement occurs when all
points on the tooth move an equal distance in the same
Rotation indicates movement of points of the tooth
along the arc of a circle, with the center of resistance being
the center of the circle.
Center of resistance
Combined rotation and translation.
Any movement that is not pure translation or rotation
can be described as combination of rotation and translation.
Effects of forces on tooth movements
The movement of a tooth depends upon the
relationship of the line of action of a force to the center of
resistance of a tooth.
If the line of action of a force passing through the center
of resistance the tooth will respond with pure translation or
bodily movement in the direction of the line of action of the
applied force. (Translation or bodily movement)
Line of action of force
If the line of action of the applied force does not pass
through the center of resistance, the center of resistance will
translate as if the force did pass through it, but the tooth will
also rotate, since the applied force produces a moment about
the center of resistance. The resulting movement is combined
translation and rotation. (Tipping movement)
The magnitude moment is equal to the force multiplied by the
perpendicular distance from the line of action to the center of
Moment = Force x perpendicular distance
= 100 x 8
= 800 gm-mm
Force = 100 gm
Tooth movement depends on
the relative degree of force
A single force (a force with a single point of contact)
cannot produce pure rotation.
A system of forces that can produce rotation of a tooth
is called force couple.
A couple consists of two forces of equal magnitude,
acting in parallel but opposite direction and having different
points of application.
A force couple produce moment (Mc) inducing rotation
about center of resistance (Cris) whose magnitude is the
product of the distance between the lines of force composing
the couple (dc) and their magnitude.
Moment = F x dc
Orthodontic Tooth Movement.
(Combined rotation and translation)
Translation or bodily movement
Optimal force level
The level of orthodontic force, just enough to
stimulate cellular activity without completely occluding
the blood vessels in the periodontal ligament.
The various force levels are required for different
types of orthodontic tooth movements.
The force applied at the crown of a tooth does not
determine the biologic reactions in the PDL.
The force is distributed to the area of the PDL that are
under compression. (force per unit area of PDL compression)
Therefore, the smaller the area of PDL under
compression, the greater the force will be distributed and the
larger the area, the lesser the force.
The area of compression of the PDL depends upon the
types of tooth movement.
Therefore different optimal force levels are required for
various types of tooth movement.
Why does optimal force level varies for different types of
Optimal force for orthodontic tooth movements.
Type of tooth movement Optimal force (gm)
Tipping 35 to 60
Translation (bodily movement) 70 to 120
Root uprighting 50 to 100
Extrusion 35 to 60
Intrusion 10 to 20
Tipping movement (combined rotation and translation)
•If the force does not pass through the CR a tooth will translate as
well as rotate around the CR.
•Orthodontically, tipping movement is produced when a single
force (from a spring or labial bow from a removable
orthodontic appliance) is applied against the crown of a tooth.
•A tooth rotates around the center of resistance as well as
translates along the line parallel to the line of force.
•The maximum pressure is created at the alveolar crest on the
opposite side of the force application and at a side near the root
apex on the same side of the force application.
•The pressure is gradually reduced to minimum towards the center
Loading diagram consists of two triangles covering in
two areas where its concentration is high in relation to the
force applied to the crown.
According to loading diagram, only one half of the
PDL area that could actually be loaded is under
Therefore optimal force required to tip the tooth is
quite low, approximately 35 to 60 gm.
The force for translation of tooth at CR would be
much below the optimal force level so that translation
movement is insignificant.
•Most orthodontic force system apply force at the bracket
cemented to the crown of a tooth. Therefore when a force is
applied at the bracket an equivalent force system at the CR
would be force (causing translation) and a moment of force
•For bodily movement the force system applied at the crown
must be equivalent to the force and moment of force at the CR.
•Therefore force system having a force and a couple at the
bracket is needed so that a tooth can be moved bodily.
Moment = Force x Perpendicular distance from the point of
•In bodily movement total PDL area is uniformly loaded from
alveolar crest to apex on the opposite side of force application,
creating rectangular loading diagram.
•Therefore to produce the same pressure in the PDL and the
same biologic response as for tipping, twice the amount of
force would be required for bodily movement.
•The optimal force level for bodily movement would be 70 to
Rotation of tooth around its long axis requires the
application of a force couple to the crown.
•There is no compression of the PDL area instead all the
ligaments are under tension.
•However, since the tooth roots are triangular or oblong
shape some areas of the PDL may be compressed.
•Therefore, optimal force levels for the rotation movement
would be about the same as tipping.
•Ideally, the extrusion would produce no area of compression
within the PDL.
•All the PDL would be under tension and stimulate bone
deposition necessary to maintain tooth support.
•Some areas of compression in PDL may occur depending upon
the morphology of root.
•Therefore, optimal force level would be about similar to tipping
movement. 35 to 60 gm. Use of heavy force should be avoided.
•When a tooth is intruded the whole of the supporting
structure is under pressure with no area of tension. The
force is concentrated over a small area at the apex
resulting in resorption of bone around the root apex.
•Therefore, extremely light force (10 to 20 gm) is needed to
produce appropriate pressure within the PDL during
•Resorption of root is likely to occur if heavy force is used.
Root uprighting (Controlled tipping)
•Mesiodistal movement of particularly of root. (more root
movement than crown.)
•Require the application of force couple to the crown in such
a way that the fulcrum lies within the crown.
•Optimal force required would be 50 to 100 gm.
• Labial or palatal movement of apices is referred to as
• Torqueing requires the application of a force-couple to the
crown in such a way that the fulcrum lies within the crown.
F = The force delivered for a given deflection
E = Elastic modulus
L= Wire length
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Types of tooth movement in orthodontics
Copyright by Ortho Department, Yangon