Download Anchorage

‫ حيدر الزرجياوي‬.‫د‬
30/4/2015
Anchorage
has been defined as the resistance to
Anchorage
the reaction forces that generated as a response
and by moving the canine lingually, an buccally
directed reaction force acts on molars and so on.
to the action of the active components of appliance.
The reaction forces are generated within the other
structures that connected by the appliance, such as
teeth, palate, head or screw. Anchorage is required
to be prepared in such way, to prevent the unwanted
tooth movements.
The amount of movement of any tooth will depend
upon its root surface area and the force system
applied on that teeth. Theoretically, the "anchorage
value" of a tooth, that is, the resistance to
movement, can be thought of as a function of its
root surface area, which is the same as its PDL area.
According to Newton's third law of motion, for
every action, there will be an equal and opposite
reaction. In orthodontics, when the appliance is
made to produce a desired tooth movement by an
active component(s), there will be an equal and
opposite reaction forces which can move other
appliance-contacted teeth. For example, if both
upper canines are being retracted with an
orthodontic appliance, which contact almost all the
erupted teeth, an equal and opposite force to that
being generated by the active canine retraction will
also be acting on the those remaining upper arch
teeth.
The larger the root, the greater the PDL area over
which a force can be distributed, in turn the tooth
has a greater anchorage value and vice versa. For
example, The PDL area for two posterior teeth is
slightly larger than the total PDL area of all anterior
teeth. Therefore, with a simple spring connecting
the segments, the anterior teeth would move
slightly more than the posterior teeth.
Each orthodontic appliance consists of two
elements: anchor unit and moving unit. The anchor
unit represents the site of delivery from which the
force is applied to move the teeth, while moving
unit is the segment of arch that move in intended
direction.
Anchorage is not merely an anteroposterior
phenomenon, unwanted tooth movements can also
occur in the vertical and transverse dimensions. For
example, when the anterior teeth have been
retracted distally, a mesial reaction force would
acts on posterior teeth, by extrusion of incisors
there will be an intrusion reaction force on molars,
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Orthodontics …..........................................................................................................................................................Anchorage
Force-movement relationship
several anchor teeth. The amount of force on each
anchor tooth in simple anchorage is equal to the
total moving force component of the appliance
divided by the number of anchor teeth. It refers to
the PDL areas of the of the anchorage units is more
than that of the teeth to be moved.
There is a relationship for pressure within the
periodontal ligament to the amount of tooth
movement. Pressure in the PDL is determined by
the force applied to a tooth divided by the area of
the PDL over which that force is distributed.
Orthodontic tooth movement is proportional to the
magnitude of the pressure, i.e. tooth movement
increases as pressure increases, up to a point, at
which it remains at about the same level over a
broad range, and then may actually decline with
extremely heavy pressure. The lightest force, & the
resulting pressure within PDL, that produces a
maximum orthodontic tooth movement is called
the optimum force.
For example, movement of central incisor against
1st permanent molar only or against all the posterior
teeth.
II. Reciprocal anchorage
Movement of two teeth or two arch units of equal
size against each other. Here the PDL area of the
anchorage units is equal to that of the teeth to be
moved. The effect of the forces exerted is equal, i.e.
the two sets of teeth are displaced in the opposing
direction but by the same amount. For example,
moving of the two central incisors to close midline
diastema or expansion of maxilla by using a
midpalatal screw.
A principal strategy for anchorage control would be
to make the force, i.e. pressure within PDL of tooth
or teeth to be moved, within the optimal limit.
While dissipate the reaction force over as many
other teeth as possible, to keep the pressure in the
PDL of that (anchor) teeth as low as possible,
below the threshold of tooth movement, so that the
pressure would produce no movement.
Types of anchorage:
I. Simple anchorage.
II. Reciprocal anchorage.
III. Reinforced anchorage.
IV. Stationary anchorage.
V. Skeletal anchorage.
I. Simple anchorage
It means the active movement of one small tooth
versus large anchor tooth, or one tooth against
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Orthodontics …..........................................................................................................................................................Anchorage
III.
Reinforced anchorage
When the teeth moved bodily, the reaction force
distributed over a larger PDL area of the anchor
teeth reduces pressure there and results in a greater
anchorage potential.
Anchorage can be reinforced by making movement
of the anchor teeth more difficult by adding more
resistance units in the anchorage areas. For
example, the use of headgears along with routine
fixed mechanotherapy, or the use of a transpalatal
arch.
In removable appliance, the base plate, however,
by its close adaptation to the palate and to the teeth
that are not to be moved, offers appreciable further
anchorage.
V. Skeletal (absolute) anchorage.
It has long been realized that if structures other than
the teeth could be made to serve as anchorage, it
would be possible to produce tooth movement or
growth modification without unwanted side
effects.
With the development of successful bone implant
techniques, it could be used, as a structure other
than the teeth ,to serve as anchorage for orthodontic
tooth movement & that they provide what could be
described as absolute anchorage (i.e. with no tooth
movement except what was desired).
IV.
Skeletal (absolute) anchorage can be provided in
two major ways. Screws placed through the
gingiva into the alveolar bone, and Miniplate
anchors placed beneath the soft tissue, usually at
the base of the zygomatic arch, so that the posterior
teeth can be intruded or the anterior teeth retracted.
After soft tissues are sutured back over the
Stationary anchorage
Stationary anchorage is can be obtained by pitting
bodily movement of one group of teeth against
tipping of another. It is made by arranging the force
system so that the anchor teeth must move bodily
if they move at all, while teeth to be moved are
allowed to tip.
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Orthodontics …..........................................................................................................................................................Anchorage
miniplate, only the tube for attachment of springs
will extend into the oral cavity.
At the time of determining the space requirement
to resolve the malocclusion in a given case it is
essential to plan for space that is likely to be lost
due to the expected movement of the anchor teeth.
The anchorage requirement depends on:
A. The number of teeth to be moved:
The greater the number of teeth being moved
the greater is the anchorage demand. Moving
teeth in segments as in retracting the canine
separately rather than retracting the complete
anterior segment together will decrease the load
on the anchor teeth.
B. The type of teeth to be moved:
Teeth with large flat roots and / or more than
one root exert more load on the anchor teeth.
Hence, it is more difficult to move a canine as
compared to an incisor or a molar as compared
to a premolar.
C. Type of tooth movement:
Moving the teeth bodily requires more force as
compared to tipping the same teeth.
D. Periodontal condition of teeth:
With decreased bone support or periodontally
compromised teeth are easier to move as
compared to healthy teeth attached to a strong
periodontium.
E. Duration of tooth movement:
Prolonged treatment time places more strain on
the anchor teeth. Short-term treatment might
bring about negligible amount of change in the
anchor teeth whereas the same teeth might not
be able to withstand the same forces
adequately.
Anchorage planning:
The importance of anchorage is perhaps most
extremely appreciated when it has been neglected.
Anchorage loss may threaten a successful result
because inappropriate movement of the anchor
teeth results in insufficient space remaining to
achieve the intended tooth movements.
In some cases, anchorage loss (unintentional
movement of anchor unit) can result in a worsening
of the malocclusion, for example, during the canine
retraction phase in treatment of a Class II
malocclusion, forward movement of the anchor
teeth can result in an increase in overjet.
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