Transcript basic principles of orthodontic treatment

BASIC PRINCIPLES OF
ORTHODONTIC TREATMENT
• The teeth and their supporting tissues show lifelong ability to reposit themselves and adapt to
functional demands.
• It is ilustrated by the fenomenon of physiological
migration. It is well known that the teeth of the
side segments tend to migrate in a mesial
direction. There is also a tendency for continued
eruption if a balance is not established with the
antagonistic tooth, or if the balance is lost.
• By these means , eruptiom and migration,
throughout life the teeth will seek to establish the
best possible relationship between the jaws.
These continuous physiological processes are
affected by the growth of the craniofacial
skeleton and are sensitive to any type of presure
( pressure from muscles, soft tissues, oclusal and
functional factors or direct external forces ).
The great potential for dentoalveolar modification is
due to:
1. an extraordinary ability of the periodontal
membrane to remodel itself and
2. an adaptability of supporting alveolar
structures in response to movement of the
teeth
What is more, the basal parts of the jaws show
adaptive reactions to stimuli directed at growth
zones.
Orthodontic treatment may involve :
1. the control of forces physiologicaly acting upon
the teeth and associated structures or
2. producing and use external forces.
The goal for orthodontic treatment may be limited to
preventing or eliminating unwanted impulses ( i.e.
dysplastic muscle function ) by restraining such
forces from acting on the teeth or adjacent
supporting structures. Such a change in the
equilibrium of forces may lead to considerable
positional changes if continued over prolonged
period of time.
A tooth can be guided into position during eruption by
being subjected to occasional contact with an
inclined plane or a lightly activated element, while
more extensive tooth movement may be obtained
by subjecting the teeth, and eventualy also the
alveolr process, to direct external forces.
During the physiological tooth migration as the
orthodontic therapeutic movement the
characteristic tissue changes take place.
The bone in direction which tooth is moving is
resorbed while on the bone wall which the tooth is
moving away from an bone apposition occurs.
Among the fundamental problems that require
elucidation are following:
• Why is the alveolar bone resorbed during tooth
movement whilst the cementum remains intact?
• What protects the root surface?
• It is known that turnover rate of the bone tissue is
high. The bone system acts as a mineral reservoir
for the whole organism and there is permanent
circulation of minerals between the bone system
and inside environment of organism. The bone
tissue shows high ability to remodel itself
following the functional pressure on it.
• On the other hand the cementum is fully
maturated tissue, built up as a permanent
depository of mineral salts. But slow apposition
continues on the cementum surface throughout life.
This fact is of great
importance for the
resorptive mechanism.
The unmineralized
precementum layer
has been considered
to be a resorptionresistant coating
layer. It protects the
root surface and
permit physiological
tooth migration and
orthodontic tooth
movement
• The periodontal ligament, the conective tissue which
attaches the teeth to the alveolar bone, has also
ability to remodel itself. However, the turnover rate is
not uniform throughout the ligament. The cells are
more active on the bone side than near the
cementum, so that major remodelling take place
near the alveolar bone.
Physiological tooth migration
During the physiological migration the resorbing cells, called
osteoclasts, are seen in the scattered lacunae associated
with the resorptive surface. Resorptive surface is the
alveolar bone wall towards which the tooth is moving.
Physiological tooth migration
Unlike the osteoclastic resorption of bone to
provide the space for tooth movements, the
corresponding remodeling processes of the
fibrous attachement is not clearly
understood. There is a meshwork of collagen
fibres of small diameter present, which explaines
this rapid reorganisation process.
Physiological tooth migration
The alveolar bone wall which the tooth is moving away
from is characterized by osteoblasts depositing nonmineralized osteoid whichlater mineralizes in the deeper
layer.
Physiological tooth migration
• The older fibres of the periodontal membrane are
surrounded by newly deposited bone matrix and
become embedded in bone. Simultaneously, new
collagen fibrils are produced by the cells on the
bone surface. The sites of active lengthening
and rebuilding of the fibrous apparatus lie in
the middle of the ligament and near the
alveolar bone side. How this comes about is
unknown.
Orthodontic tooth
movement
Orthodontic forces are
usually more powerful
than normal functional
forces so response
elicited in the
periodontal ligament
is more marked and
extensive, although it
is the same in
principles as than seen
during physiological
migration.
Pressure side:
Application of a continuous
force on the crown of a tooth
will lead to a tooth movement
within the alveolous that is
marked initially by narrowing
of the periodontal
membrane, particularly in the
marginal area. This
compresion will impede the
vascular circulation and cell
differentiation. After a few
hours a certain reduction in the
number of cells may be
observed, indicating
a temporary slowing down of
cell renewal.
Pressure side:
After a few hours a certain reduction in the number
of cells may be observed, indicating a temporary
slowing down of cell renewal.
After a certain period of time, when conditions are
favourable, the cells will increase in number and
differentiate into osteoclasts and fibroblasts.
The width of the membrane is increased by
direct osteoclastic removal of bone and
orientation of the fibres in the periodontal
membrane will change.
Pressure side:
Pressure side:
• During the critical stage of
the initial application of
force, high compression in
some areas may cause
degradation of the cells and
vascular structures. The
tissue reveals a glass-like
appearance in light
microscopy, which is
termed hyalinization. It
represents a sterile necrotic
area.
In a hyalinized zone:
• the cells cannot differentiate into osteoclasts and
• no bone resorption can take place from the periodontal
membrane
• tooth movement will stop until the hyalinized structures has
been removed and the area repopulated by cells.
The process displays three main stages :
• degeneration
• elimination of destroyed tissue and
• establishment of the new tooth attachment
The hyalinization may be limited to parts of the membrane or
may extend from the root surface to the alveolar bone. Limited
hyalinization is almost unavoidable in the initial period of tooth
movement in clinical orthodontics. However, extended
hyalinisation areas may later cause root resorptions which may
lead to permanent root shortening.
The adjacent alveolar bone is removed by indirect resorption
by cells which have differentiated into osteoclasts on the
surface of adjacent marrow spaces.
Pressure side:
When the application of
force is favourable, direct
resorption of the alveolar
bone is likely to occur.
Large number of ostoclasts
will be seen along the bone
surface and tooth
movement will be rapid. The
fibrous attachment
apparatus will to some
extent be reorganized by
the production of new
periodontal fibrils, These
are attached to the root
surface and to those part of
the alveolar bone wall
where direct resorption is
not occurring.
Pressure side:
Pressure side:
Tension side
• The main feature is the
deposition of new bone on
the alveolar surface which
the tooth is moving away
from. Cell proliferation is
usually seen after 30-40
hours in young humans.
The original periodontal
fibres become embedded in
the new layers of pre-bone,
or osteoid, which
mineralizes in the deeper
parts. New bone is
deposited until the width of
the membrane has returned
to normal limits, and the
fibrous system is
remodelled.
Tension side
Tension side
Tension side
Tension side
Tension side
In order to maintain the
dimension of the supporting
bone tissue, concomitantly
with bone apposition on the
periodontal surface on the
tension side, an
accompanying resorption
process occurs on the
spongiosa surface of the
alveolar bone.
Correspondingly, during the
resorption of the alveolar
bone on a pressure side,
maintenance of the alveolar
lamina thickness is ensured
by apposition on the
spongiosa surface.
These processes are
mediated by the cells of
endosteum, which cover all
the internal bone surfaces,
marrow spaces, Haversion
canals and dental alveoli.
Extensive remodelling,
a reaction which tends to
restore the thickness of
supporting bone, takes
place in periosteum, in
deeper cell-rich layers.
As regards control of tissue reactions many
mechanisms have been considered responsible
for the differentiation of cells incident upon the
application of an orthodontic force.
Orthodontic tooth movement shows local traits of
a damage/repair process with inflammation-like
reactions:
• high vascular activity
• many leucocytes and macrophages
• involvement of the nervous and immune
systems
The forces in orthodontics should be very
precisely controled not to damage periodontal
ligament tissue, pulp of the teeth or cementum of
the roots.
As a response to high presure and very rapid
tooth movement may occur:
• the devitalization of teeth or
• root resorption
Since we wish our terapeutic movements to stay
within physiological limits, knowledge of
orthodontic forces needed in terms of magnitude
and duration is very important.
The critical question regarding orthodontic tooth
movement is whether direct resorption without
hyalinization areas take place on the alveolar
surface
It has been observed that a ligth force acting
over a certain distance moves a tooth more
rapidly than a powerful one, because there is
no need to eliminate necrotic hyaline tissue.
What is considered a light or powerful force
depends on:
• type and anatomy of the tooth to be moved
• architecture of the periodontal ligament and
the supporting bone
• type of the tooth movement and
• mode of force application
• The size, form, number and characteristic of
the roots will influence the mechanical resistance
to an external force. Thus cuspids or molars
require stronger force to move than incisors or
premolars.
• As regards the architecture of the periodontal
ligament and alveolar bone, it is closely related
to age. The number of cementoblasts, fibroblasts
and osteoblasts is much higher in young patients
than in adults, indicating higher activity.
The necessary increase in cell numbers during the
initial phase of the application of force in adults
occurs more slowly and is more critical than in
young individuals, and the deposition of the
osteoid is similarly slower and less extensive.
In addition the type of bone
through which the tooth is
displaced must be considered
in the treatment plan. The
alveolar process consists of :
• the dense outer cortical
bone plates and
• spongious or cancellous
bone between them
The movement of the tooth is
more difficult and slower in
the cortical dense bone than
in spongious bone.
In general the bone is more
dense in side segments than
anteriorly, and in the mandible
than in maxilla.
When a tooth is moved
into the reorganizing
alveolus of a newly
extracted tooth,
remodeling is very
rapid, due to the many
differentiating cells
present and to the limited
amount of bone to be
resorbed.
Despite these facts,
individual variations in
alveolar bone architecture
are considerable
• The magnitude of the force needed depend also on
type of the tooth movement wanted. ( i.e. intrusion
or extrusion requires very light forces while bodily
movement of a tooth requires stronger force).
• The mode of application and the mechanical
arrangement of the recipient tooth units are also of
importance. A local force intended to move an
individual tooth should be only a small fraction of
a force which is applied against full dental arch,
where all teeth are united into a block.
•
The magnitude of a force depends also on its
duration.
We distinguish:
1. continuous forces
2. continuous, but interrupted after a limited period
( forces working over a short distance, typicaly
exemplified by a tooth ligated to a labial arch wire)
3. intermittent forces, mainly induced by removable
plates
4. intermittent forms of a functional type, induced
by functional appliances, transmitting muscular
activity into impulses directed at the teeth and
alveolar processes
The strong continuous force is unwanted because
it may lead to considerable injury.
Interupted continuous forces create favourable conditions for
further tissue changes.
Since the force decreases rapidly, despite inicial hyalinisation,
the tissue will readily be reorganized.
In case of intermittent application , frequent discontinuation
provokes increased vascular circulation and cell proliferation