Copy of biology and biomechanics /certified fixed orthodontic courses by Indian dental academy

BIOENGINEERING
PRINCIPLES IN
ORTHODONTICS
INDIAN DENTAL ACADEMY
Leader in continuing dental education
www.indiandentalacademy.com

CONTENTS
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INTRODUCTION
PERIODONTAL LIGAMENT
ALVEOLAR BONE
TOOTH MOVEMENTS 1. PHYSIOLOGICAL
2. ORTHODONTIC
THEORIES OF TOOTH MOVEMENTS
EFFECTS OF FORCE MAGNITUDE
FACTORS EFFECTING ORTHODONTIC TOOTH
MOVEMENT
EFFECT OF DRUGS ON RESPONSE TO
ORTHODONTIC FORCES.

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DELETERIOUS EFFECTS OF ORTHODONTIC
FORCES
TYPES OF TOOTH MOVEMENTS
MECHANICAL PRINCIPLES IN FORCE CONTROL
PROPERTIES OF ELASTIC MATERIALS
FACTORS AFFECTING ELASTIC PROPERTIES
FORCE,MOMENT AND COUPLES IN TOOTH
MOVEMENT
SYSTEM EQUILIBRIUM
SEGMENTED AND CONTINUOUS ARCH
MECHANICS
CONCLUSION.

INTRODUCTION


Orthodontic therapy depends upon the reaction
of the teeth, and more generally the facial
structures to gentle but persistent force. The
main purpose of presenting a discussion on the
biophysical principles of tooth movement is to
know the facts and histological findings that
have a bearing on practical orthodontics.


biomechanics
is commonly used in discussions of the
reaction of the dental and facial structures
to orthodontic force,

 In the orthodontic context,



whereas mechanics is reserved for the
properties of the strictly mechanical
components of the appliance system.



“Tissue consciousness ” is a vital prerequisite

to mechanics. There are available today potent
tooth-moving appliances that can accomplish
almost any desired change, but if their use is not
controlled by a profound respect for the
biological media in which they work, then
tremendous harm can be done.


The forces are applied to the teeth with the
objective of getting desired tooth movement, in
the desired direction, in the desired amount of
time. Thus it is obvious that a sound biological
understanding of the orthodontic tooth
movement is a must.

PERIODONTAL
LIGAMENT


The pdl is the soft specialized connective tissue

situated between the bone forming the socket wall
and the cementum covering the root surfaces.
It ranges in width from 0.15 to 0.38mm, with its
thinnest portion around the middle third of the root.
Like any other connective tissue it consists of cells
and an extra cellular compartment of fiber and ground
substance.



The cells include
 Osteoblasts and osteoclasts
 Fibroblasts
 Epithelial cells of malasses
 Macrophages
 Undifferentiated mesenchymal cells
 Cementoblasts

The extra cellular compartment:
 collagen and
 oxytalan fibers
 embedded in ground substance
consisting mainly of glycosaminoglycans,
glycoproteins and glycolipids






The vast majority of the collagen fibrils in the
periodontal ligament are arranged in definite and
distinctive fiber bundles. These fiber bundles are
arranged in groups and are sometimes called
the principal fibers of the ligament.
At either end all the principal collagen fiber
bundles of the pdl are embedded into cementum
or bone. The embedded portion of the fibers is
called the Sharpeys fibers.












The alveolar crest fibers : attached to the cementum
just below the CEJ and running downward and outward to
insert into the rim of the alveolus.
The horizontal group : occurring just apical to the
alveolar crest group and running at right angles to the long
axis of the tooth from cementum to bone just below the
alveolar crest.
The oblique group : by far the most numerous in the
ligament and running from the cementum in an oblique
direction to insert into bone coronally.
The apical group : radiating from the cementum around
the apex of the root to the bone, forming the base of the
socket.
The inter-radicular group : found only between the roots
of multirooted teeth and running from the cementum to the
bone forming the crest of the inter –radicular septum.

FUNCTIONS OF THE PDL
Physical functions :
–
–
–
–
–

Transmission of occlusal forces to the bone
Attachment of teeth to the bone
Maintenance of the gingival tissues in proper
relationship to the teeth
Resistance to the impact of occlusal forces
Provides a soft tissue housing to protect the
vessels and nerves from injury by
mechanical forces.

Formative functions:
The undifferentiated cells in the pdl
serve as precursors for the cementum and bone
forming cells. In fact they play a key role in bone
remodeling.
Nutritional functions :
By the way of blood vessels that
traverse, the pdl supplies nutrients to the
cementum, bone and gingival for their metabolic
activities. It also provides lymphatic drainage.

Sensory functions :
The innervations of the pdl provide
propioceptive and tactile sensation, which detect and
localize external forces acting upon individual teeth
and serve an important role in neuromuscular
mechanism controlling the masticatory musculature.
Other functions :
– Through the formation, cross linkage and
maturational shortening of collagen fibers, it helps
in eruption of teeth.
– The metabolic activities occurring within the pdl
maintain the teeth in position even though the
forces acting from extraoral and intraoral muscles
are not balanced.

ALVEOLAR BONE




The alveolar process is that bone of the jaws
that contains the sockets (alveoli) for the teeth
and consists of outer cortical plates, a central
spongiosa and bone lining the alveolus.



The cortical plate and the alveolar plate and the
bone lining the alveolus meet at the alveolar
crest, usually 1.5 to 2 mm below the level of the
CEJ of the tooth it surrounds.







The bone lining the alveolus is specifically called the
bundle bone because it is this bone that provides
attachment for the pdl fibers. It is perforated by many
foramina that transmit nerves and vessels and is
therefore sometimes referred to as the CRIBRIFORM
PLATE . It is also called as the lamina dura because
of its increased radio opacity.
The cortical plate consists of surface layers of fine
fibered lamellar bone supported by compact
Haversian system bone of variable thickness. The
trabecular or spongy bone occupying the central part
of the alveolar process also consists of fine-fibered
membrane bone dispersed in the large trabeculae.
The important part of this complex in term of
tooth support is the bundle bone.

TOOTH MOVEMENT


To a layperson the most rigid thing in the body is his
set of teeth. He accepts the fact that they can wear
down over the years but if they move he expresses
alarm. He knows nothing about the cushioning
connective tissue, the periodontal membrane that is
as vital as any tissue in the body. He does not know
that bone is a vital tissue and also undergoes
constant reorganization; that teeth move constantly
and imperceptibly through out life


Physiological tooth movement


designates primarily, the slight tipping of the
functioning tooth in its socket and, secondarily,
the changes in tooth position that occur in young
persons during and after tooth eruption.



The minor changes in tooth position observed in
growing persons and adults are usually called
tooth migration. Tooth migration in both young
and older persons is always related to definite
tissue changes that can be readily observed in
histological sections



With the wearing away process teeth continue to
erupt. Contacts are worn and contact points
become contact surfaces. Mesial drift
compensates for the space created, and as the
tooth moves the socket shifts with the tooth. Bone
is resorbed ahead of the drifting tooth and
deposited behind it.



Resorption is seen as an uneven scalloped margin,
with the presence of osteoclasts. Bone deposition
appears histologically as concentric lamella of
bundle bone laid down in the presence of and with
the aid of the bone-building cells the osteoblasts.



As the alveolus move leaving space for the tooth
and the pdl, bony reorganization outside the
alveolus occurs. Ahead of the moving tooth,
trabaculae show resorption on the side nearest
the moving tooth, deposition of bone on the side
farther away. Behind the moving tooth bone is
deposited on the side away from the tooth to
maintain a constant length of the trabecular
structure.



The osteoblast first lay down an organic matrix known
as the osteoid. This then becomes calcified as calcium
salts are deposited in the matrix. The newly calcified
tissue is called bundle bone and is basophilic in
appearance. The staining properties of bundle bone
are related to its high content of cementing substance,
consisting essentially of highly polymerized connective
tissue polysaccharides.




Cells and fiber bundles will be incorporated in bundle
bone during its life cycle. When it has reached a
certain thickness and maturity, parts of the bundle
bone will reorganize into lamellated bone with fine
fibrils in its matrix. The lamina dura will subsequently
reappear as a somewhat thinner radio opaque line.



This sequence of events is, in principle, the same as
that in bone formation after orthodontic tooth
movement.





It has been established beyond doubt that bone is
biologically plastic and adaptive to developmental and
functional forces, responding to pressure with
resorption and to tension with bone deposition. It is the
property of the teeth to move and reflect various
environmental influences by positional modifications
throughout life that the orthodontist uses to move teeth
to the desired new position. Alveolar bone has been
referred to as “the slave of the orthodontist ”.
The essential processes are there and at work before
he attempts guided tooth movement by mechanical
appliances. The bony response is primarily mediated
by the periodontal ligament, and so the tooth
movement is believed to be primarily a periodontal
phenomenon.

ORTHODONTIC TOOTH
MOVEMENTS




Theoretically it should be possible to bring about
tooth movement without any tissue damage by
using a light force, equivalent to the
physiological forces determining tooth position,
to capitalize on the plasticity of the supporting
tissues.
However most current orthodontic techniques
do not duplicate the ideal situation; most involve
some degree of tissue damage that varies
because the forces applied to move the tooth
are not equally distributed throughout the pdl

The orthodontic response to light, continuous
load is divided into three elements of tooth
displacement:


Initial strain :
occurs in about one week. The displacement
produced is about 0.4- 0.9 mm and is due to the
pdl displacement, bone strain and extrusion. The
fluid mechanics of root displacement in the pdl
probably accounts for about 0.3mm of crown
movement.



Lag phase :
the displacement of the tooth relative to its
osseous support stops in about one week. This
occurs due to areas of the pdl necrosis
(hyalinization). This phase is called the lag
phase. It varies from about 2-3 weeks and may
be as long as 10 weeks. The duration of the
lag phase is directly related to the patient’s
age, density of alveolar bone and extent of pdl
necrotic zone.



Progressive tooth movement :
after undermining resorption, vitality is restored
to the necrotic areas of the pdl, and the tooth
movement enters the secondary or progressive
tooth movement phase. Frontal resorption in the
pdl, and initial remodeling events in the cortical
bone ahead of the advancing tooth allow for
progressive tooth movement at a relatively rapid
rate

The duration of tooth movement can
be divided into two periods:

 Initial

stage :

when a constant orthodontic force is maintained on
the tooth, compression of the pdl occurs. This
causes degradation rather than causing
proliferation and differentiation. The tissues reveal
a glass like appearance when viewed in light
microscopy and is termed as Hyalinization .

HYALINISATION
 it is an unavoidable phenomenon in the

initial period of tooth movement. It is partly
caused by anatomic and partly by
mechanical factors. It is a sterile necrotic
area and is limited to 1-2 mm in diameter.


The process displays three main stages:
 Degeneration :

it starts when the pressure is the highest and
narrowing of the membrane is more pronounced.
There is retardation of blood flow followed by
disintegration of the vessel walls and degradation of
blood elements. Cells rupture, the nuclei breakdown
leaving unidentifiable cellular elements between the
collagen fibrils. In the hyalinised zone, cells cannot
differentiate into osteoclasts and so no resorption
occurs. Tooth movement stops.

 Elimination

of destroyed tissue :

Elimination of the hyalinised zone occurs by
two mechanisms
1. Resorption of the alveolar bone by
osteoclast
2. Invasion of cells and blood vessels from the
periphery of the compressed zone by which
the necrotic tissue is removed




Re establishment of tooth attachment :
this phase starts by the synthesis of new tissues
as soon as the adjacent bone and degenerated
membrane tissues have been destroyed. The
ligament space is wider than before treatment
and the membranous tissue under repair is rich
in cells. The pdl is reconstructed in the
hyalinised areas.



. Secondary stage of tooth movement :
the pdl is considerably widened. Osteoclasts attack
the bone over a wider area. Further bone resorption
occurs when force is kept constant and within
limits. New periodontal fibers are produced and the
fibrous attachment apparatus is reorganized. A
large number of osteoclasts are seen along the
bone surface and tooth movement is rapid.
Deposition of bone occurs on the alveolar surface
from which the tooth is moving away until the width
of the membrane has returned to normal limits.


THEORIES OF TOOTH
MOVEMENT


Two main theories that have been proposed and are
accepted to play a part in the biologic control of tooth
movement. They are


The Bioelectric theory that relates the tooth
movement in part to changes in the bone metabolism
controlled by the electric signals that are produced
when alveolar bone flexes and bends.



The Pressure Tension theory which relates
tooth movement to cellular changes produced by
chemical messengers, traditionally thought to be
generated by alterations in blood flow through the
pdl.

THE BIOELECTRIC THEORY


The electric signals that bring about initial tooth
movement are piezoelectric. Piezoelectricity is a
phenomenon observed in many crystalline materials
in which a deformation of the crystal structure
produces a flow of electric current as electrons are
displaced from one part of the crystal lattice to
another.
Bone is crystalline in nature and both bone and
collagen exhibit peizoeletric effect.

Piezoelectric signals have two unusual
characteristics:




A quick decay i.e.; when a force is applied, a
piezoelectric signal is created in response that quickly
dies away to zero even though the force is maintained.
The production of an equivalent signal, opposite in
direction, when force is released


When the crystal structure is deformed, electrons
migrate from one location to another and an electric
charge is observed. As long as the force is
maintained, the crystal structure is stable and no
further electric events are observed. When the force
is released, however, the crystal returns to its
original shape, and a reverse flow of electrons is
seen.
With this arrangement, rhythmic activity would
produce a constant interplay of electric signals,
whereas occasional application and release of force
would produce only occasional electric signals.

The action of any force causes minute distortions in a
bone. This leads to regional changes in configuration
involving localised surface concavities and
convexities.A concavity results in compression and a
negative surface charge, and a convexity causes
tension and a positive surface charge. This triggers
bone deposition and resorption, respectively, by the
peizo effect acting on surface cell receptors of
osteoblasts and osteoclasts. The bone thereby
remodels until biomechanical and bioelectric
neutrality is attained.

If an existing concave surface becomes more
concave, the effect is active compression and the
action response thereby depository. If an existing
concave surface becomes less concave, the action is
less compressive and a direction towards tension is
seen, the resultant response is resorption. If a convex
surface becomes either more or less convex, similarly,
the results are believed to be resorption and
deposition, respectively.




A second type of electric signal, which is called the
Bioelectric potential , can be observed in bone that
is not being stressed. Metabolically active bone or
connective tissue cells produce electronegative
charges that are generally proportional to how active
they are. Inactive cells and areas are nearly
electrically neutral.
This potential can be altered by applying an external
electric field.The effects are felt in the cell membranes.
Membrane depolarization triggers nerve impulses and
muscle contraction, but changes in membrane
potentials accompany other cellular responses as well.
The external electric signals probably affect cell
membrane receptors, membrane permeability, or both.

Experiments indicate that when low voltage direct
current is supplied to the alveolar bone, modifying the
bioelectric potential, a tooth moves faster than its
control in response to an identical spring.
Electromagnetic fields also can affect cell membrane
potentials and permeability, and thereby trigger
changes in cellular activity.


PRESSURE TENSION THEORY


The pressure tension theory, the classic theory of
tooth movement relies on chemical rather than electric
signals as the stimulus for cellular differentiation and
ultimately tooth movement.
In this theory, an alteration in blood flow within the pdl
is produced by the sustained pressure that causes the
tooth to shift position within the pdl space,
compressing the ligament in some areas while
stretching it in others. Blood flow is decreased where
the pdl is compressed, while it usually is maintained or
increased where the pdl is under tension.

Alterations in blood flow quickly create changes in
the chemical environment. For instance, oxygen
levels certainly would fall in the compressed area,
but might increase on the tension side, and the
relative proportions of other metabolites would also
change in a matter of minutes. These chemical
changes, acting either directly or by stimulating the
release of other biologically active agents, would
stimulate cellular differentiation and activity.


In essence, this view of tooth movement
shows three stages:
 Alterations in the blood flow associated

with pressure within the pdl,

 The formation andor release of chemical

messengers, and

 Activation of cells.

EFFECTS OF FORCE
MAGNITUDE


When

< 1 sec

heavy pressures are applied :
Pdl fluid incompressible, alveolar
bones bends, piezoelectric signals
generated.

1-2 sec

Pdl fluid expressed, tooth move
within the pdl space

3-5 sec

Blood vessels within the pdl
occludes on the pressure side.

Mins

Blood flow cut off to the
compressed pdl area.

Hours

Cell death in the compressed area

3-5 days

Cell differentiation in adjacent
marrow spaces, undermining
resorption begins

7-14 days

Undermining resorption removes
lamina dura adjacent to compressed
pdl, tooth movement occurs.

When
< 1 sec

light pressure is applied :
Pdl fluid incompressible,
alveolar bone bends, piezoelectric
signal generated.

1-2 sec

Pdl fluid expressed, tooth moves
with the pdl space

3-5 sec

Blood vessels in the pdl partially
compressed on the pressure
side, dilated on the tension side,
pdl fibers and cells mechanically

destroyed

Mins

Blood flow altered, oxygen tension
begins to change, prostaglandin
and cytokines released.

Hours

Metabolic changes occurring,
chemical messengers affects
cellular activity, enzyme levels
change

--4 hrs

Increased cAMP levels, cellular
differentiation begins within the pdl

--2 days

Tooth movement beginning as
osteoclastsosteoblasts remodel
bony socket.

.

FACTORS INFLUENCING
ORTHODONIC TOOTH
MOVEMENT


Local tissue reactions are influenced by
 the anatomic characteristics of the
supporting bone into which the tooth is to
be moved,
 the physiologic activity of the tissues that
surround the tooth and
 the force application


Character of bone
Remodeling processes in bone depend on the activity
of the cells that act on its surfaces. Thus alveolar
bone that is penetrated by numerous canals to
transmit blood vessels and contains cancellous bone
with marrow spaces at its deeper aspect is favorable
for tooth movement.
On the other hand, if the bone involved is compact in
nature, that is cortical bone, then the surface area
where cellular activity can take place is greatly
reduced. Here tooth movement is more difficult and
slower, and the chances of creating over
compression and greater areas of hyalinization are
much higher.

Thus it is important that when planning orthodontic
treatment, the tooth should remain in spongy bone
during movement.
Extraction spaces contain tissues undergoing
reconstruction, which is rich in cells and vascular
supply. Such an area is ideally suitable for tooth
movement, and due advantage of this should be taken
by commencing treatment as soon as possible
following extraction. Thereby one also avoids atrophy
and narrowing of the alveolar process, resulting in
bone loss and cortical bone formation at the extraction
site.

Physiologic activity
The strong relapse tendency seen after the orthodontic
rotation of teeth is thought to be the result of slow turn
over of the gingival fibers mainly the supra-alveolar
fiber bundles. Turn over varies from person to person
and depends on a number of variables such as
hormonal balances, age of the patient and health of the
patient. Therefore it is necessary to consider these
variations during treatment planning, especially if the
patient is receiving medications like steroids or anti
epileptics, as the threshold for tissue changes or
cellular reactions will be influenced.

Force applications applied force
and time
key to orthodontic tooth movement is application of
light and sustained force, which does not mean that
the force must be continuous, but it must be present
for a considerable percentage of time. Experiments
have shown that the threshold for orthodontic tooth
movement in humans is 4-8 hours.


Orthodontic force duration is classified by
the rate of decay as
 Continuous
 Interrupted
 intermittent


Continuous forces


force maintained at some appreciable fraction of the
original from one patient visit to another. Continuous
force leads to gradual compression of the pdl on the
pressure side of the tooth. If the force is within the
limitations where tissue reactions occur,
reconstructional changes of the fibrous element as
well as direct resorption of the alveolar bone wall take
place


Interrupted forces


force levels decline to zero between activations.
Here even if the hyalinised zones are
established, the pdl has the time to become
reconstructed. There is an increase in cell
proliferation, which is suitable for further tissue
changes following reactivation of the force.
Fixed appliances that are constantly present
on the tooth can produce both continuous and
interrupted forces.

Intermittent forces


force levels decline abruptly to zero intermittently,
when the orthodontic appliance is removed by the
patient or when a fixed appliance is temporarily
deactivated. On the pressure side, the circulation will
not be as easily disturbed or hindered unless the force
applied is too high. The intermittent force is thought to
act as an incitement to cell proliferation. Increase in
the cell numbers and direct bone resorptions along the
alveolar bone wall are characteristic of this type of
tooth movement. The periodontal space increases
because the tooth tends to return to its original
position following the removal of the force.

In spite of the favorable condition on the side where
resorption is seen, tooth movement often will be
slower than that seen during application of
continuous force, as the time over which the
appliance is used is a very important factor.
Formation of new tissue and apposition of bone are
seen to occur more rapidly under active or constant
stretching. Therefore, if the tooth is often allowed to
return to its original position, one can expect a
limited amount of apposition to occur.

PRE-ERUPTIVE TOOTH
MOVEMENTS
These are made by both the deciduous and the
permanent tooth germs within the tissues of the
jaw before they begin to erupt.
The deciduous teeth germs are extremely small
and have enough space in the developing jaw. But
as they grow rapidly, they become crowded
together. This is alleviated by the lengthening of
the jaws, permitting the second molar tooth germs
to move backwards and anterior tooth germs
forward.

At the same time the tooth germs are bodily moving
outwards and downwards or upwards.
The permanent tooth germs develop lingual to the
primary ones in the same bony crypt, except for the
molars which develop from the distal extensions of
the dental lamina.
The canines and incisors gradually shift to occupy a
position in their own bony crypts,on the lingual of the
roots of their predecessors, while the premolar tooth
germs are finally positioned between the divergent
roots of the deciduous molars.

Due to lack of space, the upper permanent molar
tooth germ develop with their occlusal surfaces
facing distally and later swing into position only when
the maxilla has grown sufficiently to provide room for
such movement.
In the mandible, the molars develop inclined towards
the mesial direction, which becomes vertical only
when sufficient growth has occurred.


As these preeruptive movements occur in an intraosseous location,such movement is reflected in the
patterns of bony remodeling within the crypt wall.eg,
during bodily movement in a mesial direction, bony
resorption occurs on the mesial surface of the crypt
wall, and bony deposition on the distal wall as a
filling in process.
During eccentric growth only bony resorption occurs,
thus altering the shape of the crypt to accommodate
the altering shape of the tooth germ.


BONE’S STRUCTURAL
ADAPTATIONS TO
MECHANICAL USAGE




In health and disease the architecture of a whole bone,
such as the mandible, depends on both cartilage and
bone.



Some general cartilage roles
a) In children, cartilage growth determines a bone’s
length and a joint’s shape, size and alignment.





b) During growth a cartilage layer at the bony
attachments of fascia, ligaments and tendons controls
the local growth, and migration during growth, of those
attachments. This includes the mandibular insertions of
the masseter, pterygoids and temporalis



Some general bone roles:
a)      Bone provides rigid levers for muscles to act
on, and support for joints and teeth.



b)      Lamellar and woven bones serve somewhat
different purposes and can respond differently to
mechanical and nonmechanical influences.



c)      Modeling drifts and remodeling BMUs (basic
multicellular units) can each result in bone turn
over, alter the shape and size of bone.
Each can also respond in its own way to aging,
hormones, disease, drugs and mechanical
influences.

A load (force) on a bone deforms or strains it. This
stretches intermolecular bonds in the bone that
resist with an elastic force called a stress. Living
bone may depend more on strain than stress to
generate the signals that control its biological
reactions to mechanical loads.


Modeling and Remodeling:
Two bone-biologic activities can affect a bone’s
architecture.
Modeling by resorption and formation can move a
bone’s surfaces in tissue space to shape and size it.
Remodeling by BMUs (Basic Multicellular Units) can
turn bone over in small packets. Each activity can
respond in its own way to mechanical and other
influences.


Bone modeling:
Osteoblasts in formation drifts can form new bone
on large regions of periosteal, cortical-endosteal
and trabecular surfaces. Osteoclasts in resorption
drifts can resorb bone from other similar surfaces.
Various stimuli can trigger bone modeling or drift.
These drifts usually maintain a bone’s shape while
it increases in size. Such drifts also move tooth
sockets around in the mandible and maxilla in
response to orthodontic forces.

A) Lamellar or woven bone can each provide formation
drifts on periosteal, cortical-endosteal and trabecular
surfaces, but larger stimuli are needed to make woven
bone form than lamellar bone.
B) Woven bone can appear in fracture healing, some
neoplasm, infections, and in reaction to large
mechanical loads. It can arise in the marrow cavity
ahead of a tooth socket containing a tooth subjected to
excessive orthodontic forces– undermining resorption.
Woven bone drifts can add bone much faster than
lamellar drifts.
C) Lamellar drifts can thicken or thin a cortex or
trabecula no more than about 2 mm/year, a limit that
may decrease with age.

Macromodeling, minimodeling and
micromodeling:
Drifts control if, when, where and how much bone
formation and resorption happen. The naked eye can
see their effects so they provide macromodeling. On
trabeculae these drifts provide minimodeling, since it
takes magnification to see them. During any bone
formation a different, cell-level activity determines the
microscopic organization and “grain” of the new tissue.


It organizes lamellar and woven bone differently. It
always aligns lamellar bone’s grain parallel to the
major compression or tension loads on it while it was
forming. Therefore lamellar bone’s grain in a
mandible, tooth socket, maxilla or femur can show
the orientation of the major mechanical loads on it
during its formation.
Bone’s structural adaptations to mechanical usage
(SATMU) respond to some average of many strains,
not to single ones, and large strains influence
modeling much more than small ones no matter how
frequent.

In sum: By moving bone surfaces in tissue space, global
(refers to bone as a whole) modeling can increase but
not decrease bone mass and strength. Decreased
modeling simply slows down such increases. Obviously
single resorption drift must remove bone locally.
Remodeling:
Small “packets” called BMUs (Basic Multicellular Units)
provide bone remodeling, as distinguished from the
modeling described above. In an Activation-ResorptionFormation (ARF) sequence a BMU replaces some old
bone with new bone, to create a new bone packet or
Basic Structural Unitwww.indiandentalacademy.com
(BSU).

The BMU “ rho fractions ”:
A completed BMU can resorb more, or less, or equal to
the bone than it makes. Let rho equal any such deficit
or excess of resorption over formation. When more
bone is formed than removed then it is said to be
having a positive rho and the vice versa.
Remodeling happens on periosteal, haversian, corticalendosteal and trabecular surfaces or “envelopes”
Normally rho may be positive only on the periosteal
envelope, where completed BMUs may resorb a bit less
bone than they make.

Rho approaches zero on the haversian envelope, where
net resorption and formation tend to equalize.
Rho is usually negative on cortical-endosteal and
trabecular surfaces where BMUs usually resorb a bit
more bone than they make throughout life.
Global remodeling can remove or conserve bone but
apparently cannot add to it. Increased remodeling tends
to remove bone next to marrow and make a bone
weaker. Decreased remodeling tends to conserve bone
and its strength.

Microdamage and its thresholds:

Mechanical fatigue damage (microdamage) normally
occurs in bone in life. Remodeling BMUs usually repair the
damage and keep it from accumulating. This is done by
removing and replacing the damaged bone with new bone
Overloading the bone can increase microdamage and
remodeling that repair it.


The microdamage threshold:
When loaded below about 2000 mE (microstrain),
BMUs can easily repair what little microdamage
occurs.
At and above 4000 mE enough microdamage can
occur to overwhelm the repair mechanisms,
resulting in accumulations of damage that can
cause fatigue failures of trabeculae or whole bones.
In this 2000–4000 mE range, merely doubling the
size of the strains can increase microdamage
hundreds of times.

Strains lesser than 1500mE( MESm ) trigger lamellar
drifts. The largest normally allowed peak bone strain
lies below the 1500mE range.
Strains in the range of 3000-4000mE(MESp) and
above usually trigger woven bone formation.
Strains above MESm approaching 3000mE(MESr)
increase bone microdamage, which then increases
BMU creations to repair it.
Lamellar drifts add,reshape and strengthen bone,
thus reducing future strains under the same
mechanical load towards that strain region. Woven
bone drifts suppress lamellar drifts ,but strengthen
bone faster. However strains at this range also
increases microdamage alarmingly.

Orthodontic forces above the peak bone strain can
cause damage to the teeth and the sockets.
The nonmechanical factors that can influence
modeling and remodeling include hormones, vitamins,
drugs, disease, inflammation (including infection),
genetics (including race and species), nutrition, climate
and occupation.
Errors in the bones structural adaptation to mechanical
usage by modeling and remodeling can and do cause
skeletal disease and problems encountered in
orthopedic and maxillofacial surgeries, orthodontics and
dentistry.

LATEST CONCEPTS
OF FORCE MAGNITUDE


Martina Von Bohl et al., did a study on beagle dogs
to evaluate histological changes in the periodontal
structures after using high and low continuous forces
during experimental tooth movement.
The aim of this study was to evaluate the rate of
tooth movement and tissue reaction after
standardized application of low and high orthodontic
force that lead to low and high pressures in the pdl
of different teeth within one experimental animal.


In the past many authors have described the formation
of hyalinization zone in the pdl as a result of localized
ischaemia. They reported that, after excessive
compression of the pdl, blood supply is cut off which
leads to necrosed areas and arrest of tooth movement.
Removal of the necrotic tissue and bone resorption
from adjacent marrow space allow the resumption of
tooth movement. In the absence of necrotic areas,
cells start remodeling process at the tension site, and
rate of tooth movement increases.
The outcome of the new study are contradictory to
this commonly accepted theory.

An orthodontic appliance was placed on second
premolar and first molar by exerting a continuous
and constant force of 25 gm on one side and 300
gm on the other side of the mandible. Tooth
movement was recorded weekly. Dogs were
sacrificed after one, four, twenty, forty and eighty
days for histological evaluation.
The results showed large individual
differences in the rate of tooth movement after using
high or low forces and that the force level had no
influence on the amount of tooth movement.

Hyalinization was not only found in the phase of arrest,
i,e between 4 and 20 days, but also after 40 and 80
days of tooth movement. This suggest that the
development and removal of necrotic tissue is a
continuous process during tooth displacement instead
of a single event.
It was also found that the location of the
hyalinization zone was different from those of earlier
reports. They were not found in the area of the central
plane but lingually and bucally from it. This is probably
the consequence of local stress and shear
concentrations caused by local irregularities in the
bone morphology. www.indiandentalacademy.com

The other significant finding of this study is that the
teeth on which high forces were applied did not move
faster than the ones displaced by low forces.
Areas of hyalinization were found to be more in
the tooth displaced with higher forces but these
areas were present in both situations throughout the
tooth movement. The appearance of the necrotic
tissue might be related to force magnitude, but
seems to have no significance for the rate of tooth
movement. This means that once tooth movement
has started, bone remodeling takes place at a certain
rate, independent of force magnitude.

Furthermore, the data show a large individual
variation which could be due to differences in
bone metabolic capacity. Bone density,
morphologic differences and genetic factors
could also influence the remodeling process and
subsequent tooth movement.


EFFECTS OF DRUGS ON THE
RESPONSE TO
ORTHODONTIC FORCE


Agents that stimulate tooth movement are
rare but under some circumstances vitamin
D administration can enhance response to
orthodontic forces.
Direct injection of prostaglandin into the pdl
has shown to increase the tooth movement,
but this is very painful and not practical.

Two types of drugs are known to depress the response
to orthodontic forces:
- Biophosphates - used in treatment of osteoporosis.
Osteoporosis is a problem seen mostly in postmenopausal females but may be seen in both the
sexes with aging. Thus, medication for this purpose is
seen with adult orthodontic patients. Estrogen therapy
used for the same condition has little or no impact on
ortho treatment. Therefore if a patient taking
biophosphates for treatment of osteoporosis comes for
orthodontic taerapy, it would be worthwhile to discuss
with her physician the possibility of switching over to
estrogen temporarily.

- Prostaglandin inhibitors :
Since prostaglandins play an important role in
chemical mediation of tooth movement, inhibitors of
its activity would affect movement. Drugs that affect
the PG activity fall into two main categories:
1.Corticosteroids and NSAIDs
2.Agents that have mixed agonist and antagonist

effects on various PGs.


Steroids reduce the synthesis of PG by inhibiting
formation of arachidonic acid,the precursors for
PG, whereas the NSAIDs act by inhibiting the
conversion of arachidonic into PGs.
Fortunately only potent PG inhibitors like
indomethasin used for treatment of arthritis
interfere with tooth movement, while the common
analgesics seem to have little or no inhibiting
effect on tooth movement at the dose levels used
with orthodontic patients.

DELETERIOUS EFFECT OF
ORTHODONTIC FORCE


Pain
If heavy pressure is applied to a tooth, pain develops
immediately as the pdl is literally crushed.
If appropriate orthodontic force is applied, the patient
feels little or nothing immediately. Several hours later,
patient feels a mild aching sensation which lasts for 2
to 4 days, then disappears until the orthodontic
appliance is reactivated.
The tooth is quite sensitive to pressure. This suggests
inflammation at the apex, and the mild pulpities that
usually appears soon after orthodontic force is applied
probably contributes to the pain

If the source of pain is ischemic areas, strategies to
temporarily relieve pressure and allow blood flow
through the areas should help.
If light forces are used the amount of pain to the patient
can be decreased by having them engage in repetitive
chewing of gum or plastic wafer placed between teeth
during the first 8 hours after the orthodontic appliance
is activated.
Presumably this works by temporarily displacing the
teeth enough to allow some blood flow through the
compressed areas, thereby preventing build-up of
metabolic products that stimulate pain receptors.


Mobility

Orthodontic tooth movement requires both remodeling of
bone and reorganization of the pdl itself. Fibers become
detached from the bone and cementum, then reattach
later. Radiograpically it can be observed that the pdl
space widens during ortho tooth movement leading to
some mobility.
A moderate increase in mobility is an expected response
to orthodontic tooth movement. The heavier the force,
greater the amount of undermining resorption expected,
greater the mobility that will develop. If a tooth becomes
extremely mobile during treatment, it should be taken out
of occlusion and all forces should be discontinued until
the mobility decreases to moderate levels .

Effects on pulp
Although pulpal reactions to orthodontic treatment are
minimal, there is probably a moderate and transient
inflammatory response within the pulp, which
contributes to the discomfort that the patients feel for
the first few days after appliance activation.
There are occasional reports of loss of tooth vitality
during ortho treatment. If a tooth is subjected to heavy
continuous force, there is a sequence of abrupt
movements, which could sever the blood vessels as
they enter.

Effects on root structure
When ortho forces are applied, there is usually an
attack on the cementum of the root, just as there
is an attack on the adjacent bone, but repair of the
cementum also occurs.
Rygh and co-workers have shown that the
cementum adjacent to the hyalinsed areas of the
pdl are attacked by the clast cells and can lead to
severe root resorption. It is seen that if cementum
is removed from the root surface, then it is
restored in the same way that the alveolar bone is
removed and then www.indiandentalacademy.com
replaced.

Repair of the damaged root restores its original
contours; unless the attack on the root surface
produces large defects at the apex that eventually
become separated from the root surface. Once an
island of cementum or dentin has been cut totally
free from the root surface, it will be resorbed and will
not be replaced.
Permanent loss of root structure after ortho
treatment appears primarily at the apex. Sometimes
there is a reduction in the lateral aspect of the root in
the apical region.

Types of tooth movements that may lead to apical
root resorptions include:
Prolonged tipping,notably of the anterior teeth
• Distal tipping of the molars, causing resorption of
the distal roots of the molars
• Prolonged continuous bodily movement of small
teeth such as upper lateral incisors.
• Intrusion
• Extensive edgewise torque of anterior teeth in the
more mature young and adult patients.
•


Effects on height of alveolar bone
Another effect of orthodontic treatment might be loss
of alveolar bone height. Since the presence of
orthodontic appliances increases the amount of
gingival inflammation, even with good hygiene, this
side effect might seem even more likely. Fortunately,
excessive loss of crestal bone height is almost never
seen as a complication of ortho treatment. The reason
is that the position of the tooth determines the position
of the alveolar bone. When teeth erupt or are moved,
they bring alveolar bone with them.

TYPES OF TOOTH
MOVEMENTS


Theoretically tooth movement is divided into
three types, viz,
Pure translation
Pure rotation
Combination of translation and rotation


Before we go into details about the various types of
tooth movement possible, a few concepts and
definitions have to be understood
FORCE: a load applied to an object that will tend to

move it to a different position in space. Force has both
direction and magnitude.
CENTER OF ROTATION: it is the point around

which the body seems to have rotated. The center of
rotation is not a fixed point and can be changed by the
manner of force application.



CENTER OF RESISTANCE: a point at which

resistance to movement can be concentrated for
mathematical analysis. For an object in free space, the
center of resistance is the same as the center of mass.
For an object, which is partially restrained, the center
of resistance will be determined by the nature of the
external restraints. The center of resistance for a tooth
is approximately the midpoint of the embedded portion
of the root for a single rooted tooth and at a point just
below the furcation for a multi-rooted tooth.

MOMENT: is force acting at a distance. If the line of
action of an applied force does not pass through the
center of resistance a moment is created. Not only will
the force tend to translate the object to a different
position, it will also tend to rotate the object around
the center of resistance. It is defined as the product of
the force times the perpendicular distance from the
point of force application to the center of resistance.


COUPLE: two forces equal in magnitude and
opposite in direction. A couple will produce pure
rotation, spinning the object around its center of
resistance. The combination of force and couple can
change the way an object will rotate while it is being
moved.


PURE TRANSLATION
It occurs when all points on the tooth move
an equal distance in the same direction. This
is brought about when the line of action of an
applied force passes through the center of
resistance of the tooth.


Pure translation can be of
three types:
INTRUSION: translation of the

teeth along its long axis in an
apical direction
EXTRUSION: translation of

teeth along its long axis in an
occlusal direction
They are axial type of
translation and the center of
rotation lies at infinity.

Intrusion
Intrusion is primarily done for anterior teeth. More
rapid intrusion is obtained by light continuous
forces than other types of tooth movement. Forces
applied must not act for excessively long period if
root shortening is to be avoided. A carefully
measured intruding force may cause root
resorptions, but there may be no visible
shortening of the roots. Each anterior tooth may
be intruded by forces as light as 20 to 30 gms.
This light force produces very short hyalinization
period and the teeth will intrude rapidly.

Small resorbed lacunae of the
root surface may be observed
even with this light forces. This
resorption is located between
the middle and apical thirds. It
occurs as a result of tipping of
individual tooth during intrusion.
Intruded teeth vary in their
reactions according to the
magnitude of the force exerted.
Generally teeth in young
patients are intruded more
rapidly and with less tendency to
shortening of the apical portion
of the root.

Extrusion
Extrusion of the tooth involves the
more prolonged stretch and
displacement of supra alveolar fiber
bundles than of the principle fibers of
the middle and apical thirds. Some
of the principle fibers groups may be
subjected to stretch for a certain time
as the tooth is moved, but these will
rearrange after a short retention
period(4 to 5 months). Only the
supra alveolar fibers remain
stretched for longer, leading to a
certain degree of relapse.

BODILY

MOVEMENT :

translation of teeth in mesiodistal or labio-lingual direction.
Bodily movement of a tooth is
usually produced from two-point
contact of the applied force. It
involves moving the tooth
parallel to its long axis.
Therefore the force is distributed
over relatively large areas of the
alveolar bone wall.


When small forces are used, the hyalinised zones that
occur will generally be of shorter duration than those
seen during tipping movements. The reason for this is
that the local forces in these hyalinised zones are
smaller, thus allowing resorption of the alveolar bone
wall to occur. The tooth movement following such
applied forces is quite favorable since there is steady
bone resorption as well as steady pdl fibers pull on the
tension side.


Shortly after the movement is initiated, there is no
bodily movement in the strict mechanical sense but
rather a slight tipping movement. The tooth will be
subjected to a couple. The degree of tipping varies
according to the size of the arch and the width of the
brackets. The result is compression on the pressure
side with formation of hyalinised zone between the
marginal and middle thirds of the root. Gradually
increased stretching on the tension side tends to
prevent further tipping. New bone layers are formed
along these stretched fiber bundles.

A light initial force is preferable in initial bodily
movements, especially during first 5 to 6 weeks.The
optimal magnitude of the force to be applied depends
on the resistance exerted by the stretched fiber
bundles.
During the secondary period, a force within the range
of 150- 200gms have proved favourable for bodily
movement of premolars and canines..it
may,however,become necessary to apply a force of
around 300gms during the final closure of spaces to
bring the tooth being moved in contact with the
anchor tooth.

PURE ROTATION
A displacement of the body produced by

a couple, characterized by the center of
rotation coinciding with the center of
resistance, i.e;

the movement of points of the tooth
along the area of a circle, with the center
of resistance being the center of the circle.

Pure rotation can be divided into two types:
TRANSVERSE

ROTATIONS : tooth
displacements during which the long axis orientation
changes:
a) TIPPING: the simplest type of tooth movement
in which the crown moves in one direction and the root
in the opposite direction. If a force is applied against
the crown of the tooth, and if this force has a one-point
contact, then a tipping effect is produced. Tipping
tends to concentrate compression on a small
periodontal area. Its greatest effects are seen usually
at the marginal root area. Local pressure zones and
areas of hyalinization are a common occurrence in the
marginal regions of the pdl during tipping movements.

The compressive forces generated at the root apex
can cause extensive hyalinization and therefore
increase the risk for apical root resorption.
In clinical situation, tipping movements are often
used when moving teeth in a labiolingual direction.
The labial and lingual bone plates consist of dense
cortical bone, and compensatory apposition of bone at
these sites following initial tipping movements is
comparatively slow.


During the secondary period of movement,
compensatory bone remodeling is seen in the
periosteal surfaces of both the pressure side and the
tension side.
With an increase in the thickness of the new bone
layers formed on the tension side adjacent to the
tooth moved, resorption will occur of the old bone on
the corresponding periosteal side.
This illustrates that there is a tendency for the
alveolar plate to maintain its original thickness.


Tipping movements can be further divided into
controlled and uncontrolled tipping:
1.

2.

Uncontrolled tipping: this describes a movement that
occurs about a center of rotation that lies close to or
apical to the center of resistance. Here the crown
moves in one direction and the root in the opposite
direction.
Controlled tipping:this type of movement occurs
when a tooth tips about a center of rotation at its
apex. Here the crown moves in one direction but the
root is prevented from moving in the opposite
direction.

b) TORQUE :
This can be considered
as a reverse tipping
characterized by lingual
movement of the root. The
tooth moves about a center
of rotation at or close to the
incisal edge. Much bone
undergoes resorption during
this type of tooth movement
and so root movements
require lots of time.

Torque
During the initial movement of
torque, the pressure area is
close to the middle region of
the root. This occurs because
the pdl is normally wider in the
apical third than in the middle
third. After resorption of the
bone areas corresponding to
the middle third, the apical
surface of the root will
gradually begin to compress
adjacent pdl fibres and a
wider pressure area will be
exerted.

LONG

AXIS
ROTATION :
Here the orientation of the
long axis is not altered. The
tooth rotates about its center of
resistance. Here the center of
rotation is the long axis of the
tooth.


The tissue transformation that occurs during rotation
is influenced by the anatomic arrangement of the
supporting structures. Various factors are involved in
the movement of rotation. The anatomic factor is
primarily related to the position of the tooth, its
form,and its size.
Except for the upper centrals and the lower
premolars, most teeth have an oval root form. This
implies that during rotation, a parallel movement
between the root and bone surface takes place
mainly on the buccal and lingual sides of the root. In
practice most teeth to be rotated will create two
pressure sides and two tension sides.

Rotation might cause variations in the type of tissue
response observed on the pressure sides.
Hyalinization and undermining bone resorption take
place in one pressure zone while direct bone
resorption occurs in other. It is favourable to apply a
light force during the initial period. After rotation for
3-4weeks, undermining resorption is complete and
direct bone resorption prevails on the pressure side.
Root resorptions may occur on one side of the
pressure sides and frequently on both pressure
sides, but the resorbed lacunae of the root will be
repaired over the retention period of 6-8 weeks.

On the tension side, new
bone spicules will be formed
along stretched fiber bundles
arranged more or less
obliquely. This stretch of the
pdl fibers coincide with the
formation of cellular cementum
along the root surface. Very
little cementum will be seen
on pressure side. In the apical
region less new bone will be
formed during rotation, but
some fiber groups are
frequently elongated and
arranged obliquely. www.indiandentalacademy.com

The method of treatment influences the final result
of the rotating tooth. If the tooth is moved
interruptedly with a light force that acts over a
certain distance, and then held in position by the
appliance until reactivation, more fiber bundles will
be rearranged during the treatment period. The
relapse tendency will be markedly reduced in such
conditions.
The degree of relapse is especially pronounced
when the tooth is rotated rapidly with a typical
continuous force.

COMBINATION OF BOTH
Any movement that is not pure rotation or
translation can be termed a combination of
both translation and rotation. This type of
movement is often seen in routine clinical
practice.


OPTIMAL FORCES FOR
ORTHODONTIC TOOTH MOVEMENTS
Type of movement







Tipping
Translation
Root uprighting
Rotation
Extrusion
Intrusion

force( gms)
35-60
70-120
50-100
35-60
35-60
10-20


MECHANICAL
PRINCIPLES IN FORCE
CONTROL


FORCE
Force is the load applied to an object that will tend to
move it to a different position in space. It is the
application of a force that will bring about orthodontic
tooth movement. A force is a vector, and is defined by
the characteristics of a vector. Vectors have both
magnitude and direction. Magnitude represents its size
and the direction its line of action, sense and point of
origin.
Forces in orthodontics exhibit what is known as the
principle of transmissibility. This principle says that the
external effect of a force acting on tooth is independent
of where along its linewww.indiandentalacademy.com force is applied.
of action the

When two forces are acting at the same point, the
total effect of the two can be represented as the
resultant force and can be determined by the
parallelogram of forces.


When there are more than one force systems acting
on a body,then they can be divided into
1. Co-planar
2. Non co-planar,
depending on the plane of action.
They can be further broken down into
Concurrent and non-concurrent force systems ,
depending on whether all the forces of the system
intersect at a common point. If they do then it is a
concurrent system, if not then it is non-concurrent.

SIGN CONVENTION

A positive sign is
given to all crown
movement in
•
•
•
•
•

A negative sign to

Mesial
Labial or buccal
Anterior
Lateral
Extrusive direction

•
•
•
•
•

Distal
Lingual or palatal
Posterior
Medial
Intrusive movements


FORCES, MOMENTS, AND
COUPLES IN TOOTH
MOVEMENTS


A moment is a measure of the tendency to rotate. A
moment is produced in one of two ways. If a single
force is applied to a body that does not act through the
center of resistance, the force causes the tendency for
the body to rotate. This moment, the moment of force
(Mf), is quantitatively equal to the magnitude of the
applied force times the perpendicular distance
between the line of the applied force and center of
resistance.
Mf is increased equally by either applying a larger
force to the tooth or applying the force further away
from the center of resistance.

A moment can also be applied to a tooth with a
couple,called moment of couple (Mc). The magnitude
of Mc is equal to the value of one of the forces of the
couple times the perpendicular distances between
the two parallel forces.
The magnitudes of Mc is increased by either
increasing both of the forces of the couple or
increasing the distance between the two forces


To produce different types of tooth movements, it is
necessary that the ratio between the applied

moment and force on the crown be altered. As the
moment force ratio is altered so the center of
rotation will be changed.
There are few instances in which desirable types of
tooth movement can be produced by single forces
applied to the crown alone. If this is done, the root
will move in the opposite direction.


The m/f determines the control that an
orthodontic appliance will have on both active
and reactive units.


EQUIVALENT FORCE
SYSTEMS
The application of forces and couples in orthodontics
is at the brackets and not at the center of resistance.
It is impractical to place forces and moments at the
centers of resistance, instead an equivalent force
system can be placed on the tooth at the brackets or
tube.
If two forces are to be equivalent then the sum of all
the forces and moments of each system should be
equal to that of the second system.

The control of root position during movement
requires both a force to move the tooth in the
desired direction, and a couple to produce the
necessary counter-balancing moment for control of
root position.
The simplest way to determine how a tooth will move
is to consider the ratio between the moment created
when a force is applied to the crown of a tooth(Mf)
and the counterbalancing moment generated by a
couple within the bracket (Mc).



Mc/Mf = 0

 Pure tipping



0<Mc/Mf < 1

 Controlled tipping



Mc/Mf = 1

 Bodily movement



Mc/Mf > 1

 Torque


The distance from the point of force application to
the center of resistance can and does vary, so the
moment to force ratios have to be adjusted if root
length, amount of alveolar bone support or point of
force application differs from the usual condition.
It is because of this that the Mc/Mf ratio is
believed to give a more precise description of how
a tooth will respond.


SYSTEM
EQUALIBRIUM


Newton’s third law of motion states that for every
action there is equal and opposite reaction. The
single forces and couples of orthodontic appliances
are no exceptions.
Static equilibrium requires that the sum of both the
forces and moment acting on an appliance in any
plane must be equal to zero to maintain the system
in equilibrium.
Each moment must be opposed by an equal and
opposite tendency to rotate in the opposite
direction.

Force system can be defined as statically
determinate , meaning that the moments and
forces can be readily discerned, measured and
evaluated,
or as indeterminate . Statistically
indeterminate systems are too complex for
precisely measuring all forces and moments
involved in the equilibrium

Determinate systems in orthodontics are those in
which a couple is created at one end of an
attachment, with only a force and no couple at the
other.
When the wire is tied into a bracket on both ends, a
statically indeterminate two couple system is created.
The determinate force systems are advantageous in
orthodontics because they provide much better
control of the magnitude of forces and couples.

One couple systems
In orthodontics one couple systems are found when
two conditions are met.
1) A cantilever or auxillary arch wire is placed into a
bracket or tube.
2) The other end of the spring or auxillary arch wire is
tied to a tooth or a group of teeth that are to be moved,
with a single point of force application.


Two couple system
When a wire is placed into two brackets the forces of
equilibrium always act at both brackets. There are
three possibilities for placing a bend in the wire to
activate it.
1.Symmetric V bends,
which creates equal and opposite couples at the
brackets. The forces at each bracket are equal and
opposite, and therefore cancel each other out. A
symmetrical V bend is not necessarily half way
between two teeth or two groups of teeth.

If two teeth are involved but one is bigger than the
other, equal and opposite moments would require
placing the bend closer to the large tooth, to
compensate for the longer distance from the bracket to
its center of resistance.
The same would be true if two groups of teeth had
been created by tying them into the equivalent of a
single large multi-rooted tooth, as when posterior teeth
are grouped into a stabilizing segment and used for
anchorage to move a group of for incisors.

2. Asymmetric V bend,
which creates unequal and opposite couples, and net
equilibrium forces that would intrude one unit and
extrude the other. Although the absolute magnitude of
the forces involved cannot be known with certainty, the
relative magnitude of the moments of the associated
equilibrium forces can be determined.
The bracket with the larger moment will have a greater
tendency to rotate than the bracket with the smaller
moment, and this will indicate the direction of
equilibrium forces.

As the bend moves closer to one of the two equal
units, the moment increases on the closer unit and
decreases on the distant one, while the equilibrium
forces increase.
When the bend is located 13rd of the distance
along the wire between two equal units no moment
is felt at the distant bracket, only a single force.
When the bend moves closer than that to one
bracket, moments at both brackets are in same
direction and equilibrium forces increases further.

3. Step bend
which creates two couples in
the same direction regardless of
its location between the two
brackets. The location of a V
bend is a critical variable in
determining its effect, but the
location of a step bend has little
or no effect on either the
magnitude of the moments or the
equilibrium forces.

LOAD DEFLECTION RATE

A characteristic of an ortho appliance, the load deflection
or torque – twist rate, is involved in the delivery of a
constant force. By definition the load deflection rate
gives the force produced per unit activation.
For a tooth moving under a continuous force, as the
load-deflection rate becomes lower the change in force
value is reduced. With regard to active members a low
load-deflection rate is desirable for two important
reasons.

A mechanism with low L-D rate will maintain a more
desirable stress level in the pdl.
Also a low L-D rate offers greater accuracy in control
over force magnitude.
If a low L-D rate is desirable for an active member then
the opposite is true for the reactive member. The
reactive member should be relatively rigid; that is it
should have a high L-D rate. The anchorage potential
of a group of teeth can be enhanced if the teeth
displace as a unit.

ELASTIC MATERIALS and

properties

The elastic behavior of any material is defined
in terms of its stress-strain response to an
external load. Both stress and strain refer to
the internal state of the material being studied.


STRESS: is the internal distribution of the load,
defined as force per unit area,
STRAIN: is the internal distortion produce by the
load, defined as the deflection per unit length.
When a force is applied to an appliance, its
response can be measured as deflection produced
by the force, which is bending or twisting.


For orthodontic purposes three major
properties of materials are critical in
defining their clinical usefulness:
1.
2.
3.

Strength
Stiffness/springiness
Range.


Strength
Three different points on a stress-strain diagram can
be taken as representatives of the strength of a
material.
1. Proportional limit: the point at which any permanent
deformation is first observed.

2. Yield strength: the point at which a deformation of
0.1% is measured.


3. Ultimate tensile strength:
the maximum load the wire can sustain…this point
is reached after the permanent deformation and is
greater than the yield strength.
2

Strength is measured in stress units (gms/cm )


Stiffness and springiness
are reciprocal properties. Each is
proportional to the slope of the elastic
portion of the force-deflection curve.
The more horizontal the slope, the
springier the wire,
and the steeper the slope, the stiffer the
wire.


Range
is defined, as the distance the wire will bend elastically
before permanent deformation occurs. It is measured
in millimeters or any length units.
If the wire is deflected beyond its yield strength, it will
not return to its original shape, but clinically useful
spring back will occur unless the failure point has been
reached.
In many cases orthodontic wires are deformed beyond
their elastic limit. Their spring back properties in the
portion of the load- deflection curve between the
elastic limit and the ultimate strength are important in
determining the clinical performance.

These three major characteristics are related by the
formula
Strength = Stiffness x Range.

Two other characteristics of clinical importance can
also be described on the stress- strain:
Resiliency: is the area under the stress- strain diagram
upto the proportional limit. It represents the energy
storage capacity of the wire, which is a combination of
strength and springiness.
Formability: is the amount of permanent deformation
that a wire can withstand before failing. It represents
the amount of permanent bending the wire will tolerate
before it breaks.

FACTORS AFFECTING
ELASTIC PROPERTIES


Material
Precious metal alloys :
are the first used materials for orthodontic
purposes, primarily because nothing else could
tolerate the intra-oral conditions. The introduction of
stainless steel in the 1970s made the use of
precious alloys obsolete.


Stainless steel and cobalt –chromium alloys :
both these metals have considerable higher strength
and springiness along with equivalent corrosion
resistance compared to the precious metal alloys
and so replaced them in orthodontic practice. The
properties of these steel wires can be controlled over
a reasonably wide range by varying the amount of
cold working and annealing during manufacture.


Stainless Steel is softened by annealing and
hardened by cold working.
Elgiloy, the cobalt-chromium alloy, has the
advantage that it can be supplied in a softer and
therefore more formable state, and then can be
hardened by heat treatment after being shaped.

 Nickel-titanium (NiTi) alloys.
Has proved very useful in clinical orthodontics
because of its exceptional springiness. Niti alloys
have two remarkable properties that are unique in
dentistry---shape memory and super elasticity.
Shape memory refers to the ability of the material to
“remember” its original shape after being plastically
deformed while in the martensitic form.


Nitinol was marketed in the late 1970’s for
orthodontic use in a stabilized martensitic form, with
no application of phase transition effects.
Nitinol is exceptionally springy and quite strong
but has poor formability.
In the late 1980’s new nickel-titanium wires with an
active austenitic grain structure appeared. These
wires exhibit the other remarkable property of niti
alloys--super elasticity. This group subsequently is
referred to as A-NiTi.

Over considerable range of deflection, the force
produced by A-Niti hardly varies. This means that an
initial arch wire would exert about the same force
whether it was deflected a relatively small or a large
distance, which is a unique and extremely desirable
characteristic.
The unique force-deflection curve for A-NiTi wire
occurs because of a phase transition in grain structure
from austensite to martensite, in response not a
temperature change but to applied force.
The transition is a mechanical analogue to the
thermally induced shape memory effect

 Beta-Titanium:
In the early 1980’s, after nitinol but before A-NiTi,
Beta-Ti material (TMA) was developed primarily
for orthodontic use. It offers a highly desirable
combination of strength and springiness as well as
reasonably good formability. This makes it an
excellent choice for arch wires, especially rectangular
wires, for the late stages of edgewise treatment.


Effects of size and shape
Each of the major elastic properties –strength,
stiffness and range-is substantially affected by the
change in the geometry of a beam. Both the cross
section and the length are of great significance in
determining its properties. Changes related to size
and shapes are independent of the material.


Diameter:
doubling the diameter of the wire increases its
strength by 8 times, i.e; the large wire can resist 8
times as much force before permanently
deformed,or can deliver 8 times as much force.
Doubling the diameter, however, decreases
springiness by a factor of 16 and range by a factor
of 2.


Length and attachment:
If the length of a cantilever spring is doubled, its
bending strength is cut in half, but its springiness
increases 8 times and its range 4 times. Length
changes affect torsion quite differently from bending:
springiness and range in torsion increase
proportionally with length, while torsional strength is
not affected by length.


The way in which a beam is attached also affects its
properties. An arch wire can be tied tightly or loosely,
and the point of loading can be any point along the
span. A supported beam like an arch wire is 4 times as
springy if it can slide over the abutments rather than if
the beam is firmly is attached. With multiple
attachments, as with an arch wire tied to several teeth,
the gain in springy from loose ties of an initial arch wire
is less dramatic but still significant.


CONCLUSION


The design of efficient orthodontic appliance does not
occur by trial and error. Instead, an approach based
on sound biologic and physical principles leads to
development of appliances with predictable actions.
We should be able to define and quantify forces,
moments, couples and equilibriums associated with
appliances. If the force systems acting on a tooth
cannot be defined, their effect on cells and tissues will
be difficult to understand. Biomechanics thus
analyses the reaction of dental and facial structures
to orthodontic forces.

Many variables affect the outcome of orthodontic
treatment. Some are partially or totally out of the
clinicians control such as growth, bone-pdl-gingival
responses, and neuromuscular adaptation to
changes in jaw and tooth positions. Factors that are
in the control of the clinician are the magnitude and
direction of the forces, couples,moments and
moment to force ratio exerted by the appliance. A
thorough understanding of the physical principles
operating in orthodontic appliances eliminates
appliances as an uncontrolled variable affecting the
final result.

THANK YOU


SEGMENTED ARCH MECHANICS
This is considered an organized approach to using
one couple and two couple systems for most tooth
movement so as to obtain both more favorable force
levels and better control. The essence of the
segmented arch system is the establishment of welldefined units of teeth, so that anchorage and
movement segments are clearly defined.

The desired tooth movement is accomplished with
cantilever springs where possible, so that the
precision of one couple approach is available, or with
the use of two couple systems through which at least
net movements and the directions of equilibrium
forces can be known.


Typical segmented arch treatment would call for
initial alignment within posterior and anterior
segments, the creation of appropriate anchorage
and tooth movement segments, vertical leveling,
space closure with differential movement of
anterior and posterior segments, and perhaps the
use of auxillary torquing arches.


The advantages of the segmented arch approach
are the control that is available, and the possibility
of tooth movements that cannot be achieved with
continues arch wires. The disadvantage is the
greater complexity of the appliance, and the
greater amount of time needed to install, adjust
and maintain it.


CONTINUOUS ARCH
MECHANICS
Continuous is one that is tied into the brackets on all
the teeth. An extremely complex multicouple force
system is established when the wire is tied into
place. In general, the mechanical efficiency of a
continuous arch wire system is less than that of a
segmented system, but its fail – safe properties are
better.


The advantages and disadvantages are just the
reverse of those with segmented arch approach.
Continuous arch treatment is not as well defined in
terms of forces and moments that will be
generated at any one time. But continuous arch
wires often take less chair time because they are
simpler to make and install, and because they have
excellent fail-safe property in most applications.


Required moment to force ratios for different types
of tooth movement:
•

Uncontrolled tipping

0:1 to 5:1

•

Controlled tipping

7:1

•

Translation

•

Root movement/ torque

•

Rotations

10:1
12:1

(net forces at the center of
resistance is nil, only a couple is
seen)

Thank you
For more details please visit


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