INDIAN DENTAL ACADEMY
Leader in continuing dental education
A class of appliances characterised by
the extroral positions of activating
elements and supporting structure and
having remotely located responsive
More than 100 years ago Kingsley is reported to
have used occipital anchorage during treatment.
In 1907, Angle referred to extraoral anchorage
and illustrated his occipital headgear and traction
bar, which he replaced with “Baker’s anchorage”.
In seventh edition of his text, Angle described the
use of extroral traction combined with extraction
of upper premolars.
According to Breitner, in 1911 Oppenheim
introduced the concept of center of rotation of a
tooth as the point around which a tooth would
rotate when a force was applied to the crown.
Oppenheim also recognised that if a force could
be arranged so that it passed through the center
of rotation then a tooth, such as a molar would
Since such bodily movement does not involve tipping
or rotation, the focal point for the force to produce
the translatory movement has become known as the
center of resistance.
Kloehn took up the use of extraoral traction following
a publication by oppenheim in 1936 and must be
given the credit for use of cervical traction as 1st
phase in 2 phase treatment of class II and maxillary
Phase I was concerned with distalising the maxillary
I permanent molar before pubertal growth spurt.
Kloehn – type
To translate molar distally, Kloehn
advocated alternately tipping the
molar crowns and roots. Distal
crown tipping was produced by
positioning the outer bow of the
face bow below the center of
resistance and distal root tipping
by positioning the ends of the bow
above the center of resistance.
Weber showed examples of
extraoral traction designed to
distalise mandibular teeth.
Appliances resembling chin cups have been in use
since the early 1800's. According to Graber, the early
attempts with the chin cup were not successful
because of incomplete knowledge of mandibular and
facial growth, its use on nongrowing patients, and an
inadequate understanding of the forces generated by
the chin cup.
Armstrong applied 500 Gm. of force via chin cups on
100 adolescent patients with mandibular prognathism.
He reported that half of his patients showed
improvement in the Class III profile, whereas none of
the control, nontreated patients showed any favorable
Thilander treated sixty patients with chin cups for 1 to
6 years. A significant percentage of patients did not
improve. The patients who showed improvement were
comparatively young and showed favorable dental
changes. The force generated by the chin cup in his
study was only 150 to 200 Gm.
Graber, Chung, and Aoba reported results in patients
treated with chin cups for 12 to 14 hours each day with
a force of 1.5 to 2 pounds on each side. They showed
that mandibular growth could be redirected with a chin
cup. They asserted that continuous use of the
appliance for a long period or through active growth
was necessary to achieve stable results.
Graber treated 35 Class III malocclusions in children
between the ages of 5 and 8 years with chin cup
therapy for 3 years. He found that the therapy was
particularly effective in patients with increased vertical
growth of the face.
Several clinical studies in the past have noted that
treatment of patients in skeletal Class III should include
protraction of the maxilla with or without chin cups.
Oppenheim suggested a technique for moving the
maxilla forward. He noted that restriction of growth or
distal movement of the mandible was impossible.
Kettle and Burhapp reported an appliance for cleft lip and palate
which successfully inhibited forward growth of the mandible and
simultaneously caused anterior movement of the maxilla.
Nelson described an appliance which used anterior pull on the
maxilla by means of a football-type helmet. Haas showed
downward and forward movement of the maxilla as a result of
palatal expansion. The maxillary effect was enhanced by the use
of Class III elastics from a chin cup to the distal aspect of the
Delaire, Verdon, and Floor have extensively used a facial mask to
protract the maxilla anteriorly. Elastics generating forces of 1,000
to 2,000 Gm. are used from distal of the maxillary molars to the
wires of the mask to move the maxilla anteriorly.
Types Of Headgear
According to the means of attachment to teeth
Using face - bow, which slots into tubes soldered onto
the bridge of a removable appliance crib or tubes which
form an integral part of a band attachment or tubes
which are incorporated in the design of a functional
The face – bow is an inner – outer bow. Inner bow is
available either in 0.045” or 0.51” depending on the size
of headgear tube. Outer bow is usually 0.072”
2) J – hooks which can be directly attached onto the arch
wire in a fixed appliance or attached to hooks soldered
onto the labial bow of a removable appliance.
According to force element
Elastic strap or elastic bands
According to direction of pull
High pull (Occipital)
Reverse Pull or protraction headgear
Interlandi which gives more options for force direction.
Chin cup -- occipital and vertical
Mechanics describes the effects of forces on bodies
and can be generally divided into three areas: 1)
statics 2) Kinetics 3) strength of materials.
Statics describes the effects of forces on bodies that
are at rest or have a constant velocity.
Force is action of one body on another body that
tends to change or changes the shape of that second
A force is equal to mass times acceleration (F = ma).
Unit is Newton or gram. Millisecond/s^2. Grams are
substituted for Newton in clinical orthodontics because
the contribution of acceleration to the magnitude of
force is clinically irrelevant.
A force is a vector and is defined by the characteristics of
vectors, which have magnitude and direction.
Magnitude of vectors represent its size. Direction is
described by the vector’s line of action and point of origin.
The sum of two or more vectors is called resultant.
Clinically, the determination of horizontal , vertical and
transverse components of a force improves the
understanding of the direction of tooth movement that
might be expected.
Center of resistance
A free body can be considered to have a single point
within it where all of its mass is centered.
The center of mass is the point through which an applied
force must pass for a free object to move linearly without
Tooth is restrained body in which this center of mass is
called center of resistance. It can be described in each
plane of space. Single tooth, units of teeth, complete
dental arches and the jaws themselves each have center
In other words the center of resistance is the point on the
body where a single force would produce translation i.e .,
all points moving in parallel, straight lines
The center of resistance of a tooth is dependent on the root
length and morphology, the number of roots, and the level of
alveolar bone support.
The exact location of the center of resistance is not easily
identified. Analytical studies have determined that the center of
resistance for single rooted tooth with normal alveolar bone
levels is about one fourth to one third the distance from the
cementoenamel junction to the root apex.
Miki and Hirato found that the location of the center of
resistance of the midface of the human skull was between the
first and second upper premolars anteroposteriorly, and
between the lower margin of orbitale and distal apex of the first
molar vertically in the sagittal plane.
It is distal to the lateral incisor roots for intrusive movements of
maxillary anterior teeth.
In maxillary first molar the center of resistance is
estimated to be in the middle third of the root near
the junction of cervical third or approximately at the
trifurcation of the roots.
The center of resistance should not be regarded as a
fixed point within a tooth, but rather as the composite
point of all factors, offering different components of
resistance to a certain force application.
The tooth anatomy and mass distribution within the
The structure of the periodontal attachment
The degree of bony surroundings
The adjacent teeth.
Center of resistance
The point of application of a force is simply the point of
contact between the body being moved and the applied
Direction is indicated by the body of the arrow itself and
the arrowhead. Without the head of the arrow, the body
alone indicates the line of action. The sense is
determined by which end we put the arrowhead on.
Since the movement of a tooth (or any object) is
determined by the net effect of all forces on it, it is
necessary to combine applied forces to determine a
single net force, or resultant.
There may be a force on a tooth that we wish to
break up into components.
For example, a cervical headgear to maxillary molars
will move the molars in both the occlusal and distal
directions. It may be useful to resolve the headgear
force into the components that are parallel and
perpendicular to the occlusal plane, in order to
determine the magnitude of force in each of these
Center of rotation
The center of rotation of a body is a point around
which the body will rotate or tip.
The center of rotation can be changed, being
dependent on external force application.
When a force is applied to a tooth and its line of
action does not pass through center of rotation, then
tipping will occur around a center of rotation which
may be located anywhere between the center of
resistance of the tooth and infinity.
Resolving a force into components
It is often useful to divide
a single force into
components at right
angles to each other.
Usually, the objective is to
determine how much
force is being delivered
perpendicular and parallel
to the occlusal plane,
Frankfort horizontal, or
the long axis of the tooth.
Moment of force
Forces not acting through the center of resistance do not
solely produce linear motion.
The moment of force results in some rotational
movement. The moment of force is the tendency for a
force to produce rotation.
It is unrecognized in clinical orthodontics. Awareness of
moment of force is required to develop effective and
efficient appliance designs.
Two variables determine the moment of force – the
magnitude of the force and the distance. Either one can
be manipulated by the clinician to achieve the desired
Moment of a couple
This is another method of achieving
A couple is two parallel forces of equal
magnitude acting in opposite directions and
separated by a distance.
Direction is determined by following the
direction of either force around the center of
resistance to the origin of opposite force.
Understanding how to control the direction and
magnitude of the forces produced by various headgear
designs is paramount in achieving desirable clinical
Decreasing the patient's length of treatment and
improving the treatment results would be only two of the
benefits derived from applying well-planned force
A method of analyzing force systems produced in the
anterior-posterior and vertical planes will aid the clinician.
In 1971 Armstrong demonstrated the importance of
the precise control of magnitude, direction, and
duration of extraoral force to increase its efficiency
and effectiveness in treating malocclusions in the
late mixed dentition.
Gould has shown how changes in the inclination of
the facebow affect the direction of the force and
ultimately the direction of tooth movement.
Greenspan presented reference charts elaborating
the different moments and forces produced with the
various headgear designs.
Moments (M) and forces produced by force
vectors applied at varying positions relative to
the center of resistance (CR) of a constrained
body in this case the maxillary 1st molar. The
vertical (V} and horizontal (H) forces are
proportional in magnitude
to the legs of the triangle that is constructed
. In (a) the vertical and horizontal components
of the force are approximately equal. The
moment's direction is counterclockwise since
the line of force is above CR. The magnitude of
M is the product of LF times the perpendicular
distance (identified as P)from LF to CR. LF goes
through CR in (b) thus there is no M produced.
The tooth will translate parallel to the line of
force. The posterior force component is larger
than the superior. The LF in (c) will produce a
posterior and interior movement. The moment
(P x LF) is below CRwww.indiandentalacademy.com
and is therefore clockwise.
The magnitude of the moment produced by the
headgear is calculated by multiplying the perpendicular
distance (P) from the LF to the CR by the magnitude of
the force. Thus, for a given force, the greater the
distance from the CR that the force is applied, the
greater will be the moment.
A comprehensive understanding of the potential,
limitations, and undesirable side-effects can be gained
by understanding the mechanical principles involved in
We can now apply our basic principles to assess force
systems applied by various headgear designs.
The cervical (Kloehn) headgear is a device that many
orthodontists have used routinely in the great majority
of their headgear cases.
It is composed of three basic parts: (1) molar bands
and tubes, (2) inner bow and outer bow soldered
together near the middle of the two bows, and (3) a
neckstrap that is placed around the back of the neck to
This extraoral pull is generally applied bilaterally, for
three main purposes: (1) as a restraining force, (2) as a
retracting force, or (3) as a supplementary force.
The cervical headgear is applied in early treatment of
Class II malocclusion to inhibit forward displacement of
the maxilla or maxillary teeth, while the rest of the
dentofacial structures continue their normal growth.
As demonstrated by Oppenheim, this can cause a
change in the intermaxillary relationship from Class II to
Perhaps the change in molar relationship is due not so
much to the distal force, but to the clockwise moment that
very effectively tips the molar crown distally.
The main disadvantage to the use of the cervical
headgear is that it normally will cause extrusion of the
This movement is seldom desirable except in treatment
of patients with short lower facial heights. These
patients, it should be remembered, are few and far
The decision to treat with cervical headgear needs to
be based on a complete understanding of the desired
tooth movement and the force system that is produced
with this headgear style.
Force systems with cervical
headgear. OB (Outer bow)-A
lies along the LFO and therefore
only vertical and horizontal forces
will be produced no M. The
position of OB-B will produce an
extrusive F posterior F and
counterclockwise M since it is above
CR. Outer bows located below the
LFO will produce posterior forces
and smaller extrusive forces since
they are closer vertically to the
neckstrap and clockwise moments.
The different moments and forces produced by the
cervical headgear depend on the situation of the outer
bow in relation to the LFO. By definition, when the outer
bow lies along the LFO, no moment occurs, and the force
system will be reduced to a bodily movement in a
posterior and extrusive direction.
If the outer bow is placed above this line (angle of above
20- 30 degree above occlusal plane), the moment
produced by the force will be in a counterclockwise
direction. On the other hand, if the outer bow is adjusted
below this line the moment created will be clockwise.
However, the direction of the forces are the same extrusive and posterior. It should be noted though that
If the outer bow is located below (angle of less than 20 degree
to occlusal plane) the neckstrap, the resultant force will be a
small intrusive one, instead of extrusive. Of course, a distal
force and large clockwise moment will also be produced.
The direction of pull provided by the cervical headgear is
especially advantageous in treating short-face Class II
maxillary protrusive cases with low mandibular plane angles
and deep bites, where it is desirable to extrude the upper
Also, the clockwise moment that is so readily produced with
this headgear is very effective in helping conserve anchorage
in extraction cases.
outer bow is short -- steepen the occlusal plane
outer bow is long -- flatten the occlusalwww.indiandentalacademy.com
If the teeth are banded
and stabilized, cervical
pull appliance, produces
a force below both
center of resistance of
maxilla and the
dentition.. The distances
of the force vector to A
and B determine the
center of rotation (x).
High Pull Headgear
The high-pull headgear, like the cervical-pull, is
analyzed using the same principles of force and
moment production described before. This style
headgear always produces an intrusive and posterior
direction of pull, due to the position of the headcap.
The direction of the moment that is produced is
dependent on the position of the outer bow . If the outer
bow is placed anterior to the LFO (angulated > 45
degree to occlusal plane) moment produced will be
On the other hand, if the outer bow is placed posterior
to this line (angulated less than 45 degree to occlusal
plane), the moment produced will be in a clockwise
High-pull headgears produce
intrusive and posterior forces.
Locating the outer bow in front
of the LFO (A and D) will produce
a counterclockwise M while an OB
behind (B and C) will create a
clockwise M. An OB located on the
LFO would of course produce no
The magnitude of this moment will be proportional to
the distance of the outer bow to the CR.
If a distal and intrusive movement with no moment is
desired, the outer bow must be placed somewhere
along the LFO.
This force system would be beneficial in a long-face
Class II patient with a high mandibular plane angle,
where intrusion of maxillary molars would decrease
facial height and improve the facial profile.
Short outer bow angulated
high to create the headgear
force line of action that is far
anterior to the unit’s centre
of resistance. This results in
a force system at the unit’s
center of resistance with a
moment that tends to flatten
the occlusal plane and distal
and intrusive force
With long outer bow such that
the headgear force’s line of
action passes through the
unit’s center of resistance and
therefore no change in the
cant of occlusal plane
Long outer bow. The
equivalent force system at the
unit’s center of resistance has
a moment that tends to
steepen the occlusal plane and
a force with intrusive and
distal components. May be
necessary for class II open
Straight Pull Headgear or Interlandi
or Combination headgear
This style headgear is a combination of the high-pull
and cervical headgear, with the advantage of
increased versatility. Depending on the force system
desired, the orthodontist has the opportunity to change
the location of the LFO.
The prime advantage of this headgear is its ability to
produce an essentially pure posterior translatory force.
This is accomplished by placing the LFO through the
center of resistance, parallel to the occlusal plane.
Clinically, this means bending the outer bow to the
same level as CR, and hooking the elastic to a notch
at the same vertical level.
The straight-pull headgear is versatile in
that the clinician has many optional
LFO's . In this case an OB located on
the LFO would cause translation in a
posterior and slightly superior
direction. OB's above the LFO will
produce posterior and extrusive forces
and clockwise moments. Placing the
outer bow along an LFO that Is parallel
to the maxillary occlusal plane will
produce a pure posterior translation.
The relation of the outer bow to the LFO dictates the
direction and magnitude of forces and moments.
Placing the outer bow above the LFO will produce a
posterior force, counterclockwise rotation, and most
often an intrusive force.
However, if the LFO cants up anteriorly (attachment
site of elastic is lower on headcap than at outer bow),
an extrusive force will be produced. If the outer bow is
below the LFO, the force produced will be posterior and
superior, and the moment will be in a clockwise
The straight-pull is the headgear of choice in a
Class II malocclusion with no vertical problems.
It is also the headgear of preference when the
main thrust of headgear wear is to prevent
anterior migration of maxillary teeth, or possibly
even translate them posteriorly.
Force’s line of action
passes through center of
resistance. No moment
acting to change the cant
of occlusal plane, and
there is pure distal force
passing through the
center of resistance.
This configuration is typical for redirecting maxillary
horizontal growth in class II patients and /or to move
maxillary molars distally via translation.
When a force is applied to a headgear with inner and
outer bows, one side effect is buccal expansion
component of forces, which act bilaterally.
This side effect is often helpful in class II
malocclusions because it is often necessary to
expand the posteriors to maintain proper interception
as the buccal segment class II interrelationship is
If such expansion is not required, it can be prevented
by using a transpalatal arch.
Vertical Pull Headgear
The main purpose of this headgear is to produce an
intrusive direction of force to maxillary teeth, with
posteriorly directed forces.
If the outer bow is hooked to the headcap so that the line
of force is perpendicular to the occlusal plane and
through the CR, pure intrusion may take place. Due to
the multiple notches in the headcap, this headgear is also
very versatile, as the LFO orientation may be changed.
However, upon establishing the LFO, our principles of
determining force systems produced remains unchanged.
The vertical-pull headgear is used
primarily when a large magnitude
of pure intrusion is needed. The
outer bow must be located on the
LFO to obtain pure intrusion (A).
An OB located anterior to the LFO
will produce an intrusive force and a
smaller posterior force and a
counterclockwise moment (B and
C). Locating the OB posterior to
LFO will cause intrusion a small
anterior force and a clockwise
moment (D and E).
The head is divided into two components: the anterior
component from the LFO forward and the posterior
component located behind the LFO. If the outer bow is
placed anywhere in the anterior compartment, the
moment created will be counterclockwise, and the forces
produced will be intrusive and posterior.
If the outer bow is placed anywhere in the posterior
section, the moment will be clockwise and the vertical
force will be intrusive, but the horizontal force will be
If this latter force system is desired, it will require
inserting the inner bow into the buccal headgear tube
from the distal.
The vertical-pull headgear is not as commonly
used as are the others.
However, it is very useful when pure intrusion of
buccal segments is required, as in the Class I
Adjusting Directional Pull of
Several possibilities exist but what movement is needed
in buccal segments should be analyzed.
If distal translation is needed , a distal force straight
through the center of resistance is needed. The
combination or interlandi headgear will allow this by
having equal occipital and cervical components on an
outer bow, which is angled upward to pass through the
center of resistance.
Intrusion of upper anterior segment with a base arch
would produce a undesirable side effect of eruption and a
rotation of upper buccal segments. To prevent this an
upward and backward force anterior to the center of
resistance of buccal segments is needed which can be
done by using a short outer bow and the occipital pull.
Right versus left asymmetries can be corrected using
transpalatal or lingual arches to correct asymmetric molar
axial inclinations. The same mechanism can be used to
correct asymmetric molar rotations.
If buccal occlusion is asymmetric e.g. Class I on one side
and class II on the other side, without asymmetries
either in molar axial inclinations or in rotations, then it is
most logical to achieve the correction with asymmetric
Distal forces exist on both sides, but they are three times
greater on the long outer bow side than on the short
outer bow side.
Lateral forces, directed toward the short
outer bow side exist with this headgear.
Crossbite development should be kept in
These are usually cervical or
Suggestions to be noted with regard to the use of the
asymmetric cervical gear:
1. The differential in length of arms of face-bow need not
be great, only sufficient to alter the geometry so that the
resultant bisector crosses the molar line closer to the
more anteriorly positioned molar than to the other.
Excessive difference in arm lengths could increase the
2. The diameter of wires can be increased for greater
rigidity; it is suggested that the arch wire be 0.055 inch
and the face-bow 0.075 inch (the 0.075 inch face-bow is
approximately five times as stiff as the 0.50 inch one).
3. The arms of the face-bow should clear the cheeks so
as not to introduce more undesirable lateral forces.
Rigorous force analysis of the several cervical gears
of different design using elastic straps shows that the
fundamental principle involved in the distribution of
the forces to the right and left molars is the geometry
of the direction of the right and left forces emanating
from the cervical elastic band.
If these forces are symmetrical with reference to the
midsagittal line of the head, then the distribution of
the reactionary forces at the right and left molars will
be equal, irrespective of the design of the rigid
portions of the appliance (or the point of attachment
of face-bow to arch wire).
If the direction of forces from the cervical elastic
band is asymmetrical with respect to the midsagittal
line of the head, then the anterior-posterior
components of the reactionary forces on the right
and left molars will be unequal, the molar nearest the
resultant of the two elastic band forces receiving the
2. Small lateral forces on the molars are always
developed by this eccentric design. These forces can
be manipulated to cause all lateral reaction to occur
on one side or the other by springing the labial arch
inward or outward, respectively.
Headgear to lower jaw
Headgear bracket-tube combinations can be attached
to either lower first or second molar.
If the bracket-tube combination is on the first molar, it is
advantageous to place the headgear tube occlusally.
First molar is preferred since
The lingual arch is on the first molar and gives better
It is easier for the patient.
Possible directions are:
The posterior segments tend to move back
A positive moment will be produced, which will steepen
the occlusal plane.
J- hook Headgear
J-hooks to arch wire
A line of pull attached to the incisor region of the arch wire and
passing occlusally to the center of the resistance will place a
distally directed force upon the maxillary teeth, but will also tip
the occlusal plane downwards at the incisor end of the arch.
A line of pull through center of resistance will produce distal
movement of the maxillary arch without undesirable rotational
A more vertical direction of pull, mesial and apical to center of
resistance produces an anti-clockwise moment and an
intrusive effect upon the incisor end of the arch wire
Disadvantage is that the flexibility of arch wire results in
unavoidable deformations which subject the teeth near the
attachment to diurnal reversals of force application as the
extraoral appliance is attached or disengaged. Heavy arch
wires minimize this rebound effect, but not eliminated.
J-hooks to individual teeth
If the center of resistance of a single tooth coincides
with the centroid the line of force of a J-hook
headgear intended to produce upright bodily
movement of an individual tooth should ideally pass
through this center of resistance.
Most authorities suggest an occipitally directed line
of force to move maxillary canines distally. But
straight pull is suggested as it is difficult to obey
theoretical concepts when moving mandibular
Direction of Headgear Force
Given by the line of action of force from the point of
origin to the point of application of force.
Explained by linear vectors of force. An anteroposterior
force that does not pass through the occlusal plane will
certainly have a vertical component.
a) Force directed upwards above the occlusal plane has
an intrusive effect on maxilla.
b) Force directed downwards below the occlusal plane has
an extrusive effect on maxilla.
Force passing along Center of Resistance produces
d) Force away from center of resistance( mesially,
distally, apically, occlusally) produces a moment
tending to change the occlusal cant.
e) Magnitude of moment is determined by moment
arm. Greater the moment arm – closer the Center
of rotation moves towards canter of resistance and
greater is the moment.
Medium length of outer bow is chosen for
g) Short / long outer bow chosen when moment is
a) Direction determined by the sense of the line of
b) Outer bow along the Center of resistance produces
Force apical / occlusal to center of resistance
produces moment ( Extrusive / intrusive / distal).
d) Magnitude dependent on the inclination of line of
a) Shape or length of outer Bow has no effect on force
application provided the distance of point of attachment
to the midline axis are equal. Headgear tube placed
buccal to center of resistance.
b) Hence any force applied, passes buccal to center of
resistance tending to roll the molars, buccally on
intrusion and palatally on extrusion.
This rotatory tendency is directly proportional to the
perpendicular distance of buccal tube to center of
resistance (moment arm). Clinically this moment is
1. Palatal bar
2. Rectangular headgear tubes
Magnitude of Force
When line of action is closer to center of resistance
greater force of 450 – 500 gms may be applied for
Forces away from center of resistance that produce
a moment should be restricted to 50 – 150 gm as in
J – hook headgear to prevent damage to
periodontium during dento alveolar changes that
take place during a moment.
Duration of Force
Intermittent force for 12 – 14 hrs in preadolescent
age from early evening until next morning.
Typical duration of treatment of about 12 to 18
months, depending on rapidity of growth and patient
OF HEADGEAR FORCE
There are four main uses of headgear force
Controlling the cant of the occlusal plane
In class II treatment, headgear force can play a major
role in ensuring that buccal segment teeth do not move
mesially when anteriors are retracted.
Intraoral mechanics often result in eruption of teeth.
Headgear produces a vertical force greater than the force
of side effect
Inner and outer bows can be of any shape, convolution,
Only the angle and level of the final line of action after the
strap forces have been applied to know exactly the force
of headgear system.
Vertical force on molar
tube, a side effect of
Vertical component of
occipital headgear force
negates extrusive intraoral
force side effect
The reaction force from headgear is dissipated against
the bones of the cranial vault, thus adding the resistance
of these structures to the anchorage unit.
The only problem with reinforcement outside the dental
arch is that springs within an arch provide constant
forces, whereas elastics from one arch to the other tend
to be intermittent, and extraoral force is likely to be even
For first molar extraction cases -Interlandi headgear to be
suitable and well tolerated
Adjustment of outer bow such that a horizontal force is
produced that passes through the center of resistance
of maxillary first molar and the patient wears the
headgear at a level of 14 hours each night consistently,
clinical experience shows that the first molars will move
distally 2mm in 24 months without tipping.
Distal tipping is not preferred as finite element studies
have shown that the stress levels at the periodontal
ligament-bone and tooth interfaces are beyond
acceptable limits even when tipping forces are very
Intrusion in deep bite cases
Headgear can be used in adjunct to upper utility arch. High
pull headgear allows more intrusive control permitting maximal
incisor movement whilst minimizing possible molar tipping and
also used to deliver orthopedic force on developed
120 to 150 g force is delivered.
Distalization of molars
Headgear is the obvious choice. Fill time wear is necessary.
Molar extrusion should be avoided so straight pull or high pull
is used and not cervical.
Force – 300g on each side.
Unilateral molar distalization in unilateral class II can be
achieved by asymmetric cervical headgear
Canine retraction using
direct headgear force
Headgear using four
hooks is used, which
over a base arch wire
19 x 25 steel.
200 g of force supplied
to each point of
attachment to slide the
If the headgear is applied
through the center of
resistance of maxilla, which is
in the posterosuperior part of
maxilla. Determined clinically
by dropping a line vertically
10mm from the outer canthus
of eye and making a horizontal
from that point to meet the
pupil line in front of the face.
If a preadolescent patient wears the headgear at least 12
hours each night , the forward component of maxillary
growth is redirected.
Effects of orthopedic forces on maxilla
Cervical traction produces stresses along the frontal
process of maxilla, zygomaticofrontal suture, and the
junction of the palatine bones, areas where high-pull
traction produced no observable effect. Only the high-pull
headgear produces stress at the anterior junction of
maxillae (anterior nasal spine).
Pterygoid plates of the sphenoid
High stress develops upon activation.
These stresses begin in the middle of the posterior curvature
of the plates and just superior to their anterior junction with the
palatine bone and maxilla.
As the force increases, the stresses progress superiorly
toward the body of the sphenoid bone.
Cervical and high pull both produce similar stress .
Starts at the inferior border of the zygomaticotemporal suture
and proceeds posteriorly along the zygomatic process of
Cervical force produces more intensitywww.indiandentalacademy.com
at lower load level.
Junction of the maxilla with the lacrimal and ethmoid bones
Both cervical and high pull produce a stress concentration at
the junction of the maxilla with the
lacrimal bones and with the orbital plates of ethmoid.
High stresses around maxillary molars with cervical traction.
These located around the middle third of the mesiobuccal root
and around distobuccal root at a position toward apex.
Also distal to second molar.
Frontal process of maxilla
Stresses produced anterior to nasolacrimal foramen only with
Just before maximum cervical load stress begins to
appear. Only with cervical pull.
Cervical traction produces stress in posterior region
developing in the horizontal portion of palatine bones.
High pull has no effect.
Anterior junction of left and right maxillae
Only high pull produces forces below the anterior nasal
spine and just lateral to the suture between the two
Sphenomaxillary suture- large compressive stresses.
Temporozygomatic suture- tensile normal stresses
Sphenozygomatic suture- large tensile stresses
Frontozygomatic suture- large compressive stress
Frontomaxillary suture- large tensile stress
Sphenomaxillary and sphenozygomatic sutures, in
particular, resisted the posterior displacement of the
Stresses in the nasomaxillary sutures are varied by the
direction of headgear force, and the force applied in the
direction closest to that of the CRe may produce the most
effective sutural modification for controlling maxillary
Clinical studies have also demonstrated that extraoral force is
effective at restricting maxillary horizontal growth. In fact,
several studies are also available which indicate that headgear
therapy can reposition the maxillary complex posteriorly and
inferiorly in growing patients.
Armstrong has demonstrated remarkably rapid (three to four
months) correction of Class II malocclusions in growing
patients with the use of continuous heavy forces parallel to the
Although not attached to the mandible or primarily aimed at
mandibular alteration, headgear treatment has been shown to
effect mandibular remodeling; the mandible and chin point
have been shown to relocate anteriorly in standard edgewise
treatment. Whether this represents a change which would not
In Class II malocclusions with a fault in maxilla, profile
convexity of the upper jaw can be
a) Basal – large S-N-A angle
b) Dentoalveolar – increased sell-nasion-prosthion (SN_Pr) angle.
c) Dental – increased upper incisor to S-N plane angle
Maxillary basal prognathism requires heavy orthopedic
force. When evaluating the maxillary base, the inclination
should also be considered.
An upward and forward inclination aggravates maxillary
protrusion. (Schwarz (1958) termed this as
A retro inclination (palatal plane tipped anteriorly can
actually compensate for maxillary prognathism.
The control of the vertical dimension in this type of
malocclusion often depends on the inclination of the
maxillary base, especially if it is combined with either a
deep overbite or an open bite. Combined activator –
headgear therapy is required.
Short face( skeletal Deep bite) Class II when
growth potential remains
Goal is to increase face height and correct deep bite,
while allowing more eruption of the lower than the upper
teeth so that the occlusal plane rotates up posteriorly.
Although cervical headgear tends to open the bite
anteriorly and therefore would help to correct a deep
bite problem, it differentially erupts the upper rather than
the lower molars and does not produce the desired
change in the orientation of occlusal plane.
So functional appliances are useful in these patients.
Class II with normal face height and growth potential
Clinical studies show that in these patients , many have
deep bite due to excessive eruption of lower incisors. And
can be treated successfully by two stage treatment.
Stage I using headgear or functional appliances.
Straight pull or high pull headgear is preferred over
cervical headgear, to reduce the elongation of maxillary
molars and better control the inclination of the mandibular
Skeletal open bite
Characterized by excessive AFH. Major diagnostic criteria are:
1. Short mandibular ramus
2. a rotation of the palatal plane
Typical growth pattern shows vertical growth of the maxilla,
often more posteriorly than anteriorly, coupled with downwardbackward rotation of the mandible and excessive eruption of
maxillary and mandibular teeth.
Only two thirds of the patients have actually an open bite – in
others excessive eruption of incisors keeps the bite closed –
but rotation of the mandible produces class II malocclusion
even if the mandible is normal in size and severe class II if the
mandible is small.
Successful growth modification would be restraining
vertical development and encouraging anteroposterior
mandibular growth while controlling the eruption of
teeth in both jaws.
High pull headgear to the maxillary first molars is the
least effective because it does not control the eruption
of other teeth. Furthermore use of molars as primary
handle on maxilla presents three problems:
1.Vertical component of force produces buccolingual
2. The level of force application is limited by the
tolerance and response of the supporting tissues of
these two teeth.
3. Molar movement is the predominant dental change.
High pull headgear with maxillary splint is better, as it provides
en masse dental control.
1. restraint of anteroinferior displacement of maxillary complex
2. restraint of maxillary teeth
3. Disengagement corrects occlusal interferences, facilitating
correction of functional mandibular displacements.
4. Direction and distribution of extraoral force application to the
maxilla may be adjusted over a broader range.
5. incisors can be retracted by including labial bow in the
design and full control over incisor tipping possible.
6. safety enhanced.
But this does not control the eruption of lower teeth. Eruption of
lower teeth is controlled most readily by interocclusal bite blocks,
easily incorporated into a functional appliance. If the bite blocks
separates the teeth more than the freeway space, force is created
against both upper and lower teeth that opposes eruption.
So the most effective treatment is a combination of a functional
appliance with bite blocks and high pull headgear.
If cervical (Kloehn type) headgear is used, the maxillary molars are
driven distally into the “wedge” as the molars are extruded or tipped
down and back. The mandible is rotated down and back, increasing
the apparent mandibular retrusion and allowing compensatory
alveolodental growth to stabilize this undesirable saggital change.
The maxillary incisors are usually tipped down and back at the same
time, restricting forward mandibular growth.
This result is now known as kloehn effect. www.indiandentalacademy.com
With combined activator –
headgear treatment, high pull
headgear attached to activator
exerts a retarding force on
horizontal and vertical maxillary
A high pull headgear does not tip
the palatal plane down and does
not tip up the anterior end of the
palatal plane, which tends to
enhance maxillary incisor
protrusion and upper lip
The headgear-activator has the following modes of action:
1. Intrusion and retraction of upper front teeth
2. Distalization of upper molars
3. Maxilla retraction
4. Mandibular growth stimulation, especially in the
5. Opening of the facial axis in the brachyfacial group
6. Maintenance of the facial axis in the dolichofacial group
7. Minor, if any, tilting of lower incisors
8. Stopping lower incisor eruption
9. Stopping the descent of the palate
Vertical control is obtained in two ways.
1.The untrimmed interocclusal acrylic acts as a bite block,
preventing molar eruption and clockwise mandibular rotation.
2.The inclination of the outer facebow allows precise control
over the direction of force, according to the following
a) A force passing through the center of resistance
produces pure translation in the direction of the force.
b) A force passing at a distance from the center of
resistance generates a moment, with a combined effect
of rotation (from the moment) and translation (from the
Cephalometric guidelines for headgear
Direction of growth
Broad mandibular base and ascending ramus together with a very
marked, thick symphysis suggest a change in direction toward
Narrow mandible and thin symphysis – vertical growth.
If the mandible is too small in class II in a growing individual, growth
may be expected to be quite considerable.
A well developed mandible in a posterior position must be
considered to offer poor prospects for successful correction of class
II malocclusion, except in cases with translation.
Convexity of nasomaxillary complex
SNA angle large ANS far anterior to N- Pog line.
Ante – inclination of maxilla (large J angle) will increase protrusion
(pseudo protrusion). Midface (N – Sn) is short. Extreme case –
Timing of headgear treatment
The most optimum treatment time is between
maturational stages SMI 4 to 7, a very high velocity
period of growth.
The next most desirable time to treat is during the
accelerating velocity period between stages SMI 1 to
the least desirable time is during the decelerating
velocity period between maturational stages SMI 8 to
11. This information is clinically useful for all growth
related mechanics of treatment, retention, and
Skeletal maturity index
SMI 1: third finger, proximal phalanx; width of epiphysis as wide as
or wider than diaphysis.
SMI 2: third finger, middle phalanx; width of epiphysis as wide as or
wider than diaphysis.
SMI 3: fifth (little) finger; width of epiphysis as wide as or wider than
SMI 4: ossification of adductor sesamoid of thumb
SMI 5: third finger, distal phalanx; capping of both sides of epiphysis
SMI 6: third finger, middle phalanx; capping of both sides of
SMI 7: fifth finger, middle phalanx; capping of both sides of epiphysis
SMI 8: third finger, distal phalanx; complete fusion
SMI 9: third finger, proximal phalanx; complete fusion.
SMI 10: third finger, middle phalanx; complete fusion.
SMI 11: radius; complete fusion (skeletal growth completed).
Reverse headgear or
Several authors have reported on the effects of anteriorly
directed forces on the maxillary complex both clinically and
experimentally. In his study, Dellinger examined anterior
maxillary displacement and reported that the maxilla could be
moved forward significantly in animals.
Kambara demonstrated significant maxillary changes in
suture areas and an anterior displacement with a slight
anterior rotation of the maxillary complex.
Ishii found that the relapse of facial bones was very slight
and the modality of relapse was divided into two phases—
the posterior rotation of the maxillary complex taking place 1
month after removal of the applied forces and the dental
changes following the protraction period.
Oppenheim suggested a technique to displace the maxilla
forward to achieve a cosmetic improvement without
subjecting the patient to a surgical procedure.
Irie and Nakamura reported that the acceleration of
forward growth of the midface and improvement of jaw
relationship were obtained clinically by means of maxillary
protraction with the chin cup.
Delaire and associates claimed that using the facial mask
to protract the maxilla anteriorly worked effectively in an
extensive number of young patients.
Nanda and associates and Yamaguchi and associates
introduced a modified protraction headgear bow to control
force variables such as direction and point of force
Nanda and associates also presented the theoretical
aspects and technical improvements necessary for
relating biomechanical approaches to study alterations in
Used in horizontal-vertical maxillary deficiency.
Usual effect of reverse headgear was forward movement
of the maxillary teeth with little or no skeletal effect on
maxilla, along with downward and backward rotation of
In the late 1970’s Delaire and coworkers in France
showed that forward positioning of the skeletal maxilla
could be achieved with reverse headgear if treatment
was begun at an early age.
Best data indicate that increase in maxillary growth
occurs only in young patients (below age 10). Hence a
child with maxillary deficiency should be referred for
complete evaluation as early as possible. The chance of
successful forward movement is essentially zero by the
But even in young patients, two side effects are almost
inevitable when reverse headgear is used:
1. forward movement of maxillary teeth relative to
2. downward and backward rotation of the
Hence ideal patients for this method should have both
a) Normally positioned or retrusive, but not
protrusive, maxillary teeth
b) Normal or short, but not long, anterior facial
Protraction forces applied 10 mm above the Frankfort
horizontal plane produced a posterior rotation of the
maxilla with a forward movement of nasion;
Protraction forces applied 5 mm above the palatal plane
produced a combination of parallel forward movement
and a very slight anterior rotation;
Protraction forces applied at the level of the maxillary
arch produced an anterior rotation and forward
movement of the maxilla;
All three protraction forces caused the constriction of
the anterior part of the palate.
Delaire’s facemask is simple and
well accepted by children.
The attachment is to a maxillary
splint incorporating all the teeth.
If a three dimensional deficiency
exists so that the maxilla is
narrow as well as deficient
anteroposteriorly and vertically,
slow expansion can be done
simultaneously with protraction.
Diagnostic principles for treatment of maxillary
deficiency with protraction headgear
1. Determine whether the mandible, on closure, is in centric
relation or in a "convenient" anterior position. Anterior
positioning generally results from tooth contact relationships
which "force" the mandible into a forward position.
In contrast, centric relation is determined by the muscles, the
ligaments, and the temporomandibular joint anatomy under
the control of the nervous system.
The practical implication is that a Class I problem can
appear to be a Class III malocclusion (pseudo-Class III
malocclusion) when the mandible is forced anteriorly. Even a
true Class III malocclusion can appear much more serious if
there is an anterior path of closure of the mandible.
2. Nature of the skeletal discrepancy must be defined
because treatment, to a large extent, is based on this
3.A malocclusion reflects the interplay of many conditions that
may be impossible to evaluate singularly. One important
variable is the potential growth and development of a patient
with a Class III malocclusion.
1. Maxilla retrognathic -- Treatment should be started early,
as early as 4 years of age, for two fundamental reasons.
a) One is that extraoral traction which pulls the maxilla
anteriorly functions in the same direction as the direction of
b) Second, unlike posterior movement of the mandibular arch,
anterior movement of the maxillary arch appears to have a
greater chance of remaining stable.
With this kind of treatment, we can expect to achieve
( 1) an orthopedic protraction of the maxilla with a strong
enough force (500 to 1,000 Gm per side).
(2) an increase in the inclination of the maxillary incisors
to obtain a sufficient overjet, associated more or less with
(3) bodily movement of all the teeth in an anterior
(4) both an improvement in function and a more esthetic
Studies indicate similar skeletal response can be
obtained when maxillary protraction was initiated either
before age 8 years (5 to 8 years) or after age 8 years (8
to 12 years).
Augmenting forward growth of maxilla is not as
successful as restraining the growth as seen clinically.
This is because,
1. inability to produce enough force at the posterior
and superior sutures to separate them in older children.
2. extent of interdigitation of bony spicules across the
Clinical experience suggests that more than 3mm
forward displacement of the maxilla is unlikely, probably
because of soft tissue limitations.
Chin cup therapy
Used in mandibular excess.
Recent research indicates that condylar growth is largely a
response to translation as surrounding tissues grow.
Elastic type of chin cup produces lingual inclination of lower
incisors, an undesirable side effect even with plastic chin cup.
Ideal patient with excess mandibular growth for chin cup
therapy is one who has:
a) A mild skeletal problem, with ability to bring the incisors
end to end
b) Short vertical facial height
c) Normally positioned or protrusive, but not retrusive lower
A reverse overjet of more than 4 mm inwww.indiandentalacademy.com
1.The downward vertical growth of the midface is inhibited by use of
the chin cup. Posterior vertical development is inhibited more than
anterior vertical development, resulting in a clockwise rotation of the
maxilla and midface.
2. The chin cup has no effect on the anteroposterior growth of the
3. The modifications in midfacial growth appear to be adaptational
responses to similar alterations in mandibular growth caused directly
by the chin cup. Such modifications are necessary to maintain
harmonious growth between the maxilla and mandible.
4. Force transfer from the chin cup via the mandible to the middle
cranial fossae results in a closure of the cranial flexure angle, N-SBa°, and alterations in the manner of growth of Ba (x) and S (y).
5. The maxillary molars move in a mesial direction at a rate greater
than in the control group, but are not affected in their rate of eruption.
The increased mesial movement is probably related to the increased
rate of forward movement of the mandibular molars.
Two main approaches
to chin cup therapy
Response to chincup
SELECTION GUIDELINES FOR
1. Cervical pull face bow headgear
a) A large horizontal component of force is present, but
also a vertical component, which may extrude the
b) Molar extrusion may assist the treatment of class II, low
Frankfurt-mandibular angle, increased overbite cases.
Limited molar extrusion will probably not affect class II
cases with an average FMA, particularly if angle SNB is
average. Facial changes must be monitored during
d) High FMA, class II cases, should never be subjected to
this line of force to avoid the creation of an unfavorable
mandibular rotation, with consequent ill effects upon the
Outer arms bent downwards will tilt distally the
crowns of mesially tilted molars.
Outer arms bent upwards appear to result in more
upright, but less distal movement.
2. Straight pull face bow headgear
a) A very large horizontal component of force is present.
b) A small vertical force component may produce mild
c) It will probably effect less distal movement of the root,
than of the crown.
3. High pull face bow headgear
a) Root axial control may be achieved to produce
effective upright distal movement of the molar teeth or
tilting as is required.
b) The molar will be intruded and the ratio between distal
and intrusive movement will depend upon the
steepness of the angle of pull.
c) It is a suitable line of force to move distally the fully
banded maxillary arch, intruding the molar end and
less certainly the incisor end.
d) In high FMA, class II cases, with reduced or even
average overbite, distal movement with extrusion of
maxillary molars is probably to be avoided, and high
pull anchorage may be advantageous.
e) In high FMA, class II cases, with anterior open bite, a vertical
line of force commencing occlusally and passing distally to
the center of resistance, will intrude the maxillary molars and
may rotate downwards the incisal end of a fully banded arch.
4. Cervical pull J-hook headgear
Used to the maxillary incisor region, a tipping of the incisal
end of the occlusal plane in a downward direction may result,
with a reduction of open bite. However molar extrusion is
Used to the mandibular incisor region, it may depress the
chin creating more vertical space into which maxillary teeth
may be extruded during class III treatment. The resultant
downward and backward rotation reduces the anterowww.indiandentalacademy.com
posterior basal discrepancy.
5. Straight pull J-hook headgear
It is suitable for moving mandibular canines distally.
Attached to the maxillary incisor region, distal arch
movement occurs, but a downward tipping of the incisal end
of the arch is probable.
6. High pull j-hook headgear
A line of force to the maxillary incisor region passing mesial
and apical to the center of resistance, will intrude the upper
incisors, move them distally and augment palatal root torque.
A line of force to the maxillary incisor region passing through
the center of resistance will have a large distal and smaller
intrusive effect upon the incisor region. Theoretically this may
produce the greatest orthopedic effect.
A line of force to the maxillary incisor region passing
occlusal to the center of resistance may have a mild
downward tipping effect upon the incisal end of the
d) It is the direction of choice for distal movement of
maxillary canines or to sliding jigs for maxillary molar
distal movement or anchorage
The following are to be followed with headgear treatment:
1. Routine use of Visual Treatment Objective of some
type of comparative treatment goal.
2. Routine cephalometric x-rays at six to nine month
intervals to evaluate treatment changes and progress.
3. Knowledge of normal growth and the effects of
orthodontic treatment and extraoral forces to the patient.
4. A prediction of the future skeletal pattern of the patient,
the accuracy of which can be enhanced by the control
that can be exerted upon the skeletal pattern by proper
orthodontic and orthopedic treatment.
Extraoral anchorage in Edgewise
Either occipital or cervical anchorage may be
used to supplement the intraoral Class II
elastics. Wire hooks extending from a plastic
cervical tube can be used to attach the
Class II loops on the maxillary arch wire.
Traction was derived from rubber bands
secured to the posterior ends of the wires
within the tube.
The amount of pressure is determined by the
reaction of the teeth; it should be strong
enough to cause moderate tenderness
following eight or ten hours' application.
Headgear in Pre-adjusted edgewise
Extra oral force is the most effective way to provide
posterior anchorage control in upper arch.
Combination headgear using a force level of 150 – 250 g
for occipital pull and 100 – 150 g for the cervical pull.
These force values allow for slightly stronger pull on the
occipital component of the headgear, keeping forces
directed slightly above the occlusal plane and minimizing
the tendency for vertical extrusion of upper posterior
teeth, while simultaneously allowing effective distalization
On high angle cases where little distalization of molar is
required, an occipital headgear alone can be utilized.
In very low angle cases, where musculature is strong
enough to minimize vertical extrusion of the posterior
teeth, a cervical headgear can be considered.
In controlled space closure
Combination headgear with ‘J’ hook carried directly
to arch wire hooks in the maxilla in selected cases
with the arch wire turned upside down.
Headgear in refined Begg technique
In third stage
In treatment of malocclusions such as very large
overjet, very deep overbite or severe bimaxillary
protrusion, anchorage needs to be reinforced.
A headgear may be added to the upper molars to
augment anchorage in sagittal and vertical
Components of face bow
Maxillary molar tubes are positioned
gingivally or occlusally on the molar
The advantage to gingival placement is
that the tube is closer to the center of
rotation of molar, which reduces molar
The outer Bow
The outer bow ends anteriorly to the ears. Then when
a patient wears a combination face bow, the high pull
portion will fit naturally in front of the ears and the neck
strap will attach below the ears.
In all cases, the outer bow is positioned in the
horizontal plane parallel to and even with the inner
When using a high pull retractor, the end of the outer
bow should coincide with the location of the maxillary
first molars. It is bent 60 degree angle superior to
The outer bow must be adjusted to www.indiandentalacademy.com
fit the face of the
patient. Should be 5 to 10mm away from cheeks.
Outer bow resting
passively between lips
Outer bow several
millimeters from cheek
Length of outer bow is critical to the desired
The Inner Bow
Proper adjustment of the inner bow will allow the wire to
slide in and out of the headgear tubes easily when the
posterior strap is not attached.
Adjustments to the inner bow can be made in six
directions: bucco-lingually, superior- inferiorly, anteroposteriorly.
First Bucco-lingual force is controlled.
If the bow is inserted into one headgear tube, the other
bow end should be expanded approximately 5mm buccal
to the opposite tube.
This expansion bend is made near the anterior portion of
the inner bow.
As a class II molar relationship is corrected, the relative
forward movement of the lower arch will produce a cross
bite tendency unless the upper arch width is expanded
Vertical adjustments can be made at molar
If maxillary arch expansion is desired and a face bow
is used, a greater amount of expansive force must be
built into the inner bow. Inner bow is expanded more
Secondly, in superior-inferior direction
When the patient closes his mouth and relaxes his
lips, the anterior junction of the inner and outer bows
should not be pushing either lip in vertical direction.
The bow should be in a passive position between the
lips. In order to maintain this position , the posterior
ends of the inner bow are adjusted superiorly or
Lastly, antero-posterior adjustment.
Inner-outer bow junction is just anterior to the
point where the lips seal.
It may be necessary to enlarge or constrict
the loops in the inner bow to achieve this
The potential of these devices to injure the face has
been recognized by the orthodontic community. Among
the possible sites of injury are the eyes.
With improper handling, headgear appliances can result
in penetrating ocular injuries. The removable metallic bow
contains two projections that normally fit into the mouth.
However, when pulled forward, the bow can slip from the
oral cavity, retract under tension, and strike the eyes with
Spectacles may provide protection, but it is also possible
that these metallic projections under tension could slip
beneath the frames and strike the eyes. The distance
between these two projections approximates the
interpupillary distance. Thus, there www.indiandentalacademy.com
is an added risk of
In general, bacterial endophthalmitis occurs infrequently after
penetrating ocular injuries. However, in headgear injuries, the
risk of bacterial infection is extremely high because the
penetrating object is contaminated with saliva.
The normal flora of the oral cavity consists of a multitude of
organisms, including S viridans, anaerobic and aerobic
staphylococci, gram-negative diplococci (Neisseria and
Branhamella species), Corynebacterium, Lactobacillus,
anaerobic Vibrio, and Actinomyces species. Thus, patients are
susceptible to mixed-flora infections.
In response to the occurrence of facial injuries, manufacturers
are developing new appliances with devices that prevent
disengagement or that release the elastic traction when sharp
forces are applied
which breaks apart
force is applied.
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