biomechanics of space closure in orthodonticcs / fixed orthodontics courses

Biomechanics of Space Closure

INDIAN DENTAL ACADEMY
Leader in continuing dental education
www.indiandentalacademy.com



Introduction
Every object or free body has one point on which it can be perfectly
balanced. This point is known as the center of gravity
In a restrained body, such as a tooth, a point analogous to the center of
gravity is used; this is called the center of resistance.
By definition, a force with a line of action passing through the center of
resistance produces translation.
The center of resistance of a single-rooted tooth is on the long axis of the
tooth, probably between one third and one half of the root length apical to
the alveolar crest.
For a multirooted tooth, the center of resistance is probably between the
roots, 1 or 2 mm apical to the furcation.


Relevance of the center of resistance
First, the position of the center of resistance varies with root length
The tooth movement resulting from a force delivered at the bracket depends
upon the distance of the line of action of the force from the center of
resistance;
Identical forces applied to teeth with different root lengths can have different
effects.
A second important point is that the center of resistance varies with
alveolar bone height.
The movement of teeth in adults with alveolar bone loss will be different
than in adolescents.
Smith and Burstone AJO-DO 1984



The M/F ratio is the relationship between the applied force and the
counterbalancing couple
The type of movement is dictated by the moment to force ratio (M/F) generated by
the appliance at the attachments.
Typically, M/F ratios of approximately 7:1 millimeters result in controlled tipping,
10:1 millimeters result in translational movements, and values of 12:1 millimeters
or greater accomplish root movement.

This has important implications.
It is the ratio between the applied couple and force that determines the type of
tooth movement, not the absolute magnitudes.


These ratios are based on the assumptions that
the root lengths are 12 millimeters,
the distance from the bracket slot to the alveolar crest is five millimeters,
the alveolar bone condition is normal,
the axial inclination of the teeth is normal,
and the center of resistance is located apically a distance .40 times the root
length when measured from the alveolar crest to the apex.


Manhartsberger, Morton, Charles J Burstone Angle 1989


STATICALLY DETERMINATE AND
INDETERMINATE SYSTEMS
Force systems can be defined as statically determinate when the
moments and forces can be readily determined, measured and evaluated
.
Statically indeterminate systems are too complex for precisely
determining all the forces and moments in the equilibrium.
Usually only the direction of the net moment and the appropriate net
force levels can be determined.
Determinate systems in orthodontics are those in which a couple is
created in one end of an attachment with only a force and not a couple at
the other end .e.g. a spring which is inserted to a tube or bracket at one
end and tied at the other end to only one point.

WILLIAM R. PROFFIT, HENRY W. FIELDS


Orthodontic space closure

Orthodontic space closure
should be individually tailored
based on the diagnosis and
treatment plan.
The selection of any treatment
whether a particular technique,
stage spring or appliance
designs should be based on the
desired tooth movement.



A well-designed appliance should exhibit three general
characteristics:
(1) It should deliver a known, relatively constant moment-to-force
ratio over a long range of activation;
(2) the resultant motion of the active unit (teeth being moved)
occurs about a predictable center of rotation; and
(3) the force system at the reactive unit (anchor teeth) should be
known and controllable.

Braun and Marcotte AJO-DO 1998



The six goals to be considered for
any universal method of space closure:
(1) Differential space closure..
(2) Minimum patient cooperation.
(3) Axial inclination control
(4) Control of rotations and arch width.
(5) Optimum biologic response.
(6) Operator convenience.
To achieve controlled extraction site closure, the appliance used must
deliver definable force systems regulated by the clinician and not produce
closure in some ambiguous, indeterminate way. Only when force systems
are definable are the dental movements predictable and treatment outcomes
forecast able with confidence.

CHARLES J BURSTONE AJO-DO 1982


Idealized objective of space closure:

Space closure should result in upright well aligned teeth with parallel
roots and parallel occlusal plane.
Therefore some degree of bodily or even root movement is required.



DETERMINANTS OF SPACE CLOSURE
The main factors which determine the tooth movement during space closure are:
Amount of crowding:
in cases of severe crowding, anchorage
control is very important to
maintain the extraction space for
relieving the anterior crowding
Anchorage
using the same mechanics for different
anchorage needs is very important.
Traditional anchorage methods like
lip bumpers, headgears,
transpalatal arches may be utilized
but non compliance methods for
anchorage control based on
biomechanics can also www.indiandentalacademy.com
be used.
NANDA& KULHBERG


Axial inclination of canines
the same force /and or moment applied to teeth with different axial
inclinations will result in different types of tooth movement.



Midline discrepancies and left right symmetry.
Midline discrepancies should be corrected as early as
possible in treatment as it allows the remaining space closure to be
completed symmetrically. Using asymmetric mechanics can cause
in unilateral anchorage loss, skewing of the dental arches, or
unilateral vertical forces.
Vertical dimension
Control of vertical dimension is essential in space closure.
Undesired vertical extrusive forces on the posterior teeth can result
in increased LAFH, increased interlabial gap, and excessive
gingival display. Class II elastics may potentate this problem.



FUNDAMENTAL CONCEPTS AND CLINICAL METHODS
OF ANCHORAGE CONTROL
It is the ability to prevent tooth movement of one group of teeth while
moving another group of teeth.
The problem of anchorage is rooted in Newton’s third law:
For every action there is an equal and opposite reaction.
Several anchorage control methods have been developed over the last
century.
The contributions of Angle, Begg , Case, Tweed and others have
provided a foundation for modern orthodontic mechano therapy.
Although each of them advocated different methods and philosophies, a
review of their work shows a lot of similarities.
ANDREW KUHLBERG,

DEREK PRIEBE:
SEMIN ORTHOD 2001


In 1907, EH Angle advocated 5 types of anchorage control.
Occipital anchorage depended on the use of extra oral anchorage
Intermaxillary anchorage included the use of elastics.
The remaining three were dental anchorage:
Simple, reciprocal and stationary methods for dental anchorage.

Calvin S Case also advocated stationary anchorage methods despite his
differences with Angle’s school of thought.
He also described the use of extra oral and intermaxillary anchorage as well
as the prerequisite that resistance to tipping movements was requisite for
intra arch control.
Case advocated the use of soldered firm attachment of anchorage teeth to one
another to maintain their upright positions.


20 years later Charles Tweed advocated similar techniques. His method
of anchorage preparation against unwanted tipping and extrusive side
effects were a series of tip back bends to anchor the teeth like tent stakes
to

resist

vertical

and

anterior

posterior

displacement

during

intermaxillary traction.
Although Tweed said his methods of anchorage preparation were more
mechanical than biological, the tip back bends were a further refinement
of Angle’s stationary anchorage methods.
Despite his adherence to the differential force theory, PR Begg
also used a similar technique for anchorage control. His tip back bend to
maintain the anteroposterior position

of teeth to effect preferential

movement of teeth was also supplemented by initially tipping the teeth to
be retracted followed by up righting them.


Anchorage can be classified as:
A ANCHORAGE
This category describes the critical maintenance of the posterior tooth
position.
75% or more of the extraction space is needed for anterior retraction.
Group A arches tend to be of two types:
B ANCHORAGE
This category describes relatively symmetric space closure with
equal movement of the anterior and posterior teeth to close the space.
This is the least difficult of the space closures.
C ANCHORAGE
This category describes non critical anchorage, where 75% or more
of the space closure is achieved through mesial movement of the posterior
segment; this could also be described as critical anterior anchorage.


Dividing the extraction space into quarters aids in visualizing the anchorage
classification

NANDA& KULHBERG


ANCHORAGE FROM A BIOMECHANICAL PRESPECTIVE:
The basic techniques for anchorage control basically rely on three fundamental
similarities:
extra oral forces on the anchorage unit
intermaxillary elastics
Tipping tooth movement while simultaneously discouraging
tipping of anchorage teeth.
Patient compliance is mandatory for the first two techniques.
Without co operation control of tooth movement is lost and the results may be
compromised.
The way a tooth moves is dependent on the nature of the force systems that act
on it. This includes the actual force and moments at the bracket, the force
distribution around the periodontal ligament,.
The force distribution is a function of the centre of rotation.


CONTROLLED TIPPING:
Is tooth movement with the center of rotation at the root
apex. The resultant forces are distributed at the marginal
portion of the periodontal ligament
The M/F ratio is approx. 7/1
TRANSLATION or bodily movement maintains the axial
inclination of the tooth and the centre of rotation is at
infinity. The resultant force on the PDL is equally
distributed along the pressure side of the alveolar
structures.
The M/F ratio is approx. 10/1
ROOT MOVEMENT or displacement of the tooth apex
while the crown remains stationary occurs with a M/F
ratio of approx. 12/1 here forces tend to be concentrated
on the apical third of the root.

Within these basics
lie the fundamental
principles of anchorage control.


What happens when a high M/F ratio is applied to the anchor teeth?
An applied force causes uncontrolled tipping while the applied
moment counteracts the tipping effect of the force. This applied
moment acts in the opposite direction and moves the roots to the
extraction site and if the magnitude further increases, tips the crown
distally
A low M/F ratio
noticeably greater

produces tipping with the crown moment

How can these forces be produced clinically?
If M/F ratio of posteriors> M/F ratio of the anteriors there must be
either unequal forces or unequal moments
Unequal forces can be produced by headgear, J hook headgear, and
intermaxillary elastics. Unfortunately this is co operation dependant


BIOMECHANICAL STRATEGIES FOR
DIFFERENTIAL SPACE CLOSURE:
The ideal for system for a group A space closure would have only a force
system resulting in anterior translation and no forces acting on the
posterior teeth thereby maintaining perfect anchorage control.
This is possible only with extraoral anchorage or if the opposite arch is
used as anchorage.
Two approaches:
differential forces
differential moment to force ratios
group A requires the posterior segment to have higher M/F ratios (when the
force is reduced M/F is increased) and the anterior segment to have a
decrease in M/F ratios.

NANDA& KULHBERG


Differential forces
the law of static
equilibrium ensures that
within a single intra arch
appliance the mesiodistal
forces must be equal,
thus forces can be increased
or decreased only by
utilizing extra oral forces or
the opposite arch via
intramaxillary elastics.
These
are
compliance
dependent
methods which also have
additional side effects.


differential moment to force ratios

Increasing the posterior
moment causes root movement
M/F>12/1 while decreasing the
anterior moment causes a tipping
type of moment M/F ~7/1
If the posterior moment
were large enough the M/F ratio
would
approach
infinity
consistent with the application of
a pure couple on the posterior
segment. this would result in
rotational tooth movement about
the center of resistance of the
anchor teeth moving the crowns
distally and increasing the size of
the extraction space.


Differential moments are not without side effects:
Unequal moments must be balanced by a
third moment .
This couple is a pair of vertical forces.
intrusive to the anterior and extrusive to
the posterior.
The magnitude is dependent on the
difference of the moments acting on the
teeth and the distance between the
anterior and posterior teeth.
No matter what the strategy some
biomechanical side effects will occur.
The difficulty of group C anchorage
mirrors that of group A.
The difference is that the anterior teeth
are the effective anchor unit. therefore
the anterior moment is of greater
magnitude and the vertical side effect is
an extrusive force on the anterior teeth.


BIOLOGIC VARIABLES IN ANCHORAGE CONTROL
AND DIFFRENTIAL SPACE CLOSURE:
Optimal force is the idea that there is
a force level that will promote the
most efficient treatment without
untoward side effects.
The true mechanical parameter in
tooth movement is not the magnitude
of the force per se, but rather the
magnitude of the stress generated by
the appliance in the surrounding
periodontium.
Stress is defined as force per unit
area (for example, gm/cm2) and
strain is the unit deformation that
occurs in the tissue as a result of the
stress.


. Possible hypotheses of the relationship between stress magnitude
and the rate of tooth movement are graphically represented.


Quinn & Yoshikawa AJO-DO 1998


HYPOTHESES OF THE STRESS-MOVEMENT
RELATIONSHIP (Quinn and Yoshikawa)
Hypothesis 1
shows a constant relationship between rate of
movement and stress. The rate of movement does not
increase as the stress level is increased.
The clinician operating under this assumption controls
anchorage only through interarch and/or extraoral
mechanics. To place more teeth into the anchorage unit
or extract teeth in a more anterior position in the arch
does not affect the final tooth position.

stress

If elastics are used for retraction, only one size is necessary.
Loop designs are not critical and can be simple and uncomplicated by helices.
Intrarch mechanics cannot be altered to change final tooth position.
An extraction site, regardless of where it lies in the arch, is always evenly
closed by retraction of the teeth anterior to it and protraction of the teeth
posterior to it. The periodontium is sensitive only to force direction and not to
force or stress magnitude. www.indiandentalacademy.com


Hypothesis 2 is more complex.
The relationship here calls for a linear increase in
the rate of tooth movement as the stress increases.
Operating under this hypothesis, the clinician would,
in order to shorten treatment time, use appliances
that generated the highest stress values
In this system intraarch anchorage could be
manipulated by adding teeth (second molars) to the
anchorage unit or moving the extraction site— for
example, second versus first premolars.
This would distribute the stress over a larger root surface, lowering the local
stresses and slowing the rate of tooth movement.
Arch wires designed for space closure would be fabricated from large, crosssection steel wire with closing loops activated to generate large stresses. The
appliance that delivered the highest stresses would close extraction sites most
rapidly.


Hypothesis 3
depicts a relationship in which increasing
stress causes the rate of movement to
increase to a maximum.
Once this optimal level is reached,
additional stress causes the rate of
movement to decline
This hypothesis was originally proposed
by Smith and Storey.
Some clinicians assume the validity of the
latter part of the curve in this hypothesis
where light forces move teeth "optimally"
and an increase of stress slows movement.
These clinicians use light forces, for example, to retract canines and prevent
anchorage loss while using heavy forces to protract posterior teeth and
"anchor" the canines.
The orthodontist who operates under this hypothesis theoretically has a great
deal of control over anchorage and final tooth position without resorting to
extraoral or interarch mechanics.


Hypothesis 4
is a composite of some of the foregoing
concepts.
Here the relationship of rate of
movement and stress magnitude is linear
up to a point; after this point an increase
in stress causes no appreciable increase in
tooth movement.
Because the rate of movement is
dependent on changes in stress,
anchorage can be controlled within the
arch. Change of extraction patterns,
addition of teeth to the anchorage unit,
and modification of intraarch retraction
mechanisms to fit the anchorage
requirements are all effective means to
determine the final tooth position.


For example, operating under this hypothesis, a clinician could
enhance canine retraction by
(1) extracting first premolar teeth instead of second premolars,
(2) incorporating second molars into the posterior segment,
(3) adjusting the stress delivered by the retraction mechanism
so that the stress level at the canine would coincide with the
maximal rate of movement. The stress on the posterior teeth
would be distributed over a greater root area, lowering the local
stresses and producing a rate of movement less than maximal.



EVALUATION OF HYPOTHESES
None of the studies reviewed by Quinn and Yoshikawa support
hypothesis 1. All of the results show that, to varying extents,
changing the mean stress magnitude will produce changes in the
rate of tooth movement
Hypothesis 2 is difficult to disprove because most studies used
only two force magnitudes and were unable to describe the
behavior of the curve as the stress reached higher levels. Boester
and Johnston did demonstrate, however, that in their system forces
above 140 gm produced no measurable increase in tooth
movement. This study, along with that of Hixon et al. which
suggested a 300-gm plateau, casts serious doubts on the validity of
the continuing linear relationship proposed in hypothesis


Hypothesis 3, the original Smith and Storey proposal, can no longer be
considered viable in light of subsequent data.
•A more rigorous analysis of their report shows that their data did not
justify their conclusions. The canine moved further than the molar at
both the high and low force levels and there is no evidence for the rate
of movement to suddenly reverse as the stress levels increase past a
certain "optimum" value.
• Furthermore, in all the canine retraction experiments, the rate of
canine movement was greater than that of the molar segment.
• Because of its smaller root surface, the mean stresses on the canine
can be assumed to be higher than those on the posterior unit.
•At the stress levels evaluated in these experiments then, an increase in
stress appeared to cause an increase in the rate of movement. The
available literature suggests that hypothesis 3 may not be an accurate
representation of the data.


The evidence for hypothesis 4 is more compelling.
All studies reviewed support the idea that
increasing mean stress produces a higher
rate of tooth movement. Both Smith and
Storey and Andreasen and Zanier
demonstrated greater displacement of the
posterior teeth as the force level was
increased.
This finding is consistent with the
hypothesis since the molar segment, under
less stress because the force is distributed
over a larger area, would be on the
ascending portion of the curve and would
move at a greater rate when the stress level
was increased.



Interestingly, neither study provided evidence of a statistically
significant difference in canine movement at the two force levels. This
might indicate that the canine was moving at a near maximum rate at
the lower force and that increasing the force did not increase the rate
of movement.
These data, along with those of Boester and Johnston, Hixon et al. and
Burstone and Groves, provide evidence that beyond a certain stress
level increasing stress no longer alters the rate of tooth movement.



If hypothesis 4 with a clinically useful slope and a plateau in the 100
to 200 gm range is thought to be a valid model, there are two clinical
strategies that would maximize anchorage within the arches.
The first is to lower the stress delivered to the posterior teeth.
This can be done by increasing the root surface area,
either by incorporation of second molars into the anchorage unit or
by making extractions more anteriorly in the arch. The decrease in
stress obtained by increasing the root surface area of the posterior
teeth will slow their rate of movement and allow more canine
retraction. The same rationale makes extraction of second premolar
teeth a logical choice in minimum anchorage cases.



The second strategy for minimizing anchorage loss is to
use an appliance system that can deliver relatively continuous
stresses in the range described earlier.
Excessive stress produces an increase in the rate of
movement of posterior teeth without increasing retraction of
anterior teeth.
Appliances that have a high load-deflection rate are unable
to achieve a difference in the rate of tooth movement.
Low load deflection-rate mechanics, on the other hand, can
maintain stresses in the desired range and maximize the difference
in the rate of movement.



DIFFERENTIAL FORCE SYSTEMS-VARIABLE
MOMENTS AND FORCES

The force system of an orthodontic
appliance acts in all three planes and
determines the type of tooth
movement.
ALPHA MOMENT
This is the moment acting on the
anterior teeth (also called anterior
torque)
BETA MOMENT
This is the moment acting on the
posterior teeth. Tip back bends
placed mesial to the molars produce
an increased beta moment

The components of a force system for a
space closure from a Sagittal view are:


NANDA& KULHBERG


HORIZONTAL FORCES
These are the mesio-distal forces
acting on the teeth which are equal to
each other.
VERTICAL FORCES
These are the extrusive intrusive
forces generated because of the
unequal moments. When the beta
moment is greater, a intrusive forces
act on the anterior teeth. And while
alpha moment is greater an extrusive
force acts on the anteriors while an
intrusive force acts on the posteriors.


Center of resistance of anterior teeth during retraction

The location of the center of resistance of various consolidated units of the
maxillary anterior dentition was studied using a dry human skull when
subject to retrusive forces.
The units studied consisted of
(a) two central incisors,
(b) four incisors, and
(c) six anterior teeth.
The laser reflection technique and the holographic interferometric
technique were employed to measure the displacement of the dentition
to the applied forces.
Vanden www.indiandentalacademy.com
Bulcke, Dermaut, Sachdeva, and Burstone AJO-DO 1998


Results:

1. For an anterior segment comprising two central incisors,
the center of resistance was located on a projection line parallel to the
midsagittal plane on a point situated at the distal half of the
canines.
2. For an anterior segment that included the four incisors, the center of
resistance was situated on a projection line perpendicular to the
occlusal plane between the canines and first premolars.
3. For a rigid anterior segment that included the six anterior teeth, the
center of resistance was situated on a projection line perpendicular
to the occlusal plane distal to the first premolar.



4. The centers of resistance of the anterior segments incorporating
two or four anterior teeth were within ± 2 mm of each other.
However, inclusion of the canines in the anterior segment resulted
in the center of resistance moving distally by approximately one
premolar width (7 mm). This effect may have been the result of the
resistance of bony structures at the level of the canines and some
bending of the maxillary complex as was observed on the
holograms.
5. No appreciable shift in the location of the centers of resistance of
the various segments studied was detected as varying magnitudes of
retractive force were applied.



METHODS OF CANINE RETRACTION:
Friction
Frictionless ( PG spring, Burstone T loop, Ricketts)
METHODS OF ENMASSE RETRACTION:
OF FOUR INCISORS
Friction
Frictionless
PG retraction spring,
Utility arch, Omega Loop archwire
Extraoral
Headgears
OF SIX ANTERIORS
Closing loop archwire
Burstone T loop continuous archwire
Opus loop
INTRUSION AND RERACTION OF FOUR INCISORS
Burstone’s three piece intrusion arch
Rickets retraction and intrusion utility arch
SIMULTANEOUS INTRUSION AND RETRACTION OF SIX ANTERIORS
K-sir arch


The most significant distinction between the mechanics of
standard edgewise and preadjusted appliances was observed
during space closure.
With standard edgewise appliances, rectangular archwires did not
effectively slide through the posterior bracket slots because of the
1st-, 2nd-, and 3rd-order bends.
The orthodontist normally used a closing loop arch, which was
activated in the office by opening the closing loop and moving the
archwire through the posterior bracket slots


- RICHARD P. McLAUGHLIN, JOHN C. BENNETT, JCO 1998


The level bracket slot alignment of the new appliances allowed
archwires, for the first time, to move more effectively through the
posterior slots when the patient was not in the office.
As a result, many orthodontists discontinued use of closing loops and
began using various forms of sliding mechanics— for example,
placing hooks in the anterior sections of straight archwires and tying
elastics or springs to them from molar brackets



Closing loop arches had several
disadvantages:
1. Extra wire-bending time
2. Poor sliding mechanics
3. Tendency to run out of space for activation
(after two or three activations, the omega
loop contacted the molar bracket and the
archwire had to be adjusted or remade)
4. High initial force levels

They also had advantages:
1. Precise control of the amount of loop
activation (often as little as 1mm), limiting
the amount of initial tipping
2. Adequate rebound time for uprighting
between appointments (with minimal
activations, loops closed quickly with little
tipping)


Sliding mechanics had these advantages:
1. Minimal wire-bending time
2. More efficient sliding of archwires through posterior bracket slots
3. Sufficient space for activations

But sliding mechanics at first also had disadvantages:
1. No established guidelines on amounts of force to be used during
space closure
2. Tendency for initial overactivation of elastic and spring forces,
causing initial tipping and inadequate rebound time for uprighting



To maximize the advantages and minimize the disadvantages of
sliding mechanics
•
force levels are reduced during space closure.
•
Instead of springs or over activated elastics (which
can produce 500g of force), single elastic modules are attached
to anterior archwire hooks with
ligature wires extended
forward from the molars
These "elastic tiebacks", when activated 2-3mm, generate
about 100-150g of force.
If the arches have been properly leveled, such light force allows
for effective space closure; there is little tipping with
subsequent binding of the archwires, and leveling is maintained
.


019" × .025" archwires with .022" slots provide optimum
rigidity, but adequate freedom for the wires to slide through the
slots.
Round wires and smaller rectangular wires provided less
precise control of torque, curve of Spee, and overbite.
Hooks of .024 " stainless steel or .028 " brass are soldered to
the upper and lower archwires



Effects of Overly Rapid Space Closure
Space closure typically occurs
more easily in high-angle patterns
with weak musculature than in
low-angle patterns with stronger
musculature. The rate of closure
can be increased, particularly in
high-angle cases, by slightly
raising the force level or using
thinner archwires. However, more
rapid space closure can lead to
loss of control of torque, rotation,
and tip.



Loss of torque control results in upper
incisors being too upright at the end of
space closure with spaces distal to the
canines and a consequent unaesthetic
appearance. The lost torque is difficult
to regain. Also, rapid mesial movement
of the upper molars can allow the
palatal cusps to hang down, resulting in
functional interferences, and rapid
movement of the lower molars causes
"rolling in"


Reduced rotation control can be
seen mainly in the teeth adjacent to
extraction sites, which also tend to
roll in if spaces are closed too
rapidly
Reduced

tip

control

produces

unwanted movement of canines,
premolars, and molars, along with a
tendency for lateral open bite. In
high-angle

cases,

where

lower

molars tip most freely, the elevated
distal cusps create the possibility of
a molar fulcrum effect


In some instances, excessive
soft-tissue

hyperplasia

occurs at the extraction sites
,this is not only unhygienic,
but it can prevent full space
closure or allow spaces to
reopen after treatment. Local
gingival

surgery

may

be

necessary in such cases.



Inhibitors to Sliding Mechanics
Proper alignment of bracket slots is essential to eliminate
frictional resistance to sliding mechanics. The common
procedure is to use .018" or .020 " round wire for at least one
month before placing .019"´.025" rectangular wires. Leveling
and aligning continues for at least a month after insertion of the
rectangular wires, and that space closure cannot proceed during
that period.



Therefore the rectangular wires are tied passively for at
least the first month, until leveling and aligning is complete
and the archwires are passively engaged in all brackets and
tubes
Conventional elastic tiebacks are than placed ,In some cases,
this phase takes three months.



There are three primary sources of friction during space closure
First-order
or
rotational
resistance
at the mesiobuccal and
distolingual aspects of the posterior
bracket slots is produced by
rotational forces on the buccal
aspects of the posterior teeth.
The most effective way to
counteract this resistance is to
apply intermittent lingual elastic
forces—
one month from cuspid to first
molar, the next month from cuspid
to second molar.


Second-order or tipping resistance
at the mesio-occlusal and distogingival
aspects of the posterior bracket slots is
caused by
excessive and overactivated
tieback forces, which lead to
• tipping of the posterior teeth,
• inadequate rebound time to
upright these teeth,
• and a resultant binding of the system.
The importance of light forces (50150g) and minimal activation length (to
provide time for uprighting) cannot be
overemphasized.


Third-order or torsional resistance
occurs at any of the four areas of the
bracket slot where the edges of the
archwire make contact.
Like tipping resistance, this is produced
mainly by
excessive and overactivated
tieback forces, which cause the upper
posterior lingual cusps to drop down
and the lower posterior teeth to roll in
lingually



Problems During Space Closure
Since forces are directed from the first molars to anterior hooks on the
archwire, small spaces occasionally open between the first and second
molars.
This can be managed in one of three ways:
A damaged lower premolar or first molar bracket, either from careless
use of biting sticks during bonding or from improper diet, can hinder
space closure
Interference from opposing teeth sometimes restricts lower arch space
closure, particularly if bracket placement was incorrect or a full-unit
Class II molar relationship existed.



As spaces close, the distal ends of the archwires will protrude
more and more, and these protruding wires will tend to become
bent gingivally by chewing forces
Certain tissue factors can hinder full space closure with any kind
of mechanics. Soft-tissue build-up can result from poor plaque
control or overly rapid space closure. The alveolar cortical plate,
mesial to the lower first molars, tends to narrow after extraction of
the second premolars, especially in lower-angle situations.
Retained roots, ankylosed teeth, and bone sclerosis are other
possible factors to be considered.



Constant attention is required to prevent any of the
following inhibiting factors:
1. Inadequate leveling, resulting in archwire binding
2. Posterior torque such that torquing and sliding cannot occur
simultaneously
3. Blockage of the distal end of the main archwire by a ligature wire
4. Damaged or crushed brackets that bind the main archwire
5. Soft tissue resistance from build-up in extraction sites
6. Cortical plate resistance from a narrowing of the alveolar bone in
extraction sites
7. Excessive force, causing tipping and binding
8. Interferences from teeth or the opposing arch
9. Insufficient force


BIOMECHANICS OF CANINE RETRACTION ON A
CONTINUOUS ARCHWIRE


ROBERT J NIKOLAI SEMIN ORTHOD 2001


Friction is a function of the relative roughness of two surfaces in contact. It is
the force that resists the movement of one surface past another and acts in a
direction opposite the direction of motion.
VARIABLES AFFECTING FRICTIONAL RESISTANCE DURING
TOOTH MOVEMENT
PHYSICAL

BIOLOGICAL

ARCHWIRE

SALIVA

LIGATION

PLAQUE

BRACKET

ACQUIRED PELLICLE

ORTHODONTIC APPLIANCE

CORROSSION


NANDA& KULHBERG


PHYSICAL

BIOLOGICAL

Saliva
ARCHWIRE
crossectional size/shape
Plaque
material
Acquired pellicle
surface texture
Corrosion
stiffness
LIGATION
ligature wires
elastomerics
self ligating brackets
BRACKET
material
manufacturing process
slot width and depth
first/second/third order bends
ORTHODONTIC APPLIANCE
interbracket distance
level of bracket slots between adjacent teeth
forces applied for retraction


The segmental arch technique as
developed by Burstone utilizes
T loop space closure springs for
anterior retraction, symmetric
closure or posterior protraction.
The segmental T loop as
described by Burstone is one of
the most versatile space closure
devices available.
One of the main principles of the segmental arch technique is considering
the anterior segment and posterior segment as one large tooth
respectively. The right and left buccal units are connected by a
transpalatal arch forming one big posterior unit.
The basic configuration of the TMA loops consists of a .017X.025”
TMA wire.
CHARLES

J BURSTONE AJO-DO 1982


The rate of decay of the force applied by a spring is
called the load-deflection rate, and it averages 33 Gm. per
millimeter in the Burstone’s T loop.
The low load-deflection rate is important in this spring,
since it enables the orthodontist to deliver optimal
magnitudes of force.



High-load deflection springs as vertical loops dissipate force rapidly;
hence, one must activate to very high force levels in order to produce any
significant tooth movement.
Since the load-deflection rate is so high, it would be impossible for a
clinician to activate the loop to produce an optimum magnitude of force. To
deliver 200 Gm. of force, the required activation would be 0.2 mm. Not only
is it practically impossible to activate such a small distance, the force of 200
Gm. would be dissipated rapidly over the remaining 0.2 mm. of activation.
Thus, orthodontists who use high-force load-deflection mechanisms must use
high force values that have undesirable sequelae, which include anchorage
loss, pain, and undermining resorption.



In contrast, a retraction spring with a low load-deflection rate of 33
Gm. per millimeter allows for the delivery of optimal force levels, since
an error in activation of 1 mm. results in an error of only 33 Gm.
Furthermore, as teeth move distally, the reduction in force is small,
giving greater constancy of force at optimal levels.
Early in treatment, the posterior teeth are joined together to form a
posterior anchorage unit.
The anchorage unit consists of the right and left posterior teeth which
are connected by a buccal stabilizing segment and a transpalatal lingual
arch in the maxillary arch and a low lingual arch in the mandibular arch
During space closure, it is to be considered that there are only two teeth
— an anterior tooth comprising the incisors and the canines which have
been connected and a posterior tooth which includes molars and
premolars



.

The attachment on the posterior tooth (segment) is a 0.018 by 0.025 inch
auxiliary tube on the first molar, and the one on the anterior tooth (segment)
is an auxiliary vertical tube on the canine bracket



STANDARD FORM OF .
017x.025 TMA LOOPS
WITHOUT PREACTIVATION
BENDS

PREACTIVATION FORM OF THE
SPRING DESIGNED TO
PRODUCE EQUAL AND
OPPOSITE ALPHA AND BETA
MOMENTS WHEN POSITIONED
IN CENTERED POSITION


Stanley Braun,. Sjursen, Jr., Legan, AJO-DO 1995


To understand its design one must
first understand its passive form of
the spring and then its activation.
In the passive state there are no
moments or forces acting on it. In
its active state it applies a force
system on the teeth,
The activation of a spring requires
forces and moments to engage the
spring in its brackets and tubes.


Neutral position:
The neutral position in an activated
loop is found by applying the
activation moments and without any
horizontal forces. In other words the
ends are twisted to bring the each
attachment to its horizontal position.
in this position the spring has zero
horizontal force
The horizontal force is got by pulling
the spring open from this position.



Differential moments are obtained by the principle of off center V
bends which results in unequal moments. the closer the V bend is to the
tooth the higher the moment. the segmented T loops approximated a V
bend. Clinically the spring needs to be positioned at least 1-2mm closer
to one side than another to obtain a moment differential.


With the introduction of beta-titanium wire (TMA), it has been
possible to simplify the design so that a T loop by itself will have
a relatively low load-deflection rate and a large maximum
springback. The heavier base arch which fits into the auxiliary
tube of the first molar is important, since it allows positive
orientation of the spring and, more significantly, it is capable of
withstanding, without permanent deformation, the higher
moments that are needed for anchorage control. Furthermore, the
use of a heavier base arch tends to increase the moment-to-force
ratio on the anterior teeth, since any bending in the occlusally
positioned part of the spring tends to minimize this ratio.
To aid the clinician in achieving the proper angulation, templates
are used. Rather than to measure the angles, it is more

expeditious to duplicate the shape of the spring from a template.


The

T

loop

described

in

Biomechanics by Nanda is designed
for an activation upto 6 mm.at full
6mm activation tooth movement
occurs in three phases: tipping,
translation and root movement.
For a symmetric centered spring an
initial activation produces a M/F
ratio of 6/1 which results in tipping
movement of the teeth into the
extraction space.


With 2mm deactivation or spring
activation = 4mm the

M/F ratio

is 10/1 which results in translation
of the segments towards each
other. With 1-2mm space closure
(spring activation =2mm) the M/F
ration increases to 12/1 and higher
resulting

in

tooth

movement.

Clinically the spring should not be
re activated till all three phases are
complete.



OFF CENTERED T LOOPS
This demonstrates another method
that may be used for controlling the forces
and moments produced by segmented
0.017 ´ 0.025-inch TMA T-loop springs or
closing loops in general.
Previously, the approach described for
achieving differential alpha/beta moments
with segmented T-loops used asymmetric
angulations of the preactivation bends.
However, with this method the
moment differential does not remain
constant with spring activation, i.e., the
moment differential is dependent on both
spring activation and the differences in the
preactivation angulations.
Andrew Kuhlberg, Charles J. Burstone AJO-DO 1998


Off-center positioning maintains
the constancy of the moment
differential throughout the range of
spring deactivation (space closure).
This concurs with Burstone and
Koenig

who

demonstrated

a

moment differential and vertical
forces

with

off-center

vertical

loops.



The effect of off-center placement of T-loops with a standard
shape at a standardized activation and interbracket distance.
A centered T-loop produces equal and opposite moments with
negligible vertical forces.
Off-center positioning of a T-loop produces differential
moments. More posterior positioning produces an increased
beta moment. More anterior positioning produces an increased
alpha moment.
A standard shaped T-loop can be used for differential
anchorage requirements by altering the activation and mesialdistal position of the spring.



The center position of the spring can be found by:
distance = (interbracket distance –activation)/ 2
where distance = length of the anterior and posterior arms (distance from the center
of the T loop to either the anterior or posterior tubes)
interbracket distance=distance between the canine and molar brackets.
Activation= millimeters of activation of the spring


With the use of a vertical tube at
the canine a 90 degrees gingival
bend at the calculated distance
eases placement and monitoring
throughout space closure
The T loop is places in the
molar auxiliary tube and then
inserted into the canine bracket.
The distal end is pulled back
until it is the desired length
which results in the desired
activation.


Force systems related to type A, B, and C extraction site closure



MAXIMUM POSTERIOR
ANCHORAGE:
(Group A anchorage)
The T loop is positioned closer to the
posterior segment
(1-2 mm off centering) is sufficient .
activation of 4 mm is necessary.
This reduces the horizontal forces
without
altering
the
moment
differential.
The force system acting on the anterior
segment favors tipping. The moment
difference remains as the space closes
and the spring deactivates.
The spring must be re activated when 2
or less mm of activation remains.


Because the beta moment is greater
than the anterior moment a vertical
intrusive force acts on the anterior
teeth which can exaggerate the
tipping tendency and steepen the
occlusal
plane.
Similarly
the
posterior occlusal plane can be
steeped buy the extrusive force.
Maintaining adequate horizontal
force helps to reduce this effect. A
High pull headgear can also be used
to control the posterior occlusal
plane.
It is likely that root correction will
be required at the end of space
closure.


TPA

FIRST ORDER VIEW OF A T-LOOP SPRING WITH A
V-BEND INCORPORATED FOR ROTATIONAL CONTROL


MAXIMUM ANTERIOR ANCHORAGE :( Group C anchorage)

This is the most difficult of all
space closures. The increased
alpha moment has a tendency to
deepen the overbite.
The loop must be placed 12mm closer to the anterior teeth.
Care must be taken that the
wire segment achieves full
bracket engagement because play
can
reduce
the
moment
differential.
Space closure with tipping of
the buccal segments will occur.


the activation must be around 4mm and should be
activated every 2mm.
The major side effects are loss of anchorage and
extrusion of the anteriors.
Class III elastics or protraction headgear may help
in the protration of the upper buccal segments.
For mandibular molars class II elastics may help.



Segmental T loop space closure principles can also be applied to space
closure on a continuous arch.
the force system is not as well defined a the segmental but careful use of
the alpha and beta moments helps to achieve comparable results especially
for group B and C anchorage cases.
For group A cases high pull headgear is necessary to control tooth
position.
T loops one on each side are made using preformed arch wires .017 X .
025 TMA
or .016 X .022 Stainless steel arch wire.



The activations given are for TMA wires and the Stainless
steel wires activation is reduced by half.
The T lops are made 6-7mm tall and 10mm wide and are
positioned distal to the cuspids.
Desired alpha and beta moments are place anterior and
posterior to the T loop vertical legs.
Recommended beta activations for A, B, and C anchorages are
40 degrees, 30 degrees and 20 degrees.



After the activations are placed the loops should be open
approximately 2mm before placing in the mouth. the wire is inserted
into the molar auxiliary tube and ligated to the anterior teeth. The t loop
bypasses the premolars brackets and is not inserted in them.
For TMA loops the activation can be 3mm distal to the molar tube
which gives it a range of force of 250-300gms.
The patient should be monitored but no further activations are
necessary for 2-3 months. Too frequent reactivation can prevent root
movement and cause excessive tipping



CONTROL OF THE MECHANICAL SIDE EFFECTS OF SPACE CLOSURE:

From the Occlusal view the first order side effect of
rotation of the molars and canines are observed.
Rotation of the molars can be prevented by use of a
transpalatal arch. (rectangular wires only not round wires
TPA)
Control of canine rotation can be achieved by a variety of
techniques.



For enmasse retraction a rigid anterior segment can reduce
this tendency. A canine bypass connecting he canines but
bypassing the incisors can also control rotation. thirdly a anti
rotation bend can be incorporated into the spring.
With asymmetric space closure vertical forces may be
produced. these may produce undesired extrusive or intrusive
tooth movements. these vertical forces may also produce
third order side effects.
With group A space closure the third order side effect on
the canine is troublesome the intrusive force causes a buccal
flaring which increases the overjet at the canine and/or
increases the intercanine width.


CONTROL OF THE SIDE EFFECTS OF SPACE CLOSURE:
Careful monitoring is essential during space closure.
A frequently overlooked side effect of space closure is the first order
side effects.
The mesially directed buccally located force of the molar may lead to the
erroneous supposition that there is anchorage loss.
Distalization is not necessary . A mesially out directed force is all that is
needed to regain the original molar position.
A transpalatal arch provides an excellent mean to prevent this or actively
corrects it.



CORRECTION OF THE SIDE EFFECTS
Tipping of the anterior and posterior teeth into the extraction space
Increase the alpha and beta moments
Flaring of the anterior teeth
Reduce the alpha moment or increase the distal activation
Mesial in rotation of the buccal segments
Mesial out rotation of the palatal arch, archwire or lingual arch
Excessive lingual tipping of the anterior teeth
Increase the alpha moment



Rotating moments caused by buccal forces during extraction
site closure.



The OPUS loop was designed to deliver an inherent M/F ratio
sufficient for enmasse space closure via translation of teeth of
average dimensions with no bone loss.
Because its inherent M/F ratio is high enough no preactivation
bends is needed before insertion
The neutral position is the passive position of the spring as it sits
before insertion.
Simple cinch back activations can take care of the tooth movement
thresholds to meet anchorage objectives.


Raymond E Siatkowski
Semin Orthod 2001
AJO-DO 1997


No closing loop design previously has been capable of delivering at
constant M/F of 8.0 to 9.1 mm most having inherent M/F of 4-5 mm
or less.
To achieve net translation, orthodontists have had to add residual
moments to the closing loop arch wire with
angulation bends (gable bends) anterior and posterior to the loop,
a posterior gable bend and angulations within the loop,
or a posterior gable bend and anterior wire-bracket twist (anterior
root torque).



Adding these residual moments has several disadvantages:
1. The teeth must cycle through controlled tipping to translation to
root movement to achieve net translation (lower Young's Modulus
materials go through fewer of these cycles for a given distance of
space closure).
2. The correct residual moments are difficult to achieve precisely in
linear materials.
3. The resulting ever-changing PDL stress distributions may not yield
the most rapid, least traumatic method of space closure.
If a closing loop design capable of achieving inherent, constant
M/F of 8.0 to 9.1 mm without residual moments were available, en
masse space closure with uniform PDL stress distributions could be
achieved. Such a mechanism would be less demanding of operator
skill to apply clinically and might provide more rapid tooth
movement with less chance of traumatic side effects


The opus loop achieves a M/F ratio of 8-9.1mm without addition of
activation bends in the loop or archwire itself.
Therefore its neutral position is the same as the inactivated position
before it was tied into the brackets.
Having the loops neutral position accurately allows known forces
systems to be applied to the teeth via simple cinch back activations.



The apical horizontal leg is 10mm long,
The ascending legs at an angle of 70 degrees to the plane of the
brackets
The apical helix is on the leg ascending from the anterior teeth, (that
ascent must begin within 1.5mm posterior to the most distal bracket of the
anterior teeth being retracted)
The spacing between the ascending legs especially the apical loops legs
must be 1mm or less
All these dimensions are critical to the performance of the loop.
Clinically comfort bends are not necessary.



Being free of residual moments, the design can produce a true rest period
when deactivated and therefore could be used with future technology to
produce intermittent force systems during space closure.

Wire bracket play numbers as given in the figure shows that it is important
that sufficient lingual twist exists in the arch wire engaging the incisors so
that bracket wire play is reduced for axial control of the incisors.


It is appropriate to begin with a straight wire and bend the arch wire in a
torquing turret to achieve incisor axial inclination control by inducing wire
twist ("lingual root torque") just enough to eliminate labiolingual wire-bracket
play in the incisor brackets.
The amount of such twist is dependent on the wire/bracket sizes and slot
torque used
A torquing turret has been designed for use with TMA wire.
Maximum incisor twist is appropriate for posterior protraction



The advantage of having the opus loop formed in 17X25 TMA is that it
provides a relatively long range of activation;
unfortunately it is difficult to bend the wire with sufficient incisor
torque to reduce the wire play.
It is difficult to contour the loop for comfort on one side without
altering the other side also and a large stock of wires is necessary for
preformed wires.
This can be over come by having:
An anterior wire of Niti alloy with two separate 17X25 TMA posterior
segments, which are attached by a Forestadent cross tube
This bialloy has the following advantages:
Infrequent activations
Ease of comfort bending
Incisor axial inclination control



Melsen refinement of the Quinn Yoshikawa
model of tooth movement:

The OSTEOLOGIC graphic form is
the theoretical explanation for the
mode of action of the opus loop
arch wire.
It relates the orthodontic force
systems to the stresses in the PDL
rather than the strains.
It examines the rate of tooth
movement as the loop deactivates



When a force system is applied on
a tooth initially after a quiescent
period, the initial rate of tooth
movement corresponds to no 2 on
the diagram.
This model is valid only for
uniform stress on the PDL as
produced by translation and not
tipping followed by up righting.
These arch wires by definition are
activated far less than the systolic
blood
pressure
at
which
hyalinization is supposed to occur.
Larger cinch back activations shift the curve to the right and a lesser
activation shifts the curve to the left


The various possible activations of
the opus lop cinch back as a
function of time is shown in the
figure.
Group B anchorage
Curve 1: anteriors retract
Curve 2: posteriors protract
Group A anchorage
Curve 2: anteriors retract
Curve 3: posteriors little change
Group C anchorage
Curve 4: anteriors no change
Curve 1 posteriors protract


DISADVANTAGES OF THE OPUS LOOP
Although less so than with other closing loop designs, Opus loops do
have the potential to steepen the cant of occlusal plane in the maxillary
arch and flatten it in the mandibular arch.
Although steepening occlusal plane can be useful for overtreatment of
Class III relationships (and flattening occlusal plane for Class II
relationships), that potential should be monitored for possible
intervention.
Such intervention could be reducing maximum activation force levels or
using an occipital headgear with short and high outer bows to generate a
moment tending to flatten maxillary occlusal plane.
For the most severe anchorage required to achieve treatment goals,
second molars, if available, could be included with the posteriors and/or
a Combi headgear used.
For less severe or moderate anchorage, the canines could be incorporated
with the anteriors


The configuration for posterior protraction

The closing loop arch wire generates the moments required and some of the
protraction force.
Most of the protraction force is generated by the large anterior moment and by
the intermaxillary elastics to a rigid rectangular arch wire in the opposing arch.
Intermaxillary Niti closed coil springs capable of delivering 150 gm force can
be substituted for the elastics. The potential exists for changing occlusal plane in
the opposing arch. Should such cant changes begin to be observed, the
intermaxillary force can be reduced.



In group C anchorage cases, class III elastics with a force of
150gms/side from the opposite arch which has a rigid rectangular
stainless steel archwire can be used.
Another alternative is to use TP 256 torquing auxiliary which when
overlaid over the closing loop provides an additional protraction
force to the posteriors.
It has the following advantages:
The clinician is free to continue treatment in the lower arch
Undesired vertical forces from the elastics are not a problem
Posterior arch width increases are not a problem when using a
TMA wire



SEGMENTED ARCH TECHNIQUE FOR SPACE CLOSURE IN ADULTS:
The advantages of the T-loop design over a vertical loop
is that the T-loop produces a higher M/F ratio, a lower loaddeflection rate, and delivers a more constant force and M/F ratio

Often in adult patients, where no growth is anticipated, extraction
therapy is performed. The situation is often complicated through loss
of bone. In order to maintain an assumed stress magnitude and
distribution under the condition of reduced bony support area, force
magnitude must be reduced and the M/F ratio must be increased.
The necessity of producing a lower load deflection rate in such cases
suggests the use of a wire with lower stiffness

Manhartsberger, Morton, Charles J Burstone Angle 1989


With a change in the center of resistance the M/F ratio must be modified
thus, in adult patients with periodontal loss, higher M/F values must be
attained.
To obtain higher M/F ratios a number of strategies can be employed.
The loop can be made as long as possible in an apical direction. By
increasing the height of the loop to 11 millimeters, one will approximately
double the M/F ratio. However, there are limitations to how far apically the
loop can extend before irritation is produced in the mucobuccal fold.
Another approach is to increase the amount of wire placed gingivally
at the top of the loop, as in the T-loop .
Increasing the gingival length of wire (dimension G) increases the M/F
ratio and reduces the load deflection rate.


The variation of the center of resistance with
differing levels of bony support.


The T-loop dimensions (H=7mm, D=2mm, G =10mm).



TMA T-loop with gradual
curvature angulation.

T-loop with concentrated
angulation.

A .016² × .025² TMA T-loop activated five millimeters with gradual
curvature bends in comparison to a .017² × .025² TMA T-loop
activated seven millimeters produced a 47 percent lower horizontal
force, and a 23 percent higher M/F ratio.
The actual spring must be individualized for each patient by altering
wire cross-section, angulation, and activation.


The clinical discipline of orthodontic space closure
requires complete understanding of its complexities
Just as an orthodontist is provided with innumerable
alternatives of appliance systems, it is imperative to be
aware it the merits as well as the limitations of the
techniques involved
Mechanical as well as biologic factors must be considered
in selecting the appliance best suited for the patient



Thank you
For more details please visit
www.indiandentalacademy.co
m


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