Space closure2 /certified fixed orthodontic courses by Indian dental academy

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

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.
BIOLOGIC VARIABLES IN ANCHORAGE CONTROL
AND DIFFRENTIAL SPACE CLOSURE:

. Possible hypotheses of the relationship between stress magnitude
and the rate of tooth movement are graphically represented.
Quinn & Yoshikawa AJO-DO 1998

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.
HYPOTHESES OF THE STRESS-MOVEMENT
RELATIONSHIP (Quinn and Yoshikawa)
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.
stress

BiomechanicsofSpaceClosure 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, cross-
section steel wire with closing loops activated to generate large stresses. The
appliance that delivered the highest stresses would close extraction sites most
rapidly.

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.
Hypothesis 3
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.

BiomechanicsofSpaceClosure 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.

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
EVALUATION OF HYPOTHESES

BiomechanicsofSpaceClosure 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.www.indiandentalacademy.com

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.
The evidence for hypothesis 4 is more compelling.

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.

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
DIFFERENTIAL FORCE SYSTEMS-VARIABLE
MOMENTS AND FORCES
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.

BiomechanicsofSpaceClosure 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 Bulcke, Dermaut, Sachdeva, and Burstone AJO-DO 1998www.indiandentalacademy.com

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.
Results:

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.

BiomechanicsofSpaceClosure 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 www.indiandentalacademy.com

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) www.indiandentalacademy.com

BiomechanicsofSpaceClosure 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

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Space closure2 /certified fixed orthodontic courses by Indian dental academy

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