Vertical force consideration in differential space closure /certified fixed orthodontic courses by Indian dental academy

Vertical Force Consideration in
Differential Space Closure
INDIAN DENTAL ACADEMY
Leader in continuing dental education
www.indiandentalacademy.com

Introduction
• Many class II patients are treated by extraction
to reduce overjet and obtain normal molar
relationship.
• The composite T- loop spring has been
advocated for space closure when anterior
retraction is needed without loss of anchorage.
• This article will discus how individual monitoring
of the force system delivered form the
composite T- loop can be used to minimize
“round tripping” and avoid side effects.

Differential Space Closure
• The ratio of the applied moment or torque to the
force at the bracket determines the center of
rotation.
• A simple force at the bracket will produce
uncontrolled tipping, in which the center of
rotation is slightly apical to the center of
resistance.
• If moment is added to the force, the line of
action of the resultant force is translated
apically, and the displacement in milimeters is
equivalent to the moment-to-force ratio.

• As the moment-to-force ratio increases,
the center of rotation moves apically. It
disappears into infinity when the force
system produces pure translation.
• Higher the moment-to-force ratios will
produce root movement.
• The laws of equilibrium for a retraction
spring dictate that since there is a
differential, vertical forces are produced.

• The differential between the anterior
torque and the greater posterior moment
produces an intrusive force on the anterior
which is useful in patients with deep
overbites who require intrusion.
• In open bite case require entirely different
concept in anchorage space closure –
perhaps the use of equal and opposite
M/F ratios ( without vertical force) and
headgear to the posterior segments.

Composite T-loop springs
• Composite T-loop springs made of .018” X .025”
and .017” X .025” TMA wire.
• Angulations is built into the spring either in the
form of posterior tipback bends or anterior “V”
bends to control the centers of rotation of the
anterior and posterior teeth.
• If resulting vertical force acts through the center
of resistance of the anterior segment it will not
have any effect on the total torque or moment
applied to the anterior segment.

• If the vertical force is anterior to the center of
resistance of the anterior segment, the intrusive
force times the perpendicular distance to the
center of resistance increases the moment, thus
tending to move the root lingually.
• If the vertical force distal to the center of
resistance, moment is created that reduces the
amount of lingual root torque acting on the
teeth.
• The T-loop retraction spring is usually attached
to a vertical tube on the cuspid bracket.

Patient study
• Pretreatment head films from 25 class II
patients treated by first bicuspid
extractions, with retraction and intrusion
of the incisors, were selected for this
study.
• A laser holography study has found that
the center of resistance of an upper
central incisors lies 30-40 % of distance
from the marginal bone level.

• The center of resistance for the six-tooth
unit was then located halfway between
the mid point of the four incisors’ centers
of resistance and the canines’ center of
resistance.
• In en masse retraction, the point of force
application is usually the canine bracket.
Therefore, the anteroposterior variation in
position of the center of resistance was
analyzed in relation to the canine bracket,
with the occlusal plane as the horizontal
axis.

Results
• The pretreatment estimate of the center of
resistance was, on average, close to the cuspid
bracket, but varied considerably from patient to
patient, depending on the individual’s root
length and inclination. (average .72mm).
• After the simulated 3mm retraction, the point of
force application change with an average of
1.9mm posterior to CR. Hence the cuspid
bracket moved an average of 2.6mm distally
relative to CR.

Clinical considerations
• If the anterior teeth are markedly flared,
the vertical force may be anterior to the
center of resistance, and too much
moment will be produced in the anterior
segment. Clinically, one might observe
little space closure and large amount of
intrusion. The solution is to reduce the
anterior( alpha) moment by reducing the
angulation in the anterior part of spring.

• On the other hand, the vertical force
posterior to the center of resistance at the
beginning of treatment, particularly if the
anterior segment is inclined lingually. This
problem becomes more pronounced after
partial space is closure; as space is closed.
• After 3mm closure, a full 6mm reactivation
with about 60g of intrusion will reduces
the moment on the anterior segment
about 10%- a relatively small effect.

• If they left in place too long, they can
cause intrusion of the anterior segment,
undue tipping of the anterior teeth, and
radical tipback with steepening of the
occlusal plane of the posterior teeth.
• During en masse tipping of the anterior
segment, the center of resistance moves
anterior to the vertical force.

• There are a number of possible solutions to the
problem of the vertical force posterior to the
center of resistance.
• We currently prefer to reduced the differential
between the anterior and posterior moment
would be to use a symmetrical T-loop with equal
and opposite couples.
• Still another possibility is to use the normal
retraction spring, but the point moved anteriorly
to the center of resistance. This can be done by
soldering a vertical tube to an anterior stainless
steel.

• The appropriate sequence might be for
en masse retraction:
1) Approximately 3mm of en masse tipping
around the apices of the incisors.
2) En masse root movement, with the
center of rotation around the incisor
bracket.
3) Continued en masse tipping around the
roots of the incisors, until space
completed.

Conclusion
• The vertical forces that result from
differential moments between the anterior
and posterior segments can influence the
amount of tipping of the anterior teeth
during space closure, and these vertical
forces vary from case to case.

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Vertical force consideration in differential space closure /certified fixed orthodontic courses by Indian dental academy

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