The document summarizes key concepts from the article "Common Sense Mechanics" by Mulligan T.F. It discusses how common sense is a necessary ingredient when applying orthodontic mechanics to correct malocclusions. It provides examples to illustrate that visually assessing forces based on wire position can often lead to incorrect conclusions without incorporating an understanding of mechanics. It presents a simple rule for determining intrusive or extrusive forces based on whether a wire bend is centered or off-centered. It also discusses concepts like center of resistance, translation, rotation, and differential torque. The summary emphasizes that a thorough grasp of underlying mechanical principles combined with intelligent clinical decision making is needed to achieve orthodontic treatment goals.

1. COMMON SENSE MECHANICS
Mulligan T.F. Journal of Clinical Orthodontics 1979;
12:588-683.
Guided by: Presented by:
Dr.Mridula Trehan. Dr. Deeksha Bhanotia
Head of Department of Orthodontics MDS First Year.
and Dentofacial Orthopaedics
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2.  The title “Common Sense Mechanics” is based on the
simple fact that no appliance exists which will allow an
orthodontist to treat orthodontic problems without adding the
necessary ingredient of “Common Sense” to the mechanics
instituted for correcting malocclusion.
 This means that regardless of how well we understand
mechanics and regardless of how much the appliance is
refined, we are dealing with a biologic environment whose
variation in response will continue to challenge the
orthodontist in many ways.
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3.  Common Sense is such an important part of
applying basic mechanics that without it, even the
most sophisticated knowledge of the subject
offers one little in attaining his treatment goals.
 It is a lack of a combination of the two–
knowledge of mechanics and common sense
application– that has led to the desire on the part
of many orthodontists to seek an appliance which
does the thinking.
3

4.  Visual Inspection:
When the orthodontist inserts an archwire into the molar tubes and
observes that prior to placement of the archwire into the incisor brackets,
the wire lies in the mucolabial fold, it is often concluded that this means
there must be produced an anterior intrusive force upon engagement.
This may very well be true, but likewise, it may be very untrue. There not
only may be no force present, but there might even be present an extrusive
component of force.
This visual method seems to be so obvious, but it is this method that so
often leads us down the road to faulty conclusions.
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5. Correct answers: Fig
3-A. Extrusive,
3-B. Intrusive,
Fig 4: A. None, B.
None
Correct answers:
Fig 5-A. Intrusive,
5-B. Extrusive
Fig 6: A. None, B.
None
Correct answers:
Fig 7-A. Intrusive,
7-B. Extrusive
Fig 8: A. None, B. None
Correct answers:
Fig 9-A. Extrusive
9-B. Intrusive,
Fig 10: A. None, B.
None
5

6.  In the preceding examples it is seen that one bend
was centrally located while the other bend was
located off center.
 Each time the bend was located in the center, the
answer was constant and each time the bend was
located off center the answer was constant (i.e.
either intrusive or extrusive).
Bend at Center Bend Off Center 6

7.  When the bend is located exactly at the center
there were equal and opposite moments but no
forces.
 When the bend is off-centered there were
equal and opposite forces but no longer equal
moments.
7

8. A Simple Rule:
 If the bend is located off center, there will be a long segment
and a short segment.
 When the short segment is engaged into the bracket or tube,
the long segment will point in the direction of the force
produced on the tooth that will receive the long segment.
Figure shows the long segment points apically to the cuspid meaning cuspid intrusion and
therefore molar extrusion.
Long Segment
Short Segment
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Direction of force
(intrusion)

9.  If the bend is in the center there no longer exists a long or
short segment. Therefore no force is produced.
 Some Examples:
9

10.  Center of Resistance:
When the line of force passes through the center
of a body no moment is produced and therefore no rotational
tendency. This point where if force passes, bodily moment of
the body takes place and is called Center of Resistance
10

11.  When a force acts away from the center, it causes
moment on the tooth, resulting in rotational
tendencies.
 Magnitude of moment is determined by the force
times the perpendicular distance to the center.
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12.  Translation:
If we apply a force through the center of an object (eg:
cue ball) it will move forward in a straight line.
There will be no rotation other than the forward roll due
to the friction of the table itself.
The reason there is no moment is that the line of force
has no perpendicular distance to the center; the force is
passing through the center.
When the line of force acts through the center of resistance, only translation results.
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13.  Rotation and Translation:
 If we take exactly same force and apply it on
the same body, but instead of applying it through the center,
apply it off center, then we create a situation where the line of
force has a perpendicular distance from the “ Center of Mass”.
 This means that it produces not only
translation, but also rotation, as a result of the moment
produced.
A force off center causes the object to rotate as well as move forward in a straight
line.
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14.  Pure Rotation (Couple):
If two forces are to be applied equal and opposite,
in the same plane of space the object would not translate in
any direction. Instead it would simply maintain its position
and spin.
The reason for this is that the two forces cancel
each other out, but leave a net moment due to the fact that
each of these “Lines of Force” acts at a perpendicular
distance from the center of the ball.
Equal and opposite forces(couple) produce pure rotation
14

15. Forces and Moment acting on Teeth
 When the wire is brought down from the mucolabial fold for
insertion into the incisor brackets, the force required acts at a
perpendicular distance from the center of the molar, producing
mesial root torque or distal crown thrust on each of the molars
involved.
perpendicular distance from
the center of the molar to
point of force application.
mesial root
torque or distal
crown thrust
15

16.  When the wire is engaged into the incisor brackets, the
intrusive force acts in a straight line and usually passes labial
to the centre of resistance in the incisors.
 This produces a smaller moment than molar, because inspite
of the fact that forces are equal, the distances involved are
radically different.
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17.  So, when the archwire is tied into place and tied back into
molar tubes, significantly different magnitudes of torque is
generated which can be referred to as “ differential torque”.
The system behaves as a whole and “ tug of war” is apparent
with the molar having the obvious mechanical advantage.
Fig: Differential Torque.
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18.  Observing from a distal view, if a round wire instead of
rectangular and permitted to “roll” inside the tubes, the
extrusive force present on the molar teeth then acts on the molar
tubes which lie usually buccally to the centre of resistance in
these teeth.
 This force times distance results in molar lingual crown torque.
Fig: An eruptive force at the molar tubes passing buccally to the center of resistance
produces lingual crown torque on molars.
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B L B B B
L L L
F F

19. If a wire were very rigidly attached to the tubes,
the applied force would pass lingual to the center
of resistance thereby inducing buccal crown
torque.
19

20.  Lingual Root Torque:
If we place lingual root torque into the incisor section, a
long segment and a short segment is produced.
The long segment indicates a molar intrusive force and
therefore an extrusive force on incisors.
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Short segment
Long
segment

21.  Torque produced on the incisors is a result of force times
distance, since the long segment has to be brought down to the
molar tube, and the force required to bring it down acts at a
perpendicular distance to incisors.
Lingual root torque is produced as a result of the force necessary for molar tube
engagement x the perpendicular distance to the center of resistance in the incisor.
the perpendicular
distance to the center
of resistance in the
incisor.
force necessary for
molar tube
engagement
Lingual root torque
21

22.  If the long segments are unequal in angular
relationship, then the one producing the greater angle
relative to the level of the archwire will determine
the net force present.
22

23.  If lingual root torque produces the greater angle as shown in
the figure, the net forces will be intrusive on the molar and
extrusive on the incisor..
extrusive on the
incisor.
intrusive on
the molar
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24.  So, if we know beforehand, to either increase the molar tip
back bend, decrease the amount of lingual root torque on the
incisor segment, or a combination of each, in order to assure
ourselves of a net intrusive force on the incisor segment for
overbite correction.
 Recognition of the problems and intelligent decision making
will only follow a thorough understanding of the underlying
principles.
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25. Orthodontic forces can be treated mathematically as vectors. When more than one force
is applied to a tooth, the forces can be combined to determine a single overall resultant.
Forces can also be divided into components in order to determine effects parallel and
perpendicular to the occlusal plane, Frankfort horizontal, or the long axis of the tooth.
Forces produce either translation (bodily movement), rotation, or a combination of
translation and rotation, depending upon the relationship of the line of action of the force
to the center of resistance of the tooth. The tendency to rotate is due to the moment of
the force, which is equal to force magnitude multiplied by the perpendicular distance of
the line of action to the center of resistance. The only force system that can produce pure
rotation (a moment with no net force) is a couple, which is two equal and opposite,
noncolinear but parallel forces. The movement of a tooth (or a set of teeth) can be
described through the use of a center of rotation. The ratio between the net moment and
net force on a tooth (M/F ratio) with reference to the center of resistance determines the
center of rotation. Since most forces are applied at the bracket, it is necessary to compute
equivalent force systems at the center of resistance in order to predict tooth movement. A
graph of the M/F ratio plotted against the center of rotation illustrates the precision
required for controlled tooth movement.
Related Articles 1
Mechanics of tooth Movement.
Smith RJ, Burstone CJ. Am J Orthod. 1984;85:295-307
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26. A new tool for measuring tooth movement-laser holography-offers an
accurate, noninvasive approach for determining movement in three
dimensions. This in vitro study is designed to establish the required force
system applied on the crown of a maxillary incisor that would produce
different centers of rotation, as in lingual tipping, translation, and root
movement. The relationship between moment -to-force ratios and centers
of’ rotation is shown. The experimental data are compared to theoretic
approaches. With respect to the location of the center of resistance and
centers of rotation, force systems needed to produce different centers of
rotation are given for a central incisor of average root length.
Holographic determination of centers of rotation produced by
Orthodontic forces.
Burstone CJ, Pryputniewicz RJ. Am J Orthod. 1980;77:396-408.
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Related Articles 2

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