Common sense mechanics (2) /certified fixed orthodontic courses by Indian dental academy

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

Presented by;
Dr. Binumon mani

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 the malocclusion
INTRODUCTION….

Appliance are being refined and will
continue to improve with the passage of
time. This is good, but the danger lies
with the individual who fails to recognize
that the refinement of appliances may
reduce the physical effort put forth in
treatment, but will not eliminate the need
for orthodontist to think, understand, and
apply basic principle of mechanics in a
common sense manner.

This means that regardless of how well we
understand mechanics and the appliance
is refined, we are dealing with a biologic
environment whose variation in response
will continue to challenge the orthodontist
in many ways. If we are to meet the
challenge, we must gather as much
information as possible that will allow us to
treat the patient in a practical or realistic
manner, rather than treating in a textbook
fashion..

Common sense is such an important part
of applying basic mechanics that without
it, even the most sophisticated
knowledge of the subjects offers one little
in attaining his treatment goals. Perhaps
it is a lack of a combination of the
knowledge of mechanics and common
sense application that has held to the
desire on the part of many orthodontist to
seek an appliance which does the
THINKING…www.indiandentalacademy.com

Visual inspection
“Visual inspection method” is what
often confuses the orthodontist in
attempting to determine with reliability
what forces are present.
Insertion of arch wire…
Placement of bends….

Forces and Moments
Force is nothing more than a “push or
pull” and acts in a straight line. When
ever this force passes through the
centre of a body, in ortho CR there is
no moment produced so no rotational
tendency or viz….

A moment is the product of force times
distance. If the line of force does not pass
through the CR of the tooth, then there is
a distance between the line of force and
the centre. It is the perpendicular
distance from this line of force to the
centre that causes the moment on the
tooth, resulting in rotational tendencies.

The orthodontist cannot think force and
moment are the same. One can sense a
force when you bend a wire but you
cannot sense a torque; because it is a
product of force times distance.
We might use a lot of force and may not
produce a moment or a small moment,
while some times a small force can
produce a large moment due to the
distance involved.

Through CR and away from CR LARGE AND SMALL FORCE

CUE BALL CONCEPT
If you want to move a cue ball in a straight
line (translate) with out moving to any left
or right….we got to apply the force right
through the middle of the cue ball. In
tooth it will be CR, in a free body its
centre of mass.
A force applied through the centre results in
straight line movement with out any left or
right www.indiandentalacademy.com

Forces and Moments acting on Tooth
Insertion of wire……
Forces and moments….
Differential torque…….

STATIC EQUILIBRIUM
In spite of the fact that orthodontist have heard
many times over that “every action has an equal
and opposite reaction”, Newton’s third law has
not really been understood in such a way as to
permit the orthodontist to apply the principle in
his daily orthodontic mechanics in a simple and
practical manner. The subject of static
equilibrium is so easy to understand, and yet so
very important in orthodontic mechanics, that
the practicing orthodontist cannot afford to
ignore the principles involved

REQUIREMENTS OF STATIC
EQUILIBRIUM
Sum of all vertical forces present must
equal zero…
Sum of all horizontal forces present must
equal zero…
Sum of the moments acting around ANY
point must also equal zero…

VERTICAL FORCES

HORIZONTAL FORCES

SUM OF THE MOMENTS

CROSSBITES
A discussion of static equilibrium pointed
out the three requirements which are
automatically fulfilled when ever an
arch-wire is completely engaged in
brackets and tubes

Vertical forces are usually kept as light as
reasonably possible, whereas no such
attempt is made at the horizontal level.
Infact, the forces used at the horizontal
level are often quite high. High
magnitudes of force threaten the vertical
dimension while posing little or not threat
to the horizontal dimension.

EXPANSION
If we have an individual tooth, such as a
molar, in cross bite or an entire buccal
segment, we know beforehand that we
would like to apply a force in the
necessary direction for correction on
those teeth only. But most of us realize
that there will be an equal and
opposite force applied else where as
well.

OVERLAYS
The term overlay as used here will
most often refer to a heavy wire
overlaying the main arch wire. It can
either be inserted into the headgear
tube or be designed with terminal
hooks to engage the arch wire..

INSERSION

COSMETIC OVERLAYS..
The overlay is referred to as cosmetic
because it is designed not to show
when the patient smiles. It is therefore,
ideal for the adult patient who is
concerned about the cosmetics of
appliance therapy. It can also be
removed by the patient, if necessary for
any reason such as illness or broken
appointments.

CONTROLLING VERTICAL FORCES
INTRAORALLY
Much has been said and written about vertical
dimension and the problem involved with steep
mandibular plane angles and extrusive forces,
particularly on the molar teeth. Likewise a
number of solutions have been offered,
including the use of various types of high pull
headgear. But little has been said in terms of
controlling vertical dimension problems by
controlling magnitudes of force intra orally in
the vertical plane of space.

DIVING BOARD CONCEPT
“Diving Board Concept” the advantages involved in
utilizing the factor of “length” in our archwires.
There is a formula that says stiffness or
load/deflection rate is inversely proportional to
the cube of length. Stiffness is the amount of
deflection we get from a given load. The formula
tells us that if we are dealing with a cantilever
( such as a diving board), by doubling the length
stiffness is reduced to 1/8th
. By doubling the
length, only 1/8th
the force will be required to
produce the same deflection or the same force
acting at double the length will produce 8 times
as much deflection.www.indiandentalacademy.com

CLINICAL APPLICATION
If we wish to apply the cantilever
principle and its modifications for
practical use in clinical orthodontics,
we must understand its characteristics
and possess some means of
controlling the force magnitude
involved, as these equal and opposite
forces, in the vertical plane of space,
threaten our treatment results in
various ways.www.indiandentalacademy.com

 From the “Diving Board Concept”, if
we bypass bicuspids and cuspids
during overbite correction, and use a
wire with tipback bends at the molars,
we have in ‘effect created a “diving
board”, although certain modifications
would be required in the anterior
segment in order to provide a true
cantilever system.

EFFECT ON FORCES AND
MOMENTS
Because the anterior-posterior arch length varies
from patient to patient, when bicuspids and
cuspids are bypassed the length becomes a
variable and thus, so do the magnitudes of the
intrusive and extrusive forces at each end of
the arch-wire, which we have already seen to
be greatly affected by changes in wire length.
However, the entire range of force is so low
that low magnitudes of force may pose a
greater problem than attaining higher levels of
force. In fact, it may even require going to arch-
wires of greater diameter to produce a required
force and desirable response.www.indiandentalacademy.com

The moment on the molars, however,
cannot be ignored, as it is possible to
tip back molars undesirably, if not
cautious. Be careful not to use too
large a tipback bend (angle), as this in
combination with duration (time) of use
can result in excessive tipback of the
molar teeth.

DISTALISATION WITH DIFFERENTIAL
TORQUE
The tip back bend is a off centre bend and
the long and short segment indicate the
direction of the force act. The moments
involved in this is unequal and thus
resulting in a “differential torque”
ROWBOAT EFFECT

WIRE BRACKET RELATIONSHIP
The relationship of the arch wire to the
bracket and tubes, prior to engagement
offers a valuable information. If a straight
wire is inserted over angulated brackets,
a certain angular relation develops
between the wire and the plane of the
bracket slot. The straight wire overlying
these brackets gives us a clues regarding
tooth movement.

CENTRE BEND FORCE SYSTEM
The force and moments present in the two
extremes of the wire bracket
relationships; the centre bend and the
step bend , by applying the requirements
for static equilibrium.
a) Vertical forces
b) Horizontal forces
c) Sum of all moments…

STEP BEND FORCE SYSTEM
In step relationship which is the other
extreme under discussion, we will go
through the same analysis, again using
aligned brackets with the bends placed
in the wire…..
a) Vertical forces
b) Horizontal forces
c) Sum of all moments…www.indiandentalacademy.com

EXTRACTION MECHANICS
The tip back bend is a off centre bend.
Two moments are produced, but they
are unequal. The larger moment lies at
the bracket or tube containing the
short segment. The smaller moment
may, at times clock wise or other times
counter clockwise and even disappear
producing the cantilever effect.

The important thing to remember is this bend
will result in a differential torque dominated
by the larger moment.
The technique in this discussion will depart
from exactness; but, in clinical practice
sense it works. It is simple, easy, orderly,
hygienic, reduces the need of for patient
cooperation and will make our work more
enjoyable we can think rather than follow a
cookbook and vary our procedure to fit our
schedule www.indiandentalacademy.com

CANINE RETRACTION
Since the force during retraction are equal
and opposite on the two unit- anchor and
non-anchor unit. The multi-banded unit
actually receive the lesser amount of
force per unit area along the PDL
membrane while the non-anchor unit
receives the greater. This differential
torque control the anchor unit instead of a
pure force distribution; but its not the
ideal way for anchorage control.

The initial wire may be 0.016 or 0.018
according to our choice. but should not
loose the sight of the principles. The
periodontal response that occur is
permitted to improve bracket
alignment and level, prior to placing
the bends in the arch wire. Toe-in
bends should be placed early so as to
initiate a counter rotation

BICUSPID RETRACTION
In this case a bend is placed mesial to
the tube to serve as the anchor unit.
An elastic is used to retract the
bicuspid (non-anchor). The bicuspid
will start tipping while the molars
remains upright which verifies the
anchorage and non anchorage sides
due to the unequal moments.

As the bicuspid move distally it gradually
approaches the off centered bend lying
mesial to the molar tube. As this happens
the two moments gradually becomes
more and more equal, but opposite in
direction. The gradual equalization
provides the root paralleling due to the
initial tipping.

MOLAR PROTRACTION
The wire is usually an0.018 followed by an 0.020
on occasion, as the tipping tendency for
molars is too great with a lighter wire. The
bends are placed to initiate counter rotation.
Here the molars will be the non-anchor side
and the bend is placed just distal to the
bicuspid so the differential torque is again
produced but the direction of the moment is
reversed.

MOLAR CONTROL

The molar tubes are placed in the first
molars only unlike others author don’t
band 2nd
molar unless it have to be moved
to a new position, as might occur in some
extraction cases or if the molars truly
erupt in an unsatisfactory manner.

CAUSES OF MOLAR DISPLACEMENT
Any type of tooth movement that produces
intrusive balancing forces in the posterior
area has the potential for molar crown
displacement to the buccal
Tooth movement that creates an extrusive
force in the posterior area results in the
potential for lingual crown movement

RECOGNIZING MOLAR DISPLACEMENT
If the second molar is present and
unbanded, 1st
molar displacement is
rather obvious……
The functional curves of occlusion
include the curves of Wilson and
Monson.

Extrusive force acting through the molar
tubes produce lingual crown movement
this will result in narrowing the posterior
arch width. From the frontal view, a
reduction in the normal curve of Monsoon
take place. This is the 1st
clue as to what
is taking place with out the need to use
adjacent molars for reference. This
problem is resolved with the introduction
of horizontal force through the same
molar tubes www.indiandentalacademy.com

An intrusive force acting through the molar
tube produces a buccal crown movement.
As a result the posterior arch will show an
increase, while from a frontal view, the
curve of Monson will show an increase.
This problem too will be resolved with the
introduction of horizontal force through
the same molar tubes

Intrusive and extrusive forces through the
mandibular molar tubes will result in
the buccal and lingual displacements,
with a decrease or increase in the
curve of Wilson. In both cases, the
posterior arch width also changes.

DIFFERENTIAL DIAGNOSIS
If buccal overjet is present in the molar
region, it must be determined whether
the upper molars have been displaced
to the buccal, the lower molars to the
lingual or a combination of two. From a
frontal view, any change in the curves
of Monson or Wilson will answer..

SORCES OF VERTICAL
FORCES

ARCH WIRE IN USE
After the alignment two types of arch
wires are used…..
STEP arch wires- used to increase arch
length
WRAPAROUND archwires- used to
decrease arch length

THE IN AND OUT BENDS

THE TOE-IN AND TOE-OUT
BENDS

IN BENDS MODIFICATION

OUT-BENDS MODIFICATION

CONCLUSON…..
The orthodontic profession can improve itself in
many ways by becoming more acquainted with
the principles that have been presented and by
applying them when the need arises. This does
not require using a different appliance system
which is comfortable, and it does not require that
you discard your favorite technique. It means,
making certain modification at times, but
THE CHOICE IS YOURSwww.indiandentalacademy.com

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Common sense mechanics (2) /certified fixed orthodontic courses by Indian dental academy

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