orthodontic principles for ortho treatment

1. MECHANICALPRINCIPLES
IN ORTHODONTIC
THERAPY
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2. CONTENTS
• INTRODUCTION
• ELASTIC MATERIALS AND THE PRODUCTION OF ORTHODONTIC
FORCE
• ORTHODONTIC ARCH WIRE MATERIALS
• EFFECTS OF ELASTIC PROPERTIES OF BEAMS
• CONTROLLING ORTHODONTIC FORCE BY VARYING MATERIALS AND
SIZE-SHAPE OF ARCHWIRES
• DESIGN FACTORS IN ORTHODONTIC APPLIANCES
• MECHANICAL ASPECTS OF ANCHORAGE CONTROL
• CONCLUSION
• REFERENCES
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3. Introduction
• Optimum orthodontic tooth movement is produced by
light, continuous force.
• It is particularly important that light forces do not
decrease rapidly, decaying away either because the
material itself loses its elasticity or because a small
amount of tooth movement causes a larger change in
the amount of force delivered.
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4. Elastic Materials And The Production
Of Orthodontic Force
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5. The basic properties of elastic materials:
1. The elastic behaviour of any material is defined in
terms of its stress-strain response to an external load.
2. Stress: the internal distribution of the load, defined
as force per unit area.
3. Strain: is the internal distortion produced by the
load, defined as deflection per unit length.
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6. • For orthodontic purposes, three major properties of beam
materials are critical : strength, stiffness (or its inverse,
springiness) and range. Each can be defined by appropriate
reference to a force-deflection or stress-strain diagram.
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8. • Two other characteristics of some clinical importance
can also be illustrated with a stress-strain diagram:
resilience and formability.
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9. • The properties of an ideal wire material for
orthodontic purposes:
1. High strength
2. Low stiffness(in most applications)
3. High range
4. High formability
5. The material should be weldable or solderable, so
that hooks or stops can be attached to the wire.
6. Reasonable in cost.
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Strength = Stiffness X
Range
Sspringiness= 1/ stiffness

10. Orthodontic arch wire materials
• Precious metal alloys
• Stainless steel and Cobalt-Chromium alloys
• Nickel-Titanium (NiTi) alloys
• Beta-Titanium
• Composite Plastics
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11. EFFECTS ON ELASTIC
PROPERTIES OF BEAMS
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12. Geometry: Size and Shape
• Changes related to size and shape are independent of
the material.
• In other words, decreasing the diameter of a steel
beam by 50% would reduce its strength to a specific
percentage.
• Decreasing the diameter of a similarly supported
TMA beam by 50% would reduce its strength by
exactly the same percentage as the steel beam.
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13. Cantilever Beams
• When a round wire is used as a fingerspring,
doubling the diameter of the wire increases its
strength eight times.
• Doubling the diameter, however decreases
springiness by a factor of 16 and decreases range by a
factor of two.
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15. Supported beams
• The beam size increases, strength increases as a
cubic function, while springiness decreases as fourth
power function and range decreases proportionately,
not exponentially.
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16. Geometry: Length and Attachment
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17. CONTROLLING ORTHODONTIC FORCE
BY VARYING MATERIALS AND SIZE–
SHAPE OF ARCHWIRES
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18. • Fingersprings for removable appliances are best constructed using
steel wire.
• Advantage- fingersprings behave like cantilever beams: springiness
increases as a cubic function of the increase in the length of the
beam, while strength decreases only in direct proportion.
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20. Design Factors In Orthodontic
Appliances
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21. Two- point contact for control of root
position
• Force: a load applied to an object that will tend to move it to
a different position in space.
Force, though rigidly defined in units of Newtons (mass x
acceleration of gravity), is usually measured clinically in
weight units of grams or ounces.
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Center of resistance: a point at which resistance to
movement can be concentrated for mathematical
analysis.
Center of rotation- the point around which rotation
actually occurs when an object is being moved.

23. Moment:
• Measure of tendency to rotate an object around a
same point.
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Couple
• Two forces equal in magnitude and opposite in
direction.

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25. MOMENT-TO-FORCE RATIOS AND
CONTROL OF ROOT POSITION
• Root position during movement requires :
 A force to move the tooth in the desired direction .
Couple to produce the necessary counterbalancing
moment for control of root position.
• The heavier the force, the larger the counterbalancing
movement must be to prevent tipping and vice-versa.
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27. • MC /MF = 0 Pure tipping (tooth rotates around center of
resistance)
• 0 < MC /MF < 1 Controlled tipping (inclination of tooth
changes but the center of rotation is displaced away from the
center of resistance, and the root and crown move in the same
direction)
• MC /MF = 1 Bodily movement (equal movement of crown
and root)
• MC /MF > 1 Torque (root apex moves further than crown)
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28. NARROW VERSUS WIDE BRACKETS
IN FIXED APPLIANCE SYSTEMS
• Control of root position with an orthodontic appliance
is especially needed in two circumstances:
• When the root of a tooth needs to be torqued
faciolingually and when mesiodistal root movement
is needed for proper paralleling of teeth when spaces
are closed (as at extraction sites).
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29. • The wider the bracket, all other things being equal,
the easier it will be to generate the moments needed
to bring roots together at extraction sites or to control
mesiodistal position of roots in general.
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31. • The width of the bracket determines the length of the
moment arm (half the width of the bracket) for
control of mesiodistal root position.
• Bracket width also influences the contact angle at
which the corner of the bracket meets the archwire.
• The wider the bracket, the smaller the contact angle.
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32. EFFECT OF BRACKET SLOT SIZE IN
THE EDGEWISE SYSTEM
• The use of rectangular archwires in rectangular
bracket slots was introduced by Edward Angle.
• Using undersized archwires in edgewise brackets is
a way to reduce the frictional component of
resistance to sliding teeth along an archwire.
• A reduction in slot size with full dimension steel
wires still produce slightly greater forces than the
original edgewise system.
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33. MECHANICAL ASPECTS OF
ANCHORAGE CONTROL
When teeth slide along an archwire, force is needed for
two purposes:
• To overcome resistance created by contact of the wire
with the bracket .
• To create the bone remodeling needed for tooth
movement.
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34. Frictional Effects on Anchorage
 When one moving object contacts another, friction at
their interface produces resistance to the direction of
movement.
 Friction ultimately is derived from electromagnetic
forces between atoms.
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35. • It is proportional to the force with which the
contacting surfaces are pressed together and is
affected by the nature of the surface at the interface
(e.g., rough or smooth, chemically reactive or
passive, modified by lubricants).
• Interestingly, friction is independent of the apparent
area of contact.
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36. • Whentwosolidsurfacesarepressedtogetheroroneslidesover theother,real
contactoccursonlyatalimitednumberofsmall spots,calledasperities,that
representthepeaksofsurface irregularities.Thesejunctionsshearassliding
occurs, and the force to produce this plastic deformation of the surface
irregularitiesisthefrictionalresistance.
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37. METHODS TO CONTROL
ANCHORAGE
Reinforcement:
• The extent to which anchorage should be reinforced
depends on the tooth movement that is desired.
• Satisfactory reinforcement of anchorage may require
the addition of teeth from the opposite dental arch to
the anchor unit.
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39. • This anchorage could be reinforced even further by
having the patient wear an extraoral appliance
(headgear) placing backward force against the upper
arch.
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40. SUBDIVISION OF DESIRED
MOVEMENT
 A common way to improve anchorage control is to pit
the resistance of a group of teeth against the movement
of a single tooth, rather than dividing the arch into
more or less equal segments.
 Subdivision of tooth movement improves the
anchorage situation regardless of whether sliding is
involved and where a space in the arch is located.
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41. • It would be perfectly possible to reduce the strain on
posterior anchorage by retracting the canine
individually, pitting its distal movement against
mesial movement of all other teeth within the arch.
• After the canine tooth had been retracted, one could
then add it to the posterior anchorage unit and retract
the incisors.
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43. TIPPING/UPRIGHTING
• Another possible strategy for anchorage control is to
tip the teeth and then upright them, rather than
moving them bodily.
• Require two steps in treatment:
 First, the anterior teeth would be tipped distally.
 Second step, the tipped teeth would be uprighted.
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44. ANCHORAGE CONTROL IN SPACE
CLOSURE
• It is desirable to close the extraction space 60%by
retraction of the anterior teeth and 40%by forward
movement of the posterior segments.
• Three possible approaches:
(1) one-step space closure with no sliding (via closing
loops so that segments of wire are moved with the
teeth attached, rather than sliding);
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45. (2) a two-step space closure sliding the canine along
the archwire, then retracting the incisors (as in the
original Tweed technique); or
(3) two-step space closure, tipping the anterior
segment with some friction, then uprighting the
tipped teeth (as in the Begg technique)
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46. Closure of premolar extraction space
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47. SKELETAL ANCHORAGE
• Skeletal anchorage is derived from
 implants,
 Miniplates attached with screws to basal bone of the
maxilla or mandible,
 or just a screw with a channel for attaching a spring
that is placed into the alveolar process
• These devices are referred to as temporary anchorage
devices (TADs).
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48. • Bone anchors
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49. Conclusion
• Various mechanics can often be used to achieve the tooth movements
desired for orthodontic treatments. It is important however to
understand the mechanics involved and to recognize when the
appliance will not achieve adequate results or may result in undesirable
side effects. This can help us to prevent prolonged overall treatment
time and/or compromise in the final orthodontic outcome.
The ultimate result will be a happy post treatment patient, with a
beautiful smile leaving your clinic.
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50. REFERENCES:
• Contemporary Orthodontics –5th Edition (William R. Proffit)
• Orthodontics Current Principles and Techniques – 6th Edition
(Graber)
• Orthodontics Diagnosis and Management of Malocclusion and
Dentofacial Deformities–1st Edition (Om Prakash Kharbanda)
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