2. Seminar PresenatationBiomechanics in Orthodontics
Under the guidance of :
Dr. Mohammad Mushtaq,
HOD & GUIDE
By:
Sneh Kalgotra,
2nd Year P.G.
Department of Orthodontics & Dentofacial Orthopaedics, GDC&H, Srinagar.
4. 11. Moment to force ratio.
12. Cue ball priciple, row boat efect, diving
board effect.
13. Types of tooth movement.
14. One couple , two couple system.
15. Biomechanics of leveling nad
aligning.
16. Biomechanicss of space closure.
17. Biomechnics of finishing.
18. Conclusion.
6. Introduction
• Orthodontists are biological scientists and have not
always been comfortable with the physical
sciences.
•
There is no room for the “one step forward, two
steps backward” type of treatment mechanics.
A
B
A
B
It thus behooves the orthodontist to acquire the skills
necessary to do the “A directly to B.” type of
orthodontic treatment.
Biomechanics in Orthodontics. Michael R. Marcotte, 1st Edition
7. These mechanicl principles are found within the
branch of engineering science called as mechanics.
• It is the study of mechanics as it affects the biologic
systems. It is the application of mechanics to the
biology of tooth movement – BIOMECHANICS.
• Biology + Mechanics = Bio-mechanics.
It becomes important for Orthodontists
to think, understand and apply basic
principles of mechanics in a common
sense manner.
Biomechanics in Orthodontics. Michael R. Marcotte, 1st Edition
8. The elbow joint and bending
Walking on staircase
Body
balancing
as a whole
Knee joint
9. Scalar:
When a physical property ( Weight, temperature ,force) has
only magnitude , its called a scalar quantity.
( E.g.. A force of different magnitude such as 20gm,50gm etc)
Vector: When a physical property has both magnitude and direction
its called a vector quantity.
(E.g.. A force vector characterized by magnitude, line of action, point of
origin and sense)
10. Newton’s laws of motion
1st law
An object at rest tends to
stay at rest and an
object in motion tends to
stay in motion unless
acted upon by an
unbalanced force.
12. One rock weighs 5 Newtons.
The other rock weighs 0.5
Newtons. How much more
force will be required to
accelerate the first rock
at the same rate as the
second rock?
Ten times as much
14. • A bug with a mass
of 5 grams flies into
the windshield of a
moving 1000kg bus.
• Which will have the
most force?
• The bug on the bus
• The bus on the bug
15. Force
• It is defined as an act upon a body that changes or
tends to change the state of rest or motion of the
body. Force is a vector it has both magnitude and
direction.
• The forces are indicated by straight arrows.
17. Resultant of forces
b
d
A
C
A Force Proportional To The Length Ab Operates In The
Direction Ab;A Second Force Proportional To The Length
Ac Operates In The Direction Ac;The Resultant Force
Operates In The Direction Ad And Is Proportional To The
Length Ad.This Device For Calculating The Total Effect Of
Two Forces Is Called The Parallelogram Law Of Forces.
18. CENTER OF MASS:
Each body has a point on its mass , which behaves
as if the whole mass is concentrated at that single
point. We call it the center of mass in a gravity free
environment.
• The same is called center of gravity in an
environment where gravity is present.
•
19. Center of resistance.
• Since the tooth is
partially restrained
as its root is
embedded in bone
its center of gravity
moves apically and
this is known as
Center of resistance.
Frontal, occlusal,
and mesial views
of the center of
resistance of a
tooth.
Center of resistance varies
depending up on the
- Root length & morphology
- Number of roots
- Level of alveolar bone
support.
20. A
B
Center of resistance for
A) a two-tooth segment and B) a maxilla C) Center of
resistance of Maxillary molar.
Biomechanics in clinical Orthodontics. Ravindra Nanda, 1st Edition
C
21. CENTER OF ROTATION
• It may be defined as a point about which a body
appears to have rotated as determined from its
initial to final positions.
•
A simple method of determining a center of
rotation. Draw the long axis of the tooth in its initial
and final positions; we will see that both these lines
intersect at a point. This is the point around which
the tooth rotates and is called center of rotation.
22. • The centre of rotation can be at the
centre of resistance,apical to it,at the
root apex or at infinity.
23. TYPES OF TOOTH MOVEMENT
POSITION
OF
THE
CENTER
OF
ROTATION
A.
Translation
Lies at infinity
B.
Uncontroued tipping
Slightly apical to center of resistance
A.
Controlled tipping
Apex of root
B.
Root movement of torquing
Incisal or occlusal edge
24. Moment :Defined as a tendency to rotate.
Moment of the force
• The moment of the
force is the tendency
for a force to
produce rotation. It is
determined by
multiplying the
magnitude of the
force by the
perpendicular
distance of the line
of action to the
center of resistance
• The force is not acting through the Cres
Unit– Newton. mm
25. • The direction of moment of force can
be determined by continuing the line
of action around the Cres.
26. Couple
A couple consists of two forces of equal magnitude,
with parallel but non colinear lines of action and
opposite senses.
• The result is a moment with no net force.
• The object rotates about it’s centre of resistance
regardless of the point of application of the couple.
27. Couple-clinical point
• When the tooth is embedded within the alveolar
bone,a couple can be applied only on the
exposed part of the tooth.Various tooth alignment
procedures can be achieved by this couple
mechanism. Depending on the plane in which the
couple is acting this rotational tendency is called
a.Rotation{first order} b.Tipping{second order}
c.Torque{third order}
28. M/F (moment to force ratio) is the relationship
between the force and the counter balancing
couple that determines the type of tooth movement
The ratio of the counter-balancing moment
produced to the net force that is applied to the
tooth will determine the type of tooth movement
that will occur.
29. Force applied on a tooth
Crown moves more than root
To maintain the inclination
Of the tooth
Overcome the moment
Created by the force applied
to the crown
Counter moment
30. To maintain axial inclination
Apply the force close to
the center of resisitance
Practical difficulty
Power arm
Create a 2nd moment
In the direction opposite
to the first
Counter moment
Tooth remain upright
And move bodily
31. MOMENT TO FORCE RATIO FOR VARIOUS
TOOTH MOVEMENTS
M/F
5:1
Uncontrolled tipping
M/F
8:1
Controlled tipping
M/F
10 : 1
Translation
M/F >10 : 1
Root movement
33. TIP
Bracket
system
In the PAE bracket
system, the tip
incorporated into the
bracket acts as the
counter moment in the
mesio distal direction
This prevents the
tipping of the tooth in
the mesio distal
direction.
34. Principle of power arm
The line of action of the force passes through
the center of resistance. This tooth will
translate, even though the point of
attachment to the tooth is at the bracket.
35. CUE BALL CONCEPT
A force off
center causes
the cue ball to
rotate as well as
move forward in
a straight line.
No left or right
rotation is
produced when
the force is applied
through the center
of the cue ball.
When the line of
force acts
through the
center of
resistance, only
translation
results.
Equal and
opposite
forces
(couple)
produce
pure
rotation.
Mulligan TF: Common sense mechanics 2 . Forces & moments. JCO 13:676-683,1979
36. ROW BOAT
EFFECT
If the bends produce equal angular
relationships (A), the net forces are zero. If
unequal (B), net forces occur.
Mulligan TF: Common sense mechanics 2 . Forces & moments. JCO 13:676-683,1979
37. DIVING BOARD
CONCEPT
When the length of the diving board is
doubled, only one-eighth the force is
required to produce the same amount
of deflection. B. The same force acting
at twice the length will produce eight
times as much deflection.
With a constant tipback angle,
the deflection doubles as the
wire length doubles, the force is
reduced to one fourth
Mulligan TF: Common sense mechanics 2 . Forces & moments. JCO 13:676-683,1979
39. UNCONTROLLED TIPPING
• When only a force is applied at the bracket to
move a tooth ,the equivalent force system at the
center of resistance is force plus the moment that
will tip the crown in the direction of force.as the
counteracting moment is absent, the crown moves
further than the center of resistance and the apex
moves in the opposite direction.
• When the moment to force ratio is zero, the center
of rotation is apical to the center of resistance.
40. CONTROLLED TIPPING.
• To counteract the tendency for tipping a couple
can be applied intentionally at the bracket to
produce a moment of less magnitude in the
opposite direction.the moment-to-force ratio of 7:1
is applied at the bracket,the equivalent force
system at the center of resistance is a force to
move the tooth plus a small net tendency for the
crown to tip in the direction of the force.
• Therefore ,the center of rotation,when the momentto-force ratio is 7:1, is at the apex of the tooth and
only crown movements occur.
41. TRANSLATION
• To counteract the tendency for tipping, a couple
can be applied intentionally to produce a moment
of equal magnitude in the opposite direction.when
a moment –to –force ratio of 10:1 is applied to the
bracket ,the equivalent force system at the center
of resistance is a single force with no net moment.
• In the pure translation, the center of
rotation is considered to be at infinity,
because no rotation occurs.
42. ROOT MOVEMENT.
• When the counter moment applied intentionally at
the bracket is more than the moment of force , the
root moves in the direction of force but the crown
tips in the opposite direction.when the moment-to –
force ratio of 13:1 is applied at the bracket,the
equivalent force system at the center of resistance
is a force to move the tooth plus a small net
tendency for the root to tip in the direction of force.
• The center of rotation ,when the
moment to force ratio is 13:1,is at the
crown of the tooth and only root
movements occur.
43. PURE ROTATION.
• If only a couple ,and no force is applied to a tooth
the tooth will rotate around it’s centre of resistance
and the tooth will not translate. because the action
of a couple does not depend on it’s point of
application,a pure moment always acts at the
center of resistance.
• The forces of a couple cancel out any tendency for
the center of resistance of the tooth to move but
the moment produced by the couple causes the
tooth to rotate the moment-to force ratio is infinite
and the center of rotation coincides
with the center of resistance
45. Equal and opposite force system.
• Simplest orthodontic appliance ,an
elastic band stretched between two
points of attachment is the best
example. This produces force of equal
magnitude on either end but opposite
direction.
46. ONE COUPLE APPLIANCE
SYSTEMS.
• One end of the appliance experiences couple and
the other end is tied as a point contact. It is
statically determinate because the magnitudes of
the forces and moments produced can be
determined clinically after the appliance is inserted
into the bracket / tube. This can be done by
inserting the appliance into the bracket and
measuring the force required to activate the wire to
the site it will be tied as a point contact.
47. Two couple appliance system.
• The both the ends of the appliance are engaged
into attachments{brackets or tubes}.A couple may
be generated by the wire at either or both
attachment sites. The force systems produced by
two couple appliances cannot be measured
clinically and so they are referred as statically
indeterminate.
48. • Variety of combinations of twobracket systems and their force
systems
49. Phases of orthodontic treatment.
• Stage 1: Levelling and aligning.
• Stage 2 : Bite correction and space
closure.
• Stage 3: Finishing and detailing.
50. Levelling and aligning.
We put thinner wires at the beginning of alignment ,less
applied couple - less M:F - no root moment only crown
moment (tipping)
51. The 2 central incisors are rotated
mesial in creating a symmetric V
geometry. The
desired corrective force system
involves 2 equal
and opposite moments as illustrated
Semin Orthod 2001;7:16-25.
52. The force system developed by inserting a straight wire into the brackets of
the 4 anterior teeth will create counterclockwise moments on the 2 central
incisors as well as lingual movement of the left central incisor and labial
movement of the right central incisor. The initial geometry is not favorable for
alignment.
Semin Orthod 2001;7:16-25.
53. shows a lingually placed right lateral incisor. In this case, the geometric
relationship between the right lateral and central incisors is a step geometry
and the placement of a straight wire into the brackets of the 4 anterior teeth will
align the teeth and also shift the midline to the right side
Semin Orthod 2001;7:16-25.
54. In the maxillary arch shown in Figure, the relationship between the central
incisors is a step geometry and an asymmetric V geometry is observed between
the central and lateral incisors on the right side. Analysis of the force system
shows that, although correction of the 2 central incisors will occur as
a result of straight wire placement, the right lateral incisor will be displaced
labially, which is an undesirable side effect .
Semin Orthod 2001;7:16-25.
55. During extrusion of a high canine
unilaterally. Figure A shows the
force system generated by the
placement of a straight wire
through a high maxillary right
canine. The canine will extrude as
desired, but the lateral incisor and
first premolar on that side will
intrude and tip toward the canine
space. An open bite may result on
that side of the arch, and the
anterior occlusal plane will be
canted up on the right side.
Semin Orthod 2001;7:16-25.
56. Force vectors in Cl-III elastics
Force Vectors in Cl-II elastics
Favorable in low angle deep bite
cases
Favorable in low angle cases
57. SPACE CLOSURE
ANCHORAGE CLASSIFICATION.
Group A anchorage. This category describes the
critical maintenance of the posterior tooth position.
Seventy-five percent or more of the extraction space
is needed for anterior retraction.
58. Group B anchorage. This category describes relatively
symmetric space closure with equal movement of the
posterior and anterior teeth to close the space. This is
often the least difficult space closure problem.
59. Group C anchorage. This category describes
"noncritical"anchorage. Seventy-five percent or more
of the space I closed by movement of the posterior
teeth.
62. Ideal force system for group A space closure. For
perfect maintenance of the posterior anchorage, no
forces should act on the posterior teeth; only a force
system resulting in anterior translation is desired. This
force system cannot exist unless all the anchorage units
are extra oral or in the opposite arch.
63. •
In A the blue arrow
represents an additional
force acting on the anterior
teeth (i.e. Class II elastics or Jhook headgear). In B the
blue arrow represents the
force from a headgear
acting on the posterior teeth.
In both cases the change in
the force magnitude results
in a lower moment/force
ratio on the anterior teeth
and an increased
moment/force ratio on
the posterior teeth.
64. • Force system for group B space closure. Translation
of the anterior and posterior teeth is required to
achieve ideal space closure. A moment/force ratio
approximating 10/1 is needed for translation.
65. • The difficulty of group C anchorage
mirrors that of group A anchorage. The
difference is that the anterior teeth
become the effective "anchor unit".
Therefore, the anterior moment is of
greater magnitude and the vertical force
side effect is an extrusive force on the
anterior teeth. Due to the difficulty of this
type of space closure in the lower arch,
extraction treatment should be reevaluated carefully and great awareness
of the possible side effects is needed.
66. Finishing
• The major factors involved in achieving
this proper anterosuperior incisor
inclination are the bracket and wire
coupling.
• A specific archwire with a medium-tolow load deflection rate (such as a
0.017 x 0.025) can be selected.
67. • After the third-order objectives are met, secondorder movements are addressed. Adjustments in this
plane also take a significant amount of time
because they also involve root correction. To
achieve second-order objectives, either
the same 0.016 x 0.022 beta-titanium or steel round
archwire can be used, providing all the thirdorder
objectives have been met.
68. • The final step in finishing is the correction of firstorder problems. Usually these problems are obvious
clinically and can be corrected quickly. Tn many
instances, only small correction bends in the
archwire are needed. Correction can also be
achieved with auxiliary plastic rotation wedges on
the brackets, or by making small bends in either the
0.016 x 0.022 beta-titanium or 0.016 steel archwire.
69. •
•
•
•
•
•
•
•
•
BIOMECHANICS-CORE OF CLINICAL PRACTICE
Optimization of tooth movement.
Anchorage control.
Selection of wires, brackets and clinical devices.
Explanation and evaluation of treatment results.
Research.
Minimization of tissue destruction.
Reduction of need for patient cooperation.
Development and evaluation of new appliances.
Knowledge transfer from appliance to appliance.
70. Conclusion
• The choice of appliances and techniques
used by practitioners varies radically
among individuals but the fundamental
forces and moments they produce are
universal.
• Appliance will always act according to
the LAWS OF PHYSICS. Understanding the
basic biomechanical principles involved
in effective controlled tooth movement
makes the final outcome more
predictable and consistent.
71. • Newton’s third law states that every action has a
equal and opposite reaction.
• Its important to keep this concept in mind working
with any appliance system and give adequate
importance to take steps to prevent the adverse
effects.
• In orthodontic terms, the understanding of the
moment and the application of the necessary
counter moment to bring about the optimal tooth
movement is the key to successful treatment
results.
• An application of little bit of common sense can
work wonders for the treatment outcome.
72. REFFERENCES
1. Smith RJ, Burstone CJ: Mechanics of tooth movement. AJO 85:294-307,1984.
2. Burstone CJ, Koenig HA: Creative wire bending- The force system from step &
V bends. AJO DO 93(1):59-67,1988.
3. Burstone CJ, Koenig HA: Force system from the ideal arch. AJO 65(3):270289,1974.
4. Demange C: Equilibrium situations in bend force system. AJO DO98(4):333339,1990.
5. Issacson RJ, Lindauer SJ, Rubenstein LK: Moments with edgewise appliance e:
Incisor torque control. AJO DO 103(5):428-438,1993.
6. Koing HA, Vanderby R, Solonche DJ, Burstone CJ: Force system for
orthodontic appliances: An analytical & experimental comparison. J
Biomechanical Eng102(4):294-300,1980.
7. Kusy RP, Tulloch JFC: Analysis of moment/force ratio in the mechanics of
tooth movement. AJO DO 90; 127-131,1986.
8. Nanda R, Goldin B: Biomechanical approaches to the study of alteration of
facial morphology. AJO 78(2):213-226,1980.
73. 9. Vanden Bulcke MM, Burstone CJ, Sachdeva RC , Dermaut LR: Location of
center of resistance for anterior teeth during retraction using the laser reflection
technique. AJO DO 91(5):375-384,1987.
10. Vanden Bulcke MM, Dermaut LR, Sachdeva RC, Burstone CJ: The center of
resistance of anterior teeth during intrusion using the laser reflection technique &
holographic interferometry. AJO DO 90(3): 211-220,1986.
11. Mulligan TF: Common sense mechanics 2 . Forces & moments. JCO 13:676683,1979.
12. Siatkowski RE: force system analysis of V-bend sliding mechanics. JCO
28(9):539-546,1994.
13. Tanne K, et al: Moment to force ratios & the center of rotation. AJO 94:426431,1989
14. The basics of orthodontic mechanics. Semin Orthod 2001;7:2-15
15. Leveling & aligning: Challenges & Solutions Semin Orthod 2001;7:16-25.
16. Biomechanics in clinical Orthodontics. Ravindra Nanda, 1st Edition
17. Biomechanics in Orthodontics. Michael R. Marcotte, 1st Edition
