about basic principles of orthodontics

1. Basic Principles of
Biomechanics
Presented by:
DR. ARCHIT BAHADUR
PG II YEAR
DEPARTMENT OF ORTHODONTICS
INSTITUTE OF DENTAL SCIENCES
Guided By:
DR. ANIL K. CHANDNA(Dir. PG Studies)
DR. PREETI BHATTACHARYA(Prof. & HOD)
DR. ANKUR GUPTA(Prof.)
DR. RAVI BHANDARI(Reader)
DR. SHIVANI SINGH(Reader)
DR. DHRUV TIWARI(Sr. Lecturer)

2. Recap...
• Force is defined as the action of one body on another that
changes or tends to change the shape or motion of that second
body.
• To move a tooth predictibly a force needs to be applied in the
desired direction, with the desired magnitude and at the correct
position on the tooth.

3. • Moment of the force is the tendency of a force to produce rotation.
• It is determined by multiplying the magnitude of the force by the perpendicular
distance from the center of resistance to the line of action of this force.
• Unit of measurement: Grams-mm or N-mm.

4. • A: a mesial force at the molar bracket
creates a moment tending to rotate the
tooth in a mesiolingual/ mesiobuccal
direction.
• B: expansion force on a molar creates
a moment tipping the crown buccally.
• C: intrusive force at the molar bracket
creates a moment tipping the crown
buccally.

5. • Moment of Couple is another form
of rotational movement where two
parallel forces act in opposite
directions and separated by a
distance.
• Couples result in a pure rotational
movement around the center of
resistance regardless of where the
couple is applied on the object.

6. Equivalent Force Systems
• Wires, elastics, and springs are attached to
the tooth at the bracket which is away from
the center of resistance, leading undesirable
to rotations or tipping of the teeth.
• Force equivalency is a concept that
advocates the use of a counter-moment
equal and opposite to the moment generated
as a result of the mechanics.
• This results in the application of force
system at the Cres that is equivalent to the
applied force system.

7. Types of Tooth Movements
Tipping:
• Tipping is tooth movement with a greater movement of the crown of a tooth
than the root.
• The Center of Rotation of the motion is apical to the Center of Resistance.
• Further classified into Controlled and Uncontrolled Tipping.

8. Uncontrolled Tipping:
• A single, horizontal, labio-lingually directed
force at the level of the bracket will cause the
movement of the root apex and crown in the
opposite directions.
• Often undesirable due to non-uniformity of the
stresses generated.
• Moment/Force ratio: 0:1 to 5:1
• Indications of uncontrolled tipping include Class
II Div. 2 and Class III where excessively upright
incisors need to be flared.

9. Controlled Tipping:
• Controlled tipping is achieved by the application of a force to move the crown, as done
in uncontrolled tipping, and application of a moment to control the position of the apex.
• Moment/Force ratio: 7:1
• The stresses at the root apex are very minimal, maintaining the integrity of the apex.
• Indications include retraction of proclined incisors with appropriate root apex
position,where only the crown movement is required.

10. Bodily Movement:
• A.k.a. Translatory tooth movement.
• Characterized as the crown and the root apex moving the same amount of distance and
in the same horizontal direction.
• Crot in such tooth movements is at infinity.
• M/F ratio: 10:1
• This type of tooth movement produces uniform stresses on the periodontium.

11. • Root Movement:
• A.k.a. Torquing.
• Changing a tooth’s axial inclination while holding the crown stationary.
• Crot is at the incisal edge or the bracket.
• M/F ratio: 12:1.
• Stress levels at the root apex are the highest - undermining resorption - slowing of OTM.

12. • Rotation:
• Pure rotation of a tooth requires a
couple.
• No net force acts on the Cres - only
rotation occurs.

13. Basic Principles of
Biomechanics - II
Presented by:
DR. ARCHIT BAHADUR
PG II YEAR
DEPARTMENT OF ORTHODONTICS
INSTITUTE OF DENTAL SCIENCES
Guided By:
DR. ANIL K. CHANDNA(Dir. PG Studies)
DR. PREETI BHATTACHARYA(Prof. & HOD)
DR. ANKUR GUPTA(Prof.)
DR. RAVI BHANDARI(Reader)
DR. SHIVANI SINGH(Reader)
DR. DHRUV TIWARI(Sr. Lecturer)

14. • The resultant moment due to the two
forces can be calculated as the
summation of both the moments.
• M = F1D1 + F2D2.
.

15. Manipulating the M/F ratio:
• Altering the point of force application:

16. • Altering the M/F ratio:
• M/F ratio can be altered by:
1. Applying a counter-force on the
tooth that creates a counter-moment
on the tooth.
2. Creating a couple in the bracket
using a rectangular archwire.

17. MC/MF Ratio and Root position control:
• Control of root position during OTM requires
both a force to move the tooth in the desired
direction and a 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.

18. Specific considerations in Appliance design
• An Optimal Force is the lightest force that will move a tooth in the desired position in the
shortest possible time with no iatrogenic effects.
• Force magnitudes as small as 2 g have been shown to produce tooth movements (Weinstein S
- 1967), whereas forces from headgear and orthopedic appliances often exceed 500 g.
• For tooth movements over large distances, continuity of the force levels throughout the range
of activation of the appliance is desired. This phenomenon is referred to as Force Constancy.
• Force Constancy can be obtained by reducing the load-deflection rate by:
a) Reducing wire cross-section
b) Increasing interbracket distance
c) Incorporating loops in the wire
d) Using memory alloys.

19. Reducing the wire cross-section:
• Small diameter wires - increased flexibility - eases ligation in the early stages of the
treatment of malaligned teeth.
• However, the lighter the wire, the less the control expressed on the tooth in all three
planes.
• Heavier wires provide better bracket engagement into the slot and better control in the
tooth positioning, but the load-deflection rate and the forces applied may be too high.
• As stiffness increases with wire dimension, the range of activation decreases because
of the increase in load-deflection rate.
• Heavier wires provide excellent control in the final stages of the treatment when
small, detailed tooth movements are required. Additionally, large diameter wired
provide better anchorage control.

20. Increasing the interbracket distance:
• A large interbracket distance reduces the load-deflection rate and helps to deliver
constant force magnitude, providing better directional control of OTM.
• Inc. interbracket wire length - greater flexibility.
• Incorporating loops in the Wire:
• Incorporation of loops increases the wire length, effecively reducing the load-
deflection rate, thereby providing optimal forces on the teeth.

21. • Memory Alloys:
• The use of memory alloys such as NiTi, has effectively reduced the
load-deflection rate of archwires.
• Large dimension memory alloy wires can be used much earlier in
the treatment for better tooth motion control.
• Due to NiTi having lower Modulus of elasticity than Stainless steel,
it translates to almost 1:1 reduction in the load-deflection of the
archwire.

22. Thank you