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The Bio-mechanics of Orthodontic Tooth Movement

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The Bio-mechanics of Orthodontic Tooth Movement

D 657

Dr. Shiva Shanker

Dr. Allen Firestone

Contemporary Orthodontics, Chapter 10,

3rd Edition:pp. 337-360

2nd Edition:pp. 302-315

Recommended: Smith and Burstone, Mechanics of tooth movement. American Journal of Orthodontics, 1984; 85:294-307

LECTURE READING

1. Definition of terms2. Mechanics of tooth movement3. Anchorage in orthodontic appliances

LECTURE OBJECTIVES

1. Review Laboratory Procedures:Arch Length AnalysisMolar Uprighting: Preparing typodont

LECTURE OBJECTIVES

- Force
- Center of Resistance (Cres)
- Types of Tooth Movement

- Physical property e.g. distance, weight, temperature, force.
- Physical properties are, mathematically, scalars or vectors
- Scalars have a magnitude and no direction e.g. weight, temperature
- Vectors have a magnitude and a direction e.g. force

- Line of Action

- Sense

- Magnitude

- Point of Application

- Common point of application

- Different points of application

- Resolving force into components

- A load applied to an object that will tend to move it in the direction of the applied force
- Defined in units of Newtons
- Orthodontic purposes measured as grams or ounces

- Force
- Center of Resistance (Cres)
- Types of Tooth Movement

- Free body: center of mass/gravity = point of balance
- Restrained body (tooth): center of resistance (Cres)
- By definition, a force acting through Cres moves tooth with no change in orientation = translation

- A point at which resistance to movement can be concentrated for mathematical analysis (= Cres)
- The center of resistance for a tooth is 1/2 - 1/3 (40%) root length apical to alveolar crest
- Cres varies with root length and alveolar crest height

- Cres varies with root length and alveolar crest height

- Force
- Center of Resistance (Cres)
- Types of Tooth Movement

- Translation

- Rotation

- Combination/Tipping

- Moment
- Couple
- Center of Rotation (Crot)

- A force applied in line with the center of resistance;the tooth is translated with no rotation relative to the force
- Orthodontically, the point of attachment is irrelevant; the line of action of force determines the effect on the tooth

- A force that doesn’t pass through Cres causes translation + rotation = tipping
i.e., tends to tip the tooth, movement with a rotational component.

- Force
- Center of Resistance (Cres)
- Types of Tooth MovementMomentCoupleCenter of Rotation (Crot)

- If the line of action of a force is at a distance from the Cres the force will produce some rotation. The potential for rotation is measured as a moment

- Magnitude = perpendicular distance from Cres to the line of action X magnitude of force (unit = gram mm)
- Direction: Clockwise or Counter-cw

- No single force can cause pure rotation
- Only a couple can
- Two forces: equal magnitude; parallel and non-collinear; opposite sense
- Couple is a ‘free vector’

- Two forces; equal magnitude; parallel and non-collinear; opposite sense

- Translational effects cancel each other out

- The moments are in the same direction and are additive

+

= 1000 gm mm

- The Sum of the Moments =

50 g X 10 mm

50 g X 10 mm

- The moment of a couple is equal to the magnitude of one of the forces X distance between them (50g X 20 mm)

- Only a couple can cause pure rotation

- Force
- Center of Resistance (Cres)
- Types of Tooth MovementMomentCoupleCenter of Rotation (Crot)

- The point around which rotation occurs when an object is being moved
- This point will vary depending on the force/moment/couple being applied
Bodily movement or translation

Tipping movement

- Connect the before and after positions of 2 points
- The intersection of the perpendicular bisectors of these lines is Crot

Type of Movement

Translation

Uncontrolled tipping

Controlled tipping

Root movement

Center of Rotation

Infinity

Slightly apical to Cres

Apex

Incisal edge

Point of application of force:Closer to Cres Smaller momentLess rotationMore translation

1. Definition of terms2. Mechanics of tooth movement3.Anchorage in orthodontic appliances

LECTURE OBJECTIVES

- Resistance to unwanted tooth movement
- Resistance to reaction forces
- Maximizing tooth movement and minimizing unwanted “reactionary effects”

- Reciprocal space closure
- Minimum anchorage requirements

- Differential space closure
- Intermediate anchorage requirements

1237

706

- Differential space closure
- Intermediate anchorage requirements

- Dissipate reactionary forces over as many teeth (or as widely) as possible
- Reinforced anchorage
- Maximum anchorage

- Frictional resistance to sliding archwires against brackets

- Surface of wires/bracket -
- 1) Stainless steel slides well on stainless steel;
- 2) Nickel titanium alloy wires have greater frictional resistance
- 3) Ceramic brackets also exhibit greater frictional resistance

1. Definition of terms2. Types of tooth movement3. Anchorage in orthodontic appliances

LECTURE OBJECTIVES

1. Review Laboratory Procedures:Arch Length AnalysisMolar Uprighting: Preparing typodont

LECTURE OBJECTIVES

Arch Length Analysis = Space Analysis

- Permanent Dentition
- Mixed Dentition

+Arch length available

- M/D width of 12 teeth: first molar to first molar

__________________________

= Arch length deficiency or excess

The arch circumference:

measured at the proximal contacts of posterior teeth and the incisal edges of anterior teeth i.e., the widest part of the tooth.

Measuring Arch Length

- The same procedure in themixed dentition but
- Need to estimate the size of unerupted permanent teeth (premolars and canines)
- Moyers proportionality tables, Tanaka and Johnston

+Arch length available

- M/D width of teeth mesial to first molars

_______________________

= Arch length deficiency or excess

Measuring Arch Length

To estimate the width of unerupted canines and premolars in a quadrant:

1. Get sum of the mesial-distal width of the 4 mandibular permanent incisors.

2. Locate value in Table and look up predicted width of maxillary and mandibular teeth.

To estimate the width of unerupted canines and premolars in a quadrant:

1. Get sum of the mesial-distal width of the 4 mandibular permanent incisors.

2. Divide by 2 and add

11.0 for max. canine + premolars

10.5 for mand. canine + premolars.

Tanaka &Johnston, JADA 88:798, 1974

To estimate the width of unerupted canines and premolars in a quadrant:

23mm for maxillary quadrant

22mm for mandibular quadrant

Molar Uprighting Typodont

Molar Uprighting Teeth

Molar Uprighting Wax

Molar Uprighting Wax Preparation

Molar UprightingSegment Preparation

Molar UprightingSegment Preparation