Biomechanics of tooth movements
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Transcript of Biomechanics of tooth movements
The Bio-mechanics of Orthodontic Tooth
MovementD 657
Dr. Shiva ShankerDr. Allen Firestone
LECTURE READINGContemporary Orthodontics, Chapter 10, 3rd Edition: pp. 337-3602nd Edition: pp. 302-315
Recommended: Smith and Burstone, Mechanics of tooth movement. American Journal of Orthodontics, 1984; 85:294-307
LECTURE OBJECTIVES1. Definition of terms
2. Mechanics of tooth movement
3. Anchorage in orthodontic appliances
LECTURE OBJECTIVES1. Review Laboratory Procedures:
Arch Length AnalysisMolar Uprighting: Preparing
typodont
Definitions
Force Center of Resistance (Cres) Types of Tooth Movement
Force
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
Vectors
Line of Action
Point of Application
Sense
Magnitude
Resultant of Forces
Common point of application
Resolving force into components
Different points of application
Common point of application Resultant of Forces
Different points of application Resultant of Forces
Resolving Force Into Components Resultant of Forces
Force
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
Definitions
Force Center of Resistance (Cres) Types of Tooth Movement
Center of Resistance
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
Center of Resistance
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
Center of Resistance
Cres varies with root length and alveolar crest height
Definitions
Force Center of Resistance (Cres) Types of Tooth Movement
Types of Tooth Movement
Translation Rotation Combination/Tipping
• Moment
• Couple
• Center of Rotation (Crot)
Translation = Bodily Tooth Movement
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
Translation or Bodily Tooth Movement
Tipping Tooth Movement
A force that doesn’t pass through Cres causes translation +
rotation = tipping
i.e., tends to tip the tooth, movement with a rotational component.
Definitions
Force Center of Resistance (Cres) Types of Tooth Movement
MomentCouple
Center of Rotation (Crot)
Moment
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
Moment - Magnitude
Magnitude = perpendicular distance from Cres to the line of action X magnitude of force (unit = gram mm)
Direction: Clockwise or Counter-cw
Couple
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’
Couple
• Two forces; equal magnitude; parallel and non-collinear; opposite sense
• Translational effects cancel each other out
Couple
•The moments are in the same direction and are additive
Couple
• The Sum of the Moments =
50 g X 10 mm 50 g X 10 mm+ = 1000 gm mm
• Only a couple can cause pure rotation
• The moment of a couple is equal to the magnitude of one of the forces X distance between them (50g X 20 mm)
Couple
Definitions
Force Center of Resistance (Cres) Types of Tooth Movement
MomentCouple
Center of Rotation (Crot)
Center 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 translationTipping movement
Determining Crot
Connect the before and after positions of 2 points
The intersection of the perpendicular bisectors of these lines is Crot
Tooth Movement and Crot
Type of Movement
Translation
Uncontrolled tipping
Controlled tipping
Root movement
Center of Rotation
Infinity
Slightly apical to Cres
Apex
Incisal edge
Control of Center of Rotation
Point of application of force:Closer to Cres
Smaller momentLess rotationMore translation
LECTURE OBJECTIVES1. Definition of terms
2. Mechanics of tooth movement
3. Anchorage in orthodontic appliances
Anchorage
Resistance to unwanted tooth movement
Resistance to reaction forces Maximizing tooth movement and
minimizing unwanted “reactionary effects”
Strategy 1
Reciprocal space closure Minimum anchorage requirements
Strategy 2 - Differential Anchorage Values
Differential space closure Intermediate anchorage requirements
1237 706
Strategy 2A - Differential Tooth Movement
Differential space closure Intermediate anchorage requirements
Strategy 3 Dissipate reactionary forces over as
many teeth (or as widely) as possible Reinforced anchorage Maximum anchorage
Frictional Effects on Anchorage
Frictional resistance to sliding archwires against brackets
Factors Affecting Friction
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
LECTURE OBJECTIVES1. Definition of terms
2. Types of tooth movement
3. Anchorage in orthodontic appliances
LECTURE OBJECTIVES1. Review Laboratory Procedures:
Arch Length AnalysisMolar Uprighting: Preparing
typodont
Arch Length Analysis = Space Analysis
Permanent Dentition Mixed Dentition
Space AnalysisPermanent Dentition +Arch length available - M/D width of 12 teeth: first
molar to first molar__________________________
= Arch length deficiency or
excess
Arch Length
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
Space Analysis Measuring Tooth Width
Space Analysis: Mixed Dentition
The same procedure in the mixed dentition but
Need to estimate the size of unerupted permanent teeth (premolars and canines)– Moyers proportionality tables, Tanaka
and Johnston
Space Analysis Measuring ALD
+Arch length available - M/D width of teeth mesial
to first molars_______________________
= Arch length deficiency or
excess
Measuring Arch Length
Moyers: Mixed Dentition Space Analysis
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.
Tanaka-Johnston: Mixed Dentition Space Analysis
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 add11.0 for max. canine + premolars10.5 for mand. canine + premolars.
Tanaka &Johnston, JADA 88:798, 1974
Rule of Thumb: Mixed Dentition Space Analysis
To estimate the width of unerupted canines and premolars in a quadrant:
23mm for maxillary quadrant22mm for mandibular quadrant
Molar Uprighting Typodont
Molar Uprighting Teeth
Molar Uprighting Wax
Molar Uprighting Wax Preparation
Molar UprightingSegment Preparation
Molar UprightingSegment Preparation