1P - Biomechanics

Braces setup

What is the first stage of braces?

Align and level crowns:

Crowns movement only

Round archwire

What archwires are used in the 1st stage (and diameters)

Round:

012 Niti

014 Niti

018 Niti/SS

What is the 2nd stage of braces?

Root control (and close/open space)

Rectangular archwire

What archwires are used in the 2nd stage (and dimensions)

Rectangular:

16×22 Niti/SS

17×25 Niti/SS

19×25 Niti/SS

What is biomechanics and why they are used in orthodontics?

Research and analysis of mechanical properties of living tissues and non-living objects that affect them.

Used to determine effect of orthodontic appliances on oral tissues

What are the living and non living components of biomechanics?

Non-living - Newtonian mechanical principles

Living - Biology of the tooth movement

What are the optimum forces for each type of tooth movement (forces in gm)

Tipping 35-60

Bodily movement (translation) 70-120

Root uprighting (torque) 50-100

Rotation 35-60

Extrusion 35-60

Intrusion 10-20

What can happen without proper biomechanical understanding?

Inefficient tooth movement, root resorption, bone loss, unwanted tooth movement

What force property determines whether tipping or translation?

Point of application

What are the 4 properties of a force required to move a tooth predictably?

1. Magnitude

2. Direction

3. Point of application

4. Line of action of force

Basic properties of materials (elastic and plastic)

What is elastic behaviour defined by?

Strain and stress response to load:

Stress - Internal distribution of load

Strain - Internal distortion produced by load

What is the proportional limit?

Point at which permanent deformation is first observed

Strain and stress directly proportional till that point

What is yield strength?

Point at of deformation of 0.1%

Wire wont return to origina state from here

What is meant by ultimate tensile strength?

The maximum load a wire can sustain

What is the failure point?

The point at which the wire breaks

What is the modulus of elasticity?

• Ratio between unit stress and strain

• Measured by slope of elastic region

• Describes relative stiffness/ridgitity of the material

What is meant by stiffness

Proportional to the slope of linear (elastic) portion of the curve. The more vertical the slope, the stiffer the wire

What is meant by range?

Distance the wire will bend elastically before permanend deformation occurs

What is meant by springback?

Ability to undergo large deflections without permanent deformation

What is meant by resilience

• Energy storage capacity of wire

• Area under stress-strain curve, up to proportional limit

What is meant by formability?

Amount of permanent bending wire can take before it breaks or fails.

Area under curve from yeild point to failure point

What is meatn by flexibility?

A flexible material can undergo a large deformation / strain with minumal force, withn its elastic limit

Maximum flexibility = Proportional limit/modulus of elasticity

Toughness

Force required to fracture a material

Total area under graph

Brittleness

Opposite of toughness

Brittle material cant undergo plastic deformation

Fatigue

Repeated cyclic stress of a magnitude below fracture point of wire can result in fracture.

What are the Ideal requirements of orthodontic wires?

• High strength, range, formability

• Low stiffness

• Solderable and weldable for hook or stop attachment

• Reasonable cost

Stainless steel archwire properties

• Low cost

• Excellent formability

• Good mechanical properties

• Solderable and weldable

Is used for closing gaps

NiTi properties

• Can apply light force over large range of activations

• Ideal for inital orthodontic alignment

• Shape memory

• Superelasticity

What are the 4 stages of wire progression, and the respective wire considerations?

1. Alignment - Easily engaged, round, flexible

2. Leveling - Starting to fill slot, leveling arch, filish rotations

3. Working - Rigid enough to maintain occlusal plane, achieve torque

4. Finishing - Able to accept finishing bends or be reengaged after repositioning

4 different shapes of archwire?

Round

Square

Rectangular - 19×25 fully fills slot, so is used for final wire

Multistranded

What unit is wire size specified in?

Thousandth of an inch. Example - .016 inch

Why is anchorage important?

Whenever tooth movement is attempted, there will be an equal and opposite reaction to force applied by components. (Forces also applied to anchoring teeth, may cause unwanted movement)

What are the 3 types of Intra-oral anchorage?

Simple - Active movement of one tooth versus several anchor teeth

Compound - Teeth of greater resistance as anchorage for the translation of teeth with less resistance

Reciprocal - Two groups of teeth pitted against each other, causing equal reiprocal movement of both

What are the two types of intermaxilar anchorage?

Extraoral anchorage

Skeletic anchorage

What are the 4 fundamental concepts?

1. Force

2. Center of resistance

3. Moment

4. Force systems

Force

Push/pull applied to tooth

Has magnitude, direction and point of application

What is the Centre of resistance (CR)?

Point at which a force can cause bodily movement (translation) of the tooth, without tipping or rotation.

Single root - ½ to 1/3 of length of (clinical) root

Multiradicular - 1 to 2 mm apical of furcation

What is the Moment of a force?

Measure of a forces capacity to produce rotation

Moment = force x distance from CR

Important for controling rotation, torque, tipping

What are the 3 Force systems?

Single force - tipping

Force + couple - translation without tipping

Couple of forces - Two equal and opposite forces separated by a distance - pure rotation. Form of moment

What does the effect of a force applied on a body depend on?

Relation between the force’s Line of action and gravity centre of the body:

What is meant by centre of rotation?

Fixed point around which body appears to be rotated

What are the 6 types of tooth movement and their force requirements?

What is uncontrolled tipping?

Apex moves in opposite direction to crown

What is controlled tipping?

All parts of tooth move in same direction, but different distances.

Centre of rotation at apex.

What is Torquing (root movement)?

All movement is in apex (and root)

Centre of rotation at crown tip

What is translation (bodily movement)?

Crown + root move same distance and direction with no rotation.

Centre of rotation is nonexistent/infinity

How can you acheive a mass movement of a tooth/teeth?

The force applied must pass through CR.

A) The only linear force, pass through CR

B) The sum of all forces applied pass through CR