ortho 2 zoom notes Flashcards
what is the function of chromium in stainless steel
helps with corrosion
what does nickel help with in stainless steel
Nickel also helps with corrosion but some people have allergies
what type of stainless steel is easier to melt
Soft stainless steel is easier to melt
how does stainless steel change from soft state to hard state
Changes from soft state to hard state by cold working / work hardening
The more you bend the wire as it gets manipulated this starts to break up the stainless steel
what type of stainless steel is preferred in orthodontics
hard stainless steel
explain how a wire is manipulated
- Hold adam’s no. 64 pliers in dominant hand
- Bend wire with the other hand
- Mark on wire where you want to grip the wire
- Push the wire away from your face in case it slips
- 90 degree = bending the wire
• Manipulating the wire = bending around a curvature
○ Do very small pushes
○ Keep pliers moving
○ Creates a curvature in the wire
what is URA
URA = Upper Removable Appliance
How do teeth move with a URA
teeth move on a fulcrum
They basically rotate on the midpoint
This is called tipping / tilting
= Tipping Movement
What are the advantages of URA
- tipping of teeth
- excellent anchorage
- generally cheaper than fixed
- shorter chair side time required
- OH easier to maintain
- non-destructive to tooth surfaces
- less specialised training required to manage them
- can be easily adapted for overbite reduction
what are the disadvantages of URA
• Less precise control of tooth movement
○ Just tipping and tilting
• Can be easily removed by the patient
○ Patient can remove and refuse to wear them
• Generally can only move 1-2 teeth at one time
• Specialist technical staff required to construct the appliances
○ Challenging to make these
- Rotations very difficult to correct with removable appliances
- Lots of limitations to removable appliances
What is needed if there is crowding
space to move teeth
how can we create space in an arch
- removing teeth
- expanding / widening arch
when can patients get an URA
Normally wait for the permanent dentition to be through
Very rarely treat deciduous dentition with orthodontics because the teeth are going to fall out anyway
Only tend to intervene if there are potentially functional problems
what are the problems with an increased overjet
- increased risk of trauma
- Aesthetic issues
- Significant functional problems (difficulty incising food)
- Lip trap (lower lip trapped behind the upper lips)
what is the aim in the URA appliance design
good opening statement
what it is you are trying to achieve
what is the purpose of the appliance
what does procline mean
pushing forward
what does retrocline mean
pushing back
what does ARAB stand for in the URA appliance design
Active Component
Retentive
Anchorage
Baseplate
what is the active component
= the part that is actually moving the teeth through the application of force
= any component that move the teeth through the application of force
why is it important to state the gauge of wire (this is the thickness)
○ The reason the thickness is important is because the thicker the greater the force
Using a thin wire means a gentler force
give an example of what would be written for the active component in the URA appliance design
13 & 23 palatal finger spring & guards,
0.5mm hard stainless steel wire
what is the retentive / retention
= the resistance to displacement forces
what is a displacement force
= anything that can dislodge the appliance
what are the 5 main displacement forces
○ Tongue
○ Gravity
○ Chewing / mastication / process of eating
○ Talking (creates vibration through the palate)
○ Active component (creates the largest force)
what is the most commonly used clasp for retention
Adams clasp is the most commonly used
give an example of what would be written for the retentive in the URA appliance design
16 & 26, Adams clasp, 0.7mm HSSW
11 & 21, Southend clasp, 0.7mm HSSW
what is anchorage
= the resistance to unwanted tooth movement
Making sure that the teeth we want to move are the only teeth that move and the teeth we don’t want to move stay in the same position
what are the functions of the baseplate
○ Holds all the components together ○ Helps with retention § Cohesion § Adhesion ○ Helps with anchorage § Helps with the resistance to unwanted tooth movement
what material is the base plate made from
PMMA
Can make these appliances in heat cure but there are specific reasons we use self cure
what are the advantages of heat cure PMMA
○ Can control the curing so can decide the working time
○ Better polymerisation as all the free monomer is used, little likelihood of an allergic reaction (although can still occur)
○ Considerably less shrinkage with heat cure than self cure
○ Fundamentally heat cure is stronger
Why do we use self cure PMMA if heat cure PMMA has more advantages
Can have it made from start to finish in around an hour
And then curing takes just 20 minute
= it is faster to make
Self cure is satisfactory even though heat cured is better
briefly explain the lost wax technique
- Wax up ~ 1 hour
- Use a mould (brass flask) and embed the partial denture (wax) into one side ~ 1 hour
- Something about plaster ~ 1 hour
- Put into a heating bath to melt the wax ~ 1 hour
- Heat cured acrylic then pack it into the area where the wax has been boiled from ~ 1 hour
- Heat the acrylic for 8 hours at 80˚ then to 100˚ for another few hours
- Break off plaster, then we have the right appliance
=15-20 hours = long time
what is newton’s 3rd law
Newton’s 3rd law = every action / force there is an opposite and equal reaction
what speed of tooth movement do we want with a URA
We want a mm of tooth movement per month ~ very slow
How does the adams clasp achieve retention
by utilising mesial and distal undercuts
what are the advantages of the adams clasps
- small
- unobtrusive
- can be used on permanent, deciduous and partially erupted teeth
- the bridge provides site of removal
what material is most widely used for metallic parts of orthoodntic appliance construction
Material most widely used for the metallic parts of orthodontic appliance construction
how is hard wire made
The hard wire is made by drawing the metal in a cold state through a series of dies in successively smaller diameter
This process also causes work hardening which gives the wire its spring properties
how is stainless steel further work hardened
Stainless steel is further work-hardened during the bending process of constructing component parts for the appliances
This can be utilised to increase the spring properties of the wire
when does fracture of stainless steel wire occurs
- Overworked
○ The wire has been excessively overworked by bending and then straightening the wire at the same point
○ This creates extreme stresses within the immediate area which can result in fracture - Mechanical abrasion crushed or marked
○ This can occur if the wire has been damaged by burs or stones in the finishing process of removable appliances
○ Or if the wire has been marked or crushed during the fabrication of components - Fatigue
○ This can be caused by repeated straining action
○ Eg continually strained to engage a deep undercut with an Adams clasp - Weld Decay
○ Is an intergranular corrosion created by overheating the alloy
○ This causes the chromium carbides to precipitate at the grain boundaries
○ The oral fluid can now access the surface of other metals resulting in galvanic action which weakens the area sufficiently to result in fracture
what type of stainless steel is commonly used in orthodontics
18/8 austenitic stainless steel alloy
This material is corrosion resistant when cold worked:
what is austenitic
austenitic is a crystalline space lattice structure of the alloy
The alloy cannot be heat hardened and remains in a soft condition unless it has been cold worked
what are the constituents of 18/8 austenitic stainless steel allloy
• 72% Iron
○ Main constituent
○ When combined with carbon it forms steel
• 18% Chromium
○ When added to steel lowers the content of carbon present and lowers the temperature at which martensite forms
○ Forms a passive oxide film over the surface of the metal that enables the stainless steel to be corrosion resistant
• 8% Nickel
○ Along with chromium assist in achieving the austenitic structure at room temperature by lowering the critical temperature at which the austenitic structures breaks down on cooling
• 1.7% Titanium
○ Added to the steel primarily to prevent the precipitation of chromium carbides at the grain boundaries when the alloy is heated during welding or soldering
○ Carbon combines with the titanium in preference to the chromium
• 0.3% Carbon
what is martensite
- The principal component of hard steel
- Formed by quenching from high temperatures
- Consists of intergrown plate-like crystals with a distorted cubic structure arising from the presence of carbon atoms in iron structure
what is the difference between austenite and martensite
- In austenite the average unit sell is a perfect little cube
- The transformation to martensite sees this cube distorted by interstitial carbon atoms that do not have time to diffuse out during displacive transformation (quenching)
- So that it is a tiny bit longer than before in one dimension and a little short in the other two
- A microscopic crystallites of steel is millions of unit cells long
- Since all these units face the same direction, distortions of even a fraction percent become magnified into a major mismatch between neighbouring materials
• This creates crystal defects within the material similar to work hardening
These defects prevent atoms from sliding past one another in an organised fashion, causing the material to become harder
