12. Impression Materials: Linking Of Lab To Clinics Flashcards
LOs
INDIRECT RESTORATIONS
- What are indirect restorations
- Accuracy + fit of indirect restorations depends on?
- Stages needed to make an indirect restoration
- EG of a current development which may decrease/ eliminate the use of impression taking?
INDIRECT RESTORATIONS
- When is indirect restoration indicated?
- Advantages of indirect restorations?
- What is a disadvantage? (Hint: M….)
INDIRECT RESTORATIONS
What must be made for indirect restorations to be made?
INDIRECT RESTORATIONS
What must be made for indirect restorations to be made?
INDIRECT RESTORATIONS
INDIRECT RESTORATIONS
Indirect restorations are mainly constructed using rigid materials. WHY?
- as significant tooth tissue loss, materials used should be able to provide mechanical support + thus allow appropriate function
- hence materials with high modulus of elasticity that can resist high stress without permanent distortion are used
- Many behave as a brittle material
INDIRECT RESTORATIONS
EG of rigid materials used for indirect restorations + what are there properties like? (Hint: brittle, ductile, etc)
- ceramics
- pure gold
- prefabricated composites
What are impression materials used for?
What are the 2 types of impression trays?
What are they made of?
What are the 2 main classes when looking at set properties?
What are there subcategories + EGs of them?
EXTRA FOR UNDERSTANDING
- there’re many diff impression materials
- diff criteria’s are used to classify these materials
- EG - can consider chemical type of the material, the initial viscosity, viscosity of the unset material when 2 phases have been mixed
- low viscosity material on mixing the two phases = usu tend not compress the soft tissues in the oral environment = called mucostatic materials
- can also have mucocompressive materials = initial viscosity of these are much higher - hence likely to compress oral tissues
- most widely used criteria for classifying impression materials are there set properties
- 2 main properties that are of interest are the elasticity + rigidity
- if you are taking an impression where there are undercuts, one of the important aspects is removal of those impressions
- if the material is elastic + can be removed, these materials will show good recovery to original shape + size & hence an accurate impression
- other important aspect = materials should be fairly rigid
- so that when you remove materials they should not undergo any deformation or tear
- hence impression materials that belong to this class as ‘elastic materials’
- elastic materials are further grouped into 2 categories
(ON SLIDE) - other group = non elastic materials
- technically should be called as a non rubbery material
- since some of the materials are flexible + some are not
these materials do not show any rubbery nature hence are called non - elastic materials
(SLIDE)
TIP
- won’t go over each of these non-elastic type materials
- should read about these materials in own time
- these materials are not widely used BUT it’s important that you have an idea about the properties + how these materials undergo setting
IMPRESSION MATERIALS
There are 2 main set properties that are of interest
1
what are they?
2
Why are they important?
1
- elasticity
- rigidity
2
ELASTICITY
- if you are taking an impression where there are undercuts, one of the important aspects is removal of those impressions
- if the material is elastic + can be removed, these materials will show good recovery to original shape + size & hence an accurate impression
RIGIDITY
- so that when you remove materials they should not undergo any deformation or tear
What are ideal requirement factors when looking at impression materials
what are the important physical + mechanical properties of impression materials?
EXTRA
These materials are going to be placed in the mouth so they should be:
- biocompatible + they should not cause any irritation to tissues of the mouth, be toxic, cause discomfort to mouth, should be fairly pleasant
- Be an Acceptable taste + odour
What is rheology?
What does it involve?
Viscosity formula?
EXTRA FOR UNDERSTANDING
- IM should exhibit good flow properties
- Since these materials are required to flow over oral tissues (hard + soft)
- Rheological prop of these materials = important + govern the accuracy of an impression
- 2 aspects we are concerned with in impression taking = how does the viscosity change with Factors such as shear rate + time
1
What is viscosity? (Layman’s + sciencey meaning)
2
What is a shear rate?
3
What is shear stress formula and what do the letters mean?
1
SIMPLE
consistency of a fluid
SCIENCEY
measure of a fluids resistance to flow
2
function of the flow of the material
EXTRA FOR UNDERSTANDING
- Since impression materials are used in a flowable state
- Need to understand how a fluids viscosity varies with factors such as rate of shear + time
- to understand think of impression material in syringe in fluid state
- to extrude material / fluid, need to apply pressure to depress it + fluid to come out of nozzle
- If you if you extrude the material at a constant rate, the shear stress is related to the pressure you apply to depress the barrel of the syringe
1
Explain the graphs
2
Common EG of a fluid that exhibits shear thinning property used in everyday life and how it applies?
3
How is this relevant to taking impressions?
1
- If K is a constant & n = 1 that means the shear stress is directly proportional to the shear rate
- so if a fluids internal flow resistance is independent of the external flow, that is your shear rate, acting on the fluid, then these fluids are said to exhibit a Newtonian behaviour
- BUT if there are viscosity changes with the shear rate, these fluids then exhibit a non-Newtonian behaviour
- when shear stress is directly proportional to shear rate , the viscosity changes with shear rate
- if value of n is less than 1, the shear rate is not proportional to shear stress any longer - this causes a decreasing viscosity with increasing shear rate
- 3rd case = n>1 - viscosity increases with increasing shear rate
- Common flow behaviour of fluids, can see that Newtonian behaviour - viscosity does not change with shear rate
- 2nd case viscosity decreases with increasing shear rate
- 3rd case viscosity increases with increasbg shear rate
2
- ketchup - if you take a bottle filled with ketchup and turn it upside down, the ketchup doesn’t flow automatically BUT when you squeeze the bottle (you are applying some shear rate) the material exhibits shear thinning properties + the ketchup is extruded from the bottle
3
- This is also relevant in taking impressions - since its possible that your paste exhibits shear thing property - in which case the viscosity will decrease when you increase the shear rate - (depends on what sort of shear rate you’re applying using the plunger)
- Decreasing the viscosity may make the materials flow better over oral tissues
- since impression materials undergo setting over time, the viscosity also changes over time
- Hence we need to consider how the viscosity changes over time
- here is a graph that shows you 3 different behaviours of viscosity over time
1
What is happening in the graphs with viscosity?
2
What curves materials would be best to use?
1
CURVE A
- in this case, viscosity over time has not changed for a substantial amount of time
- As a result since the viscosity is not changing, it is allowing you enough time to mix the pastes + mix it homogenously, put it in the tray + place it in the mouth = THIS TIME + WORKING TIME (when the change is viscosity is minimal or there’s no change in viscosity)
- Ideally what a material with adequate working time to manipulate the material + place it in the mouth
- However once placed in mouth, you would like the material ot set immediately or as quickly as possible
- Can see in the curve that after some time the viscosity has increased quite dramatically hence the material is almost setting
- That is an ideal profile for an impression material
CURVE B
- materials such curve B - the viscosity is changing quite steadily - have a very short window of time where the viscosity is only changing gradually (In th early part of the curve)
- Soon after, can see the viscosity is changing more and risinhg
- These materials = more difficult to handle as you don’t get adequate working time + changing viscosity will not allow good flow properties and thus you won’t get a good recording of the oral tissues
CURVE C
- not really any time to manaipulate material
- As soon as the paste is mixed - can see the viscosity has started rising V rapidly - not allowing any time of flow HENCE these type of materials are unsuited for use as an impression material
2
- would use a time dependence viscosity material such as A
- as have adequate working time
Different viscosities of pastes = high, medium + low
What are there advantages + disadvantages?
- It is clear that the flow of a material is directly related to the viscosity hence you can get these impression materials in different viscosities
LOW VISCOSITY
- will flow V well over soft tissues
- record surface details V accurately
- this material can drip (out of the tray, all over the mouth)
- This is a disadvantage as the patient can gag and result in bad consequences
- You have to have certain viscosity (don’t want water like viscosity as wont be suitable for impression taking)
MEDIUM VISCOSITY
- good as will flow - will be better in terms of consistency + be used to take impressions
- BUT may nit be adequate for V accurate impressions
HIGH VISCOSITY
- High viscosity materials may not be able to flow over undercuts + may not be able to record details required in getting an accurate impression
- BUT in certain cases (eg if wanted to make mouthguard or nightguard) in this case the fine detail accuracy is not needed of the oral tisssues hence can select a high viscosity material - it gives you the body + good impression
EXTRA
- Depending on the purpose of the material can choose a suitable viscosity of impression material
- Low viscosity materials are not really considered suitable in any case
How can wettability of a material affect whether you choose them?
EXTRA FOR UNDERSTANDING
- have seen how flow is affected by the viscosity
- Another important parameter to consider if a material will flow in a certain environment = wettablilty
- Mouth = moist environment = V hydrophilic environement
- So for impresion materials to flow, have to consider how these materials interact with saliva
- If material is hydrophobic, then it will exhibit limited flow (hydrophobic material will tend not to go around moist area ad hence get a blowhole = inaccurate impression)
- BUT if have hydrophilic material, will exhibit good flow
- BUT one of the problems of hydrophilic materials = problems with storage as materials tend to absorb moisture from environment (Depends on humidity of environment) hence undergo dimensional changes - thus resulting in inaccurate fixture when cast up
- Need to have a balance between hydrophilicyty + hydrophobic to so that the material still flows but does not undergo dimensional changes on storage
- It’s all about achieving a balance
1
An impression material can sets by either a?
2
Are elastic, plastic or viscoelastic materials the best to used?
(MAKE BETTER Q’s)
1
- chemical change
Physical change of state
2
- ability of a impression material to replicate an intraoral surface details governs the accuracy of the impression - have seen that good flow characteristics help in recording an accurate impression BUT should also consider the properties of the set material
- One of the most important properties to consider = the nature of the material ( is the material elastic in nature, is it plastic material or is it a viscoelatsic material)
- Impression materials set by either a chemical or physical change + resulting materials = set material which will have certain properties so this material, if is elastic, will tend to show the least distortion
- When you take an impression + remove it from the mouth, you’re entering some external forces so mainly tensile forces + some compressive forces so that you can take the impression out
- Especially when you have taken an impresion where ther are undercuts present
- If material is elastic, then it will recover , and all the deformation suffered when applying the external forces to remove it from the mouth, will recover + recoil back to original shape + size, thereby giving rise to an accurate impression
- In contrast if materials were plastic in nature, you will get a considerable amount of deformation + distortion Hence not an accurate impression
- If have viscoelastic material - has a viscous component + elastic component
- if this material has an elastic component, which is greater than the viscous component, then this material can recover + go back to original shape + size BUT distortion may still occur - distortion depends on amount of elasticity + the how long you have applied these external forces for + how thick your material was
- Cannot always get 100% elastic materials but can get viscoelastic materials (with 99or more % elasticity + the viscous component plays a small role + the distortion you get is V small)
- EG that shows you what happens if you had .1% shrinkage
- This is just a calculation, for your information
How can we help manage shrinkage?
EXTRA FOR UNDERSTANDING
SLIDE 20
- impression materials undergo setting either by a chemical reaction or through a physical change in state
- These processes are accompanied by a certain amount of shrinkage
- To minimise the shrinkage that arises, there are ways of overcoming or managing the shrinkage so that it is kept out a minimum
- For instance, if you take a heavy body impression material, that has a high filler content + therefore would contract less + place it on the tray, whilst you covet the tooth with a light bodied imporession mateiral so that the material is able to flow, but your only placing a thin amount of the impression material to cover the tooth + then place the impression tray containing the heavy body impression material - then you can actually minimise the amount of shrinkage that occures
SLIDE 21
- diagram shows you the shrinkage that can occur on setting of an impression material placed in a tray
- Impression material = blue
- Tray holding it = grey
- Arrows shows that shrinkage is towards the tray
- Since the materials is shrunk towards the tray, an extra space is created therefore dies tend to be oversized + thus casting may be oversized
elastomers are sensitive to strain rate
Therefore what is recommended to do when removing elastomer impression materials?
Why?
- Hence recommended that when you remove an elastomers impression material from mouth you use a snap action + move it as rapidly as possible
- If you remove the impression rapidly from the mouth, then the stress you’re applying is applied V quickly hence the energy is stored elastically
- If the energy is stored elastically then the energy will also be an elastic recovery and not cause any plastic deformation HENCE there will be no distortion or change in shape or size
GRAPH EXPLANATION
- graph oh RHS has hashed line
- Line 1 = fast removal = high stress but low strain hence this material will be recovering V quickly + undergoing no distortion
- But if stress applied slowly - line 4 - then will get a certain amount of deformation
- Anything under blue line will cause a certain amount plastic deformation
- elastomeric material → remove quickly with snap action → get mostly elastic recover → no distortion → = accurate impression
What is working time?
Why should setting time ideally be short?
Another factor that affects the dimensional accuracy of the impression = the thickness of the impression material
How does it affect the dimensional accuracy?
Factors that affect the dimensional stability of impression materials once set?
ANY CHANGES IN DIMENSIONS WILL CHNAGE ACCURACY OF IMPRESSION
1
Thermal properties can also effect on accuracy of materials why?
2
How can you minimise this effect?
1
- also has an effect on accuracy of materials
- - impression materials = mainly polymeric in nature hence have a fairly high value of coefficient of thermal expansion
- the material has undergone setting in temp environment of 32-27 degrees - then taken out to 21-25 degrees
- drop in temp - so material undergoing cooling - can lead to inaccurate impressions
2
- suggested that you can minimise this effect by having the coefficient of thermal expansion if the material closely matching to the stock tray
mechanical properties that are relevant in terms of impression materials?
Why?
EXTRA FOR UNDERSTANDING
- REMEMBER - these materials are not going to be placed under any load bearing conditions HENCE do not require very high modulus of elasticity
- but these materials are going to be removed from mouth + should resist tearing (especially when you have undercuts)
- when taking impression out from the mouth, you apply some external forces + that shoul be such taht it doesn’t cause material to tear
- sometimes if use low viscosity IM then tend to but high body IM to give it some body to prevent tearing on removal
- tear strength should be fairly high
- need adequate compressive strength - doesn’t need to be similar to those of restorative materials BUT hsould have some body + resist some compressive forces
EG of why you should use a material with a sufficient tear strength
if any material tears and remains in periodical region can lead to severe periontal reaction HENCE impression should be completely intact + not leave any debris or remenants in mouth
EGs of how you can measure tear strength
- tear strength of materials can be determined in diff ways
- some EG of how you can measure tear strength
- EG D470 test - material in rectangle + cut, see how much load required to tear apart
Minimum biocompatibility tests required by regulatory bodies before impression materials can be placed in mouth?
Biocompatibility requirements + advantages
1
Impression material delivery systems - current available options
2
Advantage of auto mix system?
3
Main aim of delivery systems of impression materials?
3
several ways of mixing + delivering these IM
- main aim = reduce the amount of voids so don’t cause problems
- when taken impression = voids can lead to tearing or even uptake of moisture
- HENCE important that air bubbles are not incorporated + mixing is homogenous to minimise effect of any voids
Diff mixing tips
if take automxing tip + use syring to extrude material, REMEMBER that amount of stress applied can affect viscosity of extruded materials (depending on type of material being used)
Extra - be careful about moisture in environment?(hydrophillic or hydrophobic material) - how to store materials?
- type of setting reaction?
- eg if buying sillicone - need to identify if condensation cured silicone or addition cured sillicone as shrinkage in bothe of these = quite different - the way you store these materials + what time you send them off also vary HENCE IMPORTANT FOR YOU TO ASSESS: what sort of chemical type + what type of setting reaction occurs for mateiral to undergo setting
- BP3 - if using light body material, what sort of tray would be most suitable
- overall should use material you’re most comfortable in using + overall cost
Non elastic impression materials EGs
