Impressions Flashcards
What is required to get a gook impression?
Impression tray choice Size of tray Rigid tray - maintains pressure on teeth and soft tissues, preventing distortion Good Soft tissue management Appropriate technique Appropriate choice and use of material
What is meant by accuracy in terms of impression?
ability to replicate the hard and soft tissues
What is meant by the resolution/precision?
ability to reproduce surface detail
What is the dimensional stability?
Maintenance of the material in terms of size and shape over a prolonged time
What are hydrophobic materials?
dont like water, lead to bubbles, voids, inaccuracies if inadequate moisture control
What is the order of different impression materials available?
1800’s dental plaster
1930’s elastic hydrocolloids: agar and alginates
1950’s synthetic elastomers polysulfide materials
Then condensation silicones - dimensional stability better than hydrocolloids but release condensation byproducts and shrinkage
Special trays and fillers in putty silicone reduced the effect of shrinkage
1965 polyethers - no condensation by-products and better dimensional stability
1975 addition silicones no condensation by-products
What are examples of hydrocolloids?
Reversible - agar
Irreversible - alginates
What are examples of synthetic elastomers?
Polysulphides
Polyethers
Silicones - addition or condensation
What are the ideal properties of impression materials?
High accuracy Good dimensional stability Good tear strength Elastic Easy to use Good working and setting times Acceptable taste and smell Tolerant to disinfection Cheap
What are the disadvantages of alginate?
Insufficient reproduce of surface details
Poor dimensional stability
Not suitable for definitive indirect restoration impressions
Do not use it for opposing arch impression
What are the advantages of polysulphides?
Long working time if have multiple preps
Flexible, so easy to remove
Good tear resistance
Long shelf life
What are the disadvantages of polysulfides?
Long working time Poor dimensional stability Sticky to handle Unpleasant smell Need special tray
What is the by-product in Condensation silicones?
Alcohol
What are the advantages of condensation siicones?
Easy to use
Good surface detail reproduction
Wide range of material choice and techniques
Good tear resistance
What are the disadvantages of condensation silicones?
Dimensional stability
Hydrophobic
What are condensation and addition silicones also known as?
Polyvinyl siloxanes
What are the advantages of addition silicones?
Excellent dimensional stability Excellent surface detail reproduction Wide range of material choice (viscosities) techniques Biocompatible Simple mixing Reasonable working time
What are the disadvantages of addition silicones?
Expensive
Hydrophobic - getting better - does lead to poorer wetting of the surfaces and increased air entrapment
What are examples of different addition silicones?
Putty Heavy-bodied silicone medium bodied silicone Light-bodied silicone Super light-bodied
Why have so many different addition silicones?
Putty = not very accurate, but good dimensional stability and viscosity with more filler
Light/medium bodied = high detail reproduction, thermic contraction, setting contraction, lower viscosity and filler
What are polyethers (impregum) used for
fixed prosthodontics
Impressions of implant copings
What are the advantages of polyethers?
Excellent dimensional stability
Good surface dtail reproduction
Only one viscosity but undergoes shear-thinning allowing it to flow
Hydrophilic
What are the disadvantages of polyethers?
Very stiff/rigid so difficult to remove from the mouth and cast - can pull out periodontally involved teeth
Swells with water if kept for a long time in damp environment
Bitter taste
How do you do mechanical soft tissue management?
Retraction cord or retraction paste
How do you do chemical soft tissue management?
Haemostatic agents
Electrocautery
How do you do surgical management of soft tissues?
Soft tissue laser
Rotary curettage
What is gingival retraction?
The downward and outward movement of the free gingival margin
What does gingival retraction allow?
Syringe impression material beyond the prep margin
What is the problem if have gingival inflammation when taking impression
The techniques do not control inflammed or allow us to take an impression when gingivae are inflammed - manage before hand
Need equi- and subgingival prep finish lines
What is retraction cord?
A cord that is packed into the gingival sulcus to retract the gingivae to improve visibility, access and moisture control
What is the single cor technique?
Cord soaked in aluminium chloride
packed into gingival sulcus and removed before impression is taken
What are retraction pastes, and what are they used for?
Viscous agents syringed into gingival sulcus to provide retraction and haemostasis
useful in simple cases with equigingival finish lines
What is electro-surgery and soft tissue lasers used for?
Widen or re-contour gingival profile Widen gingival sulcus Remove overgrown tissue Used in combination with cord or paste usually Haemostasis
What is the onestep impression technique
Select tray and apply adhesive
Select material - putty, heavy body with light body
Soft tissue management - retraction cord/paste, electrocautery
Remove retraction cord
Dry teeth - silicone hydrophobic
Syringe light-body around the prep
Place some light body onto the heavy in the tray
Seat impression
Remove in one direction
Inspect, if good, rinse and disinfect
Send to lab
What is different in the 2-stage technique?
Putty or heavy body is mixed
Spacer is places - thin sheet of plastic
Take impression
This creates a special tray
Then remove retraction cord, dry teeth, syringe light body around the prep, reseat impression
What is the risk with the 2-stage?
Not sitting the tray exactly where placed first time or it will not be accurate
What is the problem with 1-stage?
with putty which has a high viscosity, a lot of pressure is needed to seat the tray, this can distort or push away the lower viscosity material
What do you look for when assessing the impression?
Well defined margins Good surface detail No drags or voids No tray exposure All prep in the appropriate material Impression adhering to tray
How can digital impressions be taken?
Intra-oral scans in teeth and preparations
