impressions for indirect restorations Flashcards
What are the principles of a good impression?
Impression tray choice
Appropriate technique
Good soft tissue management
Appropriate choice and use of materials
What tray should you use?
Size- anything impeding? Room for rotation?
Rigid tray- supports material, maintains pressure, prevents distortion
Retention?
What is accuracy?
Ability to replicate hard and soft tissues
What is resolution/precision?
Ability to reproduce surface detail
What is dimensional stability?
Maintain material in size and shape over prolonged period of time
What are hydrophobic materials?
Don’t like water, may lead to bubbles, voids, inaccuracies if inadequate moisture control
What are 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 is the classification of impression materials?
Hydrocolloids
-reversible (agar)
-irreversible (alginate)
Synthetic elastomers
-polysulphides
-polyethers
-silicones (addition and condensation)
What is alginate?
Poor resolution and dimensional stability
Not suitable for definitive
Used for opposing arch
What are polysulphides?
Better materials now available
Still used for secondary imps for CDs
Long working time, flexible, good tear resistance, long shelf life
Poor dimensional stability, sticky to handle, unpleasant smell, need special tray, long working time
What are condensation silicones?
By product is alcohol
So poor dimensional stability
Limited use but cheaper than addition
Used in labs to form matrices and for copying techniques
Easy to use, good resolution, wide range, good tear resistance
Dimensionally unstable, hydrophobic
What are addition silicones?
Aka PVS
Most commonly used
Addition reaction between silane and vinyl groups
Good dimensional stability
Not as hydrophobic as past (poorer wetting and more air entrapment)
Low viscosity- gun, cartridge, automixing tip
High viscosity- putty, catalyst and base
Great dimensional stability and resolution, wide range, biocompatible, simple mixing and reasonable working time
Expensive, hydrophobic, setting time affected by temp, difficult to remove from casts, latex gloves can inhibit setting
What are polyethers?
Good option for fixed prosthodontics
Good accuracy and hydrophilicity
Rigidity makes difficult to remove from mouth (but good for implant copings)
Great dimensional stability and resolution, only one viscosity but undergoes shear thinning for flow, hydrophilic
V rigid, swells w water if kept damp for long time, bitter taste
Why is it important to disinfect?
Contamination w saliva and blood
Risk of infecting surgery/lab staff
Rinse under tap to remove visible debris
Immerse in disinfectant bath for 10 mins (if too long, can affect accuracy esp. alginate)
Rinse and cover in cloth (dry cloth for silicone/polyether, wet for alginate)
Put in bag
How can you manage soft tissue?
Retraction cord
Retraction pastes
Haemostatic agents
Electrocautery
Soft tissue laser
Rotary curettage
Why is gingival retraction important?
Allows to syringe impression materials beyond prep margin
Doesn’t control inflam or allow to take impression when gingivae are inflamed
What is retraction cord?
Cord that is packed into gingival sulcus to retract gingivae to improve visibility, access and moisture control
Cord is often soaked in haemostatic solution (aluminium chloride/epinephrine) to control gingival bleeding
Triple 0 smallest size
3 largest size
What are cord techniques?
Single cord- most common
Double cord- better retraction and useful for subgingival margins
What are retraction pastes?
Viscous agent syringed into gingival sulcus to provide retraction and haemostasis
Useful in simple cases w equi-gingival prep margins
Expensive
Why are surgical options for gingival retraction used?
Used to widen or recontour gingival profile
Widen sulcus
Remove overgrown tissue
W cord or paste usually
Haemostasis
What are techniques for addition silicone?
Twin mix/one stage- at least 2 viscosities used and seated in mouth together
Two stage- first impression taken in putty or heavy body w a spacer, then reline w lower viscocity
What is a good impression?
Well defined margins
Good surface detail
No drags or voids
No tray exposure
All prep in appropriate material
Impression should adhere to tray
