Impressions (for indirect restorations) Flashcards
How to get a good impression?
Impression tray choice
Good soft tissue management
Appropriate technique
Appropriate choice and use of material
Which tray should I use?
Size of tray - trim the tray in the mouth, adequate seating? Room for rotation?
Rigid tray - supports material, maintains pressure on teeth and soft tissues, prevents distortion, retention with perforated holes
Define accuracy
Ability to replicate hard and soft tissues
Define resolution/precision
Ability to reproduce fine surface detail
Define dimensional stability
Maintenance of the material in terms of size and shape over a prolonged period of time
Define hydrophobic materials
Don’t like water, may lead to bubbles, voids, inaccuracies if inadequate moisture control
Types of hydrocoloids?
Reversible (agar)
Irreversible (alginates)
Types of synthetic elastomers?
Polysulphides
Polyethers
Silicones - addition silicones, condensation silicones
Ideal properties of impression materials?
High accuracy Elastic Good dimensional stability Good tear strength Easy to use Acceptable taste and smell Cheap Tolerant to disinfection Good working and setting times
Why is alginate not used for indirect restorations?
Insufficient reproduction of surface details
Poor dimensional stability
Not suitable for definitive indirect restorations
Used for the opposing arch
Uses of addition silicones/polyvinyl siloxanes (PVS)? How does it work?
Impression material for fabrication of indirect restorations
Addition reaction between silane and vinyl groups when catalyst and base pastes are mixed
No byproducts produced - very good dimensional stability
Hydrophobic - increased risk of air entrapment
Advantages of addition silicones?
Excellent dimensional stability
Excellent surface detail reproduction
Wide range of material choice and techniques
Biocompatable
Simple mixing and reasonable working time
Disadvantages of addition silicones?
Expensive Hydrophobic - but getting better Setting time affected by temp Can be difficult to remove from casts Some latex gloves could inhibit setting
Formulations of addition silicones?
Putty Heavy bodied silicone Medium bodied silicone Light bodied silicone Super light bodied silicone
Positives of heavy bodied/putty addition silicone?
High filler
High viscosity
High dimensional stability
Positives of light/medium bodied addition silicone?
High detail reproduction
High thermal contraction
High setting contraction
Polyether features?
Good for fixed prosthodontics
Good accuracy and hydrophilicity allows it to compete with addition silicones
Rigidity - difficult to remove from the mouth and may remove periodontically involved teeth
Rigidity is favoured - popular material, commonly used for impressions of implant copings
Advantages of polyethers?
Excellent dimensional stability
V good surface detail reproduction
Only one viscosity but undergoes shear thinning allowing it to flow
Hydrophilic
Disadvantages of polyethers?
Very stiff/rigid difficult to remove from mouth and cast
Swells with water if kept for long time in damp environment
Bitter taste
How to disinfect impressions?
Trim off excess
Contaminated in water and blood
Risk of infecting surgery and lab staff is cross infection procedure not followed
Rinse under tap to remove debris
Put in disinfectant bath - soak for 10 mins - longer can affect the accuracy of the impression
Soft tissue management?
Retraction cord Retraction pastes Haemostatic agents Electrocautery Soft tissue laser Rotatory curettage
How to retract the gingivae?
Downward and outward movement of the free gingival margin
Allows us to syringe impression material beyond preparation margin
Does not control inflam or allow us to take an impression when the gingivae are inflammed - manage this beforehand
What is a retraction cord?
Cord that’s packed into the gingival sulcus to retract the gingivae to improve visibility, access and moisture control
Single cord technique most common (removed before impression)
Cord often soaked in haemostatic agent to prevent bleeding
What are retraction pastes?
Viscous agent syringed into gingival sulcus to provide retraction and haemostasis
Useful in simple cases with equi-gingival finish lines
More expensive
Electrosurgery and soft tissue lasers features?
Widen or re-contour gingival profile Widen gingival sulcus Remove overgrown tissue Used in combination with cord or paste usually Haemostasis
Techniques for addition silicones?
Twin mix or one stage (most common) = at least 2 viscosities used, seated in mouth together
Two stage
- First impression taken in putty, removed just prior to full setting
- Then reline with lower viscosity (light body alone or in combo with medium body)
Protocol for the one stage addition silicone impression?
Adequate tray, adhesive applied
Select material: combo of putty or heavy body with light body
Soft tissue management - retraction cord, retraction paste, electrocautery
Place putty/heavy body into tray
Remove retraction cord (damp) if used
Dry teeth
Syringe light body around preparation - starting within gingival sulcus and ending at coronal part of prep - use controlled pressure to force material against tooth
Place some light body into putty/heavy body on tray
Seat impression
Cafeful removal in 1 direction - do not rotate
Inspect, rinse and disinfect
Send to lab
Two stage addition silicone impression protocol?
Adequate tray, adhesive applied
Select material - combo of putty/heavy body with light body
Spaced placed (thin sheet of plastic)
Take impression
Essentially creates a special tray
Follow previous steps: remove retraction cord, dry teeth, syringe light body around prep, reseat impression
Negatives of the two stage addition silicone impression technique?
If you dont sit the original impression in the same way - get a step or inaccurate
Should you use the single stage or two stage addition silicone impression technique?
Case dependent
Single stage easier, quicker
Putty - high viscosity so a lot of pressure is needed to seat the tray which can distort or push away the lower viscosity material
If you don’t have a rigid tray and you can see the lower viscosity material is being pushed from the prep margin - you could chose the 2 stage technique - however ensure you fully seat the original impression
What to look for when assessing your impression?
Well defined margins Good surface detail No drags or voids No tray exposure All prep in appropriate material (light body) Impression adhering to tray
Why may you not be able to see your finish line?
Problem with prep, may need refining or smoothing to improve the surface feature
Or it may be problem with impression - excessive bleeding or gingivocrevicular fluid affecting the set of the material? Are the gingivae healthy enough? Is my gingival retraction technique suitable?
Why may your impression have bubbles/voids/drags?
Alginate mixing
Drying the tooth is important to allow adequate wetting and to avoid bubbles/voids
Keep tip of syringe within the material and starting from the finish line and ending occlusally should minimise the problem
Whole impression tray needs to be filled with material so the impression is supported in all directions
Chose the correct tray size
