Impressions Flashcards
Terminology: accuracy
Ability to replicate the hard and soft tissues
Terminology: resolution/ precision
ability to reproduce surface
detail
Terminology: dimensional stability
maintenance of the material in terms size and shape over a prolonged period of time
Terminology: hydrophobic materials
don’t like water, may lead to bubbles, voids, inaccuracies if inadequate moisture control
Classification of impression materials
Hydrocolloids -reversible (agar) -irreversible (alginates) Synthetic elastomers -polysulphides -polyethers -silicones (addition and condensation silicones)
Ideal properties of impression materials
High accuracy Good dimensional stability Elastic Good tear strength Easy to use Good working and setting times Acceptable taste and smell Tolerant to disinfection Cheap
Alginate?
Insufficient resproduction of surface details
Poor dimensional stability
Not suitable for definitive indirect restoration impressions
We do use it for opposing arch impressions
Addition silicones
Also known as PVS (polyvinyl siloxanes)
Most commonly used imp material for fabrication of indirect restorations
Addition reaction between silane and vinyl group when catalyst and base pastes are mixed
No by-products: v good dimensional stability
Historically silicones have been very hydrophobic. This
leads to poorer wetting of the surfaces and increased risk of air entrapment - new materials better
Advantages and disadvantages of addition silicones
Advantages -biocompatible -simple mixing and reasonable working time -excellent surface detail reproduction -wide range of material choice (viscosities) and techniques -excellent dimensional stability Disadvantages: -not very flexible (can be difficult to remove from casts) -expensive -setting time affected by T -hydrophobic - but getting better -some latex gloves could inhibit setting
Addition silicones - formulations
Putty Heavy-bodied silicone Medium-bodied silicone (monophase) Light-bodied silicone Super light-bodied silicone *low viscosity vs high viscosity*
Why do we have so many formulations of addition silicones?
Light/ medium bodied: -setting contraction -thermic contraction -detail reproduction Putty/ heavy bodied: -filler -viscosity -dimensional stability
Polyethers
Good options for fixed prostho
Good acuracy and hydrophilicity allows it to compete with addition silicones
Rigidity makes it difficult to remove from the mouth and
may remove periodontally involved teeth.
This rigidity is favoured and therefore this material is
popular and commonly used for impressions of implant
copings
IMPREGUM?
Advantages and disadvantages of polyethers
Advantages:
-excellent dimensional stability
-very good surface detail reproduction
-only one viscosity but undergoes shear-thinning allowing it to flow
-hydrophilic
Disadvantages
-very stiff/ rigid difficult to remove from mouth and cast
-swells with water if kept for a long time in damp environment
-bitter taste
Disinfection
Imps contaminated with saliva and often blood
Risk of infecting surgery and lab staff if strict-croos-infection procedure not followed
Rinse under tap to remove visible debris.
In CCDH we disinfect by immersing the impression in a disinfectant bath
Soak for 10 mins, no longer as this can affect the
accuracy of the impression (particularly alginates)
Soft tissue management
Mechanical: -Retraction cord -Retraction pastes Chemical: -Haemostatic agents -Electrocautery Surgical: -Soft tissue laser -Rotary currettage
Gingival retraction
Downward and outward movement of free gingival margin
Allows us to syringe imp material beyond our prep margin
These techniques do not control inflammation or allow to take imp when gingivae are inflamed - this should be managed beforehand
need to equi- and subgingival preparation finish lines
Retraction cord
& most common technique
Cord packed into gingival sulcus to retract gingivae to improve visibility, access and moisture control
Single-cord technique most common (removed before imp)
Retraction pastes
Viscous agents 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
Used to widen or re-contour gingival profile
Widen gingival sulcus
Remove overgrown tissue
Used in combination with cord or paste usually
Haemostasis
Imp technique for addition silicones
Twin mix or one stage (most common)
-at least two viscosities used, seated in mouth together
Two stage:
-first imp taken in putty, removed just prior to full setting
-then reline with lower viscosity (light body alone or in combination with medium body)
Twin mix/ one stage imp (addition silicones)
Adequate tray selected, adhesive applied
Select material: usual combination putty or heavy body with light body
-however medium and light body can be used
Soft tissue management
Place putty/ heavy body into tray
Remove retraction cord (dampen) - if used
Dry teeth (still hydrophobic material)
Syringe light body around prep: starting within gingival sulcus and ending at coronal par of prep
-use controlled p to force material against tooth
Place some light body onto putty/ heavy-body on tray
Seat imp
Careful removal in 1D - don’t jiggle or rotate
Inspect, if adequate, rinse and disinfect
Send to lab
Two stage impression (addition silicone)
Adequate tray selected, adhesive applied.
Select material: usual combination is putty or heavy body
with light body
Should i use one stage or two stage impression?
Case dependent
Single stage easier, quicker
Problem esp with putty (high viscosity), 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 prep margin - you could chose the two stage technique - however ensure you fully seat the original impression!
Assessing imp
Well defined margins Good surface detail No drags or voids No tray exposure All prep in appropriate material Impression adhering to tray
