Primary imps - edentulous dentures Flashcards
What is the conventional method for making edentulous dentures?
Start with primary impressions and then secondary impressions.
What are the stages of making a edentulous dentures?
- Primary impressions -> Lab: cast models and create special trays.
- Secondary impressions -> Lab: cast models, create record rims.
- Jaw registration. Lab: Mount on articulator. Set up teeth.
- Try in -> Lab: process to finish
- Fit
What makes a good impression?
Impression that records the entire functional denture bearing area to ensure maximum support, retention and stability for the denture during use.
What makes a good impression?
- Doesn’t distort the soft tissues.
- Free from significant defects
- Completed with minimal discomfort to pt
- Covers maximum denture bearing area.
What makes a bad impression?
> Air blow, significant voids
> Under / over extension
> Impression material not adhering to tray
> Not covering the full denture bearing area.
What are the clinical decisions to make before taking impressions?
Tray selection: edentulous tray in this case
Choice of impression material.
Need for tray modification
List the common primary impression materials
Alginate
Compound
Silicone
What are the properties of impression compound?
Softens with heat (55-60 degrees), hardens when cool. Can repeat process until impression is correct.
What are the ingredients in impression compound?
Resins - paraffin wax, beeswax, shellac
Filler - talc, chalk
Plasticiser - stearic acid (can leach into the water if left in too long. Won’t soften)
What are the adv / disadv of impression compound?
- Cheap, easy to use, non toxic
- Can resoften
- Mucodisplacive
- Can correct gross tray problems
- Poor surface detail (can take alginate wash)
- Poor dimensional stability
What is alginate?
Irreversible hydrocolloid - forms a gel in the presence of water
Can alginate be used on its’ own for impressions?
Not good at reaching the parts that the tray doesn’t extend to.
Need to extend tray (e.g. with compound / silicone) then use alginate. If the tray is more than 3mm short.
What are the disadv of alginate?
poor stability after setting. Needs to be cast within 30 mins.
Shrinking and synerisis - the long chains of polymer material contract and squeeze water out of material. So need to cover with wet paper towel.
Can also absorb water if stored in water- swelling and imbibition.
Poor wear resistance - esp embrasures of teeth
Requires adhesive to stick to tray.
Unsupported alginate likely to distort.
What are the adv of alginate?
Easy to use, cheap, well tolerated, elastic, viscosity can be altered with water, good surface detail, mucostatic so doesn’t distort ST.
What is silicone putty?
Made of Polydimethyl siloxane polymer + platinum catalyst
Reaction can be addition polymerisation or condensation polymerisation
What are the adv of silicone putty?
Easy to mix.
Relatively easy to use.
Moderate surface detail – can be improved with washimpressions either light body silicone or alginate
Well tolerated.
Elastic
Supports its own weight
Mucocompressive
What are the disadv of silicone putty?
Expensive. (platinum catalyst)
Needs tray adhesive.
Long setting time. (several mins)- may not be comfortable for pt e.g. esp if they have a gag reflex
Hydrophobic – need a dry mouth to take impression
What needs to be considered for the lab prescription for edentulous dentures in primary imp stage?
Draw tray outline approximately 2 mm short of periphery on impression.
Decide on the need for spacing or close fitting design.
Upper handle slightly labially proclined.
Lower vertical handle and finger rests in premolar region
Also inform technician what material you are using for secondary impressions to allow the correct amount of space to be added.
