Complete Dentures - Primary and Secondary Impressions

Question 1

What area does a primary impression have to cover?

Answer 1

Full extent of denture bearing area.

For lowers, just over half the retromolar pad should be taken.

Question 2

What makes a good impression?

Answer 2

• Minimal discomfort to the patient (not too hot)
• Minimal distortion of tissues
• Full extent of denture bearing area recorded
• Free of significant defects.

This helps to ensure stability, retention and support for final denture.

Question 3

What makes a poor impression?

Answer 3

• Air blows.
• Underextension
• Overextension and alteration of sulcus
• Not got full extent of denture bearing area
• Peeling off from stock tray

Question 4

Which tray do you use for primary impressions?

Answer 4

Rounded stock trays to fit shape of edentulous ridge. Use handle in a way that doesn’t interfere with patient’s soft tissues.

Question 5

What materials are there for primary impressions?

Answer 5

Impression compound
Alginate
Silicone putty

Question 6

Describe impression compound, how you use it and the advantages and disadvantages of it.

Answer 6

Thermoplastic material that becomes soft when heated and sets when cools down. This can be redone over and over.

Comes in a variety of shapes.

Made of resin (beeswax, paraffin wax, shellac), fillers (talc, chalk) and plasticiser (stearic acid). So don’t leave in water bath for too long because stearic acid leaches out and it won’t be flowable anymore.

Heat in a 55-60 degree water bath and soften and use the compound. Can place alginate wash over it. Macro-mechanical retention: doesn’t need chemical retetnion.

Pros:

• Muco-displacive
• Easy to use
• Can be reheated and added to
• Non-toxic

Cons:

• Poor surface detail (so alginate wash used)
• Poor dimensional stability - so shrinks so needs to be cast asap.

Question 7

Describe alginate, how you use it and the advantages and disadvantages of it.

Answer 7

Alginate is an irreversible hydrocolloid. This means once it sets, it sets.

Pros:

• Easy to use
• Tolerable
• Good surface detail
• Can be made more or less viscous

Cons:

• Doesn’t hold its own weight well so can flick off in unsupported areas and needs another material to extend tray if need be.
• Poor tear resistance
• Sets fast so needs to be cast quickly.
• Can shrink and dry and syneresis
• Can swell in presence of water

Question 8

Describe silicone putty, how you use it and the advantages and disadvantages of it.

Answer 8

Silicone putty is two putties mixed together: platinum catalyst and polydimethyl seloxane. Addition or condensation polymerisation reaction occurs.

Pros:

• Can hold its own weight
• Tolerable to patients
• Easy to use
• Muco-compressive

Cons:

• expensive
• Sets slowly so can be uncomfortable for patients
• Hydrophobic
• Needs tray adhesive

Question 9

What should you remember when making a lab prescription?

Answer 9

Use indelible pencil 2mm short of sulcus on inside.

Handle proclined upwards for upper dentures.

Handle vertical and premolar rests for fingers on lower trays.

Indicate which material will be used for secondary impression for close-fitting or spacing (for ridges with undercut or flabby ridges, you want perforations, otherwise none).

Do you want stops?

Question 10

Why do we want the special tray to be made 2mm short?

Answer 10

To allow space for greenstick/pinkstick/ heavy bodied silicone putty, for boulder moulding.

Question 11

What should we check when we get a special tray from lab?

Answer 11

Stops
2mm short
Perforated?

Question 12

What are stops for? What would we do if the technician forgot to add them on?

Answer 12

Stops - used to create space between tray and mucosa to allow impression material to flow and not get stuck in one area.

If not placed, use green stick and form a tripod of stops.

Question 13

Describe how to make a secondary impression for uppers.

Answer 13

• Use pinkstick/greenstick for postdam area first. Be careful not to go too laterally.
• Remove and then use pinkstick/greenstick for border moulding on posterior borders. Ensure it’s well heated and that it’s softened in the middle, ALWAYS put in water bath and then place in the patient’s mouth.
• Manipulate cheeks and lips to record functional sulcus.
• For any pinkstick/greenstick on the inside, use a hot wax knife to remove it.
• Place adhesive
• Then place alginate inside and take an impression.
• Use indellible pencil on outside of the impression for the lab to know where to trim excess to.

Question 14

How is making an impression different for lowers?

Answer 14

• Want retromolar pad, not tuberosities.
• No post-dam.
• Use light bodied silicone rather than alginate.

Question 15

What makes a good secondary impression?

Answer 15

• Full denture bearing area recorded
• No air blows significant
• Full depth and width of sulcus recorded
• Post-dam in upper recorded
• Good surface detail and muscular attachments

Question 16

When are perforations placed?

Answer 16

Not preferred for upper.

Question 17

What should you remember when making prescription for record rims?

Answer 17

Use indelible pencil to record 2mm OUTSIDE of the impression as we want to preserve the functional sulcus but remove excess.

Question 18

What is the difference between greenstick and pinkstick?

Answer 18

Pinkstick is more likely to distort tissues.

Gets softer at lower temperature, stays soft longer, is stickier, and more flowable.

Is polymers rather than waxes.