Preliminary Impressions and Custom Tray Impressions

What is an impression?

an impressed reverse/negative reproduction of a surface of any object in plastic substance where a positive reproduction can be made

What is a preliminary impression?

Used to diagnose and make a study model/diagnostic cast

What is a cast?

a positive reproduction of both jaw, and its hard/soft tissue

What is normally used to take a preliminary impression?

Alginate (Irreversible Hydrocolloid)

What are the 4 main reasons for using alginates?

• Moderate soft tissue displacement

• Quick Setting Time

• Moderately high ability to capture detail

• Cost-effective

How much of alginate is water?

80%

What does high water content make alginates highly susceptible to?

• Syneresis

• Imbibition

What is syneresis?

characteristic of a gel to secrete from its surface and cause shrinkage

What is imbibition?

Tendency of gel lacking in water to absorb any contact with water causing expansion

What are impression compounds?

A thermoplastic material that softens when heated and hardened when cooled

What is an impression tray?

used to carry impression material to and from a patients mouth

What are the 2 basic types of trays?

• Prefabricated/stock

• custom

What are the 3 types of stock trays?

1) Rimlock

2) Perforated

3) Perforated Rimlock

What are custom trays made from?

• light cured resin

• self-curing resin

• vacuum method

What are 3 reasons that custom trays more exact impressions?

• fit a patient exactly

• provide a uniform thickness of impression material

• use the most accurate materials, elastomeric

What are the 5 requirements for impression trays?

1) Tray should be rigid but not overly thick

2) Should retain its shape throughout the construction and pouring

3) method of construction should be simple

4) easy to trim/thin tray

5) must be smooth

How short should the border of the tray be?

2-3mm

How many layers of baseplate wax do you use for a completely edentulous patient?

one

How many layers of baseplate wax do you use for a partially edentulous patient?

two

What do wax relief/spacers provide for the tray?

Room for controlled thickness of impression material

What are tissue stops made for?

Hold the tray off the cast by a distance equal to the thickness of the spacer