QUIZ 3: Primary Impressions and Diagnostic Casts

Define impression

Negative likeness or copy in reverse of the surface of an object

Imprint of the teeth and adjacent structures

Define primary impression

Preliminary / Diagnostic Impression / Study model impression

An impression for the production of a Diagnostic Cast (study model)

Define final impression

an impression for the production of a Working or Master cast

What are the armamentarium necessary for a primary impression?

Impression Trays

Alginate Powder

Material

Water

Mixing Spatula

Rubber Bowl

Water Measure

What is the technique for a primary impression?

1. Select tray size and fit

2. Locate convenient operator's position

3. Position the patient safely and allow ease of access by the operator

4. Familiarize with the properties of the alginate impression material (working, initial setting, setting time)

5. Use mouth mirror to deflect the cheek on one side and edge of tray on the other side

6. Trim excess material

7. Disinfect

What is the working time?

mixing + loading time

max time to mix: 1 min

max time to load tray and seat: 30 sec

What is the initial setting time?

2 min 15 seconds

What is the setting time?

time from start of mix until material has set

3 min

When should the cement be poured and why?

within 15 minutes of taking the alginate impression to maintain appropriate dimensions bc it can shrink

What makes an acceptable impression?

Properly mixed

Tray adherence

Complete coverage

Accurate reproduction of detail

No voids

No tray exposure

What makes an unacceptable impression? (look at slides)

Too thick

Incomplete coverage of anatomical landmarks

Under-extended

Too runny

Patient compromised / uncomfortable

Short of functional

Separation from tray

Presence of voids / inaccuracies in critical area

Unmixed alginate

Tray material exposure

What is a diagnostic cast used for?

Poured in microstone and trimmed

A life-sized reproduction of parts or parts of the oral cavity and/or for the purpose of study and treatment planning

Used as a guide to managing patient care

What can be made from a diagnostic cast?

Bleaching Trays or Mouthguards

What is a master cast used for?

Definitive Cast

A replica used to fabricate a dental restoration

What are the types of diagnostic casts?

Plaster (Beta hemihydrate)

Stone (Alpha hemihydrate)

High Strength Stone (Alpha hemihydrate)

Describe Plaster (Beta hemihydrate). What type of impression is it?

Heating gypsum at atmospheric pressure

Study Model

Describe Stone (Alpha hemihydrate). What type of impression is it?

Heating gypsum under pressure

Study Model / Diagnostic Cast

Describe High Strength Stone (Alpha hemihydrate). What type of impression is it?

Heating gypsum under pressure with the addition of chemicals

Master Cast

When does initial setting occur for a diagnostic cast and what type of strength is this?

after 1 hour

wet strength

What is complete with a diagnostic cast after 24 hours? What type of strength?

reaction setting is complete and excess water

dry strength

What is the technique for a diagnostic cast?

1. Vacuum-mix labstone

2. Use vibrator when pouring up

3. Eliminate bubble formations

4. Provide adequate base height

6. Trim cast

What is the adequate base height for a diagnostic cast?

10-12 mm measured from the deepest area

What can the alginate impression material be used for making?

Athletic Mouthguards

Night Guards

Bleaching Trays

Occlusal Guards

What should the alginate impression material NOT be used for making?

Crowns, bridges, implants

Primary vs Final Impression

Diagnostic Cast vs Master Cast