CLASSIFICATION OF IMPRESSION MATERIALS BASED ON MODE OF SETTING & ELASTICITY

RIGID

a. Reversible (thermoplastic)

Impression Compound

Type I:

low fusing compound – green stick compound.

medium fusing compound – impression compound.

Type II:

high fusing compound – tray compound.

Type I:

For making primary impressions.

For individual tooth impression

For peripheral tracing or border moulding.

To check undercuts in inlay preparation.

Type II:

To make a special tray.

Advantages:

1. cheap2. reusable3. does not produce irritation to the patient4. impression can be remodified & resoftned again till an accurate impression is obtained.

Disadvantages:

1. difficult to record details accurately.2. Soft tissues are compressed due to pressure applied while taking the impression.3. Distortion.4. Difficult to remove undercuts.5. Does not have an pleasant taste.6. Can be uncomfortable in patients because of the rigidity.

b. Irreversible (thermosest)

ZOE Impression paste

Type I or hard

Type II or soft

Setting time:

Initial setting time final setting time

Type I 3-6 min. 10 min.

Type II 3-6 min 15 min.

Advantages:

1. Good dimensional stability2. good detail reproduction3. impression surface can be modified by adding in deficient areas4. easy to manipulate5. long working time, so border moulding can be done while relining dentures.

Disadvantages

1. burning sensation due to eugenol.2. messy to work with3. sticks to skin & instruments , difficult to clean.4. rigid so tends to break off in areas of undercuts.

Impression Plaster

ELASTIC

a. Hydrocolloid1. Reversible

Agar2. Irreversible

Alginate

Types:

Type I fast setting : 1-2 min.

Type II normal setting : 2-4.5 min.

Advantages:

1. ease of mixing & manipulation.2. no elaborate equipment required.3. material is elastic & comes out easily from undercuts.4. economical5. impression can be removed easily & has pleasant taste & odor, so comfortable for

the patient.6.

Disadvantages:

1. can’t be electroplated.2. poor tear strength especially in thin sections.3. dimensional stability is poor.4. distortion may occur if tray is shifted during setting.5. cant be refined.

b. Elastomeric impression materialsPolysulfides

Available in 3 viscosities:

Light bodied

Medium bodied

Heavy bodied

Advantages:

1. long working time.2. proven accuracy3. high tear resistance.4. inexpensive to use5. less hydrophobic6. longer shelf life.

Disadvantages:

1. must be poured with dental stone immediately.2. potential for significant distortion.3. odor is offensive.4. messy & stains clothes.5. second pour is less accurate.

Polyether

Advantages:

1. short working & setting time2. proven accuracy 3. adequate tear strength4. hydrophilic5. long shelf life6. less distortion on removal7. good dimensional stability.

Disadvantages:

1. stiffness requires blocking of undercuts2. slightly more expensive3. multiple casts can’t be poured due to stiffness of the material.

Addition silicones

Advantages:

1. short setting time2. adequate tear strength3. extremely high accuracy4. minimal distortion on removal 5. dimensionally stable even after 1 week 6. if hydrophilic, good compatibility with gypsum.

Disadvantages:

1. hydrogen gas may cause dimensional change.2. hydrophobic & hence requires a very dry field.3. expensive.

Condensation silicones

Advantages:

1. adequate working & setting timer.2. pleasant odor & no staining.3. adequate tear strength4. better elastic properties on removal.5. less distortion on removal.

Disadvantages:

1. adequate accuracy if poured immediately.

2. poor dimensional stability.3. potential for significant distortion.4. putty-wash method is technique sensitive.5. slightly more expensive.