Dental Impression is a negative replica of the oral cavity, many impression materials are present which are capable of getting the perfect impression if the proper guidelines are followed

IMPRESSION COMPOUND

It is a rigid, reversible impression material which sets by physical change.

Classification:

Acc to ADA sp. No.3:

Type I:

  • Low fusing compound – green stick compound.
  • Medium fusing compound – impression compound.

Type II:

  • high fusing compound – tray compound.

Supplied as:

1. as sticks, cakes, & cones.

2. colors – green, brown, red.

Applications:

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.

Requirements of an ideal impression compound:

  1. Harden at or little above mouth temp.
  2. Be plastic at a temp. not injurious or harmful to oral tissues.
  3. Not containing irritating or toxic ingredients.
  4. Harden uniformly when cooled without distortion.
  5. Have a consistency when softened which will allow it to reproduce fine details.
  6. Be cohesive but not adhesive.
  7. Not undergo permanent deformation or fracture while withdrawing the impression from the mouth.
  8. Be dimensionally stable after removal from mouth & during storage.
  9. Exhibit a smooth glossy surface after flaming.
  10. Withstand trimming with sharp knife without flaking or chipping after hardening.

Composition

Rosin 30

Copal resin 30

Carbuna wax 10

Stearic acid 5

Talc 25

Coloring agent app.

Significance:

  • During softening of the material , the outside will soften first & inside last. So to ensure uniform softening the material should be kept immersed for a long time in water bath.
  • The layer adjacent to the mouth tissue swill remain soft. Therefore Its imp to cool the compound thoroughly before removing the impression.

Properties:

Thermal properties

i) thermal conductivity:

has low thermal conductivity.

ii) Coefficient of linear expansion. (CTE)

high CTE , 0.3% acceptable.

iii) Flow

  • high fusing compounds – < 2% at 37 .C & < 70-85 % at 45.C
  • low fusing compounds – < 3% at 37 C & < 80-85% at 45 C

iv) Glass transition temp:

  • It is the temp. at which the material loses its hardness or brittleness on heating or forms a rigid mass.
  • App. 39.CMaterial removed from the mouth after glass transition temp. is reached.

v) Fusion Temp.

  • It’s the temp. at which the impression compound becomes plastic.
  • Its 43.5.C

vi) Dimensional stability.

  • Poor dimensional stability.
  • To prevent distortion pour the cast within 1 hr.

vii) surface detail reproduction is less because of high viscosity & low flow.

Manipulation

  • Sticks: Small amount softened over flame
  • Cakes: Softened in a thermostatically controlled waterbath.

Precautions:

  • Prolonged immersion in a water bath is not indicated overheating
  • In water makes the compound sticky & difficult to handle.
  • Avoid incorporating water while kneading.

Advantages:

  • cheap reusable
  • does not produce irritation to the patientimpression
  • can be remodified & resoftned again till an accurate impression is obtained

Disadvantages

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

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