impression

IMPRESSION MATERIALS USED IN ORTHODONTICS

 

  • Boiling section
  • Storage section
  • Tempering section.

In boiling section, the material is kept for 10 min for liquefaction.

Then material is stored in storage section at 65deg.cel. in sol form until it is required.

Material is tempered at 45deg.cel. For 3 to 10 min so as to be tolerated by the patient.

Just before the tempering process for the tray material is completed, the syringe material is directly taken from the storage compartment and applied to teeth.  The water soaked on the layer of the tray hydrocolloid is removed from the container and gauge is removed.

The tray is immediately brought into position and seated with tight pressure and held with a very light force.

Gelatin is accelerated by circulating cool water (18-21deg.cel.) through the tray for 3-5 min.  After the gelatin is over, the tray is removed suddenly with a snap.

—  Advantages:

  1. Accurate dies can be prepared if handled properly.
  2. Good elastic property and reproduces undercuts  properly
  3. Well tolerated by patient
  4. Used for duplicating models.

—  Disadvantages:

1)      Flow is less Compared to recent elastic impression materials

2)      Gelatin may be painful to the patient.

3)      Tears relatively easily.

4)      Extensive equipment is required

Colloids exist as sol or gel.

A sol has the appearance and many characteristics of a viscous liquid.

A gel is a jelly-like elastic semisolid and is produced from a sol by a process called gelation.

It occurs in two ways

1) By temperature changes

2) By chemical means

TEMPERATURE CHANGES

Gelation in this case is a reversible process

Eg. Agar

The fibrils are held together by secondary molecular forces— they break at slightly elevated temperature and become re-established as the hydrocolloids cool to room temperature.

The temperature at which these changes occur is GELATION TEMPERATURE.

Gelation occurs at approx 37 deg. C temperature whereas liquifaction occurs at a higher temperature, i.e. 60-70 deg C higher than the gelation temperature.

This temp lag between liqueifaction and gelation is known as hysteresis.

 

CHEMICAL CHANGES

  • Conversion of sol           into       gel
  •  The fibrils formed are held together by primary bonds And is unaffected by  temperatures.

Hence, they are called IRREVERSIBLE HYDROCOLLOIDS

eg. ALGINATE

DIMENSIONAL EFFECTS

A majority of gel volume in hydrocolloid is made of water.

Gel may lose water by:-

a)Evaporation- from the surface

b)Syneresis- Exudation of fluid on the surface.

The exudate is not pure water, may be alkaline or acid depending on the composition of gel, both these process leads to SHRINKAGE.

Gel may absorb water if placed in water by a process called IMBIBITION causing gel to swell.

Both SYNERESIS and IMBIBITION should be avoided

As former can cause SHRINKAGE and later EXPANSION.