Preliminary Impressions and Study Casts

Preliminary impressions materials are used in preparation for study and diagnostic casts, in the fabrication of mouth guards, whiting trays, temporary restorations, orthodontic appliances, custom impression trays, and as opposing cats in fixed and removable prosthetics. Alginate material is probably used more often than any other single type of impression material in dentistry.

 < that is a preliminary impression

which turns into....

 < these lovely impressions

One type of impression material is called irreversible hydrocolloid, and it cannot return to the sol state after it becomes a gel. Alginate is the irreversible hydrocolloid most widely used for preliminary impressions.

Sol= a liquid colloid

Gel= a colloid is its semi-hard rubbery consistency

Synerisis= happens when the impression material will lose water and begin to shrink if it is not in a humid environment

Imbibition= happens when the impression material gain extra water and expands (submerged in water)

Hydrocolloid (hydro=water, colloid= gelatinous substance) is the material, and a larger portion of it is water, therefore, making them very susceptible to dimensional changes.

The second type of preliminary impression material is called reversible hydrocolloid

•  1. Irreversible Hydrocolloid (or alginate)
• is supplied as a powder to be mixed with water to form a viscous gel
• once placed in the mouth, the sol forms an elastic gel through a series of chemical reactions. Once the gel has formed, the impression is removed from the mouth. The gel cannot be changed back to the sol consistency because a chemical reaction cannot be reversed.
• the chief ingredient is soluble potassium alginate. The irreversible hydrocolloid is obtained from sea kelp!
• it dissolves in water to form a viscous sol. To transform the sol to gel, you add calcium sulphate. A third soluble salt, trisodium phosphate is added to slow down the reaction
• you have two different kinds of irreversible hydrocolloid: normal set and fast set
• Normal set
• total working time is 2 mins
• 1 min mixing time
• 30 sec loading time
• 30 sec placing time
• Once placed in the mouth is is held up to 4 and a half mins until the firm gel consistency is reached
• ** BAD FOR GAGGERS
• Fast Set
• total working time is 1 min 15 sec
• 30 sec mixing time
• 30 sec loading time
• 15 sec placing time
• Once placed in the mouth it is held for 1-2 mins until firm gel consistency is reached.

• 2. Reversible Hydrocolloid (or agar)
• it is supplied as a gel, it is heated to change it to a sol and as it cools, it gels again. It changed back and forth from sol to gel by heating and cooling. This is rarely used in dentistry.

Mixing the alginate