impression

IMPRESSION MATERIALS USED IN ORTHODONTICS

   REMOVAL OF THE IMPRESSION

Alginate impression develops effective seal which can be freed by running the finger around periphery.  Impression should be removed suddenly with a jerk. Torquing or twisting should be avoided. After removal impression should be washed with water to remove saliva, covered with wet gauge to prevent synerisis and cast should be poured as soon as possible.

PROPERTIES

ADA specification No. 18 for alginate sets following standards

Mixing time:

Fast setting– 45 Sec

Normal setting – 60 Se

Working time:

Fast setting – 1.2 to 2minutes

Normal setting – 2-4.5minutes

Setting time:

Fast setting – 1.2 minutes

Normal setting – 2-4.5minutes

Over mixing Causes:

Reduction in final strength as gel fibrils are destroyed

Reduction in working time.

Under mixing Causes:

Lack of homogeneity and reduced strength

STRENGTH

—  The compressive strength of alginate ranges from 5000-8000gm/cm2.

—   The tear strength ranges between 350-700gm/cm2.

—              Both these properties are time dependent. If removal is delayed, strength increases.

—       The final strength depends on

—  W/P ratio: Too much or too little water reduces strength

—  Mixing time: Over and under mixing reduces strength

DIMENSIONAL STABILITY

—  Alginate impression lose water by evaporation and shrink in air. If placed in water it absorbs and swells. So cast should be poured immediately

Advantages

Inexpensive

Hydrophilic

Contact Angle 37 degrees (which means they will displace blood and saliva & pour well with gypsum stones)

Easy manipulation

Minimum equipment

Low cost

Accurate if properly handled

Disadvantages

Low tear strength

Poorer reproduction of surface details

Not Dimensionally stable on storage due to syneresis

RECENT ADVANCEMENTS

Zhermack advertised that the alginate impression can be kept up to 48 hours prior to pouring in their long life Hermetically sealed bags

The Vanilla flavoured Orthoprint is Anti Nausea.

Vival NF: Dimensionally stable for up to 100 hours.

High tear strength alginates: Silicone reinforced two paste system one with alginate, other with calcium reactor. It has higher tear strength

Self disinfecting alginates containing iodophor.

Dust free alginates: Organic glycol is used to agglomerate the dust particles

LAMINATE TECHNIQUE/ALGINATE AGAR METHOD

In this technique agar in the syringe is injected on to the area to be recorded, an impression tray containing chilled alginate is placed over it.