Lecture 5- Impression Material

The correct usage of impression materials is crucial for creating accurate dental models.

Scoop to Water Ratio: 1 line in the cup = 1 scoop of powder; most mixtures require a 1:1 ratio.
Three Scoops: If using three scoops of powder, fill the cup to the indicated line with water.
Technique: Water should be dispensed first to avoid powder clumping.
Mixing Process: Mix in a bowl using spatula; use a figure-eight motion for efficiency.

This ingredient acts as an accelerator for gypsum (stone) setting, creating hard, dense casts when mixed with impressions.
It's not primarily for alginate but improves the cast's properties.

Sodium Phosphate: Controls and lengthens setting time to allow for adequate working time.

Mixing alginate powder with water creates a reaction:
ext{soluble alginate} + ext{calcium sulfate}
ightarrow ext{calcium alginate (gel)}
The gelation process is rapid; sodium phosphate as a retarder allows for more effective working time.

Working Time: Time available for mixing before setting begins in the oral cavity.
Setting Time: The time during which the material solidifies in the mouth.
Typical working time varies based on the alginate type:
- Regular set: 3-4.5 minutes
- Fast set: 1.5-3 minutes

Water Temperature: Cold water extends working time; hot water speeds up setting.
Each 10°C increase in temperature shortens setting time.

Syneresis: Loss of water from the impression material causes deformation.
Inhibition: Absorption of water leads to a swollen impression, compromising accuracy.
Deformations result in inaccuracies in the cast; retaking the impression is necessary if deformation occurs.

After creating an impression, the gypsum is poured to create a positive cast.
Avoid long contact of impression with cast after gypsum sets to mitigate inaccuracies due to water absorption from the impression.

Rinse and disinfect the impression before pouring the gypsum to avoid interference.
Ensure the impression surface is dry to prevent microbubbles from forming in the cast.

Polysulfide: Two-paste system with good tear resistance, but has a characteristic smell due to sulfur.
Polyether: Hydrophilic, sets chemically without requiring cooling, maintains accuracy for a longer period (7-10 days).
Condensation Silicone: Releases ethyl alcohol as a byproduct, effect on dimensional stability, low odor.
Addition Silicone: No byproduct release, best for prosthodontic procedures due to high accuracy and detail capture.

Comprises a base, catalyst, fillers, and plasticizer. Setting influenced by temperature and humidity.
Advantages: Good working and setting time, accurate reproduction.
Disadvantages: Strong sulfur smell, byproduct of water may cause shrinkage.

Known for dimensional stability; contains base and catalyst. Hydrophilic properties help in capturing details during impression making.
Advantages: Durable, precise, reduces distortion compared to other materials.
Disadvantages: Rigid; can be costly.

Characterized by a base of poly-dimethylsiloxane, common catalyst is stannous octoate.
Advantages: Less expensive compared to other elastomers, captures detail well, pleasant odor.
Disadvantages: Byproduct ethyl alcohol can cause inaccuracies; must pour within an hour.

Base consists of polyvinyl siloxane. It's one of the most accurate types and widely used in modern dentistry.
Advantages: Excellent accuracy, no byproduct release, good for retakes.
Disadvantages: Cost is higher than other materials; potential for hydrogen gas release affecting quality if not handled properly.

Plaster: Basic impression material for initial records.
Impression Compound: Comes in sheets or sticks for higher temperature use.
Zinc Oxide Eugenol Paste: Utilized primarily for final impressions and bite registrations; excellent detail with a strong smell due to eugenol.

Trails and Use: Custom trays made from impression compounds for making precise dentures; border molding ensures better fit.
Dimensional Stability: Adequate for clinical procedures, rigid after setting.

Hydrocolloids: Include agar (reversible) and alginate (irreversible).
Elastomers: Polysulfide, polyether, condensation silicone, and addition silicone; varied properties based on composition and usage.
Inelastic Materials: Less common, used primarily for specific applications in dentistry, including bite registration and denture fabrication.