Many of dental procedures rely on the use of elastomeric impressions material. Advancements in the delivery system and improved properties of materials have made it easier to predict impression techniques. Impression materials are extremely useful for dentists trying to obtain an exact replica of hard and soft oral tissues. To obtain the most accurate impression, a dentist must carefully choose a proper technique and heavy body impression material.
The best impression materials must fulfill the following conditions:
- During polymerization, transportation, or storage, the material must not shrink.
- The impression material must have an excellent flow.
- The impression material must have a saturated colour so that it is easy to detect that whether the tooth margin was captured or not.
- Impression material must have a detailed reproduction, an excellent tear strength and must not distort when removed from the mouth.
- It must be non-toxic.
- It must have good taste and odour.
- It must have a long lifetime, must settle down quickly in the mouth and must have a long shelf life.
- An ideal heavy body impression material can be used more than once. The material does not deteriorate in accuracy.
No impression material will comply with all of the above-mentioned standards. Mostly there are compromises for impression materials and techniques used depending on the requirements of the patient.
Impression Materials Basics
Polyvinylsiloxane (PVS) is a 2-paste system. One is a base and the other is a catalyst. The base paste is made of polydimethylsiloxane polymer. The catalyst paste is made of a pre-polymer in which some terminal methyl groups are replaced by vinyl groups. As the two pastes mix together, an addition reaction occurs resulting in silicone rubber. Platinum is also added to the mixture to remove any hydrogen that might have been produced during the reaction.
PVS is an excellent impression material due to:
- Excellent dimensional stability,
- Good detail reproduction,
- High tear strength,
- High recovery time from deformation.
The only drawback is that PVS are hydrophobic but this has been dealt with by combining cosmetic grade surfactants.
Wearing latex gloves while mixing impression materials will set addition silicone very slowly. The sulphur present in the powder of latex will contaminate the chloroplatinic acid catalyst. It is recommended to use vinyl gloves.
Only a small amount of catalyst is needed for the setting reaction. Hydrophobic PVS material makes harder stone dies as compared to polyether and hydrophilic PVS impression materials. Despite their shortfalls, hydrophilic PVS impression materials have had a higher clinical success rate compared to hydrophobic PVS impression material.
Impression materials and equipment for dental office are always chosen very carefully. The flexibility of impression material is measured by its strain during compression. If an impression material has a medium (5% to 8%) or high (>10%) strain value this shows that impression material is more flexible when removed from teeth.
Similarly, if an impression material has a high value (>99%) of elastic recovery, then it means there will be very little distortion.
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