Dental impressions are negative molds of the teeth and surrounding oral tissues. This negative imprint is poured with gypsum to create a positive model, or cast, which accurately represents the patient’s mouth. These models are fundamental tools for diagnosis, treatment planning, and the fabrication of items such as crowns, bridges, dentures, aligners, and mouthguards. The material selected must precisely capture the fine details of the oral structures to ensure the final restoration fits correctly and functions properly.
Understanding Material Categories
Dental impression materials are primarily grouped based on their mechanical behavior after setting: Elastic Materials and Inelastic (Rigid) Materials. Elastic materials are flexible enough to be withdrawn from undercuts—areas where the tooth or gum tissue flares out—without permanent distortion. These are generally used when a patient still has most or all of their natural teeth.
Inelastic materials set to a rigid, hard mass and cannot be removed from undercuts without fracturing or causing permanent deformation. Their application is limited to situations where undercuts are absent, such as a completely edentulous, or toothless, arch. The choice of material depends on the procedure and the required level of detail and dimensional stability.
Water-Based Flexible Materials (Hydrocolloids)
The most common elastic material is Irreversible Hydrocolloid, widely known as alginate. This material is derived from alginic acid, a polysaccharide extracted from brown algae, and is supplied as a powder mixed with water. The mixture sets into a gel through a chemical reaction that cannot be reversed, hence the term “irreversible.”
Alginate is popular because it is cost-effective, easy to manipulate, and has a quick setting time, often between two and five minutes. It is typically used for preliminary impressions, diagnostic study models, and the fabrication of orthodontic appliances or custom trays. The material is hydrophilic, meaning it is compatible with the moist environment of the mouth.
The main limitation of alginate is its poor dimensional stability due to its high water content. It can shrink through water loss (syneresis) or swell through water absorption (imbibition). Therefore, alginate impressions must be poured with gypsum immediately after being taken to prevent distortion.
High-Accuracy Rubber-Based Materials (Elastomers)
For procedures requiring the highest level of detail and long-term accuracy, such as crowns, bridges, and implant restorations, synthetic rubber-based materials called Elastomers are used. These materials offer superior tear strength and dimensional stability compared to hydrocolloids. They are capable of reproducing the very fine surface details of prepared teeth with minimal distortion.
Polyvinyl Siloxane (PVS)
PVS, also known as addition silicone, is widely used for high-precision work due to its exceptional dimensional accuracy and stability. PVS impressions often remain accurate for one to two weeks after being taken.
Polyether
Polyether is another highly accurate material known for its superior hydrophilicity, allowing it to flow and capture detail even in the presence of minor moisture or saliva. Polyether tends to be more rigid than PVS when set, which can make it slightly more challenging to remove from the mouth.
Both PVS and Polyether are typically supplied in a two-paste system that is mixed, often using automated dispensing guns, to initiate the setting reaction.
Rigid and Highly Specialized Materials
The inelastic or rigid materials are reserved for specific applications, primarily involving patients without any remaining teeth. Zinc Oxide Eugenol (ZOE) paste is one such material, typically used for final impressions in complete denture fabrication.
ZOE paste is supplied as two tubes, a base paste containing zinc oxide and a reactor paste containing eugenol, which set through a chemical reaction. It is highly advantageous because it is mucostatic, meaning it records the soft tissues in their uncompressed state, and it exhibits good dimensional stability. Another inelastic material is Impression Plaster, which is essentially a form of gypsum. Impression plaster is also mucostatic and provides excellent detail, but its use is limited due to its brittleness, which risks fracturing upon removal. These rigid materials are used in conjunction with a custom tray to capture the precise contours of the edentulous arch.