Alginate is a common dental impression material used to create a negative replica of the teeth and surrounding oral structures. The resulting model, known as a preliminary impression, is used by dental professionals for diagnostic purposes, such as fabricating study models or custom trays. The material is popular because it is cost-effective, easy to manipulate, and sets quickly, which improves patient comfort. To translate the oral structure into an accurate model, the impression must be kept moist from the moment it is removed from the mouth until the model is poured. Maintaining this moisture balance is paramount because the material is highly susceptible to dimensional change when exposed to air.
Alginate’s Hydrocolloid Nature
Alginate belongs to a class of materials called irreversible hydrocolloids, meaning it is a water-based gel that cannot return to its liquid state once set. It is primarily composed of alginic acid salts, derived from brown seaweed, which are mixed with water and calcium sulfate powder. This mixture undergoes a chemical reaction where calcium ions cause the alginate chains to cross-link and form a three-dimensional polymer network. The final set impression is a gel structure in which a significant amount of water is physically trapped. This high water content gives the impression its elasticity and flexibility, but the material’s dimensional stability depends directly on maintaining the precise concentration of water trapped within the gel matrix.
The Distortion Caused by Water Loss
The primary reason for keeping the impression moist is to prevent syneresis, which is the expulsion of liquid from the gel. When an alginate impression is exposed to dry air, the water within the gel matrix begins to evaporate from the surface. As water leaves the structure, the polymer network contracts, similar to how a sponge shrinks as it dries out. This shrinkage leads directly to a loss of dimensional accuracy, distorting the fine details captured from the patient’s mouth. Even a small amount of water loss can result in a significant dimensional change, making the resulting gypsum model an inaccurate representation of the oral anatomy.
Proper Storage for Moisture Equilibrium
The goal of post-removal care is to maintain moisture equilibrium, balancing keeping the impression moist without soaking it. While water loss causes shrinkage, completely submerging the impression causes imbibition, which is the absorption of fluid that makes the impression swell and distort. The most effective storage technique is to place the impression in a sealed container or plastic bag with a damp paper towel. This creates a 100% relative humidity environment, preventing water loss while avoiding direct contact with standing water. The time between taking the impression and pouring the gypsum model should be as short as possible, ideally within 12 minutes at room temperature, though storing it at 4°C can extend the acceptable pouring time up to about 45 minutes.