Alginate is a widely utilized material in dental practice, serving as the primary compound for creating molds, known as impressions, of a patient’s mouth. Its ease of use and ability to accurately capture the shape of teeth and surrounding tissues make it a standard material in general dentistry. These impressions are foundational for numerous procedures and appliances that require an accurate model of the dental arches.
Defining Alginate and Its Source
Alginate is classified as an irreversible hydrocolloid, meaning it sets through a non-reversible chemical reaction involving water. The fundamental ingredient is sodium or potassium alginate, a polysaccharide extracted from brown seaweed. This natural origin allows the substance to transition from a powder to a gel-like state.
The dental alginate material is supplied as a powder, which contains the soluble alginate salt, a reactor like calcium sulfate, and a retarder, typically trisodium phosphate. When the powder is mixed with water, a chemical reaction occurs where the calcium sulfate initially reacts with the retarder, controlling the working time. After the retarder is consumed, the calcium sulfate reacts with the sodium alginate to form an insoluble calcium alginate gel, which is the final set impression material.
Primary Function: Dental Impressions
The primary application of alginate is to create a negative reproduction, or impression, of the teeth and adjacent soft tissues. The process begins with mixing the powder and water according to the manufacturer’s specifications to form a smooth paste, which is then loaded into a specialized tray. This filled tray is seated over the dental arch and held steady until the material sets, which typically takes a few minutes.
The resulting impression serves as a template from which a positive model, or cast, can be poured using dental stone. These models are used extensively for diagnosis and treatment planning, often referred to as study casts. Alginate impressions are also used to fabricate various custom appliances, including orthodontic retainers, mouthguards, bleaching trays, and preliminary models for dentures or partial dentures.
Key Material Properties and Limitations
Alginate’s popularity in dentistry stems from several beneficial properties, including its relatively low cost, ease of manipulation, and patient comfort compared to more rigid materials. The setting time can often be controlled by adjusting the temperature of the water used for mixing, offering flexibility in the clinical setting. Because it is hydrophilic, alginate can also capture sufficient detail even when moisture, such as saliva, is present in the mouth.
Despite its advantages, alginate has a significant drawback known as poor dimensional stability. As a hydrocolloid, the material is highly susceptible to dimensional change when exposed to air or water. If the impression is left in the open air, it can lose water through a process called syneresis, causing it to shrink and distort. Conversely, if stored in water, the impression may absorb excess moisture through imbibition, leading to expansion and inaccuracy.
To minimize these distortions, dental professionals must pour the dental stone into the impression to create the model as soon as possible after removal from the mouth. Delays in pouring compromise the accuracy of the final cast, affecting the proper fit of any fabricated dental appliance. Other limitations include a relatively low tear strength, making the material prone to tearing in thin sections or deep undercuts, and a lower level of detail reproduction compared to more advanced elastomeric materials.