Making of impressions for partial dentures is more difficult compared to those for complete dentures. Most of the failures occur as a result of the rigid nature of the impression material used, which results in an inaccurate impression of the oral tissues and undercuts surrounding the teeth. A material which is sufficiently elastic and that can be retrieved from the mouth without any defect is a solution for this issue. As a result, researchers invented flexible impression materials which can retain the elastic nature even after a complete set. The idea evolved from the making impressions of statutes using jelly molds. In 1925, an Austrian, Alphons Poller, invented and patented a material known as "Nogacoll." After Alphons Poller's death in 1931, the formula was attained by a company and given a new name: "Denticole."
Many companies started manufacturing a similar material. All of these compounds were reversible hydrocolloids with agar-agar as their fundamental ingredient. This substance, which was a jelly-like substance, became soft when heated and hardened when cooled. The use of agar-agar was not convenient as it required special equipment such as heaters, syringes, and water-cooled rim lock trays. During the World War II, agar-agar was no longer available. Chemical treatment of brown algae led to the formation of a new elastic impression material which had an alginate base which is an irreversible hydrocolloid and was much easier to use. The alginate powder, when mixed with water, forms a gel, which is known as alginate. Agar-agar impression materials are no longer used in clinics because of the inconvenience of manipulation; whereas, alginates are still popularly advocated. Alginates remain an economical option and easily manipulated impression material.
Alginate impression can be used for the fabrication of a primary cast in prosthodontics, study models in orthodontic, and duplication of casts. In one study, it was shown that alginate impressions could also be used as final impressions for indirect restorations with chamfer finish lines.
There have been reported cases of allergic reactions to alginate impression material. Alginate impression material is contraindicated in patients with a history of severe allergic reaction to crystalline silica, calcium sulfate, or potassium titanium fluoride. In these cases, it is better to use an alginate-free impression material.
Alginate can be mechanically mixed. Alginate mixed using these devices is same each time. Recommended mixing time for mixing by hand is 60 seconds, and for mechanical mixing, it is 15 seconds.
The mixing method can significantly affect the elastic recovery and tensile strength of the alginate impression material. Mixing the alginate impression material can be done in two ways: manual mixing or mechanical mixing. In the manual procedure, the proper mixture of water and powder cannot be accurately determined. Ideal water to powder ratio is 38 ml of water for 16 gm of powder. But it can vary according to each manufacturer. Ideally, the powder should be weighed and not measured volumetrically using scoops. The measured powder is to be shifted to the premeasured water in the rubber bowl. The powder is incorporated into the water by mixing with a curved metal spatula. A quick figure-eight motion while wiping or stropping against the side of the rubber mixing bowl to express air bubbles is best. The mixing time is particularly important. Generally, 45 seconds to 60 seconds is sufficient depending on the brand and type of alginate impression material taken. The mixing time for regular alginate is 1 minute and the time should be carefully checked because both undermixing and overmixing affects the strength of the set impression. Fast-set alginate powder should be mixed with water for 45 seconds. The product's instructions for mixing time, working time, and setting time should be followed strictly.
To reduce the dust encountered after tumbling, manufacturers have introduced a “dustless” alginate. The powder becomes thicker and less prone to become airborne. Color indicators have been added in some formulations to identify setting reaction. This allows the operator to decide when to proceed to the next step of impression making.
The amount of impression material required is loaded in either a plastic or metallic tray. Alginate adhesives also can be used instead of perforations in the impression tray. The alginate impression material is filled to the borders of the impression tray, and any material excess should be removed using the spatula. The impression material surface is moistened with the help of wet, gloved fingers. The perfectly mixed impression material should be gently placed on the occlusal surface, for obtaining the occlusal anatomy of teeth. A small amount of alginate must also be placed on the palate. Using a mouth mirror or gloved finger, retract the patient's lips, and put the impression tray near the mouth. Rotate the tray and place it into the mouth on the opposite side. The tray should be centered in position and the impression tray held in place with light pressure. The soft, tissue-like, labial flange should be relieved and molded by manipulating the lips. Constant, gentle pressure must be applied while holding the tray in position until the material sets. Alginate impression materials start setting from the tooth surface to the impression tray.
After obtaining a complete set of the impression material, remove it with a quick snap. The seal between oral tissues and the impression tray is broken by pushing gently with the finger or by applying air using air syringe directly into the buccal sulcus and then pulling with a downward pressure. The impression must be rinsed using only cold water to remove saliva, debris, or blood from the surface. Using damp gauze or napkins, the impression should then be covered to prevent syneresis which results in dimensional change. Disinfect all alginate impressions before pouring with gypsum. Antiseptic alginates are available with alginate containing antiseptic such as glutaraldehyde and chlorhexidine gluconate of the cured product of alginate impression material per 100 parts by weight. Unsupported alginate excess should be removed using a sharp knife. Finally, using gypsum, the impression should be poured for a cast.
Making an impression is an irreplaceable step in the fabrication of indirect restorations. Digital impression making provides excellent precision and eliminates many conventional steps in denture fabrication. Until these digital techniques are perfected, alginate impressions can be relied upon by following the appropriate technique suggested. Alginate impressions remain an irreplaceable part of prosthodontics.