Impact of Storage Environments on the Dimensional Stability of Irreversible Hydrocolloid Alginate Impression used in Dentistry

Abstract

Background: Alginate impression material is widely used for recording accurate impressions to fabricate various oral appliances such as crowns, bridges, partial dentures and complete dentures. Alginate impressions are dimensionally unstable and hence it should be poured immediately to form exact cast. The impressions are sometimes sent to the nearby laboratories for making cast and models. For maximum dimensional stability, the cast should be poured immediately or it can be stored in lower concentration NaCl or K2SO4 solutions during transport to distant laboratories. Objective: To evaluate the effect of various storage environments on the dimensional stability of irreversible hydrocolloid alginate impressions and to select a suitable medium for long time storage. Materials and Methods: Alginate impression material is supplied in powder form and is mixed with distilled water and poured on cylindrical hollow plastic mold. After setting, the sample is removed, weighed and then stored for 24 hours in a) exposed to atmosphere at 28+2°C(60 +5% relative humidity), b) refrigeration at 8+2°C, c) refrigeration in closed plastic sachets at 8+2°C, d) humidity chamber at 28+2°C and 8+2°C (100% relative humidity), e) distilled water f) solutions of NaCl at three different concentration and h) solutions of K2SO4 at three different concentration. The dimensional stability is measured by weight loss method and ion released during syneresis and imbibitions in storage media by using ion selective electrode method. Results: Alginate impressions were more stable at lower concentrations of NaCl & K2SO4¬ and dimensional changes are less than +0.6%. At higher concentrations more syneresis occurs, hence causes shrinkage of impressions. Impressions stored in open atmosphere showed maximum dimensional change followed by refrigerated sample, closed plastic sachets in refrigerator, humidity chamber and distilled water. Conclusion: For maximum dimensional stability, the impression should be poured immediately with cast material or the impression can be stored 24 hours in lower concentrations NaCl or K2SO4 solutions during transport to distant laboratories. This can also be stored in humidity chamber or zip lock polythene sachets in refrigerator at 8+2°C up to 6 hours. This saves chair side time and helps to produce accurate results.