Introduction

Alginate-based impression materials have been used in dentistry since 1947.¹ Originally used as precision impression materials in fixed prosthodontics¹ they are more commonly used for the initial impression to obtain a preliminary model used for diagnostic purposes, treatment planning, and for the fabrication of a provisional prosthesis or custom tray. The dimensional stability of alginate-based impression materials has been studied since the 1970s.² In particular, the phenomena of syneresis has received a great amount of attention.^3-47 Syneresis is intended as the expression of fluid onto the surface of gel structures.⁴⁸ The clinical consequence is the shrinkage of the material due to the loss of water by evaporation.⁴⁸ The actual ISO 1563⁴⁹ states the requirements for the alginate-based impression materials, however, no information is provided on the methodological test valid for the evaluation of their dimensional stability. In recent years different tests have been developed for analyzing this property.^{4-6,36,42,50-54} The most common one consists of taking the impression from a master cast (usually made of acrylic or stainless steel) and measuring the discrepancy between the latter and the obtained cast. Although the measurement of a single die can be performed with greater accuracy,⁶ the distance between various dies is an important variable that should be considered in order to perform the test in clinically relevant conditions. Shrinkage occurring in the palatal zone of the impression might reduce the distance between the teeth of different semi-arches.

The aim of this study was to verify the dimensional stability of five different alginate-based impression materials stored in a 100% relative humidity environment after different periods of time. The null hypotheses tested were: (1) there is no difference in dimensional stability among different alginate-based materials and (2) the dimensional stability is not affected by storage time.