Alginate Case Study

Alginate is an irreversible hydrocolloid impression material commonly used due to its ease of handling with minimal equipment [1], fast set, reliability and low price. However, its disadvantages include poor tear strength, permanent deformation of up to 1.5% and dimensional change [2].
In this scenario, dimensional change takes place during disinfection of the alginate, affecting the subsequent cast and denture, and I will be exploring how this has happened and what can be done to avoid it.
Dimensional Stability
The hydrocolloid structure of alginate is responsible for its dimensional instability, as the hydrophilic gel contains a large amount of water allowing evaporation, syneresis and imbibition. Syneresis, where an exudate forms on the
However, the problem ensues when the alginate impression is left in disinfectant for a long period, as is the case in this scenario.
As Figure 2 shows, alginate initially swells by imbibition and then shrinks after a longer period of time [6]. A disinfection period as long as overnight would therefore cause shrinkage of the alginate impression.
Table 1 shows the initial expansion of alginate in water after 10 minutes and its consequential shrinkage after 1 hour. This not only emphasises the effect of lengthened disinfection time, but also how different alginate thicknesses are affected: the thinner the alginate, the more percentage shrinkage takes place, highlighting how an impression of the mouth, with sections of differing thicknesses, would be unevenly distorted by overnight disinfection. [5]
Avoiding Dimensional Change
To avoid what has happened in the scenario, the alginate impression should not be immersed overnight in disinfectant. After rinsing the impression under cold water to remove saliva and/or blood, the alginate can be disinfected via various methods, as shown in Table 2