Don’t you hate it when your absolutely favorite impression material is no longer available? Occasionally, it happens. For example, not too long ago, the Silhouette impression material was discontinued, leaving many of my colleagues with empty DM-50 cartridge guns wondering what available product would be most comparable. To help pick a product that best approximates your favorite but no-longer-available impression material, choose a substitute based on its viscosity.
Viscosity:
Viscosity is a term used to describe the thickness or fullness of impression material once the base and the accelerator have been mixed prior to curing. Impression material viscosity is either low, medium, or high. High viscosity impression material is thicker and more firm whereas lower viscosity impression material is thinner and less firm.
Impression material viscosity will influence the accuracy in which an impression will accurately reflect the dimensions of the ear canal. The tissue and texture of an ear canal will differ from patient to patient. In general, most patients exhibit firm ear canal tissue and texture although ear canals exhibiting softer ear canal tissue and texture is often evident in the geriatric population because as we age, skin losses elasticity. Why is this 
relevant? Soft textured ears are more easily stretched by impression material injected in the ear canal. When a soft textured ear canal is overstretched by impression material, the end result is an earmold impression that is much bigger than the actual size of the ear canal, resulting in discomfort due to too tight of a fit and/or the need for significant modifications. To have better control over how much pressure is exerted by the impression material on the ear canal, the clinician can use viscosity of impression material to their advantage.
When to use low versus medium/high viscosity
Low viscosity silicones are of a softer consistency and are probably most appropriate in those situations where the patient is older and/or has a softer ear canal texture. By using lower viscosity, the impression material will fill the ear canal but not expand it to the point where it gets exaggerated. In contrast, using thicker material as is the case with medium or higher viscosity impression silicones in a soft ear will overstretch the ear, resulting in an earmold impression that may be bigger than it should be. Medium and higher viscosity silicones are firmer and are probably most appropriate in those situations where the patient has a normal or firm ear canal tissue and texture. Because the ear canal is already firm, it is not susceptible to stretching and thicker material will not exaggerate the ear mold impression. In contrast, using low viscosity impression material with poor technique on firm or normal ear canals can potentially result in an earmold that is too loose.
Now, if you want to sound really smart at work or at school, become familiar with the term shore value. Shore value is another term used to describe impression material and refers to the hardness of the material once it has fully cured. Values range from 20 to 40. The higher the shore value of the impression material, the more durable the material. This durability is a factor that mainly concerns the earmold manufacturer and has no relevance to the clinician in terms of making the actual impression. Shore value does not influence impression taking techniques. In addition, it is not correlated to viscosity in any capacity; in other words, viscosity may not be predicted on the basis of shore value and vice versa.
