elastomeric impression materials

• polyether - addition silicones

When removing impression material (IM) from contact with a tooth - after its set - the IM has to stretch, flaring out at the slides to overcome the bulbous aspect of the tooth Ideally it recovers to original dimensions, replicating the shape of the tooth ~ that is assuming it is perfectly elastic

• Material characteristics ○ Need to be aware of the degree of accuracy needed for your particular purpose • Clinical performance ○ Patient acceptance § Working time § Setting time § Taste / smell of material ○ Ease of use for clinician • Need to make a choice that best suits the specific circumstances

• Need to know how specific IM perform • That means evaluating the product information available ○ For some manufacturers (eg 3M ESPE, Heraeus Kulzer), substantial documentation is provided on their websites ~ this can just be claims, better to have actual robust evidence • Need to know what characteristics are important so you can interpret the data they give in a meaningful way • Need to relate the data to other materials and the ideal properties

Elastomers are formed by polymerisation with cross-linking of polymer chains Cross-linking generates elastic properties - Causes fluid to solid transition • Polymerisation may produce by-products (H2O, H2, alcohol) which affect dimensional stability and cast compatibility

• Polysulphides (1950s) ○ We don’t use these now • Silicones ○ Addition curing (1975) § This is the type we will focus on ○ Condensation curing (1950s) - ie conventional • Polyethers (1965)

• Surface detail (reproduction) • Flow / viscosity • Contact angle / wettability These 3 affect the accuracy by which the surface features are recorded • Elastic recovery (%) • Stiffness (flexibility) • Tear strength These 3 affect the accuracy of the dimensions and shape of the final impression • Mixing time (min) • Working time (min) These are practical considerations - they affect the clinician’s skill and working method, and will influence the choice, as will patient preferences • Shore A hardness ○ This is a specific hardness test for IM • Shark fin test (flow under pressure) ○ Relates to the ability of IM to deal with undercuts • Setting shrinkage ○ Fundamental property • Dimensional stability ○ Critical for producing an accurate positive replica using gypsum • Thermal expansion coefficient ○ Plays a pivot role, as we can’t eliminate the temperature gradient between mouth and room temperature • Biocompatibility ○ An essential requirement ○ It relates to patient’s safety

addition silicone

Do you understand it Does it make sense What else do you want to know

how the material interacts in the presence of water;

• Viscosity • Surface wetting • Contact angle

• Surface reproduction (ISO) - Visco-elasticity / elastic recovery

• Flow under pressure (“shark fin” test) • Tear / tensile strength • Rigidity

• Setting shrinkage • Thermal expansion / contraction • Storage

A measure of material’s ability to flow Vital for it to reach all the dental tissue’s surface area • Determines a material’s potential for making close contact with hard / soft tissue surface –> so how well it records surface detail - Has to flow readily without too much pressure being required • Clearly viscosity influences the surface texture that is replicated • Range: • Low • Medium • High

Must make intimate contact with teeth / mucosa

• Determines how well material envelops the hard / soft tissue surface (to record fine detail) • The contact angle is an objective measure of how well the IM envelops the hard and soft tissue surfaces

The contact angle indicates how readily the IM “wets” the tooth surface Ie how closely the IM envelops the tooth surface

Large contact angle at surface, results in spaces between globules of impression material, so some of tooth surface not replicated an IM has a high contact angle with a tooth surface is more likely to have gaps between each globule of material so the IM won’t replicate the whole tooth surface = not ideal

means a large percentage of its volume will make contact with the target surface - that is ideal Small contact angle, no spaces between globules of impression material, so all of surface is replicated In contrast, this IM has a low contact angle with the tooth surface, ensuring there are no gaps between each globule of the material So the whole tooth surface is recorded = ideal

• Incorporate non-ionic surfactant ○ Wets tooth surface ○ More easily wetted by water containing die materials • Initial addition silicones had some difficulties in making good contact with moist tooth surfaces ○ To overcome this, manufacturers introduced a non-ionic surfactant component ○ These are referred to as hydrophilic silicones

• There is an international standard measure for surface reproduction of an IM - ISO 4823 This measures how accurately the surface is represented

• This test involves placing IM along a surface which has grooves of specified width: 20, 50, 75um - Pretty narrow To conduct the test, a uniform pressure is applied across the width of the IM if the IM cannot fill all the grooves / notches then it cannot reach narrow niches so the IM will not give the most accurate surface detail

when a load is applied at T=0 (as when removing an impression tray) the material stretches instantly to the strain required This level of strain is maintained Until the load is removed, and the IM returns instantly to its original dimensions -