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Impression Materials (One)

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Required Properties of Dental Impression Materials

Properties of AlginateImpression Materials as Examples

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The Importance of Impression Materials

Dental appliances can be made outside the patient’s mouth

They must have accuracy of ‘fit’ and functional efficiency

That depends on the model closely replicating the natural

oral tissues

Accuracy of models depend on accuracy of impressions

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Classification of Impression Materials

Chemical types allow dentist to quickly select for particular applications

Properties either before or after setting can also be usedMost used property before setting is Viscosity

Effects ability to recording of fine detail of hard tissues

Degree of soft tissue compression or displacement

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Mucostatic and Mucocompressive Properties

Materials initially fluid are termed MinimallyDisplacive (Previously called Mucostatic)

More viscous materials are termed Mucocompressive

Note that some materials have pseudoplastic properties(become more fluid on mixing)

Initially fluid, all impression materials need trays for support

Thickness will also affect pressure transmitted to the tissues

Thinner sections of fluid materials can act as if mucocompressive

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Set Properties of Impression Materials

Important factors are rigidity (non-rubbery) and elasticity

Flexibility for withdrawal from hard and soft tissue undercuts

Elasticity to recover original form for accuracy

Note some materials are plastic (pastes) and some rigid

(plaster)

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Introduction to Hydrocolloids in Dentistry

Colloidal suspensions of polysaccharides in water

Behave neither as solutions nor as true

suspensions

Small electrical charges repel the colloid molecules

Sol forms (fluid) random polysaccharide chains

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Setting of Hydrocolloids in the Mouth by Cooling

Conversion from sol form to gel form with elasticity

Alignment of chains due to one of two mechanisms

Cooling allows bonding between adjacent chains

Mostly Van der Waals forces but also hydrogen bonds

These are be reversed to gel form by heating (Agar Gel)

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Irreversible Hydrocolloids (Alginates)

Supplied as powders which are mixed with water

Composition of supplied powers is varied

Some have reduced filler to give more fluidity

Some set faster due to lower trisodium phosphate content

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Rheological Properties:Study of Flow and Deformation

P = Shear StressQ = Shear Rate

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Rheological Properties:Study of Flow and Deformation

Viscosity

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Requirements of Impression Materials

A.Factors which affect the accuracy of the impression

B. Factors which affect the dimensional stability (time)

C. Manipulative variables - handling,

setting characteristics, etc

D. Additional factors such as cost, taste, colour etc

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Accuracy (One)

Low viscosity or a degree of pseudoplasticity on insertion

Ideally hydrophilic; Hydrophobic materials require dry tissues

Wettable by freshly mixed dental plaster of stone

Minimal small change of dimension on setting (contraction)

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Setting reaction of Alginate Impression Materials

Mixing and spatulating forms an aqueous alginate sol

Gypsum is only sparingly soluble (solubility about 0.2%)

Sodium in alginate readily exchanges with calcium ions

Cross-links of the alginate chains result producing a gel

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Extension of Setting Time of Alginate Material

Sodium phosphate is dissolving readily in the water

Reacts with calcium ions being released from gypsum

When all the sodium phosphate is used up setting occurs

The effect is to extend the period of low viscosity

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Accuracy (Two)

Minimal thermal contraction from 32-37oC to about 23oC

Also minimal thermal contraction of impression tray

Must remain attached to impression tray (adhesive)

Sufficient rigidity to allow removal of material

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Accuracy (Three)

Adequate elastic propertiesNot so much that removal is prevented or teeth damaged

Viscoelastic materials removed rapidly to reduce deformation

Adequate tear resistance – tensile strengthEspecially in thin section

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Dimensional stability

Best if casting into impression can be when convenient

Ideally the impression retains its original accuracy indefinitely

Setting reaction may continue and cause change

Slow elastic recovery of viscoelastic materials (? Beneficial to wait before casting?)

Loss of volatile substances either primary or products

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Dimensional Stability of Alginates

Poor due to syneresis and imbibition of water

Cast model as soon as possible

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End of

IMPRESSION MATERIALS (ONE)