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Impression Materials Chapter 14

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Impression Materials

Chapter 14

Impression Trays (cont’d)

Some trays are made of metal, others are made of acrylic or plastic.

Impression trays may cover the whole arch or just a segment of the arch.

A check-bite or triple tray is used to take a registration of the patient’s bite, that is, to show how the teeth come together.

Hydrocolloids

A colloid is a glue-like material composed of two or more substances in which one substance does not go into solution but is suspended within another substance.

Hydrocolloids are water-based colloids that function as elastic impression materials.

Hydrocolloids used in dentistry include reversible and irreversible hydrocolloids.

Irreversible HydrocolloidAlso called alginate hydrocolloid or

just alginateMost widely used impression material

in dentistryEasy to manipulateRequires no special equipmentReasonably accurate

UsesDiagnostic

modelsPartial denture

frameworksRepair of broken

prosthesesFabrication of

provisional restorations

Mouth guards Preliminary

impressions for working models

Opposing modelsCustom fluoride

traysBleaching trays

Problems

One of the reasons that alginate is not used for permanent prostheses is that it does not flow well.

Alginate does not capture the fine detail necessary for accurate appliances.

It is too thick to flow into the embrasures or to accurately show margins of preparations.

CompositionThe main active ingredient in alginate

is potassium or sodium alginate. It accounts for approximately 15% to 20% of

the powder content. It is produced from seaweed derivatives.

Other components include Calcium sulfate dihydrate Potassium sulfate Trisodium phosphate

Composition (cont’d)When alginate powder is mixed with

water, calcium sulfate dihydrate reacts with sodium alginate to form calcium alginate.

Calcium alginate is insoluble and causes the solution of mixed powder and water to gel.

Typically, the chemical reaction is rapid, but alginate manufacturers add a retardant to slow down the process.

Working TimeRegular-set alginates have a working

time of 2 to 3 minutes.Fast-set alginates have a working time

of 1½ to 2 minutes.Working time starts when the

materials—powder and liquid—are joined.

The longer one takes to mix the material, the less time one has to load and seat the tray.

Setting Time

Regular-set alginates take from 2 to 5 minutes to set completely.

Fast-set alginates take from 1 to 2 minutes to set completely.

Setting time may be adjusted by altering water temperature.

The impression is left in the mouth beyond the appearance of set to reduce tearing and distortion.

Deformation

Alginate will be compressed when it is removed from undercuts in the mouth.

The greater the compression, the more likely it is that the alginate will be permanently deformed.

The ideal thickness of the impression is 2 to 4 mm.

Thin alginate deforms and tears easily.

Removal

When removing the impression from the mouth, one should use a rapid snap movement to decrease deformation.

The impression must be kept wet until the assistant is ready to pour up the impression in dental plaster (gypsum).

Impressions also must be disinfected.

Tear

Alginate mixed with too much water will be weaker and more likely to tear.

Thin sections are also prone to tear.Slow removal from the mouth may

cause the material to tear.

Tray Selection (cont’d)

A properly selected tray will cover all of the teeth and will extend into the facial and lingual vestibules without impinging on tissues.

For a mandibular impression, the tray will extend over the retromolar area.

In the maxilla, the tray should cover the maxillary tuberosity and extend into the hamular notch.

AdhesiveEven though most trays have retention

holes in them, tray adhesive should be used to help keep the material in place.

Adhesive should cover the entire inside of the tray right up to the borders.

Be careful not to get the adhesive on you or your patient because it does not come off easily.

Do not cross-contaminate the adhesive.

Seating the TrayOnce the tray is loaded, the operator

should take some of the alginate from the bowl and wipe it across the occlusal surfaces.

Always take the lower impression first.Stand in front of the patient and insert

the tray, first one side and then the other, into the patient’s mouth.

Seating the Tray (cont’d)Place the front of the tray first, and

work to the posterior.Have the patient raise and extend the

tongue and then relax the tongue.Mold the borders by gently pulling on

the lips and cheeks in an upward motion.

Leave in place about 1 minute beyond setting.

Remove quickly.

Seating the Tray (cont’d)When seating the maxillary tray, stand

behind the patient with arms around the back of the chair.

Seat the anterior of the tray first and then the posterior.

Have the patient tilt the head forward.Muscle trim the borders by gently

pulling down on the cheeks and lips.Remove the impression quickly.

Criteria for Clinically Acceptable Impressions

All teeth and alveolar processes recorded

Peripheral roll and frenum includedNo large voids or tray burnGood detailNo debrisNo distortionPalatal vault recordedRetromolar area or tuberosity

recorded

DisinfectionDental impressions should be

considered contaminated.After the impression is removed from

the mouth, rinse with cool water to remove debris.

Place the impression in a plastic bag and then spray with a disinfectant.

Seal bag and let set for 30 minutes before pouring.