Impression MaterialsO B J . 5 A , B , C

C H A P T E R 1 5

Z C , B S C . R D H

Impression Material

Used primarily to reproduce the form of the teeth in a negative reproduction◦ This includes existing restorations, hard and soft tissues,

and preparations

How to take Alginate Impressions

http://www.youtube.com/watch?v=ZfMt4Bq-Yjc

8 Keys to take Alginate Impressions

http://www.youtube.com/watch?v=M5L_o98ZtZk

Types of ImpressionsDental impressions can be categorized into three basic types based on how they will be used:

1. Preliminary impressions

2. Final impressions

3. Bite registration (occlusal) impressions

Preliminary impressions are made as precursor to other treatment.

Final impressions are more accurate in their replication of the oral structures. They are used to make final restorations, such as crowns, bridges, partials, and complete dentures.

Bite registration is a replication of the patient’s bite and is used to establish the proper relation between a restoration or prosthesis and the opposing teeth.

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Types of Impression Materials Two major groups:◦ Elastic materials

◦ Inelastic materials

Elastic impression materials include the hydrocolloids (agar and alginate), polysulfides, silicone rubber materials, polyethers, and a hybrid of polyether and polyvinyl siloxane.

Inelastic materials are the older impression materials and include dental compound, impression plaster, zinc oxide eugenol, and impression wax.

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Key PropertiesAccuracy:When the impression is made, the impression material must closely adapt and flow over the surface of the tooth preparation and tissues to record the minute details in order to be accurate.

Tear resistance:

After the impression material sets, it must have good tear resistance to prevent tearing during removal from the mouth.

Dimensional stability:After the impression is removed, the set material must remain dimensionally stable.

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Classification of Impression MaterialElastic Impression Materials -Hydrocolloids

1. Agar Hydrocolloid also known as reversible hydrocolloid

2. Alginate Hydrocolloid or irreversible hydrocolloid.

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

-It has two phases, a liquid phase called sol and a semisolid phase called gel.

-Hydrocolloids are a water based colloid used as an elastic impression material

Common Uses Study models (Diagnostic Casts)

Preliminary impressions for dentures

Partial denture frameworks

Opposing casts for crown and bridge treatments

Provisional restorations

Custom trays for fluoride or bleaching

Sports protectors and or night guards

Removable orthodontic appliances

Irreversible HydrocolloidAlso known as alginate hydrocolloid, most widely used impression material.

It is inexpensive, easy to manipulate and requires no special equipment.

When the alginate powder is mixed with water, calcium sulfate dihydrate reacts with the sodium alginate to form calcium alginate which is insoluble and causes the water and the sol to gel due to this chemical reaction the mixture cannot reverse back to the sol state.

Working/Setting Time There are variations in material: regular set and fast set.

The ingredient that controls this is trisodium phosphate, this is added to slow down the chemical reaction and the amount added determines the difference between regular and fast set alginates.

Regular set 2 to 3 minutes for working time

Fast set is 1.25 to 2 minutes for working time

Setting time is the time it takes to completely set in the oral cavity and dependent on the temp of the water used, regular set is usually 2 to 5 minutes, fast set is 1 to 2 minutes.

Question to considerClient in the chair requires impressions taken and has moderate sensitivity throughout to cold temperature. How do you manage this situation?

For patients with sensitive teeth, alginate mixed with cool water can be painful. Use regular-set alginate with warm water. The working and setting times will be shortened, but the patient will be more comfortable.

Impression Trays Used to carry the impression material to the mouth and support it until it sets, is removed from the mouth and is poured in dental plaster or stone.

Trays come in a variety of sizes ( pre- manufactured) they should be rigid to prevent distortion of impression

Trays can be made for arches with or without teeth (edentulous)

Trays can be metal, plastic and solid (rim-lock trays) or perforated for better retention. Solid trays may require an adhesive for retention.

Metal trays are cleaned and sterilized and plastic trays are discarded.

Impression Trays

•Stock trays •Custom trays•Bite registration trays

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Tray selection

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

It will extend posteriorly to include the retromolar area for a mandibular tray and the hamular notch area for a maxillary tray.

It will be deep enough to provide at least 2 mm of space for alginate beyond the incisal and occlusal surfaces of the teeth.

Tray selectionIt will extend posteriorly to include the retromolar area for a mandibular tray

Utility wax may be added to extend

Criteria for an acceptable alginate impression

Troubleshooting Alginate Impressions

Troubleshooting Alginate Impressions

Irreversible Hydrocolloid (Alginate) (Cont.)

Permanent deformation◦ Compression: 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 to some degree.

◦ Ideal thickness: 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 ready to pour up with dental plaster (gypsum).

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Irreversible Hydrocolloid (Alginate) (Cont.)

Dimensional stability◦ Moisture – Very sensitive to moisture loss and will shrink as a result.

The impression must be kept wet (100% humidity).

Once removed from the mouth, the impression should be rinsed and disinfected.

Wrapped in a damp paper towel and sealed in a zippered bag. Alginate can imbibe water and swell.

If the impression loses too much moisture, it will shrink and become distorted.

Some moisture will be lost from the impression even in 100% humidity from syneresis.

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Irreversible Hydrocolloid (Alginate) (Cont.)

Tear strength◦ Thin mixes

◦ Slow removal

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.

Alginate ImpressionsMaking alginate impressions◦ Objective: to reproduce the oral structures with acceptable accuracy

while practicing good infection control and maintaining patient comfort

◦ Tray selection◦ Dispensing◦ Mixing – One unit of water is required per scoop◦ Loading the tray◦ Seating the tray◦ Removing the tray◦ Handling the impression

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Tear StrengthAlginate mixed with too much water will be weaker and more likely to tear on removal from the mouth

Thin sections of alginate are also prone to tearing

In addition, slow removal from the mouth can contribute to tearing.

Disinfecting Impressions

Impressions are considered contaminated due to saliva and blood

After the impressions are removed from the mouth, it should be rinsed with water to have the saliva, blood and debris. Excess water should be removed. Evaluate immediately.

Spraying with disinfectants has two disadvantages:◦ 1. airborne particles may be inhaled by the staff or patients

◦ 2. not effective as it may not reach all surfaces

Immersion for up to 30 minutes in 1% sodium hypochlorite or 2% glutaraldehyde has been shown not to significantly affect the dimensions or surface detail of alginate

Disinfecting at school? - In the clinic at school the impressions are rinsed and then placed in a solution of bleach and water.

Disinfecting CastsCasts may need to be disinfected if impression cannot be disinfected before pouring.

Casts may be sprayed with disinfectant but not immersed.

Sterilizing Impression TraysAll plastic trays should be disposed of as recommended

All chrome plated, stainless steel trays can be sterilized by heat, steam or dry heat.

Handling impressionsLab knife is used to remove excess, unsupported alginate from the back of the tray.

Pooled liquid is drained.

Impressions are placed into a zippered bag in a damp paper towel. Marked with patient’s name until ready to pour.

Alginate impressions should be poured within 30 minutes to 1 hour

Impressions – continuedCustom TraysThese are fabricated in a laboratory with chemical-cured or light-cured or thermoplastic resins on casts of teeth.

They may be made as a full arch or partial arch/quadrant tray

Triple Trays

A sectional tray or full arched trays used to make an impression of teeth being treated and opposing teeth at the same time, it allows for accuracy in the occlusion

http://www.youtube.com/watch?v=Wao_dLy8zrc

Bite registration trays have a thin piece of mesh and material is placed on both sides. Client closes into normal bite until the material sets.

Elastomers

Highly accurate elastic impression materials that have qualities similar to rubber and are often referred to as rubber materials

Used extensively in restorative dentistry for construction of castings, indirect esthetic restorations, bridges, implant restorations, partial dentures and complete dentures.

Four types of elastomers:

Polysulfides, polyethers, condensation silicones, and addition silicones (PVS).

Most widely used – PVS and polyether

Four Common Types of Elastomers - Polysulfides

Polysulfides – oldest type of elastomer, referred to as rubber base, more accurate than alginates but not as accurate as other elastomers. It is comes in two pastes a white and brown, base and catalyst dispensed equal amounts and mix together.

Commonly used for crown and bridge, requires a custom tray with adhesive, comes in a light body, regular and heavy viscosities, the light body is applied directly onto the prep area to obtain accuracy and the heavy body is placed in a tray and pushed over the light body material.

Set time is about 8 to 14 minutes and is set time is sensitive to moisture, temperature and correct ratios of material mixed. (unpleasant taste and rotten egg odour)

Four Common Types of Elastomers -PolyethersPolyethers- rubber impression that is very accurate with good flow and tear strength and excellent for use in crown and bridge. Comes in light, medium and heavy body viscosities

Supplied in two tubes and involves mixing equal parts

It is the stiffest of the rubber impression materials and can be difficult to remove once set in the oral cavity.

Short working time of 3 – 5 minutes and sensitive to moisture and temperature

Four Common Types of Elastomers - Condensation Silicones

Condensation Silicones- alternative to polysulfides, easy to mix, pleasant taste, odorless, shorter working and setting time

Two tubes, a base and catalyst, light, medium and heavy body viscosities, sets in 5 – 7 minutes

The dimensional change within the first 24 hours is greater and therefore needs to be poured ASAP for accuracy.

Has been used in crown and bridge but since been replaced by addition silicone

Four Common Types of Elastomers -Addition Silicone

Addition Silicone- also known as polyvinyl siloxane (PVS), improvement over the condensation silicone as it provides more dimensional stability and accuracy

Clean and easy to use, no bad taste or odour, the most popular material used for crown and bridge, most costly, manufactured in light, medium and heavy body viscosities, dispensed in a cartridge with two chambers.

Sets in 3 – 7 minutes, must be used in a dry field, can be used as a bite registration, material can be directly applied to the teeth or used in a disposable plastic tray with the mesh.

Dimensionally stable for at least a week.

http://www.youtube.com/watch?v=ZJYrC6MpQKMHow to take one step PVS impressions:http://www.youtube.com/watch?v=MgdHWSUb2UAhttps://www.youtube.com/watch?v=DrUIcoKgReE

Inelastic Impression MaterialsDental Impression Compound- rigid thermoplastic material, softens when heated and becomes firm again at mouth temperatures, commonly used for complete dentures

Impression Plaster-seldom used but if used only for denture impressions

Zinc Oxide Eugenol Impression Material- not commonly used again for edentulous clients, two tube system, unpleasant to the taste

Impression Wax Used for bite registration

Stiff at room temperature and moldable when heated

Lacks accuracy for final impressions and distorts easily

Used in the early years of dentistry

Digital Impressions - Scanning devicesThe digital impression removes many of the requirements needed for traditional impressions. Complete and stand-alone intraoral scanners are available.

A big advantage is the ability of the clinician to view the preparation magnified on a computer monitor and see it from multiple angles by rotating the image.

A major disadvantage to the digital impressions is the significant cost involved in purchasing a digital scanner and some training and practice that is needed to efficiently operate it.

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Review and KNOW!

Mandatory Homework – to be posted

Taking alginate impression

Must Read:

Procedure 15 – 1

Pg. 269 – 272