IMPRESSION MATERIALS IN ORTHODONTICS

UMAR MOHAMED

CONTENTS• Introduction• History• Ideal requirements

• Classification of impression materials

• Hydrocolloids

• Elastomers

• Causes for failure of impression

• Ora scanner

• Conclusion

• References

INTRODUCTION

Impression

• A mould of the teeth used to make models of the teeth.

HISTORY

• PHILIP PFAFFS (1756) was the first to describe taking an impression of

the jaw .

• DELABARRE (1820) INTRODUCED THE IMPRESSION TRAY

• E.C. Stanford, 1881 carried out the experimental studies on the extraction of alginate from brown seaweeds.

• 40 years later S. WILLIAM WILDING USED ALGIN AS

DENTAL IMPRESSION MATERIAL

  In orthodontics impression are primarily made to

obtain CASTS, which in turn are used for

 1.   STUDY PURPOSE

2.   FABRICATION OF APPLIANCES

3.   MAINTAINING RECORDS

  In dentistry various types of materials are used for making an

impression. NO ONE MATERIAL will be used in all the scenarios rather

based on the properties, ease of use, accuracy and economy. Various

material will be used according to the particular need of the practitioner.

IDEAL REQUISITES OF AN

IMPRESSION MATERIAL

•They should be fluid enough to adapt to the oral tissue

•They should be viscous enough to be contained in the tray that

seat in the mouth

•While in the mouth they should transform in to rubbery or rigid

solid in reasonable amount of time

•The seat impression should not distort or tear when removed from

the mouth

•They should dimensionally stable

•They should be biocompatible

•Cost effective

Phillips .science of dental materials 11th edition-Anusavice

No impression material fulfills all these requirements

And the selection of the material best suited for a

particular Clinical situation and technique rests with

Dentists.

CLASSIFICATION OF THE IMPRESSION MATERIALS BASED ON:

SETTING REACTION

THERMOPLASTIC THERMOSET

ELASTICITY ELASTIC INELASTIC OR RIGID

NATURE OF THE SETTING REACTION

REVERSIBLE IRREVERSIBLE

PRESSURE EXERTED ON THE TISSUE

MUCOCOMPRESSIVE MUCOSTATIC

DENTITION STATUS

EDENTULOUS DENTULOUS

In orthodontics…

The most commonly employed materials include

I. Hydrocolloid – Agar Agar (reversible)

- Alginate (irreversible)

II. Elastomeric impression materials.

HYDROCOLLOIDS

Derived from two Greek words ’Kolla’ meaning glue and ‘oid’

meaning like. The term was originally applied to substances that form

solutions with properties that fall between solution and suspension,

Thomas Graham[1805-1869]

They are the fourth state of matter because of their differences in

structure, constitution, and reactions .

Particles size ranging from 1 to 200 nanometers.

Colloid: Suspension of 2 phases

Colloidal materials used for making impressions are either

algin or agar dissolved in water. Hence the name

HYDROCOLLOIDS

Colloids , have two distinct phases

A dispersed phase + dispersion phase

LIQUID COLLOIDS ARE CALLED SOL

SOLID COLLOIDS ARE CALLED GEL

Sol to gel transformation

In the gel state, the dispersed phase agglomerates, forming chains or fibrils

called “MICELLES”. The fibrils may branch and intermesh to form a brush

heap structure

Water being entrapped between the fibrils

SOL GELGELATION TEMPERATURE (370 – 500C)

Fibrils are held together by secondary molecular forces

This process may also occur through a

chemical reaction the structure of the fibrils

formed is very similar but the process is very

different and is irreversible

The fibrils of the gel formed are

chemically bonded by primary forces and

therefore they are not affected by the

temperature.

SYNERISIS

The process of synerisis may be explained as

the squeezing of water from between the

polysaccharide chains. As a result of which one

may often observe a small droplets of water

on the surface of impression. Which results in

shrinkage of the impression.

IMBIBITION

In the presence of excess water the hydrocolloids

may absorb water by a process, which is opposite

of synerisis that will cause the separation of the

polysaccharide chains and thus the swelling of

the impression.

IRREVERSIBLE HYDROCOLLOID

ALGINATE (ADA no 18)

At the end of 19th century the chemist from Scotland noticed

that certain brown seaweed produced a mucous extraction that he

termed as ‘ALGIN’. Later S William wilding used this algin as dental

impression. This became the chief ingredient in our popularly used

dental ALGINATE, which is

A SODIUM SALT ANHYDRO-ß-D-MANURONIC ACID/ ALGINIC ACID

Structural formula of alginic acid with molecular wt of 32000-200000

The factors causing its successful use as an

impression material include:

• Ease of manipulation

• Comfortable to the patients

• Relatively inexpensive not requiring elaborate

armamentarium.

TYPES

TYPE I – RAPID SETTING

TYPE II – NORMAL SETTING

COMPOSITIONNO COMPOSITION FUNCTION %

I. Salt of Alginic acid Dissolves in water & reacts with calcium ions

15%

II. Calcium sulphate Reacts with pottasium alginate to form calcium alginate

16%

III. sodium phosphate Reacts in preference with calcium sulphate - retarder

2%

IV. Pottasium Titanium fluoride

Gypsum hardener and accelarator

3%

V. Zinc oxide Filler 4%

VI. Diatomaceous earth Filler 60%

VII. Flavouring & coloring agents

Makes the material more palatable

traces

Dental materials 8th edition Robert G Craig

SETTING REACTION.

Soluble alginate reacts with the calcium sulphate resulting in the

formation of an insoluble calcium alginate.

This process occurs quite rapidly and thus would not allow sufficient

working time, so to compensate for this problem a water soluble salt namely

Tri-sodium phosphate is added to prolong the working time.

The basic idea behind adding this salt is to allow calcium sulphate to

initially react with tri-sodium phosphate in preference to the soluble alginate

K2nAlg + CaSO4 nK2SO4 + CanAlg

Properties

Working time-for fast setting materials-1.25-2 min (mixing time-

45 sec) for regular set-3-4.5 min (m.t.-60 sec)

Setting time-1-4.5min(regular) 1-2min(fast set)

Permanent deformation- The A D A specification requires that

the recovery from deformation be more than 95% when

material is compressed 20% for 5sec

Flexibility A D A specification permits a range of 5% to 20% at a stressof 1000 gms/cm2 and most alginate have a typical value of 14%.

Strength

The compressive strengths range from 5000 – 9000 gms/cm². The Tear strength vary from 380- 700 gms/cm.

DIMENSIONAL STABILITY

For maximum accuracy the model material must should be poured into alginate impression as soon as possible.

If it is delayed , the impression should be stored in 100% relative humidity in a plastic bag or wrapped in a damp paper towel

It is satisfactory for some materials for periods up to 2 hours

BIOCOMPATIBILITY

Inhaling fine airborne particles from alginate impression materials Can cause SILICOSIS and PULMONARY HYPERSENSITIVITY

Crosslinking of Na alginate molecules to form Ca alginate

STEPS IN MAKING IMPRESSION 

• Selection of the tray

• Preparation of the patient

• Seating of the tray

• Disinfection of the impression 

• Practice the placement of the empty tray into the patient’s mouth

• Assume the correct operator positioning

Maxillary impression - 11 o’ clock

Mandibular impression - 7 o’ clock

MANIPULATION

The recommended water powder ratio should be used. In general it

is 38 ml of water per 16 gms of powder but slight variation can be

present depending on the manufacturer. The weighed power is

incorporated into the water by carefully mixing with the spatula.

Care should be taken to ensure that air is not incorporated into the

mix by spatulating the mix against the side of the bowl while using

the vigorous figure of 8 motion till a smooth creamy homogeneous

mix is obtained.

IMPRESSION MAKING

Manipulation

Maxillary tuberosity

Palate

Rolled round & thick borders

Maxillary impression

IMPRESSION MAKING

Mandibular retromolar area

Detailed along gingival anatomyPosterior lingual areas

Rolled round & thick borders

Mandibular impression

IMPRESSION MAKING

SEQUENCES OF IMPRESSION MAKING

It is advisable to make mandibular

impression first as there is less chance of

the patient to gag .This allows the patient to

become accustomed to the taste and the

feeling of the material in the mouth which in

turn reduces the chances of gagging while

making an impression of the maxillary arch.

DISINFECTING THE IMPRESSION

Disinfection of impression is a concern due to viral diseases such as Hepatitis B,A I D S and herpes simplex

• Gently rinse the impression under the tap water to remove any

debris that may be retained in the impression.

• May immerse the impression in a disinfectant such as :

gluteraldehyde solution

iodophors

1:10 dilution of sodium hypochlorite

synthetic phenols

for less than <10 min

CRITERIA FOR ACCESSING THE

IMPRESSION

• Lack of voids.

• No distortion , clear and distinct impression

• Adequately extended.

• Free of debris and extraneous material.

• Adequate details and all structure must be recorded.

• Stable material which is sufficiently attached to the tray.

MODIFIED ALGINATE• Dust free alginate – GLYCOL COATED particles

•A new system in which the SOL contains water but not calcium ion

so that the second component added is actually Plaster of Paris.

• A two component paste system

(I) Alginate Sol

(II) calcium reactor

• Chromatic alginate actually changes the color once set.

• Alginates modified with silicon/polymers for fine detail but these

have the disadvantage of having a very poor dimensional stability.

CAUSES OF FAILURE

1. Grainy material •Improper mixing •Prolong mixing •Undue gelation•Water : powder ratio too low

2. Tearing •Inadequate bulk•Moisture contamination•Premature removal from mouth•Prolong mixing

3. External bubbles •Undue gelation, preventing flow•Air incorporated during mixing

4. Irregularly shaped voids •Moisture or debris on tissue

5. Rough and chalky stone model

•Inadequate cleansing of impression•Excess water or potassium sulfate solution left in impression•Premature removal of model

6. Distortion •Impression not poured immediately •Movement of tray during gelation•Premature removal from mouth

REVERSIBILE HYDROCOLLOIDS

AGAR-AGAR

This is a organic , hydrophilic colloid

(polysaccharide) extracted from certain type of sea

weed. It is a sulfuric ester of a linear polymer of

galactose

The temperature at which the

LIQUIFACATION TEMPERATURE (700 – 1000C)

GEL SOL GELATION TEMPERATURE (370 – 500C)

INGREDIENTS FUNCTION COMPOSITION %1. AGAR BASIC CONSTITUENT 13-17

2. BORATES STRENGTH 0.2-0.5

3. SULPHATES GYPSUM HARDNER 1.0-2.0

4. WAX FILLER 0.5-1.0

5. THIXOTROPHIC MATERIAL

THICKNERS 0.3-0.5

6. WATER REACTION MEDIUM BALANCE

7. ALKYL BENZOATE PRESERVATIVE 0.1%

THE MATERIAL

TYPES AVILABLE

The hydrocolloid is supplied in two forms:-

The more fluid “cartridge form” available for use in syringes.

the more viscous type used in trays

Special water cooled trays are used to rapidly cool the

material in the patients mouth

AGAR

At 1000C – rapidly converts Gel to Sol and the contents of the

tube become very fluid.

At 650c- transfer the material to second bath, this temperature

will maintain the material in its SOL form leave the material in this

temperature until the material is required for use

At 450c a few minutes prior to the use the material is cooled to

this temperature before placing in the patients mouth.

CONDITIONING UNIT

The cooling of the material in the mouth to promote

rapid gel formation may be achieved by circulating cold water on

the tray or by using special water cooling impression trays. These

trays are stock trays with a narrow bore metal tube attached to the

outer surface, the tube is connected to the cold water supply and

circulatory water reduces the temperature of the tray.

WATER COOLED TRAYS

DUPLICATING MATERIAL

Alginate and Agar –Agar may be used successfully as a

duplicating material. In this way multiple casts can be duplicated from a

single master cast, which may have been formed using some other

material. This process is essential in ones orthodontic practice as it

enables the use of one master cast to form several casts subsequently.

NON AQUEOUS ELASTOMERIC

IMPRESSION MATERIAL (ADA no 19)

These materials are classified as

synthetic rubbers, developed to mimic natural rubber

when it became difficult to obtain during WORLD WAR

II .They are liquid polymers and can be converted to rubber at

room temperature by mixing with a suitable catalyst they

undergo

polymerization and or cross linking by condensation/addition reactions to produce a firm elastic solid.

CLASSIFICATION

• POLYSULFIDES

• SILICON POLYSILOXANES

CONDENSATION POLYSILICON

ADDITION POLYSILICON

(POLYVINYL SILOXANE)

• POLYETHER

THEY ARE SUPPLIED IN FOUR VISCOSITY

• LIGHT BODY

• MEDIUM /REGULAR)

• HEAVY BODY

• PUTTY (VERY HIGH)

HIGH, MEDIUM AND LOW CONSISTENCIES ARE SUPPLIED IN

TWO FORMS

• BASE

• ACCELARATOR / CATALYST

CATALYST SOME TIMES AS LIQUID

PASTES ARE SUPPLIED IN COLLAPSABLE TUBE.

Condensation silicones

Supplied in a low and putty type consistency. The base contains poly dimethyl siloxane which have reactive terminal hydroxyl groups.

The accelerator maybe liquid that consists of stannous octoate suspension and alkyl silicatesor it may be supplied as paste by addition of thickening agent

The reaction proceeds producing 3 dimensional network rubber withethyl alcohol as a byproduct Stock trays are generally used

Addition silicones It is available in low,medium,heavy and putty consistencies and is also Poly siloxane. No by product.Pinpoint voids

Base contains low molecular wt polymer with silane group and filler. The accelerator contains a moderately low molecular wt polymer with vinyl terminal groups and filler

Polyether

Supplied as base and catalyst systemBase- low molecular weight polyether

Polyether + sulfonic ester- cross-linked rubber

Base paste- polyether polymer, colloidal silica as filler and plasticizer such glycolether or pthalate

Accelator paste-alkyl aromatic sulfonate, filler and plasticizer

Better mechanical propertiesLess dimensional changesGood wetting properties

limitationsShort working timeHigh stiffness

Biocompatibility

Comparing the cell cytotoxicity for different impression materials reveals that polysulfide results in the lowest cell count and set polyether impression material produces the highest cell cytotoxicity

Foreign body of impression material can cause severe gingival inflammation if it is left in a gingival sulcus

Contact dermatitis from polyether especially to the dental assistant has been reported due to polyether catalyst

MANIPULATION

Equal lengths of two pastes are dispensed on a paper

pad for normal mixes. Initial mixing is accomplished with

circular motion and final mixing to produce a mix free from

streaks is done with broad stroke of spatula.

Mixing is readily accomplished in 45 seconds.

When catalyst is supplied in liquid form a specific number of

drops per unit length is indicated in the instructions and

mixing is accomplished in the manner similar to the two

paste system

The putty system use scoops supplied by the manufactures

for dispensing and may be mixed with heavy spatula or kneaded

with hand until free from streaks.

FOR MAKING DIGITAL ORTHODONTIC STUDY MODELS

The impression can be taken in a high quality ALGINATE orPOLYVINYL SILICONE or POLYETHER material

High quality alginate such as ORTHOPRINT taken in a rigid plastic impression tray The alginate impressions are sanitized, wrapped in a damp paper towel and Packaged in seal able plastic bags This ensures that impression will remain stable for 5 daysIf the impression will not be delivered by the 5 days then we considerUsing a polyether such as IMPREGUM

Orthodontic Products update :Impression materials by B.Doubleday British journal of orthodontics Vol 25 No2 may 1998

Impression of dentate archesAlginate No benefit using putty or heavy body silicon Xantalgin(bayer) Ideal(GAC) Blend a print

Impression for neonates and cleft patientsCatridge delivery systems of silicon materials perfexil Elite H-D

Bite registrationDedicated bite registration materials

ORASCANER

It is a revolutionary new process. The orthodontist uses 3-D

images taken by the OraScanner™ to build a computer model of the

teeth. The OraScanner functions much like a video camera, taking

pictures of the teeth and sending them to the computer to build a

complete 3-D model

CONCLUSION

One must not depend on a material to provide good impressions. Any material is as good as the dentist using it and any material must be used for sometime before the operator is familiar with its characteristics Selecting the impression material depends on the convenience of the dentist

ALL NEED IS AN ACCURATE IMPRESSION.

REFERENCES

I Anusavice Philips science of dental materials XI Edition

2 Restorative dental materials X EDITION By Robert G. Craig 3 Contemporary Orthodontics III Edition By William R. Proffit

4 Removable appliance Fabrication By Emil Witt, Dr Med. Dent

5 Hand book of Orthodontics IV Edition By Robert E. Moyers

6 Cleft lip and palate Persectives in Management Vol II By SAMUEL BERKOVITZ

7 Orthodontics for dental students By T.C. White and J.H. Gardiner

8 Journal of Orthodontics Vol 31 Nov 4 Dec 2004 ISSN 1465-3125 (AJODO)