Thermoplastic impression materials

 

Silicone impression materials. Thermoplastic impression materials.

   Impression is a negative image of the hard and soft tissues of the prosthetic bed and its borders obtained by using special materials.

Plastic impression materials.Before considering these materials, we shall define concept “plasticity” and “elasticity”. Plasticity is a property of the material to keep the changed form after cessasion of external deforming force. An example of plasticity of the substance can be waxes, plasticine, dough, clay. Elasticity is a property of the material to take the initial form under the influence of rather small force from the outside. Rubber, gutta-percha and other similar materials are related to elastic substances.

      Plastic impression materials are divided into two subgroups: impression thermoplastic materials and impression plastic compositions. Thermoplastic materials should meet the following requirements:

1) to be softened at the temperature which does not cause burn of the mucous membrane of the oral cavity (55°);

2) to possess necessary plasticity at the set temperature;

3) not to be sticky at the set temperature;

4) to harden at the temperature a little exceeding temperature of the oral cavity;

5) to be easy to be processed by instruments, not to be stratified;

6) not to change the size in hardening and storage after removal from the oral cavity;

7) to be easy to be separated from the model.

Thermoplastic materials.Thermoplastic materials soften and harden under the effect of temperature. They are divided into reversible and irreversible. The former keep the plastic properties in repeated heating and cooling during taking of impressions, the latter gradually lose plasticity. Weinstein’s masses are related to reversible materials, stens – to irreversible.

Their composition includes paraffin, stearin, gutta-percha, beeswax, ceresin and other materials. A mixture having a temperature of softening of 50-70° is received by various combinations of components.

Except for resins and waxes, being glue substances, their composition includes fillers:chalk, talc, zinc oxide, pumice, etc. Dyes, correcting taste substances are also introduced in them.

Of domestic materials of this group dentafol is best known, it is applied to constructed dentures with the expanded borders for edentulous jaws, Weinstein’s masses N 1, 2, 3. Masses N1 and N2 are applied in taking the impressions of the edentulous jaws, the mass N3 – for taking the impressions by means of a ring in prosthesis with inlays, semicrowns. Of foreign materials of this group adhesial (Austria), mycodin (Germany) are most known which represent complex compositions of resins and waxes.

Plastic impression materials.Plastic impression materials possess high plasticity at the temperature of 18-20°. These materials do not harden in the oral cavity and reflect functional features of the mobile and motionless mucous membrane of the orthopedic bed well. Plastic impression materials are recommended to be applied: for specification of denture borders of the maxilla and mandible in absence of the teeth and taking functional-sticking impressions in significant atrophy of the alveolar processes, for correction of the denture (relocation) by a laboratory method.

Orthocor is a representative of this group of materials, produced in the form of plates, isolated with a polyethylene film on either side.Orthocor put on the denture surface can be left in the oral cavity of the patient for some hours. For this period the impression is formed with functional edges.

     Thermoplastic materialsare forgotten too. Some masses are heated up in special small pot, and then are put on the tray by a brush. To receive an ideal impression with such mass is impossible. There was such mass referred to as “Stens-03”. This mass looked unappetizingly, like a piece of sealing wax by all parameters.

     Thermoplastic materialsof Stens type have noadvantages, but cheapness and simplicity in use. The piece of the mass is put in hot water, after a while it softens, is put on the tray and introduced into the oral cavity at once. Earlier it was written that this mass can be used repeatedly and it is one of advantages. But sterilization of such mass by heat is impossible, and unreal at low temperature.

There are a lot of drawbacks:

1.  Insufficient accuracy of the impression.

2.  Bad retaining of the form at temperature drop.

3.  Impossibility to remove an impression from the mouth if the temperature of hardening is passed.

4.  Impossibility of sterilization.   

     A variety of conditions encountered in the clinic for prosthesis including not only a condition of organs of the oral cavity, the chosen construction of the denture, but also general somatic status of the patient, his age, presence of allergy and ability to adaptation. It dictates necessity of individual approach to a choice of the material and a method of taking the impression. There is no uniform approach acceptable to all clinical situations. The way and material for taking the impression should be chosen according to a concrete clinical situation.

Now the industry of all countries manufactures impression masses of various chemical composition and assortment. Each of them has positive and negative properties. In each concrete case the dentist chooses such impression material which application will cause the patient minimum inconveniences and will allow to take a qualitative impression of the orthopedic bed tissues. Dental technicians should know well properties ofimpression materials with which they should work in the laboratory.

I.M.Oksman’s classification of impression materials is widely used in our country:

Impression materials:

1. Crystallizing (plaster and zinc oxide eugenol)

2. Thermoplastic

3. Elastic (agarinic)

4. Polymerizing (silicon and others)

Now impression materials on the basis of silicon organic polymers –silicon rubbers are more widely used in dental practice.

Introduction to Elastic Impression Materials

These materials can be stretched and bent to a fairly large degree without suffering any deformation. These are used for recording the patient’s mouth where undercuts are present. Usually used for partial dentures, over dentures, implants and crown and bridge work .The elastic impression materials are:

Introduction to Elastomers

These are used where a high degree of accuracy is needed, especially in crown and bridge work. They have two main advantages over the Hydrocolloids – good tear resistance and dimensional stability.They are mainly hydrophobic rubber based materials. All of these materials come in different viscosity’s ranging from low to high viscosity. The light bodied material maybe used as a wash impression over a medium or heavy-bodied material. There are two ways this can be carried out as described below.

ONE STAGE IMPRESSION

Light bodied impression material is placed in a syringe, and placed over the areas where high detail is required (e.g. over a crown preparation). Some is then squirted over the heavy-bodied impression material which has been loaded into an impression tray. The impression is then taken as normal. This technique saves time, but it can be very labour intensive because the two need to mixed at the same time often requiring more than one DSA.

TWO STAGE IMPRESSION

An impression is taken with the heavy-bodied material. This is then removed from the mouth and inspected. The light bodied material is then prepared and again placed in a syringe. This is then squirted over heavy-bodied material and then impression relocated in its original impression.

Polysulphides

Silicones

Polyethers

Polysulphides

CLINICALLY

Used for crown and bridge work mainly, but also used for partial dentures, overdentures and implants. Two equal lengths are mixed together with a spatula for about a minute. The tray needs to be treated with an adhesive (rubber solution in acetone) to provide retention for the polysulphide. Taking the impression is delayed by 5 minutes before the impression is placed in the patients mouth – the final setting time is usually about 10 minutes from the start of mixing – this delay therefore decreases the amount of time the impression tray is in the patients mouth. A one or two stage impression technique may be used. Although dimensionally stable, the impression should be cast within 24 hours.

CHEMISTRY

Other names : Thiokol rubbers, rubber base or mercaptan.
Supplied as two pastes mixed in a 1:1 ratio.

BASE PASTE

 Polysulphide (forms rubber on polymerisation)

 Filler (to give body)

 Plasticiser (control viscosity)

ACTIVATOR PASTE

 Inert oil (forms a paste)

 Sulphur (facilitates the reaction)

 Lead oxide (causes polymerisation and cross-linking)

The active constituent in the base paste is the polysulphide and the active constituents in the activator paste is lead oxide and sulphur which cause further polymerisation of the polysulphide.
On mixing crosslinking and chain lengthening causes the material to become an elastic solid after about 5-8 minutes. Setting is more rapid in the presence of moisture.
They come in three types: light, regular and heavy bodied (viscosity increasing from light through to heavy bodied). The light bodied polysulphides are used as wash impressions on heavier bodied impression materials. The medium and heavy-bodied impression materials may be used on their own.

PROPERTIES

• Dimensional stability

• Excellent surface detail (is only used in special trays)

• Viscosity depends on the brand used

• Very small setting contraction (0.3-0.4% over the first 24 hrs)

• Contraction on cooling from mouth to room temperature

• Very good tear resistance

• Good shelf life

• Viscoelastic

ADVANTAGES

1.     Dimensional stability

2.     Accuracy

3.     Comes in a number of different viscosity’s

4.     Long working time (although this may be a disadvantage in some clinical situations)

5.     Long shelf life

DISADVANTAGES

1.     Lead oxide in base paste may have toxic effects

2.     Staining of clothes due to the Lead oxide

3.     Messy to work with – unpleasant rubbery smell

4.     Can only be used in a special traY

Silicones

The silicone impression materials are classified according to the type of chemical reaction by which they set.

Addition

Condensation

Addition silicones

Can be used as a one or two stage technique. May be used in special or stock trays. The very heavy bodied materials are measured in scoops and are mixed by hand until homogeneous in colour.

 

 

Properties of Addition Silicones

CHEMISTRY

These materials are often termed vinyl polysiloxanes.
Supplied in 2 pastes or in a gun and cartridge form as light, medium, heavy and very heavy bodied.
One paste contains a polydimethylsiloxane polymer in which some methyl groups are replaced by hydrogen. The other paste contains a pre-polymer in which some methyl groups are replaced by vinyl groups, this paste also contains a Chloroplatinic acid catalyst.
On mixing, in equal proportions, crosslinking occurs to form a silicone rubber. Setting occurs in about 6-8 minutes.

PROPERTIES

• Good shelf life

• Dimensionally stable

• Moderate tear strength

• Excellent surface detail

• No gas evolution

• Non toxic and non irritant

ADVANTAGES

1.     Accurate

2.     Ease of use

3.     Fast setting

4.     Wide range of viscosity’s

DISADVANTAGES

1.     Hard to mix

2.     Sometimes difficult to remove the impression from the mouth

3.     Too accurate in some circumstances (cast produced is not sufficiently oversized)

Condensation Silicones

CLINICALLY

Used for crown and bridge work mainly, but also for partial dentures, implants and overdentures. Used in stock trays or special trays. One or two stage impression stage. Although dimensionally stable the impression should be cast within 24 hours.

CHEMISTRY

Supplied as a paste and liquid or two pastes, in light, medium, heavy or very heavy bodied (putty).

BASE PASTE

• Silicone polymer with terminal hydroxy groups

• Filler

CATALYST PASTE

• Crosslinking agent (organohydrogen siloxane)

• Activator (dibutyl-tin dilaurate)

On mixing the two pastes react, cross linking occurs and setting takes about 7 minutes.
The setting reaction is a condensation reaction.
Hydrogen gas is evolved on setting which leads to surface pitting, and a roughened surface to the resulting model.

PROPERTIES

• Hydrophobic

• Hydrogen gas evolution on setting

• Moderate shelf life

• Moderate tear strength

• Good surface detail

• Shrinking of impression over time

• Non toxic and non irritant

• Very elastic (near ideal)

ADVANTAGES

1.     Accurate

2.     Ease of use

3.     Can be used on severe undercuts

DISADVANTAGES

1.     Hydrogen evolution

2.     Liquid component of paste/liquid system may cause irritation

Polyethers

Used for crown and bridge work, partial dentures, implants and overdentures. Mixed in a 1:1 ratio until homogeneous colour, the amount of catalyst used can be used to control the setting time. Used in special or stock trays with an adhesive. A one or two stage technique can be used. Although dimensionally stable the impression should be cast within 24 hours.

Properties of Polyethers

CHEMISTRY

Based on imine chemistry
Supplied in two pastes

BASE PASTE

 Polyether

 Filler

CATALYST PASTE

 Sulphonic acid ester (enhances further polymerisation and crosslinking)

 Inert oils

When mixed the polymer and sulphonic acid ester react to form a stiff polether rubber.
Setting time occurs in about 6 minutes.
Usually only comes in one viscosity – regular bodied, but can also come as light + heavy bodied (Diulent)
Heat and moisture speed up the setting reaction.

PROPERTIES

• Hydrophillic (ie absorbs water)

• Good shelf life of up to 2 years

• Good elastic recovery

• Non toxic

• Low setting contraction

• Low tear strength

• Excellent surface detail

• Good dimensional stability

ADVANTAGES

1.     Accuracy

2.     Good on undercuts

3.     Ease of use

DISADVANTAGES

1.     May cause allergic reaction due to the sulphonic acid ester

2.     Poor tear strength

3.     Rapid setting time (ie short working time)

4.     Stiff set material (sometimes hard to remove from mouth)

 

Advantages ofsilicon impression masses:

1. Very high accuracy in reproduction of a relief of the orthopedic bed tissues;

2. Low shrinkage;

3. High mechanical durability;

4. Elasticity;

5. Stability to deformations;

6. A possibility of choosing a degree of viscosity (consistence) of the material;

7. Simplicity of disinfection;

8. Good adhesion to the tray.

Drawbacks:

1. High cost;

2. A possibility of toxic effect (C-silicons);

3. High sensitivity of A- siliconcatalysts to external factors.

Silicon materials became one of the first polymeric impression materials. They appeared in the 50s years after discovery of silicon mixtures of cold polymerization. By its nature they are silicon organic polymers. Today fillers – fine-dyspersated oxides of metals (ZnO, MgO), white soot, diatol, silica are added in the structure of materials for giving them necessary properties. The size of the filler particles do not exceed 5-10 microns. All mineral fillers strengthen the structure of silicon impression materials considerably, increase their durability and reduce shrinkage. Various combinations of dyes, aromatizers as well assofteners (plasticizers) are applied.

Viscosity of the material is determined by a percentage share of the filler and a long chain of polymer. At present silicon materials of a various degree of viscosity are manufactured by the industry:

– Dough-like consistence (for a primary impression);

– Viscous consistence (for individual trays);

– Liquid consistence (correcting mass);

– Fluid consistences (correcting mass).

The amount of the filler in the materials from I to IV group decreases from 70 up to 35 %.

Process of vulcanization of various silicon impression materials proceeds by one of two reactions: polycondensation or polyconnection. On this basis silicon impression materials are divided into two groups:

–C-silicons (polycondensation);

–A-silicons(polyaddition).

The belonging of the material to this or that group is necessarily indicated on the packing by phrases: condensation tуpe in case of C- silicons or addition type in case of A- silicons. Besides, materials of these groups differ in the form of  manufacture that will be described below.

C-silicons

Materials which basic structure consists of molecular chains of groups Si – Metal – O (silicons). Both end free valencies of the molecules are saturated with groups of OH (the chemical name – polydimethylzilanol). The hardener consists of the organic compound of tin and orthoethyl silicate. Under the effect of vulcanizing agents of the activators and catalysts linear polymers “are crossed”, forming cross-linked polymer. As a result the mass is structured and gets necessary elastic properties.

Polycondensation is a reaction of synthesis of polymer in which there is a chemical interaction when collateral low-molecular substances are formed (ammonia, alcohol, water) besides polymers. This reaction is the basis of hardening of C- siliconsand polysulphidic materials. The hardener is added to the basic mass. Thus the ready mass and residual substances (gas, alcohol, water) are formed, i.e. dimensional stability is short-lived.

Structurization of the material occurs due to cross-link by the end hydroxyl groups by means of hardeners in presence of vulcanizing agents. During vulcanization there is a condensation of molecules of alcohol (hence the name polycondensation), which then evaporate. Thus shrinkage of the material develops progressing in time.

Condensed materials include the basic and catalyst pastes. The basic paste consists of silicone of rather low molecular weight, dimethyl siloxan, having reactive final hydroxylic groups. Carbonate of copper or silicon may be the fillers. Liquid consisting of suspensions of tin octate and andalkyl silicate or pastes with addition of the condensed agent may be the catalyst.

Chemical reaction with formation of firm silicon proceeds with formation of rubber with three-dimensional structure, release of ethyl alcohol and exothermal rise in temperature by 1Со with presence of shrinkage.

Advantages of silicon impression materials:

– Good adhesion to the tray and excellent – between the layers;

– Exact enough in reproduction of fine details;

– Inexpensive for traditional two-stage technique;

– Are applied to take impressions in construction of high precision denture;

– Are neutral in taste and smell.

It is possible to influence cementation rate of the given material by the catalyst by reducing or increasing its amount.

Drawbacks:

– Materials demand casting of model within an hour, some materials – in 2 hours, at least no more than 24 hours;

– Hardened materials are afraid of pressure as the size of model may change;

– Shrink at long storage;

– Require thorough mixing of the base and catalyst;

– Highly hydrophobic, demand the control over casting;

– Possessing high hydroscopicity, absorb moisture from the air, changing the properties, therefore cans with a hardener should be closed at once after use;

– In presence of crystal formations in liquid it is undesirable to use the given material;

– Soaking in soap solutions before casting of model is recommended;

– It is undesirable to cast model by the impression for the second time.

To minimize shrinkage of the material the model construction should be made within a day after taking the impression. It is necessary to know that in removing from the oral cavity the material undergoes significant overloads, therefore to provide elastic reset, the model is recommended to cast not at once, but in 2 hours after taking the impression.

Fillers as inorganic substances are not subject to shrinkage, therefore its degree does not depend on their structure and quality. More viscous silicons due to a lot of the filler have less expressed shrinkage than silicons with average and especially low viscosity.

Optimum properties of the material can be achieved only by exact observance of the proportions specified by the manufacturer. Therefore the universal requirement while working with any impression masses is the exact dosage of their components.

Excess of a hardener (catalyst) leads to very fast formation of a polymeric net and substantial growth of internal pressure. Because of early polymerization of the material the dentist may not have time for qualitative and high-grade mixing of the components. As a result, the catalyst spreads irregularly in the mass causing the internal pressure and breaking the process of polymerization. The application of smaller amount of a hardener causes incomplete polymerization of the material and is the cause of bad elastic properties and sharp impairment of accuracy of the taken impression.

Now the hardener for C – siliconsis issued in tubes in the form of gel for materials of dough-like consistency and in the liquid form for materials of liquid consistency.

Cans with catalysts should be closed after work immediately. The catalyst possesses high sensitivity to moisture and absorbs it from the air, thus changing the reactivity. Crystal formations in the can with a hardener is evidence of its poor quality.

It is necessary to avoid construction of repeated impression because of the nature of these materials.

At present C- siliconsare practically safe, but some of these materials can cause growth of staphylococcus on the mucous membrane, therefore after taking the impression material from the oral cavity it is recommended to the patient to rinse his mouth with plenty of water.

It is necessary to mix up the material only in gloves.

Today the most known C-silicon impression masses are:Plast/Bisiko/Germany; Optosil, Lastic, Сstomaflex /Kulzer/Germany;Rapid, motion, Spidex, Dimension/Espe/Switzerland; Septosil, Condensil /Septodont/France.

In duplication of models in construction of fixed and removable dentures silicon materials:Degufrom Дегуформ, Viprosil /Degussa/Germanyare applied in orthopedic dental laboratories.

A-silicons.

In hardening of materials of this group there is a specific reaction of polymerization in which there is no formation of by-products. Unlike polycondensation, the reaction of connection does not create a low-molecular product, and is of other kind of polymerization, therefore at present they are most stable materials.

The basic properties are associated with hydrophobia of polyvinyl siloxan chains. Reactive groups are both vinylic groups at the end of siloxan chain, and Si-H-groups in a cross-section link. Pt-complexes are used as a catalyst. The reaction of polymerization occurs due to formation of cross-section connection between chains by joining Si-H-groups to vinylic parts.

The drawback of polyvinyl siloxans is that hydrophilia of the material can be achieved only by addition of a surfactant. The surfactant improves hydrophilia of the impression material. It has a lipophilic head and a hydrophilic tail. Both properties are determined by hydrophilic –lipophilic balance. In traditional A- siliconshydrophilia of polyethers can’t be achieved.

The additional (joining) type of silicon material is presented by pastes of a low, average, dense consistency and is also polysilicone. The basic paste consists of polymer with a moderately low molecular weight and silicon groups (-Si-H) from 3 up to 10 molecules as well as the filler. The catalyst is presented by a polymer with a moderately low molecular weight and vinylic final groups as well as the catalyst – chloroplatine acid.

It is inadmissible to influence adjustment of the catalyst (increasing or reducing its amount) during cementation in the given material.

A- silicon impressionmaterials are manufactured in all viscosity and used for all techniques of taking the impression. Identical pastelike consistency of the catalyst and base substance is typical of them, it provides accuracy of the dosage and convenience of mixing. Polymerization rate depends on the temperature – the higher the temperature, the higher is polymerization rate.

Advantages:

– Good reproduction of details;

– Dimensional accuracy;

– Stability to pressure;

– Excellent layer-by-layer connection;

– Tolerate disinfection in any solutions;

– Have no taste and smell;

– Are galvanized;

– Optimum compatibility with the skin and mucous membrane;

– Ideal final hardness;

– Contours are clear and accuracy of details.

It is possible to cast some models by impressions from A-silicons.

The model can be cast within 30 days (better till 7 days).

Drawbacks:

– Hydrogen peroxide, anesthetics, retraction solution damage and inactivate the catalyst – it is necessary to work in thoroughly washed out and dried up oral cavity;

– in application it is necessary to use adhesive for the tray;

– clinically the material gives insignificant shrinkage;

– is costly.

It is necessary to avoid direct contact of the latex gloves in mixing the material as it can inhibit reaction of polymerization.

For removal of the internal pressure impression before casting the model it is necessary to wait for 2 hours.

If there is no time for this purpose it is necessary to hold the impression under warm water for 2 min.

The materials possess excellent mycostatic properties that is necessary in taking the impression for removable constructions when it is undesirable to crush the mucous membrane.

It is impossible to combine C-and A-silicons in taking the impression as there is no adhesion between the layers.

A- silicons are intended for taking one-stage or biphase impressions, some masses provide a high-grade and precise reproduction of the orthopedic bed in real conditions of the oral cavity in presence of moisture and blood, can be applied in construction of dentures in partial and full absence of the teeth. Expressed thixotropy of some materials enables to work on the maxilla as easily as on the mandible, not being afraid that the material will flow down in its drawing from a syringe. The same material can be used for removable dentures in relocation. Properties hydrophilia are kept after polymerization of the material that allows to cast precision models easily.

Both components of A- silicons (the basis and the catalyst) are contrastly coloured without dependence on a degree of viscosity and thus have identical consistency. They are mixed up in equal amount till getting a mass of homogeneous colour.

Materials of tough-like consistency are produced in identical plastic cans, and masses with lower viscosity are produced in a cartouche with a double chamber and squeezed out by means of a pistol-dosatorthrough a special needle-mixer. Thus errors in the dosage and negative influence of moisture in the atmospheric air are excluded.

Extremely important factor is considerably smaller toxicity of A- siliconsin comparison with C- silicons. Characteristic features of C- silicons such as burning, tingling, reddening of the mucous membrane of the oral cavity are practically absent in A-silicons.

It is necessary to adhere precisely to recommendations on duration of mixing materials. Reduction of this period leads to development of heterogeneity (lamination) of the impression mass. In increasing the period of mixing the process of vulcanization starts in the material resulting in formation of internal pressure. It is caused by the fact that elastic zones are formed in formation of the polymeric net and it inevitably leads to deformation of the impression.

At present the most known silicon masses are:S1, S1 soft(Germany);President, Permagum (Switzerland);Formasil-A, Contrast, Sielast, Silasoft (Germany);Vigalen (Russia);Express, Reprosil (USA);Septoflex (France);Sielast 20, 21(Ukraine).

Silicon materialsare recommended to be appliedwhile constructing crowns, inlays, bridge dentures from metal ceramics and porcelain, arch prosthesis.

Techniques of application of silicon impression materials.

Accuracy of the impression is of paramount importance in making orthopedic construction. There are various techniques of taking a two-layer impression by means of silicon impression materials. However, correct performance of the following preparatory stages is of great importance:

1. Retraction of the gingival edges in the area of the prepared teeth.

2. Cleaning of the oral cavity before taking the impression.

3. Provision of qualitative adhesion impression material to the tray.

Having taken the preparatory measures, start taking the impression by means of one of the listed ways: 

Two-stage method

– Traditional technique of the double-layer impression;

– “Isolating” technique.

The essence of the two-stage method of taking the double-layer impression is that the impression is twice introduced into the oral cavity.

At present there are several such ways which are described below.

One-stage method

Technique with application of a syringe;

Technique of a two-phase one-stage impression.

Traditional technique of taking the double layer impression (double-layer technique, two-stage mixing).

The primary impression is taken by the mass of dough-like consistency. After structurization it is removed from the oral cavity, washed out and dried up. The purpose of the given stage is formation of the individual tray for more fluid correcting mass.

While taking this impression it is necessary to pay attention to a number of the moments:

I. Interrelation of the tray and dentition. If the distance from the equator of any tooth up to the tray edge is not enough (less than 2 mm) the material is irreversibly deformed in this place while removing it from the oral cavity; it sharply worsens quality of the impression.

II. Provision of unobstructed repeated introduction of the impression with the correcting mass into the oral cavity. To achieve this it is necessary to remove (to cut off) all possible retention points after taking the primary impression, such as: impressionof the dystopic teeth, interdental septa, zones of the expressed anatomic undercuts, etc.

III. Creation of the conditions allowing to avoid a compression of the  primary layer while taking the final impression. The compression of the primary layer while taking the final impression is often the cause of noncorresponding of the whole piece of orthopedic constructions to the orthopedic bed tissues. It accounts for the fact that after removal from the oral cavity due to elastic forces it comes back to the initial condition and breaks accuracy of the correcting layer.

Therefore it is necessary to form discharge canals which will allow excess of the correcting mass free to be squeezed out from the deepest sites in taking of the second layer on the primary impression in the area of all prepared teeth. Creation of discharge canals allows to exclude zones of the increased pressure leading to distortions in the final impression.

The discharge canals are cut by means of special instruments. They should begin in the area of the cutting edge or chewing surface of the prepared teeth and come to an end at some distance from them, for example, in the area of the hard palate.

In taking the correcting impression it is necessary to press it for some seconds to enable the correcting layer to be distributed and more precisely reproduce a relief of the tissues in regular intervals. Longer compression is undesirable, as in this case polymerization of the correcting mass can take place against the background of elastic deformation of the primary impression. In removal from the oral cavity it comes back in the initial condition and deforms the correcting layer, aw well as the whole impression.

Two-layer technique, fast mixing.

The main mass is introduced into the oral cavity. Not waiting for full hardening of the impression mass, the tray is removed by slightly shaking movements (the impression is a little bigger). The correcting mass is applied and removed after full hardening.

Isolating (wash) technique.

For more uniform distribution of the correcting mass and removal of excessive pressure at the second stage of taking the two-layer impression, the so-called “isolating” technique has been offered.

The mass of dough-like consistenct in the plastic condition is put on the tray and covered with a polyethylene film. The material is introduced into the oral cavity, centred and slightly pressed. Then, not waiting for polymerization, the  primary impression is removed from the mouth, the film is taken off, and the material with low viscosity (correcting mass) is put on its place After that the impression is introduced into the oral cavity again and slightly pressed.

The polyethylene film leaves significant spaces for flowing of the correcting mass and does not give a possibility for undercuts to be formed preventing repeated introduction of the impression into the oral cavity. In this case, in comparison with a traditional technique, the correcting mass is brought to necessary sites without pressure and it increases accuracy of the impression. Besides, this technique allows to achieve stronger connection between layers of the material as their polymerization occurs simultaneously.

Technique of taking a two-layer impression by the individual tray.

For making fixed constructions it is possible to prepare the individual tray and correcting masses to take the impression

In stage-by-stage taking the impressions it is necessary to adhere to the following criterion of estimation of the impression: the thinner the layer of the correcting mass, the more precise is the impression.

A one-stage method (monophase).

A one-stagemethod is based on reception of the impression by basic and correcting masses in one step. Ways of taking such impressions are given below.

Reception of a double-layer impression with the use of a syringe

This method consists in application of the correcting mass to the prepared teeth by a special syringe with a needle-mixer. Special attention should be paid to cervical areas of the teeth. Simultaneously the dentist’s assistant or the nurse should mix and put on the tray the mass of dough-like consistency. After application of the correcting mass, the tray with a basic material is introduced in the oral cavity, pressed and fixed. While taking the impression by this technique it is necessary to dry up the teeth carefully before application of the correcting mass to them, as adhesion of the material is possible only to a dry surface of the tooth. In this case retraction strings should be removed before taking the impression.

While using this method there is no necessity for preliminary preparation of the primary impression and maximally strong connection of the basic and correcting mass is achieved. It is more preferable to use A- silicons for this technique.

Sandwichtechnique

The given technique assumes simultaneous application of the basic and correcting mass to the tray. The plastic basic material is put on the tray, and the flute is formed in the teeth or the alveolar process area. The tray prepared in this way is filled with a mass of low viscosity. After that the impression is immediately introduced into the oral cavity, centred and fixed. For reception of the best result it is recommended to put a material of low viscosity on the teeth by means of a syringe before introduction of the tray with the basic amount of the mass, that is to combine both one-stage techniques. In this case A- silicons are supposed to be also used for achievement of optimum result.

The basic impression mass can be mixed by tips of the fingers.

The correcting mass can be mixed:

a) on the sheet of paper by means of the spatula.

b) in a special glass

c) by means of the special device for automatic mixing of polyethers and A-silicons.

Polyesterimpression materials.

Polyester impression materials are one of new and perspective groups of elastomer impression materials. They consist of the basic paste and a hardener which are mixed up in definite ratio. The molecular structure of these impression materials is based on the linear chain built by tetrahydrofuran etylene oxide polyethers. The basic paste contains polyether with reactive amine groups (a ring of aziridine at the end of molecules, various fillers and softeners, and the paste of hardener – aromatic ethers of sulfacid. In their interaction there is a splitting of aziridine rings and formation of polymeric network. The chain has hydrophilic properties which can be received by a proportion tetrahyfrofuran and ethylene oxide. Aziridine parts at the end of the linear polymeric chain are the reactive groups, and strong acids (Lewis’s acids) serve as the catalyst. Crossing bounds between chains are formed by ring opening of the aziridine rings. The reaction goes as polyconnection, without release of volatile substances. Consequently polyethers as well as A-silicons have strain stability and possess low shrinkage. However, unlike silicon impression materials, polyethers actively absorb moisture at storage. Therefore, to avoid swelling (puff-up) of impressions they should be kept dry.

Advantages:

– Stable to deformation;

– Have good moistening abilities in working hours;

– Dimensional accuracy and accuracy in reproduction of details.

Drawbacks:

– Too high hydrophilia in long contact with water that leads to swelling of the impression material;

– Strong acids can cause irritation of the skin and soft tissues of the oral cavity;

– Very strong intramolecular interactions create excessively firm polymer which is difficult for removing from the oral cavity of the patient;

– They are not mycostatic, can displace mobile soft tissues, demand thorough preparation of the gum (but come under the gum well);

– They are not completely polymerized in presence of blood;

– It is difficult to mix them up to a homogeneous consistence.

The model should be cast in 2 hours but not later than 7 days (by some innformation – within 24 hours). The impressions are kept dry. The increased humidity at storage and disinfection can cause change in size due to absorption of moisture.

Polyethers cause allergic reaction very often. Polyester impression materials are of unpleasant taste and smell. They are right for taking the impression in bite because of simple technology.

Polyethers are usually applied in the form of paste of moderate consistence (the basic and catalyst). The basic paste represents polyether with a moderately low molecular weight and ethylene rings in the form of end groups. The filler is silica, the softener –glycol etherflotate. The catalyst paste contains dichlorobenzene sulphonate as a cross-linking agent as well as a filler. The separate tube contains softener –octyl flotate and about 5 % of methyl celluloseas as a filler. Dyes can be added to the basic and catalyst pastes. The polyester system may be of high and low viscosity.

The rubber is formed as a result of ionic polymerization and development of imine rings. Copolymer tetrahydrofuranand ethylene oxide is a basis of the material. The reaction is more exothermic with increase of temperature by 4°Сthan in other rubber-like materials.

Catalysts of polyester impression materials may cause toxico-allergic reaction.

Now only some materials of this group are presented in the market. Impression materials of the firm ESPE (Germany) Impregum FandPermadynehave received the widest application. These are single-phase materials of low (Permadyne) and moderate (Impregum F) viscosityintendedfor taking functional and specified anatomic impressions in various kinds of removable prosthesis. However, after construction of the individual tray single-phase materials can be partially applied in construction of the whole piece fixed orthopedic designs.

Material on the basis of synthesis of polyethers andsilicons.

A quadrofunctional hydrophilic structure combines cross-linkedpolymeric network with the included surface-active substance. It is advanced А-silicon. The polymeric network provides high breaking strength, and the included surface-active substance makes moistening abilities of “Aquasil” equal to those of polyethers. The patented QM-polymer which several times increases branching and density of the polymeric chain is introduced into the structure of “Aquasil”,  therefore breaking strength exceeds all known materials in “Aquasil”. This unique modifiedvinyl polysiloxan chemical structure provides high accuracy of reproduction of details in the moist environment that cannot be achieved in using traditional impression materials. Therefore it is marked in recommendations on application of the given material that elements of the orthopedic bed are not dried up, but left damp.

“Aquasil”is a synthesis of the most preferable properties of polyethers and silicon with additional reaction of polymerization.

Advantages of the materials made of polyethers:

– Hydrophilia during taking of the impression;

– Viscosity and fast hardening;

– Absence of taste and smell;

– High accuracy of the size;

– Good resistance to deformation;

– Accuracy of reproduction of details;

– Absence of swelling or shrinkage;

– Possibility of disinfection/sterilization.

Advantages of traditional materials on the basis of vinyl polysiloxans:

– Easy removal from the oral cavity

– Absence of taste and smell;

– Keeping of long disinfection/sterilization.

It is characteristic only of the system “Aquasil”

– Very high breaking strength which is achieved due to high density of crossing bounds in the hardened elastomer;

– Rheologic properties change from correcting masses of ultralow viscosity (Ultralow Viscosity) up to materials of very high density (Soft Putty) that enables the dentist to use this ideal material for any clinical situation, for any technique of taking the impression;

– Owing to the best ergonomics the force necessary for impression is reduced;

– Reduction of amount of waste.

– Impression materials “Aquasil” suit for all exact techniques of taking the impression where it is required excellent hydrophilic properties, volumetric accuracy, high breaking strength, good stability to constant deformation.

– There are some versions of materials in density, each of which is intended for special techniques depending on various clinical conditions.

Contraindications:

It is impossible to use “Aquasil” in combination with polyester, condensed silicons (C- silicons)or polysulphidic impressionmaterials.

Safety measures:

The liquid contains toluene and is inflammable substance. It is necessary to work with the material only in well aired premise since it has irritating effect on the respiratory tract.

Interaction with other materials:

Astringents on the basis of aluminium salts can influence reaction of hardening of vinyl polysiloxan.

– The material doesn’t tolerate a direct sunlight;

– Gloves from the material containing sulfur can influence reaction of hardening of the impression material “Aquasil”, therefore:

– It is impossible to mix up a base material in them;

– It is impossible to touch a retraction cord with hands, and it is necessary to manipulate only by means of tweezers;

– It is impossible to touch the prepared tooth before taking the impression;

– It is necessary to have adhesives for all types of the trays;

– Disinfection is made by standard disinfectant solutions.

While using modern impression materials it is necessary to follow precisely the instruction on their storage and application. Such approach allows to receive high-quality impression that in many respects determines a successful outcome of orthopedic treatment and promotes economical expenditure of expensive impression materials.

The knowledge of technological possibilities of modern impression materials and features of their application allows to understand clinical tasks and ways of their solution and be successful during rehabilitation of patients with defects of dentitions. Plastic impression materials. Before considering these materials, we shall define concept “plasticity” and “elasticity”. Plasticity is a property of the material to keep the changed form after cessasion of external deforming force. An example of plasticity of the substance can be waxes, plasticine, dough, clay. Elasticity is a property of the material to take the initial form under the influence of rather small force from the outside. Rubber, gutta-percha and other similar materials are related to elastic substances.

      Plastic impression materials are divided into two subgroups: impression thermoplastic materials and impression plastic compositions. Thermoplastic materials should meet the following requirements:

1) to be softened at the temperature which does not cause burn of the mucous membrane of the oral cavity (55°);

2) to possess necessary plasticity at the set temperature;

3) not to be sticky at the set temperature;

4) to harden at the temperature a little exceeding temperature of the oral cavity;

5) to be easy to be processed by instruments, not to be stratified;

6) not to change the size in hardening and storage after removal from the oral cavity;

7) to be easy to be separated from the model.

Thermoplastic materials.Thermoplastic materials soften and harden under the effect of temperature. They are divided into reversible and irreversible. The former keep the plastic properties in repeated heating and cooling during taking of impressions, the latter gradually lose plasticity. Weinstein’s masses are related to reversible materials, stens – to irreversible.

Their composition includes paraffin, stearin, gutta-percha, beeswax, ceresin and other materials. A mixture having a temperature of softening of 50-70° is received by various combinations of components.

Except for resins and waxes, being glue substances, their composition includes fillers:chalk, talc, zinc oxide, pumice, etc. Dyes, correcting taste substances are also introduced in them.

Of domestic materials of this group dentafol is best known, it is applied to constructed dentures with the expanded borders for edentulous jaws, Weinstein’s masses N 1, 2, 3. Masses N1 and N2 are applied in taking the impressions of the edentulous jaws, the mass N3 – for taking the impressions by means of a ring in prosthesis with inlays, semicrowns. Of foreign materials of this group adhesial (Austria), mycodin (Germany) are most known which represent complex compositions of resins and waxes.

Plastic impression materials.Plastic impression materials possess high plasticity at the temperature of 18-20°. These materials do not harden in the oral cavity and reflect functional features of the mobile and motionless mucous membrane of the orthopedic bed well. Plastic impression materials are recommended to be applied: for specification of denture borders of the maxilla and mandible in absence of the teeth and taking functional-sticking impressions in significant atrophy of the alveolar processes, for correction of the denture (relocation) by a laboratory method.

Orthocor is a representative of this group of materials, produced in the form of plates, isolated with a polyethylene film on either side.Orthocor put on the denture surface can be left in the oral cavity of the patient for some hours. For this period the impression is formed with functional edges.

     Thermoplastic materialsare forgotten too. Some masses are heated up in special small pot, and then are put on the tray by a brush. To receive an ideal impression with such mass is impossible. There was such mass referred to as “Stens-03”. This mass looked unappetizingly, like a piece of sealing wax by all parameters.

     Thermoplastic materialsof Stens type have noadvantages, but cheapness and simplicity in use. The piece of the mass is put in hot water, after a while it softens, is put on the tray and introduced into the oral cavity at once. Earlier it was written that this mass can be used repeatedly and it is one of advantages. But sterilization of such mass by heat is impossible, and unreal at low temperature.

There are a lot of drawbacks:

1.  Insufficient accuracy of the impression.

2.  Bad retaining of the form at temperature drop.

3.  Impossibility to remove an impression from the mouth if the temperature of hardening is passed.

4.  Impossibility of sterilization.     

 

Once you have all your materials standing by, take a few minutes and explain to the patient what is involved in the impression procedure. The key to taking good impressions is to have the correct size impression tray fit the arches, to mix the alginate, position the tray correctly in the mouth, have the patient relax and breathe through the nose, let the alginate set, and correctly remove the impression tray.

 

Use the following steps to take preliminary impressions:

1.     Select the correct size impression tray and ensure its fit in the patient’s mouth is correct. Allow 3-4 mm of space between the tray, teeth, and soft tissues when the tray is positioned in the mouth.

2.     Place utility rope wax around the top border of the maxillary and mandibular trays to extend their height. This will also act as a “pad effect” on the soft tissues. Place the trays in the mouth again to ensure the fit.

3.     Mix the alginate and water together in the mixing bowl with a spatula. Follow the manufacturer’s instructions. Mix into a creamy, smooth consistency. Use the sides of the mixing bowl and press the mixture against it to eliminate air bubbles. Total mixing time is usually 1 minute depending on the type or manufacture of alginate used.

4.     Have the patient rinse with mouthwash vigorously. This aids in removing food particles and thick saliva that may cause voids in the impression.

5.     As the patient is rinsing, load the maxillary tray with the mixed alginate. Load the tray with one large portion of alginate on the spatula using a wiping movement to avoid air being trapped in the material.

6.     Wipe off any excess alginate, and smooth the surface of the tray with your index finger. Use the excess alginate from the mixing bowl and place some directly onto the palate with your index finger before seating the impression tray. This prevents a large void within the palatal vault.

7.     Have the patient open his/her mouth about halfway. Using your left index finger, retract the patient’s right cheek. Carefully place the filled tray into the patient’s mouth and use the tray to move the left cheek out of the way.

8.     Guide the tray in the mouth and center it over the maxillary teeth. Pressing up with the posterior border of the tray, raise the tray to the hard palate area to form a seal.

9.     Keeping the posterior border of the hard palate in place, next raise the tray up over the maxillary teeth. The tray should be seated so that it is parallel with theocclusal plane.

10. Holding the tray in place with your right hand, use your left hand to gently lift the patient’s lips and cheeks away from the tray until it is completely seated.

The maxillary arch should now be completely embedded in the alginate material.

11. While keeping the tray parallel with the occlusal plane, pull the upper lip over the anterior border of the tray to form the anterior section of the impression. The average working time from mixing the alginate material to this step is 1 1/2 minutes. After this time, the alginate begins to gel and set up.

12. Still holding the tray in place, look in the patient’s mouth and ensure that no alginate material is running down into the throat area. If needed use a mouth mirror to remove any excess. Have the patient relax and tilt the head down and breathe through the nose as the material is setting up. A saliva ejector or patient napkieeds to be in place to catch any excess saliva while the alginate is setting up.

13. After the alginate has set, place one of your index fingers along the lateral border of the tray and press down to break the seal formed by the set alginate material. Once the seal is broken, carefully remove the tray from the patient’s mouth and wrap the tray in a moist paper towel. Have the patient rinse his/her mouth out to remove any excess material left from the impression. Have the dental officer inspect the maxillary impression for accuracy.

14. Next, take the mandibular arch impression using the same basic steps as with the maxillary arch technique. When seating the mandibular tray onto the lower arch, have the patient raise the tongue to allow the alginate in the tray to take an accurate impression of the lingual aspects of the alveolar process.

15. Once the tray is seated, gently pull the lower lip over the anterior border of the mandibular tray to form the anterior section of the impression.

16. After the alginate has set, remove the mandibular tray in the same fashion as with the maxillary arch, except push up to break the seal.

17. Have the patient rinse his/her mouth again, and have the dental officer inspect the mandibular impression for accuracy. Wrap in a moist paper towel.

18. The dental assistant will now disinfect the maxillary and mandibular impressions. While in the DTR, remove the moist paper towels. Rinse and disinfect using an accepted phenyl disinfectant on the impression material and trays. Wrap in moist paper towels again and place impressions in a headrest cover for transportation to the dental laboratory.

Many dental materials are unique to prosthodontic procedures. The improper use of any of these materials could cause a delay in the treatment and an inconvenience to the patient. You should be familiar with the use, handling, reaction time, and storing procedures for these materials. This knowledge is necessary for your successful performance as a prosthodontic assistant.

Many types of impression materials are used in the dental clinic. However, no one material fulfills all requirements for making a perfect negative reproduction of the oral structures. The dentist will determine which material will best meet the requirements for each case. The two commonly used impression materials are alginate hydrocolloids and synthetic rubbers.

 

Alginate Hydrocolloids.

 Hydrocolloids that change state because of thermal changes are known as reversible hydrocolloids because the process can be changed back and forth by altering the temperature. Those that are altered through a chemical change are known as irreversible hydrocolloids. Once the chemical change has taken place, it cannot be reversed or turned back to the previous state.

Irreversible hydrocolloids, more commonly known as alginates, were developed from seaweed during World War II. Alginate impression material has largely replaced the reversible type for impressions. The advantages of alginate material are that it is easy to prepare and handle, it does not require excess equipment and advanced preparation, it is comfortable for the patient, and it is inexpensive. Alginate is used in making preliminary impressions for all study casts and most final impressions for RPD working casts.

According to the American Dental Association (ADA) specifications, alginate materials are divided into two types based on gelling time:

• Type I—Fast set material, must gel in 1 to 2 minutes.

• Type II—Regular set material, must gel in 2 to 4.5 minutes after the beginning of the mix.

Also, under ADA specification, the manufacturer is required to include detailed instructions for use. The dental assistant should read and follow these directions carefully.

Thermoplastic materials of imprints

 

The name means “thermoplastic” ability of mass to acquire plastic properties under act of certain temperature. Materials of imprints of this group contain combination of the varied matters with thermoplastic properties (paraffin, stearin, bee beeswax, gutta-percha) and fillers which provide a certain structure and thermal properties. In addition, resin and some synthetic matters which give to material of the proper hardness after cooling enters in their composition, and also dyes are the aromatic which give to mass of the proper tastes internals.

Thermoplastic materials of imprints must answer such basic medico – technical  requirements:

1)to damage not fabric of cavity of mouth, be economic advantageous and accessible;

2)to grow soft at a temperature higher temperatures in the cavity of mouth, but to cause not the burn of mucus shell and feeling of discomfort at a patient;

3)to dissolve not and swell not under act of secret of cavity of mouth, quickly to harden at a temperature, something more high from the temperature of cavity of mouth;

4)to have good plasticity;

5)exactly to reproduce the imprint of fabrics of prosthetic bed, not becoming deformed at the leading out from the cavity of mouth and in a next period to founding of models;

6)it is easily to be processed by a sharp instrument without twisting, to crack or flocculation;

7)to be added to sterilization without worsening of properties.

Two types of thermoplastic the masses of imprints are known: circulating – at reusing does not lose plastic properties, can be sterilized to heating; irreversible – at the repeated use become less plastic as a result of change of properties or disappearance of separate components. Irreversible thermoplastic the masses are subdivided into two types: 1st type – usually painted in green, red or grey colors, grow soft.

Defenition:

Is a material used to reproduce the form of teeth and other supporting hard and soft tissues.

The impression material is loaded into a tray to carry it to the mouth and support it

Within a specific time the impression set to semisolid, elastic or rigid state

When plaster or stone is poured into the impression and hardened, the replica of teeth and tissues is formed              ( Cast or Model)

Types of impressions:

Primary(preliminary) impression:

It shows teeth and surrounding tissues for the use of(1) diagnostic aids, (2) fabrication of appliances and (3) pretreatment and post-treatment records.              Study(diagnostic cast)

Secondary(final) impression:

It extremely accurate and used in fabrication of cast restoration.              Master cast

Occlusal registration impression:

It provides an accurate registration of normal centric relationship of the maxillary and madibular arches at occlusion.

Impression trays:

They are used to carry the impression material to the mouth and to support it until it sets.

Function of trays:

v Carrier.

v can stabilize the set impression material.

Types of impression trays:

 

1-    Stock tray(prefabricated)

2-    Custom tray                     

3-    Bite registration tray

4-    Triple tray

1-stock trays(prefabricated tray):

Trays that come in different:

Materials            Plastic disposable trays

                             Metal trays

Shapes                edentulous mouth

                            mouth with full complement of teeth                 

 

Sizes                 Large for adult                       

                          Small for pediatric patients

                             

                         Solid

                         Perforated

2-Custom trays:

They are made on a model of the patient’s arch with acrylic or other resin.

3-Bite registration trays:

They record the occlusal surfaces of both arches and used to relate the upper and lower casts in the lab in the same manner as in the patient’s mouth.

4-Triple tray(double bite tray):

It takes an impression of the prepared teeth, opposing teeth and a bite registration at the same time.

 

Requirements of an impression materials:

Types of impression materials:

 

ACCORDING TO the ability of the set material to be withdrawn from undercuts

Inelastic impression materials:

1-Plaster:

Composition:

Plaster of paris.

Potassium sulfate or potassium chloride.

Uses:

-Mainly for complete denture impression.

-As wash material inside tray made by softened compound in aluminum tray.

Advantages:

-Rapid set.

-Inexpensive.

Disadvantages:

-Hard and brittle.                -Complexity of its technique.

-Unpleasant taste.

2-Wax:

Is a thermoplastic material(that soften on heating and harden on cooling , process is reversible.)

Forms:

Sticks, strips, tubes and others.

Advantages:

-Clean.    –cheap.    –Easy to use.

Disadvantages:

–Very weak. –Lack of accuracy.  -Distort easily.

Uses:

-primary impression for complete denture.

-Bite registration.

-To correct minor voids in impression for complete denture.

3-Impression compound:

Is a rigid thermoplastic material that soften when heated (by water bath or direct flame) and become firm at mouth temperature.

Forms:

Thick sheets or sticks.

composition:

Thermoplastic resin and waxes, filler, plasticizers and pigments.

Uses:

-Primary impression for complete denture.

-To mold the peripheral borders of a custom tray for complete denture impression.

 

 

4-Zinc oxide-Eugenol:

It comes as two pastes:

          1                                                                                                  2

ZnO powder+ vegetable oil                                                    Eugenol+ inert filler

Mixed together until one homogenous color is obtained.

uses:

Impression of edentulous ridges for removable denture.

Advantages:

-Accurate for its application in complete denture.

-Inexpensive.

-Does not require immediate pouring.

Disadvantages:

-unpleasant taste.    -irritate the tissues of some patients

Aqueous elastomeric impression material:

There are two colloid impression materials:

Reversible hydrocolloid(Agar):

It is made mainly of water with agar and other components as colorants, flavors and sulfate compound.

It needs:

 -special equipment to heat , store and temper the material.

-Special impression trays that circulate cooling water.

   Gel                           Sol                              Gel

  (solid)           heating             (solution)           cooling                     (solid)

                         

Hydrocolloid conditioner:

                     

                       It has three water bath chambers:

         1                                                 2                                         3

     100ْ C                                          65ْ C                                     45ْC               

   Heating chamber                storing chamber              tempering chamber

Reversible hydrocolloid(Agar):

Advantages:

1-It can be used in a moist field.

2-It poured easier than elastomeric impression materials.

Disadvantages:

1-It needs to be poured immediately.

2-It caot be poured than once.

3-It is not dimensionally stable for long time.

4-It requires expensive equipment.

5-It requires the use of water cooling hoses that sometimes leak.

Tray used with agar:

Special stock tray with water cooling system.

Phenomena with hydrocolloid material:

-Syneresis:

A characteristic of gels to contract and squeeze out some liquid which then accumulates on the surface.

-Imbibition:

It is the act of absorbing moisture.

Irreversible hydrocolloid(Alginate):

They are termed irreversible because they will not reverse to a sol once they react and become a gel.

  Sol                                gel

          (chemical reaction)

Composition:

The powder is an inert silica filler, potassium alginate, reactor, retarder, colorants, flavors and antimicrobial agent.

Handling of alginate :

-It supplied in preweighed envelopes or in bulk container with a calibrated scoop.

-The water is dispensed using a calibrated water measure.

-Alginate is very sensitive to moisture loss and will shrink as a result. Once the impression is removed from the mouth it should be rinsed ,evaluated ,disinfected, wrapped in a damp paper towel and sealed in a zippered plastic bag marked with the patient’s name.

Then it should be poured within an hour because it is not dimensionally stable.

Factors affect the manipulation of alginate:

1-The temperature of the water controls the rate of the setting reaction:

(Warmer water increase the setting rate while cooler water slows the setting reaction).

2- Powder to water ratio can affect the reaction:

P\w ratio(more powder)           increase setting reaction

p\w ratio (more water)             slow setting reaction     

Criteria for an acceptable alginate impression:

Both maxillary and mandibular impression:

-All teeth and alveolar processes recorded.

-The impression tray is centered over the incisors.

-Peripheral roll and frenums included.

-No large voids and few bubbles present.

-Good reproduction of details.

-Free of debris and No distortion.

-Alginate firmly attached to tray.

Maxillary impression:

-Palatal vault recorded.

-Hamular notch area included.

Mandibular impression:

-Retromolar areas included.

-Lingual extension recorded.

Advantages of alginate:

-Inexpensive.      

  -Easy to manipulate.

-Requires no special equipment.

-Reasonably accurate for many dental procedures.

Disadvantages:

-It is not accurate enough for the final impression for many prosthesis.

-It does not capture the fine detail of the preparation.

Tray used with alginate:

Perforated stock tray.

Common uses of alginate impression:

-Diagnostic casts.

-Primary impression for complete dentures.

-Partial denture frameworks.

-Opposing casts for crown and bridge treatment.

-Repairs of partial and complete dentures.

-Provisional restorations.

-Custom trays for fluoride or bleaching.

-Sports protectors and night guards.

Clinical tips:

v -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.

v -Water and Powder measures can vary in size among manufactures. If your office uses more than one brand of alginate, color code the measures so they are not intermixed.

v -Be sure to wear a mask while dispensing and mixing alginate because alginate dust is hazardous to inhale .

Alginate-agar hydrocolloid combination technique:

It used for crown and bridge procedures.

A low viscosity agar is liquefied and placed in impression syringe then put it around the prepared teeth while alginate is mixed and placed in tray then seated in patient’s mouth, the two materials will fuse together.

 

Advantages of this technique:

-Accuracy of the agar.

-No need for special equipment.

-low cost of each material.

 Disadvantages:

-It must be poured immediately.

-It caot be repoured from the same impression.

Problems and solutions of alginate impression:

Problem	cause	solution
1)Premature set	-Too much powder. -Prolonged mixing or loading time. -water or room too warm.	Use correct measure. -use timer to gauge working time. -use cool water to slow the set.
2)Slow set	-water too cold. -too much water.	-use warm water. -use correct  P\w ratio.
3)Grainy , lack of surface detail	Incomplete mixing of powder and water.	Mix well to creamy consistency.
4)Incomplete coverage of teeth or tissues.	-Tray too small or short for arch. -Tray incompletely seated.	-select large tray or extend the borders. -check complete seating of tray.
5)Voids on occlusal surfaces.	Trapped air when seating tray.	Wipe alginate on occlusal surface before seating tray.
6)Large voids at vestibule or midpalate.	-trapped air. -not enough alginate in the tray. -improper seating of tray. -lip in the way.	Put some alginate in vestibule before seating tray. -use adequate amount of alginate. -set tray properly. -pull lip out to create room for alginate.
7)Small voids throughout	Air trapped in mix during spatulation.	Press alginate against sides of bowel when mixing.
8)Distortion or double imprint.	-impression removed too soon. -tray moved while alginate was setting.	-check residual alginate in bowel, add 1-2 min. -hold tray steady until set, don’t let patient hold tray.
9)Torn alginate	-impression removed too slowly. -Thin mix.	-remove impression quickly. -use proper P\W ratio.