IMPRESSION MATERIALS First put you patient in a hot baking oven or a searing furnace, then turn them round and round in a centrifugal machine and pour hot metal on them. Or better still, heat them above 1000°C in a ceramic furnace and fine till well done. Don’t look shocked, - not a very pleasant idea right. Well, that what impression materials same is fromLet’s confabulate on this topic under these myriad subheadings: Introduction History Definition and Classification Ideal requisities Impression materials and Respective techniques -
Inelastic or rigid. o Latest Advances o Summary and conclusion
INTRODUCTION One of the cornerstones of modern dental practice is the accurate recording and reproduction of tooth tissue details. Material science plays a pivotal role in dentistry and impression materials form a vital core in that foundation. Mimicking the intricate details required in dental practice in the demanding and challenging oral conditions is an acid test for any material. Right from the advent of initial wide impression materials to the modern sophisticated elastomeric chemistries the quest for the perfect replicating material has spanned a diverse range of materials, techniques and devices. From dentures to implants, inlays to crowns, orthodontic to pedodontic to prosthodontist to restorative and endodontists, there is no branch of high quality dentistry untouched by the magic of these materials and no dentist who has not marveled at their ingenuity. Let us delve deeper into this fascinating and vitally important class of material science and understand then for their successful and ideal use. History: The history of dentistry has been paralleled by the development of impression materials. From the cumbersome and highly unpredictable materials of yore, replicating materials now match highly exacting standards. 2
The earliest dental materials were waxes used in the 18 th and 19 th centuries. Beeswax is reported as the material first used in making oral impressions. In 1782, William Rae reported use of wax in the movement of jaws, using plaster of paris reproducing the cast. In 1842, Montgomery discovered gutta percha. In 1848, Colburn and Blake described its use for impressioning by soaking in boiled water, moulding in the same was as wax and firmly pressing it is place. In 1930, a series of true physiologic waxes were developed by the cooperative effort of Dr. G.C. Bawles and S.G. Applegate. The first real impetus was the use of zinc oxide eugenol as an impression material by 2 pioneer dentists – A.W. Ward and E.B. Kelly in the early 1930s. In the mean while various other waxes and impression compounds and materials were being experimented. A further boost was the development of hydrocolloids. Firstly, in 1925, Alphous Poller patented a radically different impression materialAgar, his “Negacol’ was introduced to the dental profession as ‘Dentacol’ in 1928.
Algin and alginate type materials, were being experimented with S. William Wilding in 1936 receiving a basic and irreversible hydrocolloid being introduced in the early 1940s. The major breakthrough was the introduction of elastomeric impression materials in the middle 1950s. Polyether was further introduced in Germany in late 1960s. The latest addition to this class are the new polyether urethane dimethacrylate photoinitiated elastomeric impression materials. Truly, impression materials have come of age. Definition and Classification: Impression material is a dental material whose function is to accurately record the dimensions of the oral tissues and their spatial relationship. An impression is essentially a negative or reverse replica of any entity. In dentistry, this replica is of the teeth and / or supporting and surrounding structures. Classification: Impression materials have been classified utilizing numerous criteria. They are:
1) By their generic or chemical name: For example, one may refer to silicone materials or zinc oxide eugenol materials or even particular commercial brands of these materials. 2) According to the manner in which they harden or set: Set by chemical reaction or irreversible -
Plaster of paris.
Set by temperature change (reversible) Thermoplastic materials
3) According to the ability of the set material to be withdrawn over undercuts: Elastic impression material
Non-elastic or rigid impression material
4) According to the use of the material in dentistry: A) Materials used for obtaining impression of dentulous mouth. -
Used in crown and bridge, partial denture and operative dentistry. B) Materials used for obtaining impression of edentulous mouth: -
Zinc oxide eugenol.
5) According to their viscosity or tissue displacement: Mucostatic (more fluid, decreased viscosity)
Mucocompressive materials (increased viscosity)
Ideal Requirements of Dental impression materials: Impression materials should ideally fulfill the following criteria: 1.
Pleasant taste, odor and esthetic color.
Not contain any toxic or initiating ingredient.
Have adequate shelf life for storage and distribution.
Be easy to use with minimum equipment.
Have adequate setting characteristics that meet clinical requirements.
Possess satisfactory consistency and texture.
Adequate strength so that it will not break or tear while removing from the mouth.
prominent deformation after strain. 10.
Exhibit dimensional stability over temperature and humidity ranges.
Readily wets oral tissues.
Compatibility with cast and die materials
Accuracy and faithful reproduction of details.
Ability to be electroplated.
No release of gas during setting of impression or cast and die materials.
Impression materials of all types may be used either with stock trays which are available in standard sizes or special trays which are fabricated exclusively for that patient. Trays may be metal, plastic, polymer, acrylic, shellac etc. and may be perforated or non-perforated, dentulous or edentulous etc. Now coming to the individual impression material: Firstly, the hydrocolloids: ‘Colloid’; is derived from ‘kola’ meaning glue and ‘oid’ meaning ‘like’. Thus a colloid has a glue like character. Colloids are classified as the fourth state of matter. They lie between suspensions and solutions, characterized by this unique dispersion of particles held together by primary or secondary forces. The size of the particle ranges from 1 to 200nm. Types of colloids: Colloids can be combination of any of the states of matter with the exception of gaseous (i.e. 2 gases):. For example: -
Liquid / solid in air (aerosol).
Liquid / solid in liquid (Lysol). 8
Gas /liquid / solid in solid.
All colloidal dispersions are termed as sols. The colloidal materials that are dissolved in water are termed hydrocolloids. If the concentration of the dispersed phase is the hydrocolloid is sufficient it changes to a ‘gel’. This is formed by dispersed phase agglomerates called ‘micelles’ resembling a “brush heap structure”. Here they are more viscous and have elastic properties compared to the low viscosity and fluidity of the sol forms. This change of sol to gel is thermal and reversible for agar and hence it is called reversible hydrocolloid while it is chemical and irreversible for alginate, hence called irreversible hydrocolloid. An important characteristic of gels is the process of syneresis and imbibition. Syneresis in the process of loss of water by evaporation from its surface or by exudation of fluid onto the surface. Imbibition is a process by which a gel absorbs water if placed in a humid atmosphere. These both alter the original dimension of the gel. 9
Now let us have a look at the 2 hydrocolloids used as dental impression material: Agar and Alginate: Agar: Introduced by Alphous Poller of Vienna in 1925 and adopted commercially as “Dentacol’ in 1928, it was the first successful elastic impression material. Agar is an organic hydrophilic colloid extracted for certain ‘seaweed’. It is a sulphuric ester of a linear polymer of galactose. Though highly accurate, it has been largely replaced by alginates and elastomers due to its cumbersome manipulation. Composition: 1. Agar – Basic constituent comprising 13-17% for tray material and 6-8% for syringe material form the dispersed phase. 2. Borates 0.2-0.5% - Improves the strength of the gel. 3. Potassium sulphate 1-2%. It counters retarding effect of borates, ensuring proper setting of the cast or die. 4. Wax, hard – 0.5-1% Acts as a filler. Filler affect the strength, viscosity and rigidity of the gel.
5. Thixotropic material 0.3-0.5% - Acts as a plasticizer. E.g. glycerine and thymol. Thymol acts as a bactericide too. 6. Alkyl benzoates – 0.1% acts as a preservative and prevents growth of molds. 7. Traces of coloring and flavoring agents – for patients comfort, to improve appearance and tests and differentiate various phases of setting in chromatic materials. 8. Water – 84-85% the major compartment while acts as the dispersion or continuous phase. The material is supplied as: -
Gel in collapsible tubes (for impression).
Number of cylinders in a glass jar
In bulk containers (for duplication).
Properties – According to ADA No: 11. 1. Viscosity – Has a low viscosity and is sufficiently fluid as a sol allowing detailed reproduction of tissues. It is mucostatic and a viscoelastic material.
2. Strength – The compressive strength is 8000gm/cm2 (0.245 Mpa or 35.6Ps). More important is the tear strength, which is generally around 700gm/cm2. This is time dependent and higher tear strength is seen at higher rates of loading. Hence these material are removed with a quick jerk. 3. Gelation temperature : gelation occurs at 37-50°C. liquefaction of the gel to sol occur at 70-100°. The temperature lag between liquefaction gelation is hysteresis. 4. Dimensional stability. Storage
100% relative humidity
Inorganic salt solution
Depends on relationship of Expansion / shrinkage electrolytic in gel and solution. Shrinkage (minimal)
Hence impression are best stood in 100% relative humidity for not more than 1 hour and best poured immediately. 5) Flexibility – ADA specification allows a range of 4-15% for a stress of 14.2 psi which is met by most agar materials. Avg. flexibility of 11% is desirable.
Manipulations and impression taking: It requires special equipment: -
Water cooled rim lock trays.
Conditioner consists of : A) Boiling or liquefaction section – 10 mins in boiling water (100°C). The sol should be homogenous and free of lumps. Every reliquefaction requires added 3 minutes. B) Storage section – 65-68°C is used to store sol till needed. C) Tempering section – 40°C for about 2 minutes with the material loaded in the tray to make the material tolerable to oral tissues. This material is placed in special rim lock trays with water circulating devices. The rim lock is beading on the inside to retain material. There is an inlet and outlet for water tubes the tray should have a space of 3mm occlusally and laterally to cover all teeth. The material is conditioned in the trays placed into the mouth with the help of guiding stops and post dam and cooled with
6) These materials are highly elastic and elastic recovery occurs to the extent of 98.8%. 7) Reproduction of detail of upto 25Âľ is achievable with agar. 8) Working time ranges between 7-15 minutes and setting time is about 5 minutes controlled by the water flow of the trays. Material sets from the periphery towards the teeth. Water of 18 to 21Â°C until set. (Ice cold water or rapid cooling is contraindicated as it can induce distortion. The tray is removed with a quick snap or single stroke and cleaned. It can be stored in 100% relative humidity but is best poured immediately. Casts should be removed as soon as set for best results. It can be disinfected with iodophor, bleach or glutaraldehyde. Alternative techniques: A) Laminate techniques: Agar alginate combination technique: In this modified technique, the tray hydrocolloid is a mix of chilled alginate that lands to the synergy agar. The alginate gels by a chemical reaction whereas the agar gels by means of contact with the cool alginate. Here a syringe type agar in a cartridge is heated in boiling water for 6 minutes and stored in 65Â°C, 10 minutes before use. The tray alginate of 14
regular set type is mixed with 10% more water then recommended and placed in a tray. The agar is injected around the preparation and the mixed alginate seated on top of the agar. The alginate sets is about 3 minutes and the agar also sets within the tissue, forming a bond. The impression may be removed in 4 minutes. Advantages include good surface details, elimination of water cooled trays, simplification of heating equipment, faster set of agar and better completely with model. B) Wet field technique: This is a relatively new technique that has become popular. The areas to be recorded are actually flooded with warm water. Then the syringe material is introduced and tray material seated. The hydraulic pressure of the viseous tray forces the fluid syringe material into the areas to be recorded. This motion displaces materials as well as blood and debris. Advantages: 1. Hydrophilic, hence responds less initially to moisture, fluids are well tolerated. 2. Good elastic properties, good recovery from distortion. 3. Palatable and well tolerated by patients.
4. Can be removed as a duplicating material. 5. Long working time. 6. Mixing is eliminated, hence potential for errors minimized. 7. high accuracy and fine detail recording. 8. Compatible with die and cast materials. Disadvantages: 1. Low tear resistance. 2. Low dimensional stability. 3. Only one model can be poured. 4. Extensive and expensive equipment required. 5. Gypsum casts may have softer surfaces. 6. Impossible to sterilize for reuse. 7. Cannot be electroplated. Uses: 1. Widely used for cast duplication (especially for cast partial dentures). It is economical and allows large quantities to be prepared easily. Also it follows easily around the casts.
2. Full mouth impression. 3. Crown and bridge impression 4. Tissue conditioner. Irreversible hydrocolloid or alginate: Alginate comes from â€˜Alginâ€™ a peculiar mucous extract of brown seaweed. Algin was coined by a Scottish chemist. In England, 40 year later, Williams Wilding received the patient for alginate as impression material. It was developed basically as a substitute for agar while it became scarce during the II world war (Japan being its prime source). It is a linear polymer of anhydro beta- D mannuronic acid with numerous carboxylic acid groups. Composition of alginate powder: Ingredients:
Potassium or sodium or triethanolamine alginate
To dissolve and water and react with calcium ions
React with potassium alginate to form the insoluble calcium alginate gel.
Counteract inhibition effect of hydrocolloid on the setting of gypsum---hardness.
Calcium sulfate dihydrate (reactor) Potassium sulfate, potassium, zinc fluoride, silicates or borates, potassium titanium fluoride
Sodium phosphate (retarder)
To react preferentially with calcium ions to provide working time before gelation
As a filler
Diatomaceous earth or silicate powder
Control consistency and flexibility acts as filler
To make the powder dustless.
Wintergreen, peppermint, anise, pigments
Flavors and colors agents for pleasant taste coloring.
Disinfectants (quaternary ammonia salt and chlorhexidine)
To help in disinfection.
Types: Type I â€“ Fast setting. Type II â€“ Normal setting. It is available as a powder in bulk or preweighed sachets. Modified alginates or latest advances: A) In the form of sol contains water with a reactor of plaster of paris supplied separately. B) As 2 paste system, one containing the alginate sol, the second the calcium reactor. These are said contain silicone and have superior tear resistance.
C) As chromatic alginates, containing acid/base indications that change color at different critical points indicating mixing time, loading into mouth and setting. D) Dustless alginates that contain a coating of glycol that prevents silicon dust during fluffing. E) Silicone alginates – incorporates of silicone polymers improve physical properties. Setting reaction: When mixed with water, the calcium sulphate (reaction) first reacts preferentially with the retarder (sodium phosphate). It then reacts with the sodium or potassium alginate to form the final gel. -
2Na 3PO 4 + 3 CaSO 4 Ca 3 (PO 4) 2 + 3 Na 2 SO 4 .
Sodium alginate + CaSO 4 + H 2O Ca Alginate + Na 2 SO 4 (Gel)
This is the characteristic brush heap gel network with calcium alginate, excess water, filler particles, sodium alginate etc. calcium is responsible for the cross-linking. Properties: According to ADA specification No. 18. 1. Flexibility – Approx 14% at a stress of 1000gm/cm2. 19
2. Elastic recovery – Highly elastic with about 97.3% elastic recovery. Removal should be quick from the mouth. 3. Tissue detail reproduction – Specification of 0.075mm width reproduction. Most products exceed this value. 4. Strength – Compressive 5000-8000 gm/cm2 (0.313-0.70 Mpa). Tear strenth – 350-700 gm/cm2. 5. Dimensional stability – Poor dimensional stability with 100% relative humidity (humilar) being best storage media. 6. Try retention is by mechanical interlocking in perofations or adhesive use. 7. Silica particles arising from fulfilling alginate are a potential health hazard addition of glycol has resulted in ‘dust free’ alginates. Manipulation: After
manufacturers supplied scoop and add water according to retraction (usually 2 scoop of powder and one measure water) 5-15g powder and 40ml of water. After a figure of 8 stirrring motion to every wet the particles, rapid spatulation by swiping and stropping against the bowl and done to obtain a 20
smooth creamy mix. It can be done with mechanical devices too. Mixing time ranges from 45-60s and working time 1 Âź minute 2 minutes. Setting time is usually 1-2min for the fast set which 2-4.5 min for regular set. Can be altered using cooler water and retards. Load into a mechanically retentive tray providing at least 3mm space all sides and smoothened with a moistened finger. Some material may be directly applied on the tooth tissue surface and tray is then seated with firm pressured and held stable. After setting, indicated by loss of tackiness and rebounding on probing or by color change, impression is removed after 2-3 minutes with a quick snap or jerk and poured immediately removing cost after 1 hour. Disinfection is done using 10 minutes immersion of sodium hypochlorite or glutaraldehyde. Advantages: 1. Easy to mix and manipulate. 2. Minimum requirement of equipment. 3. Flexibility of set impression. 4. Accuracy of properly handled. 5. Low cost. 6. Comfortable to patient.
7. Hygienic. 8. Good surface detail even in saliva. Disadvantages: 1. Cannot be electroplated. 2. Distortion occurs easily. 3. Poor dimensional stability. 4. Poor tear strength. Hence not recommended for high accuracy applications like crown bridges, cast partial denture etc. Uses: 1. Impression making in complete dental prosthesis and orthodontics. 2. In undercut and in excessive salivary flow. 3. For impression for mouth protection. 4. For impression in study models and working cast. 5. Limited usage in crown and bridge inlay procedures. 6. For duplicating models. Types of failures: 1. Distortion.
2. Grain impression. 3. Tearing. 4. Bubbles. 5. Irregular voids. 6. Rough and chalky. 7. Stone cast. Next we come to the most important categories of impression material: Elastomers or rubber base impression materials: Non â€“aqueous elastomeric dental impression materials as per ADA sp. No. 19 are liquid polymers that cross link or polymerize with various reagents to become solid elastic rubber at room temperature. They are essential in todayâ€™s high technique dental age of metal free ceramics and high precision castings.
I) According to Chemistry: 1. Polysulfides. 2. Polyethers. 3. Silicones or polysilicones
Condensation Addition polyvinyl silicones.
II) According to Viscosity: 1. Light body or syringe. 2. Medium or regular body. 3. Heavy body or tray consistency. 4. Very heavy or putty consistency. III) ADA Classification : Based on selected elastic properties and dimensional changes: 1) Type I. 2) Type II. 3) Type III. General properties: 1. Excellent reproduction of surface details.
2. Generally hydrophobic (polyethers are hydrophilic). 3. Good elastic properties. 4. High coefficient of thermal expansion resulting in thermal contraction or removal from mouth to room temperature. 5. Dimensional inaccuracies are lower but exist due to myriad reasons. 6. Excellent tear strength. 7. Electroplatable. 8. Require try adhesives and mechanical interlocking. 9. Extended shelf life. 10.Generally high cost. Generally supplied in 2 paste or jaw system though automix cartrides and gums are now becoming increasingly popular: USES: 1. Impression material for all applications including: -
Fixed parital dentures.
Dentulous and edentulous impression.
2. Border moulding applications.
3. Bite registration. 4. As duplicates material for refractory casts. Now lets explore the individual systems: I) Polysulfides: This was the first elastomeric impression material to be introduced. Also known as Mercaptan or Thickol. Composition: Base paste: -
Liquid polysulfide polymer – 80-85%.
Inert fillers (titanium dioxide, zinc sulfate, copper carbonate or silica) –16-18%.
Reactor paste: Lead dioxide – 60-68%. Dibutyl phthalate – 30-35% Sulfur – 3% Other substances like magnesium stearate (retarder) and deodorants-2% Tray adhesives like butyl rubber or styrene / acrylo nitrile dissolved in a volatile solvent such as chloroform or a ketone are used with polysulfides. 26
Available as 2 system – box accelerator and 3 viscosities light, medium and heavy bodied. Chemistry and setting reaction: On mixing, the lead dioxide react with the polysulfide polymer resulting in: -
Chain lengthening by oxidation of terminal – SH groups.
Cross linking by oxidation of the pendent – SH groups.
The reaction is exothermic – 3-4°C rise in temperature and is accelerated by heat and moisture PbO2 + S HS – R – SH HS- R- S- S – R – SH + H2O OR Mercaptan + Lead dioxide Polysulfide + water Organic hydroperoxide (t-butyl hydroperoxide) can be used as alternative to lead dioxide. Other cross linking systems like complex inorganic hydroxides (e.g. copper).
Properties: 1. Unpleasant odor and color. Stains linen and is mercury. 2. Exhibit pseudoplasticity filler particle size is 0.3µ. 3. Mixing time is 45 seconds. 4. Long setting time of 12.5 seconds (at 37°C). 5. Excellent reproduction of detail. 6. Dimensional stability – curing shrinkage is 0.45% and has highest permanent deformation (3-5%). 7. It has high tear strength (4000g/cm). 8. It has good flexibility (7%) and low hardness 2mm spacing of tray is required. 9. It is hydrophilic. 10.Can be electroplated –more with silver then copper. 11.Shelf life is good (2 yrs). II) Silicone Rubber impression material: Developed to overcome disadvantages of polysulfide materials, those are based on silicone technology and are of 2 types: -
Condensation silicones. 28
Available in various colors and viscosities. Condensation silicone Also known as conventional silicones, available as pastes and in putty in contrasting colors. Composition: Base: -
Poly dimethyl siloxane (hydroxy-terminated).
Colloidal silica or microsized metal oxide filler35-75%.
Ortho ethyl silicate â€“ crosslinking agent.
Stannous octoate - catalyst.
Chemistry and setting reaction: It is a condensation reaction. Polymerization occurs as a result of cross linkage between the orthoethyl silicate and the terminal hydroxy group of the dimethyl siloxane to form a 3D network. Reaction is exothermic (1Â°C). 29
CH 3 | OH – Si – OH + C 2H 3 O –Si –OC 2H 5 | | CH 3 OC 2H 5
Silicone + CH3 CH2 OH Silicone Rubber
Ethyl alcohol (the one we drink)
Dimethyl silicone + Orthoethyl silicate
Ethyl alcohol is a reaction by product that evaporates gradually leading to shrinkage and dimensional instability. Tray adhesives contains poly dimethyl siloxanes and ethyl silicate. Properties: 1. Pleasant odor and color. 2. Mixing time of 45s, setting time is 8-9 minutes. 3. Excellent reproduction of surface details and highly elastic. 4. Lener dimensional stability because of high curing shrinkage (0.40.6%) and due to ethyl alcohol evaporation, permanent deformation is also high (1-3%). 5. Tear strength is 3000gm/cm, lower then polysulfides. 6. Needs a tray with 3m spacing on all sides. 7. Hydrophobic, hence needs a dry field.
8. can be electroplated and has adequate shelf life. 9. biologically inert. 10.Compatible with all gypsum product. Addition silicones: Also called as polyvinyl silicones, these care later than condensation sterilization but are the most widely used elastomers. Available in various viscosities and pastes and jars with contrasts colors. Composition: Base: -
Poly (methyl hydrogen siloxane).
Other siloxane prepolymers.
Other siloxane prepolymers.
Platinum salt catalyst (chloroplatinic acid).
Palladium or hydrogen absorber.
Filler particles size is optimally 5-10Âľm. These are surface treated to make it compatible with and reinforce the silicone rubber.
Chemistry and setting reaction : It is an addition reaction. The base polymer is terminated with vinyl groups and cross linked with silane (hydride gps) and reaction is activated by platinum salt.
CH 3 | Si – H + CH 2 | CH 2
Vinyl silicone + Silane siloxane
CH 3 CH 3 | | Si – CH 2 + CH 2 – S | | CH 3 CH 3 Silicone rubber
Usually there are no byproducts, but imbalance may cause release of hydrogen gas which causes air bubbles in the stone models. To avoid this, palladium is added. One important point to note for silicone is that sulfur compounds can retard the setting of silicones. Latex gloves and rubber dam material can be a potential source of sulfur contamination thus prolonging and distorting setting times. Thus care should be taken to avoid this error. Properties: 1. Pleasant odor and color. 2. Excellent reproduction of surface details. 3. Mixing time of 45s, setting time of 5-9 minutes.
4. Best dimensional stability. Low curing shrinkage (0.17%) lowest permanent deformation (0.05-0.3%). 5. Pouring of stone is delayed by 1-2 hrs if non-palladium containing mixes are used and can be poured for upto 1 week later with dimensional accuracy. 6. Good tear strength (3000gm/cm2). 7. Though extremely hydrophobic, thus repairing dry fields and precautions while pouring, the latest formulations have surfactants and other chemicals added to make them more hydrophilic and adapt to even slightly moist surface very well, rendering them less technique sensitive. 8. Can be electroplated with silver and copper. 9. Regions trays with 3mm spacing on all sides for best results. Lower flexibility. 10.Good shelf life of 1-2 years.
III) Polyethers: Introduced in Germany in late 1960s, it has good mechanical properties dimensional stability but is expensive. Again available in different viscosities and as base and accelerator. Composition: Base: -
Colloidal silica and filler.
Glycolether or phthalate – plasticizer.
Aromatic sulfonate ester – cross linking agents.
Colloidal silica –filler.
Phthalate or glycolether – plasticizer.
Chemistry and setting reaction: It is cured by the reaction between aziridine ring which are at the end of branched polyether molecule. The main chain is a copolymer of ethylene oxide and tetrahydrofuran. Cross linking is via the aromatic sulfonate ester via the imine end gps. Reaction is exothermic (4-5°C released). 34
H O O | | | CH 3 – C – CH 2 – C –O – R – O – C – CH 2 – | N | CH 2 – CH 3
H | C – CH 3 + SO 3 R | N Cross linked rubber | CH 2 – CH 3
Poly ether + Sulfonic ester Properties: 1. Pleasant odor and taste. 2. Mixing time of 30 s, setting time of 8.3 minutes. 3. Curing shrinkage is low (0.24%) remnants deformation is also low (1-2%) can absorb water and change dimension. 4. Very stiff (flexibility of 3%) require extraspacing of upto 4mm. 5. Tear strength is good (300gm/cm2). 6. It is hydrophilic, so moisture control is not a critical has best compatibility with stone. 7. Can be electroplated silver and copper. 8. Excellent shelf life above 2 years.
TECHNICAL CONSIDERATIONS: Basically, for mixing putty, the base and accelerator are taken from jag in scoop and kneaded by heat to obtain a uniform, stream free mix. For pastes, equal lengths are dispensed or if directed, more base and less accelerator is taken and mixed vigorously and evenly with a stiff metal spatula to obtain an even, strink free mix and then loaded into syringe or tray as required. After checking the tray with appropriate spacing and using tray adhesives if necessary â€“ various techniques can be employed â€“ 1) Single mix technique : A resin custom tray with 2-4 spacing with regular viscosity elastomer is used. The paste is mixed, graded into the tray and syringe and injected onto the prepared area. Try material is seated over it and allowed to set. 2) Multiple mix technique: A custom tray with 2-4mm spacing and heavy and light body elastomers are used. The 2 viscosities are mixed simultaneously on separate prob. Heavy bodies is carried on a tray, light body is injected around the area of preparation and the heavy body seated on it till set to produce a single impression.
3) Reline technique (2-stage putty-wash technique): Here a preliminary impression is taken with a putty silicone is a stock impression tray. This forms a custom made tray in which by cutting away some of the tray silicone or by using thin resin, rubber or wax sheet as spaces between the teeth and the silicone. This area is then filled with a thinner consistency silicone and the tray is repeated into the mouth for reproduction of sharp angles accurately, a light bodied silicone is injected. The latest technique is mixing is the use of automatic dispensers and mixers. These consist of a double barrel caulking gun with mixing tip. The tip contains spirals on the inside. Forcing of the base and accelerator through these spirals results in mixing. Advantages include improved properties, more uniform mix, lesser air bubbles and reduced working time. Finally, the impression is removed after checking is set by providing with a blunt instrument. It become firm and returns to its original contour. Removal is done quickly and is one motion for best result disinfection is done by 10 minutes in 2% glutaraldehyde or 3 minutes in chlorine dioxide solution. Phenols or iodophors can also be used.
RECENT ADVANCES IN ELASTOMERS: Visible light cured polyether urethane dimethacrylate: In early 1988, a visible light cured impression material was introduced (Genesis, L.D. Caulk). Available in 2 viscosities â€“ Light and heavy bodied. Composition: 1. Polyether urethane dimethacrylate. 2. Photoinitiates. 3. Photo-accelerators. 4. Silicone dioxide filler. Properties: They have long working but short setting times. Blue light is used for curing along with transparent impression trays. Highest resistance to tearing among the elastomers (tear strength of 6000-7500gm/cm2). Dimensional stability, flow, detail reproduction, permanent deformation, wettability, compatibility with cast materials and electroforming is similar to addition silicones. The material is rigid and severe undercuts should be blocked to ease impression removal.
Manipulation: Light body is syringed into the sulcus and over the preparation while heavy body is loaded onto the clear tray and seated over the light body. Both are simultaneously cured with a visible light curing unit having an 8mm or larger diameter probe. Curing time is approximately 3 minutes. Advantages include â€“
Controlled working time. Excellent properties.
Disadvantages include; 1. Need for special transparent trays. 2. Difficult to cure in remote areas. It is contraindicated in patients with a known allergy or sensitivity to urethanes, acrylics or methacrylates. Lastly, we come to the inelastic impression materials â€“ due to this limited use in operative dentistry, we shall have a brief overview of those: 1. Impression plaster â€“ Type I gypsum i.e. calcium sulfate with modifier was used earlier but is really used now. It is brittle and rigid. It may be used as a final or wash impression in complete denture prosthesis.
2. Impression compound or modeling plastic â€“ A thermoplastic material, it is used primarily for edentulous complete denture primary impression and for single tooth tube impression with a copper band, greenstick compound, a type of impression compound is used for border moulding. It is composed of thermoplastic resins, copal resins, carnauba wax, steam acid, talc, coloring agents and fillers along with plasticizers. The compound softens at 39Â°C(glass transition) and is manipulate at 43.5Â°C (fusion temperature). It can be softened over a flame or in warm water. It is then loaded on to tray and binding seated till rigid. Dimensional stability is less with distortions occurring and surface details reproduction is comparatively less. Casts should be poured immediately. Advantages include repeated cure and reparability. Disadvantages include distortions and difficult manipulation as well as rigidity.
3) Zinc oxide eugenol pastes: Available as 2 pastes, composition is as follows: Base paste
Zinc oxide – 87%
Oil of cloves or eugenol – 12%
Fixed vegetable or mixed oil – Gum or polymerized resin – 50% 13% Filter (silica type) – 20% Lanolin – 3% Resinous balsam – 10% Accelerator solution (CaCl2) – 5% Coloring agents. Setting reaction is an acid base reaction forming zinc eugenolate. Final setting range from 10-15 minutes. Manipulate – equal length are dispersed, mixed quickly (1 minute mixing time) and loaded onto the tray, seated till set and removed. Advantages
compatability with casts. Disadvantages include requirement of special tray, burning reaction of eugenol and inability register undercuts. Non eugenol pastes have been developed to overcome eugenol initiation by adding carboxylic acids like orthoethoxy benzoic acid. They can also be used as bite registration pastes.
4) Lastly, a material not strictly an impression material but used as suchinlay wax -used in direct or indirect techniques to record single tooth impressions. It has type I and II and is composed of paraffin wax, gum dammar, carnauba wax and coloring agents. Candellila wax, natural resins and other waxed are also added. The wax softens at about 40-45°C and flows at 56°C or higher and vaporizes at 500°C. it is heat flamed, softened and manipulated as desired, and invested immediately to avoid distortion. CONCLUSION: The famous saying goes “The first impression is the best impression”. That should also be the endeavor of every dental surgeon. Realizing that a restoration or prosthesis can only be as good as the preparation and the impression will encourage dentists to master the art and science of impression making and recording. This can only be fulfilled by having an indepth information of material science and unraveling their intricacies as well as being update on the driving technologies and techniques governing those materials. Only this holistic knowledge will enable clinicians deliver ideal dental care and “impress” the patient.
Bibliography: 1. Philipps Science of Dental Materials. 2. Restorative Dental Materials – Craig. 3. Materials in Dentistry – Jack L. Ferracane. 4. Basic dental material – John J. Manappalhl. 5. Notes on dental materials – C. Combe. 6. Dental materials. – Richard Van Noort.