Impression materials/ rotary endodontic courses by indian dental academy


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Impression materials/ rotary endodontic courses by indian dental academy

  1. 1. IMPRESSION MATERIALS First put your 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 us from. Let’s confabulate on this topic under these myriad subheadings: Introduction History Definition and Classification Ideal requisities Impression materials and Respective techniques - Hydrocolloids. - Elastomeric materials. - Inelastic or rigid. Latest Advances Summary and conclusion 1
  2. 2. Bibliography 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 crude 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 in delve deeper into this fascinating and vitally important class of material science and understand them for their successful and ideal use. An impression, in general terms is a mark produced on a surface by pressure. The word impression is divided from latin word “impression”. 2
  3. 3. From impression, it is possible to produce an exact replica of the dental structures of interest using a cast or die material such as dental stone or some type of plastic. There are various categories of impression materials. Each type processes characteristics which influence the purpose to which it is best suited, i.e. different types of impression materials have been developed few different application. These materials have different physical properties and each has certain advantage or disadvantages. Thus, an understanding of physical characteristics and limitation of each material is necessary for its successful use in clinical dentistry. It would be virtually impossible to perform high quality restorative dentistry without impression materials. 3
  4. 4. History: The history of dentistry has been influenced a great deal by development of impression materials. From the cumbersome and highly unpredictable materials of yore replicating materials now match highly exacting standards. In fact the earliest dental impression materials was waxes that was used in 18th and 19th century. Beeswax was apparently the materials first used in making impression in the mouth. In 1782 William Rae said that he get the measurement of jaws in a piece of wax pushed into the gums, afterward making a cast of it with plaster of paris. In 1842, Montgemery discovered gutta-percha. In 1848 Colburn or Blake said that it should thoroughly soaked in boiling water, then kneaded and moulded in the same way as wax and immediately by placed in the mouth and firmly pressed to its place. In 1930 according to Applegate, a series of true physiologic waxes was developed by cooperative effort of Drs. G.C. Bawles, S.G. Applegate. 4
  5. 5. The first real impetus in the use of the zinc oxide eugenol for impression materials came from two pioneer dentists, A.W. Ward and E.B. Kelly, during early 1930’s. Alginate type materials were experimented with over many years, however, the first patient was awarded in 1936. This in early 1940’s the first irreversible hydrocolloidal (alginate) impression material was developed. In middle 1950’s the elastomeric impression materials were introduced. Polyethers were introduced to dental profession in Germany in late 1960’s. Recently, a new polyether urethane diamethacrylate photo- initiated elastomeric impression material has entered the market. Definition and Classification: Impression material is a dental materials whose function is to accurately record the dimensions of the oral tissues and their spatial relationship. An impression essentially is a negative replica of some structure. In dentistry, this replica usually is made of teeth or gingival tissue of maxillary such as mandibular arch. 5
  6. 6. Classification: Impression Materials: Elastic Non-elastic Hydrocolloid Rubbers ZOE Impression compound Agar Alginate Mercaptam Silicones Polyether Lead peroxide Clean Condensation addition Catalyst catalyst Many criteria may be used to classify impression materials. They are listed as follows: 1) By their generic chemical same: 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: A) Set by chemical reaction (Irreversible)  Plaster of paris  Zinc oxide eugenol.  Alginate  Non-aqueous elastomers. 6
  7. 7. B) Set by temperature change (reversible) These materials are again subclassified as: 1) Thermoplastic materials. - Impression compound. - Wax. 2) Non-thermoplastic material and agar 3) According to the ability of set material to be withdrawn over undercuts. A) Elastic impression materials: - Alginate. - Agar. - Non-acqueous elastomers. B) Non-elastic or rigid impression materials: - Impression compound. - Impression plaster. - Zinc oxide eugenol. - Wax. 4) According to the use of material in dentistry: A) Materials used for obtaining impression of dentulous mouth. - Alginate. - Agar. - Non-acqueous elastomers. 7
  8. 8. These materials are used in crown and bridge, partial denture and inoperative dentistry. B) Materials used for obtaining impression of edentulous mouth: - Impression compound. - Impression plaster. - Zinc-oxide eugenol. - Wax. 5) According to the viscosity or the tissue displacement: Materials which are initially vary fluid are often classified as mucostatic impression materials, because they are less likely to compress soft tissues, while materials which are initially more viscous are classified as mucocompressive. A) Mucostatic materials: - Impression plaster. - Agar. - Alginate. B) Muco-compressive materials: - Impression compound. 8
  9. 9. Ideal Requirements of Dental impression materials: To produce an accurate impression, the materials used to produce replicas of intraoral and some extraoral tissues should fulfill the following criteria. 1. Pleasant taste, odor and esthetic color. 2. Not contain any toxic or initiating ingredient. 3. Have adequate shelf life for storage and distribution. 4. Be economical. 5. Be easy to use with minimum equipment. 6. Have adequate setting characteristics that meet clinical requirements. 7. Possess satisfactory consistency and texture. 8. Adequate strength so that it will not break or tear while removing from the mouth. 9. Possess elastic properties with freedom from prominent deformation after strain. 9
  10. 10. 10. Exhibit dimensional stability over temperature and humidity ranges. 11. Readily wets oral tissues. 12. Compatibility with cast and die materials 13. Accuracy and faithful reproduction of details. 14. Ability to be electroplated. 15. Readily disinfected. 16. No release of gas during setting of impression or cast and die materials. - They should be fluid enough to adapt to the oral tissue and viscous enough to remain content in the tray that delivers impression to the mouth. - While in the mouth they should transform (set) into a rubbery solid in a reasonable amount of time (should be less than 7 minutes). - The set impression should not distort or tear when removed from the mouth. Material should dimensionally stable so the cast can be poured. 10
  11. 11. - In this seminar I will be discussing about the following impression material: Hydrocolloids Reversible Irreversible 1) Hydrocolloids – It is a suspension of time part less the 1µm). Colloids are often classified as a faster state of matter, the colloidal state, because of their difference in structure, constitution and reaction. If the particles are large and can be seen by the naked eye as through a microscope, the system is turned a suspension or emulsion. There suspended particles do not readily diffuse and tend to fall out of the suspending medium unless some type of bonding is employed to maintain the suspension or emulsion. The molecules of the colloid remain dispersed nature of fact that they carry small electrical changes and repel one another within the dispersion medium. Types of colloids: With the exception of the gaseais state (two gases), colloidal sol may be composed of combination of any other states of matter. For example: 11
  12. 12. - Liquid / solid in air (aerosol). - Liquid / solid in liquid (Lysol). - Gas /liquid / solid in solid. All solids are termed as sols, and not just more in which a liquid is the dispersion medium. Since hydrocolloid impression materials are solids (polysaccharides) suspended in liquid (water) they are lyophilic (liquid leaving) solutions. In general, organic colloids are lyophilic, whereas the metallic dispersion tend to lyophobic (liquid heating). Dental hydrocolloid impression materials exist in two form: sol or gel form. In the sol form they are fluid with low viscosity and there is random arrangement of polysaccharide chains. In gel form the materials are more viscous and may develop elastic properties if the long polysaccharide chain become aligned. Gels: If the concentration of the dispersed phase in the hydrocolloid is proper amount, the sol may be changed to a semisolid material known as a gel or jelly when the temperature is decreased. The temperature at which this change occurs is known as gelation 12
  13. 13. temperature is about 37°C as slightly higher few agar. The process is reversible. Thus agar is known as reversible hydrocolloids. Reversible hydrocolloids: These impression materials are compounded from reversible agar gels, when heated, they liquefy argo into the solution state and on cooling they return to gel state. Since this process can be repeated, a gel of this type is described as reversible. Agar impression are dimensionally unstable on standing and so models should be made so soon as possible after the impression is taken. If the agar type of impression material is used carefully with an understanding of its physical properties it is an excellent elastic impression material of high accuracy in registering fine detail. Chemical Ingredients: The main active constituent of a reversible hydrocolloid impression product is agar-known commercially as agar-agar, which is a sulfuric ester of a galactan complex. This material forms a colloid with water which will liquefy between 71°C and 100°C and set a gel again between 30°C and 50°C. 13
  14. 14. Composition and function: 1) Agar (12.5%) – To prevent the dispersed phase of the solution and the continuous fibril structure of the gel. 2) Potassium sulfate (1.7%) – To counteract the inhibiting effect of beewax and agar on the setting of gypsum model materials. 3) Borax (0.02%) – To produce intermolecular attraction in order to improve the strength of the gel. 4) Alkyl Benzoate (0.1%) – To prevent the growth of the mold in the impression material during storage. 5) Water (85.5%) – To provide the continuous phase in the solution and second continuous phase is the gel, the amount centrals the flaws properties of the solution and the physical properties of the gel. 6) Colour and flavour (trace) : To improve the appearance and taste. The agar content is reduced in the syringe type of materials so that it is much more fluid at the time of injection than the tray material at the time of insertion. 14
  15. 15. Properties: 1) Viscosity of the sol: In sol form agar is sufficiently fluid to allow detailed reproduction of hard and soft tissues. Its low viscosity classifies it as a mucostatic materials as it does not compress or displace soft tissues. Agar is a visco-elastic material. 2) Strength: The strength values of importance for agar impression are tear strength and the compressive strength. Since the agar impression are viscoelastic the strength preparation are time dependent and higher compression and tear strong occur at higher rates of loading. The compressive strength of a typical agar impression material is 8000gm/cm2 (0.245Mpa or 35.6Psi). The tear strength is about 700gm/cm though there is no ANSI/ADA specification requirement. 3) Gelation temperature: The temperature at which the hydrocolloid impression material sets to a gel is important. The gel must be heated to a higher temperature. Known as liquefaction temperature (70°C to 100°C) to return to its sol condition. 15
  16. 16. It transforms into a gel between 37° and 50°C. The exact gelation temperature depends on several factors, including the molecular weights, the purity of agar and the ratio of agar to other ingredients. 4) Dimensional stability: Storage condition Dimensional change Causes 1) Air Shrinkage Evaporation of water foam gel 2) Water Expansion Inhibition and absorption of water 3) 100% relative humidity Shrinkage Syneresis 4) Inorganic salt solution Expansion / Shrinkage Depends on relationship of electrolyte in gel and solution. When stored in air, agar gels losses water and contract, when they started in water it results in absorption and swetting. So agar impression are best stored in 100% relative humidity not more than 1 hour. 5) Flexibility: The ADA specification requirement for flexibility allows a range of 4% to 15% when a stresses of 14.2 psi (1000gm/cm2 ) and 16
  17. 17. most agar impression materials meet this requirement. A few hard setting materials have a flexibility of 1% to 2% agar gel has very poor mechanical properties and tears at very low levels of stress. Inter proximal and subgingival areas are very difficult to record with this type of impression materials. Advantages: 17. Hydrophilic impression materials since it respondless critically to moisture, fluids in the sulcus are less tolerated. 18. Long working time. 19. It does not require any custom tray. 20. Because there is no mixing of separated components, the potential for errors of measurement are eliminated. 21. It is clean and pleasant. 22. It is compatible with die stone, enabling bubble fice casts. 23. It is a material of high accuracy and registers fine detail. 17
  18. 18. 24. Casts are easily removed. 25. Reliability. 18
  19. 19. Disadvantages: 1. Low tear resistance. 2. Low dimensional stability and the special handling to prevent dimensional change. 3. Potential hazard of a bown to the patient. 4. Initial purchase of the conditioning unit. 5. Easily distorted as a result of movement during gelation. 6. Rapid cooling can cause concentration of stresses near the tray during gelation. Clinical Presentation: The agar impression material is supplied as a gel in a metal, plastic or other types of collapsible disposable or as a number of cylinders in a glass jar. The first form is used with a water cooled impression tray and the second for injector with a syringe. The syringe material may be used in combination with a tray material or a copper-band technique as with impression compound. 19
  20. 20. Types of failure: Type Cause 1. Grainy material. A. Inadequate boiling. B. Storage temperature too low. C. Storage time too low. 2. Seperation of tray and syringe material A. Water- soaked layer of tray material not removed. A. Gelation of either syringe or tray material. 3. Tearing A. Inadequate bulk. B. Premature removal from mouth. C. Syringe material partially gelated when tray seated. 4. External bubbles A. Gelation of syringe material preventing flaw. 5. Irregularly shaped voids A. Material too cool or grainy. Laminate technique (Agar alginate combination impression): In a recent modified procedure, the tray hydrocolloid is replaced with a mix of chilled alginate that will bond with the syringe agar. The alginate gels by a chemical reaction, where the agar gels by 20
  21. 21. means of contact with the cool alginate rather than the water circulating through the tray. The equipment needed for taking an agar impression can be minimized by use of agar alginate syringe tray combination impression. In this procedure a syringe type of agar in a cartridge is heated in boiling water for 6 minutes and stored in 65°C water bath 10 minutes before use. The tray alginate of the regular set type is mixed with 10% more water than normally recommended and it is placed in a tray. The agar is injected around the preparation, and the mixed alginate is promptly seated on the top of agar. The alginate sets in about 3 minutes and agar sets with this time as a result of being cooled by alginate. During the setting of alginate and gelling of the agar a bond forms between them. The impression may be removed in about 4 minutes. The accuracy of the agar-alginate impressions was determined with a laboratory model. Impressions were taken and paired in high strength stone. The accuracy of 1) The interpreparation distance, 2) buccolingual diameter and 3) the preparation height of the models were measured and compared to values obtained with polysulfide condensation silicone, polyether and addition silicone impression 21
  22. 22. materials. Except for the interpreparation distance the agar-alginate system had same order of accuracy as rubber impression materials. The advantage of agar-alginate combination impression compared to agar system alone is the simplification of the heating equipment, the elimination of water cooled impression trays and the overall simplification of the procedure. In addition, the agar is more compatible with gypsum model materials than alginate, the accuracy is acceptable and the cost of the material is low. Wet field technique: Another recent technique has become popular for making impression in a wet field. It differs in than the tooth surface and tissue are purposely left wet. The areas are actually flooded with warm water. Then the syringe material is introduced quickly, liberally and in bulk to cover the occlusal and / or incisal areas only. While the syringe material is still liquid, the tray material is seated. The hydrolic pressure of the viscous tray material forces the fluid syringe hydrocolloide down into the areas to be restored. This motion displaces the syringe material, blood and debris with the stronger tray material throughout the sulcus. Irreversible hydrocolloide: 22
  23. 23. At the end of the last century, a chemist from Scotland noticed that certain brown seated (algal). He named it algin. This natural substance was later identified as a linear polymer with numerous carboxyl acid groups. When agar impression material become scarce because of World war II (Japan was a pumice scarce of agar), research was done to find a suitable substitute. The result was of cause the present irreversible hydrocolloid, or alginate. Alginate is an elastic mucostatic impression material. It is more widely cured than any other elastic impression material. The principle factors responsible for the success of this type of impression material are: 1. Early to manipulate. 2. It is comfortable to the patient. 3. It is inexpensive. 4. Does not require elaborate equipment. Uses: 1. Used widely in complete and partial denture prosthesis and orthodontics. 23
  24. 24. 2. To prepare study models of either the entire dental arch or a segment of it. 3. For master impression in rigid or in divided trays. 4. Whenever there is undercut and not suitable for rigid materials. 5. Useful to a limited extent in inlay, crown and bridge procedures. 6. In mouth where there is excessive flow of saliva. 7. As a duplicating mateial. 8. To prepare gypsum models of patients for the preparation of alternate in mouth protectors. Composition: Ingredients Function 1. Potassium alginate 18% To dissolve in water and react with calcium ions. 2. Calcium sulfate dehydrate 14% To react with potassium alginate to form an insoluble calcium alginate gel. 3. Potassium sulfate, potassium zinc fluoride silicate or borates 10%` To counteract the inhibiting effect of the hydrocolloids on the setting of gypsum, giving a high quality surface to the die. 24
  25. 25. 4. Sodium phosphate 2% To react preferentially with calcium ions to provide working time before gelation. 5. Diatomaceous earth or silicate powder 56% To control the consistency of the mixed alginate and the flexibility of set impression. 6. Glycols-small amount To make powder distlys. 7. Winter gum, peppermint trace To produce a pleasant taste. 8. Pigments – Trace To provide colour. Change in the water / powder ration will alter the consistency and setting times of the mixed material. Mixing time for regular set is 1 minute. Fast set alginate should be mixed with water for 45 seconds. According to ADA specification No 18 for alginate impression material requires that it no less than 1.25 minutes. Flexibility: The ANSI/ADA specification permits a range of 10% to 20% at a stress of 1000gm/cm2 . The compressive strength ranges from 5000- 8000gm/cm2 . ADA specification requires a compressive strength of atleast 3500gm/cm2 . The tear strength vary from 350 to 600gm/cm. Types of failure: 25
  26. 26. Type Cause 1. Grainy material a. Improper mixing. b. Prolonged mixing. c. Undue gelation. d. Water / powder ratio too low. 2. Tearing a. In adequate bulk. b. M oisture contamination. c. Pr emature removal from mouth. d. Pr olonged mixing. c. Bubbles a. Undue gelation, preventing flow. 4.Irregularly shaped voids a. Air incorporated during mixing. b. Moisture or debris on tissue. 5. Rough or chalky stone cast. a. Inadequate cleaning of impression. b. Excess water left in impression. c. Premature removal of cast. d. Learning cast in the impression too long. 6. Distortion a. Impression not paired immediately. b. Movement of tray during gelation. c. Premature removal from 26
  27. 27. mouth. d. Improper removal from mouth. e. Tray held in mouth too long. Development in alginate: 1) Dustless alginate: Many materials have been formulated which give off little or no dust particles, so avoiding dust inhalation. This can be achieved by coating the material with a glycol / glycerine. 2) Chromatic alginates: During the setting reaction, the pH value of the fluid mass changes. Because of this, some manufacturers include acid/base indications in their formulation so that a colour change of the setting mass indicates that a certain point has been reached, usually the point at which the tray should be loaded or inserted into the mouth. 27
  28. 28. 3) Modified alginate: The traditional alginate is supplied as powder and water. There is no reaction until the water is added to the powder to initiate the reaction. There is yet another modification of this concept. The two component system may be in the form of two pastes. One certain alginate sol, while the second contain the calcium react or impression materials of this type are said to contain (9/50) silicone and humeetants to stabilize. It is said to be better than water / powder alginate. 4) Silicone alginates: Alginate modified by the incorporation of silicone polymers have been developed. There are supplied as two parts which are mixed together. The materials are considered as hybrids of alginates and silicone elastomers but their properties are closely related to more of alginates. 28
  29. 29. Next we come to the most important categories of impression material: Elastomers or rubber dam impression materials: Non-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 today’s high teeth dental age of metal free ceramics and high precision castings. Rubber impression materials: Three major types of rubber impression materials are used to record dental impressions. They are polysulfides, silicones (poly siloxanes) and polyethers. The silicone type is subdivided into two classes, condensation and addition or vinyl polysiloxane. Polysulfide: These materials are supplied in three consistencies: 1. Low (syringe or wash). 2. Medium (regular). 3. High (tray). 29
  30. 30. These are supplied as two pastes in collapsible when, one labeled base and the other labeled accelerator or catalyst. Base: - Polysulphide polymer – 80-85% weight. - Titanium dioxide, zinc sulfate, copper carbonate or silica – 16-18% weight. Accelerator: - Lead dioxide – 60-68%. - Dibutyl or dioctyl – 30-35%. - Sulfur – 3% Other substances such as Magnesium stearate and deodorants – 2% Properties: 1. The elastic properties of these rubber impression materials improve with curing time (i.e. the longer the impression can remain in the mouth before removal, the greater the accuracy). 30
  31. 31. 2. Polysulfide ranks as one of the best stiff of elastomeric impression materials. 3. They have the highest resistance to tearing (thin sections of polysulfide impression material are less likely to tear than similar thickness of polyether or silicone impression material). 4. The stone should be poured immediately because, the impression in the most accurate immediately after removing it from mouth. 5. The ADA specification for testing biocompatibility includes dental impression materials, despite the fact that the probability of allergic or toxic reactions from impression materials or their components is small. 6. Perhaps the most likely elastomer induced biocompatibility problem occurs when a piece of the impression material is left in the gingival sulcus. The irritation can range from minor to severe. The radioopacity of the load containing polysulfide materials is an advantage in these situations, as in the materials resistance to tearing. 31
  32. 32. 7. One way to minimize the effects of polymerization shrinkage, less reaction by-product and deformation associated with distortion is to minimize the amount of material that is used to make the impression. The most accurate polysulfide impression are made by using a custom acrylic tray. 8. The polysulphide polymer has a molecular weight of 2000 to 7000 with terminal and pendant mercaplan groups. The terminal and pendant groups of adjacent molecules are oxidized by the accelerator to produce chain extension and crosslinking. The reaction results in a rapid increase in molecular weight, and the mixed part is converted to a rubber. The weight percent of the filler in the base increases from low medium to high consistencies. The particle size of the filler is about 0.3µm. Although the most common active ingredient in the accelerator is lead dioxide, some magnesium oxide may also be present. 32
  33. 33. Other oxidizing agents such as hydrated copper oxide or organic peroxides such as amine hydroperoxide have been used as a substitute for lead dioxide. Properties: 1. Condensation silicone impression materials are more ideally elastic than polysulfides. They exhibit minimal permanent deformation and recover rapidly when strained. Like polysulfides, there materials are not very stiff, which means it is not difficult to remove them from undercuts without distortion. 2. The viscoelastic characteristics of these materials suggest that they can respond elastically as an viscous liquids that easily sustain permanent deformation. 3. Tear resistance is low for condensation silicone impression materials. Although they do not tear as easily as alginates or agar hydrocolloids, they must be handled carefully to avoid relining a margin of a crown preparation when it is run. 4. The excessive polymerization shrinkage of the condensation silicones requires a modification of the impression making 33
  34. 34. technique to produce accurate impressions. A putty-wash technique is used for condensation silicones. 5. Silicone is one of the most biologically inert materials. 6. The alkyl silicates are slightly unstable, particularly if they are mixed with a tin compound to form a single catalyst liquid. Thus a limited shelf life may result because of oxidation of the tin component within the catalyst. 7. The condensation silicones are compatible with all gypsum products. II) Silicone Rubber Impression materials: Developed to overcome disadvantages of polysulfide material, these are based on silicone technology and are of 2 types: - Condensation silicones. - Addition silicones. Available in various color and viscosities. Silicones (Polysiloxane): Condensation silicones are usually supplied in low and putty like consistency. These are supplied as base and an accelerator. The 34
  35. 35. base contains a moderately low molecular weight silicone called a dimethyl siloxane which have reactive terminal hydroxyl groups. Fillers may be copper carbonate or silica having particle size from 2 to 8µm in concentrations from 35% to 75% for low to putty consistencies. The accelerator may be a liquid that consists of stannous octoate suspension and alkyl silicate or it may be supplied as a paste. The reaction proceeds a three dimensional network rubber with the liberation of ethyl alcohol and an exothermic temperature vice of about 1°C. Besides viscosities common to the polysulfides the condensation silicones are also supplied in an extremely high viscosity or putty material. These materials are used in the putty wash techniques. Polyethers: Polyether impression materials are supplied as a medium consistency type in a base and an accelerator tube. The base is a moderately low molecular weight polyether with ethylene imine rings. Properties: 1. The polyethers have always been considered the stiffest of the impression materials, excluding the high viscosity putties. 35
  36. 36. Some of the new formulations of regular or medium-bodied material are actually less stiff. 2. The pseudoplastic characteristics allows the original single- viscosity materials to be used as both syringe and tray materials. 3. Tear resistance is better than that of the condensation silicone impression materials. However, polyether is more prone to tearing than polysulfide. 4. The dimensional change of the polyether impression material is small. Like the addition silicones, polyether have no by- product. 5. The most likely elastomeric induced problem for the patient arises from pieces of the impression materials being left in the sulcus. The irritation can range from mild to severe. 6. Storing in a cool, dry environment prolongs the shelf life. The terminal groups. The catalyst paste contains 2.5-dichloro benzine sulfonate as a cross-linking agent, along with a thickening agent. A separate tube contains a thicker that includes actylpindate and about 5% methyl cellulose as a thickening agent. The rubber is 36
  37. 37. formed by cationic polymerization and opening of imine rings. The setting reaction is slightly more exothermic than that of the other rubber impression materials with a temperature rice of about 4°C. Addition silicone (poly vinylsiloxanes): The addition type is available in low, medium, heavy and very heavy putty consistencies and is also polysiloxane. The base contains a moderately low molecular weight polymer with siloxane (-Si-H) groups and filler. The accelerator (or catalyst) contains a moderately low molecular weight polymer with vinyl terminal groups, pulls filler and chloroplatine acid catalyst. Several products contain finely divided palladium or platinum, which absorbs hydrogen. A retarder, a liquid low molecular weight polymer of the same type as the base polymer is available to extend the working and setting time. Silicone rubber impression are hydrophobic when mixes of gypsum products are poured into them. Properties: 1. As one of the most pseudoplastic impression materials, the effect of increased strain rate on the unset material is quite pronounced for vinyl polysiloxane. 37
  38. 38. 2. The resistance to tearing is adequate, similar to that of condensation silicone. If not handled correctly, these materials will fear rather than stretch like polysulfides. 3. Vinyl polysiloxane impression materials are most dimensionally stable of all existing materials. No volatile by- product is released to cause the material to shrink. The base and catalyst putty of addition silicones are mixed by hand. If they are mixed by the operator while later gloves are being warm, the setting time is lengthened or the material will not set. Sulphur compounds used in the vulcanization of latex rubber gloves can migrate to the surface on storage during mixing of the two putties, there compounds are incorporated into the mix and poison the platinum containing catalyst resulting in retarded or no polymerization. In general polysulfides have the longest working time, followed by silicones and polyethers. 38
  39. 39. Comparison of certain characteristics of elastomeric dental impression: Polysulfide Condensation silicone Polyether Addition silicone Mixing Fair to easy Fair to easy Easy Easy Flow Variable Good Good Good Stock Fair to good Fair Good Fair to Good Elastic recovery Fair Very good Very good Excellent Advanced Inpleaus Acceptable Acceptable Acceptable Clean up Difficulties Easy Easy Easy Types of failures: Type Cause 1. Rough or uneven surface on impression a. Inc omplete polymerization caused by premature removal from mouth. b. To o rapid polymerization from high humidity or temperature 2 Bubbles a. Too rapid polymerization preventing flow. b. Air incorporated during mixing. 39
  40. 40. 3. Irregularly shaped voids a. Inadequate cleaning of impression. b. Excess water left on the surface of the impression. c. Premature removal of cast. d. Improper manipulation of stone. 4. Distortion a. Lack of adhesion of rubber to the tray caused by not enough coats of adhesive, filling tray with material too soon after applying adhesive, or using wrong adhesive. b. Lack of mechanical retention for more materials where adhesive is ineffective. c. Excessive bulk of material. d. Movement of tray during polymerization. 6. Improper removal from mouth a. Premature removal from mouth Advantages of the elastomeric impression material: 1. They are vary elastic in nature. 2. They have good dimensional stability. 40
  41. 41. 3. The impression material do not have to be paired within half hour unlike hydrocollides. 4. More than one cast can be made successfully within or half hour. 5. Exceptionally smooth dies can be made. 6. Gum retraction or gingival retraction is not always necessary. 7. Accuracy is comparable to that of the hydrocollides. Visible light cured impression material: In early 1988, a visible light cured impression material was introduced (Genesis, L.D. Caulk). This material is available in two viscosities. The light body material is packed in disposable syringe and the heavy body material is packed in tubes. This material has excellent elasticity and very low dimensional shrinkage upon storage. It may be poured immediately or upto 2 weeks later. The material is rigid and it is recommended that severe undercuts should be blocked out to case removal of the impression. 41
  42. 42. Manipulation: No mixing as syringe loading is necessary. The light body material is syringed into the sulcus, around and over the preparation and portion of the adjacent teeth. A clear tray is loaded to the full time with heavy body material. After the tray is seated in the mouth, both viscosities are cured simultaneously using a visible light curing unit having an 8mm or larger diameter probe. The curing time is approximately 3 minutes. Advantages: 1. Control over the working time. 2. Curing time is relatively short (3 minutes). 3. Excellent clinical, physical and mechanical properties. Disadvantages: 1. Need for special tray which should be transparent to the visible light received to case the material. 2. If delay occurs before placement, the material should be stored in a dark place away from light. 3. Difficult to light cure the remote area. 42
  43. 43. III) Polyethers: Introduced in Germany in late 1960’s it has good mechanical properties and dimensional stability but is expensive. Again available in different viscosities and a base and accelerator. Composition: Base: Polyether polymer. Colloidal silica – filler. Glycolether or phthalate – plasticizer. Accelerator: Aromatic sulfonate ester – cross linking agents. Colloidal silica-filler. Phthalate or glycolether – plasticizer Chemistry and setting reaction: It is cured by the reaction between azinidine rings 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 43
  44. 44. aromatic sulfonate ester via the imina end gps reaction is exothermic (4-5°C↑). Properties: 1. Pleasant odor and taste. 2. Mixing time of 30 seconds, setting time of 8.3 minutes. 3. Curing shrinkage is low (0.24%) permanent deformation is also low (1-2%) can absorb water and change dimension. 4. Very stiff (flexibility of 3%) requires extraspacing of upto 4mm. 5. Tear strength is good (3000gm/cn2). 6. It is hydrophilic, so moisture control is not a critical. Has best compatibility with stone. 7. Can be electroplated with silver and copper. 8. Excellent shelf tip above 2 years. The material should not be used with patients with a known always or sensitivity to urethanes, acrylic or methacrylates. Impression techniques: There are two techniques: 44
  45. 45. - Multiple mix technique. - Reline technique. a) Multiple mix technique: Two consistencies of material are provided one for use with the tray and other for use with the syringe type has longer working and setting time, and a greater polymerization shrinkage and thermal contraction. In the multiple mix technique, both the syringe and tray material are used for same impression. The tray material is usually mixed first and filled into the tray to a uniform thickness and set aside. The syringe material is mixed on a separate mixing pad, loaded into a syringe and injected into the prepared teeth. The filled tray is then carried to place. b) Reline technique (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 45
  46. 46. the mouth for reproduction of sharp angles accurately, a light bodied silicone is injected. The latest technique is mixing is the use of automatic spanars and mixes. These consist of a double barrel caulking gun with mixing tip. The tip contains spirals on the inside. Foving 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 chocking 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, phobe iodophor 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 – Light and heavy bodied. Composition: 46
  47. 47. 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 – Controlled working time. Excellent properties. 47
  48. 48. 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 wethers, acrylics or methacrylates. Lastly, we come to the inelastic impression materials – due to this limited use in operative dentistry, we shall have a brief overview of those: 1. Impression plaster – 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 – 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, talu, coloring agents and fillers along with plasticizers. 48
  49. 49. The compound softens at 39°C(glass transition) and is manipulate at 43.5°C (fusion temperature). It can be softened over a flame or in warm water. It is then baded 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 Accelerator Zinc oxide – 87% Fixed vegetable or mixed oil – 13% Oil of cloves or eugenol – 12% Gum or polymerized resin – 50% Filter (silica type) – 20% Lasolin – 3% Resinous bulsam – 10% Accelerator solution (CaCl2) – 5% Coloring agents. Setting reaction is an acid base reaction forming zinc eugenolate. Final setting range from 10-15 minutes. 49
  50. 50. Manipulate – equal length are dispersed, mixed quickly (1 minute mixing time) and loaded onto the tray, seated till set and removed. Advantages include good accuracy, dimensional stability and 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 orthoetching benzoic acid. They can also be used as bite registration pastes. 4) Lastly, a material not brief an impression material but used as such inlay 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 damer, canaculi were and coloring agents. Candidia 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: 50
  51. 51. 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. 51
  52. 52. 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. Manappallil. 5. Notes on dental materials – C. Combe. 6. Dental materials. – Richard Van Noort. 52