Impression materials

impression materialsplay an important role in dentistry so here is the basic information about various materials used in dentistry

1. Impression materials By Dr Vijyanta suman I MDS

2. contents Introduction Definition History Classification Properties Impression making Prosthodontic considerations Review of literature Conclusion Reference 2

3. Introduction Impression materials are used to register or reproduce the form and relationship of the teeth and oral tissues. Hydrocolloids and synthetic elastomeric polymers are most commonly used to make impressions of various areas of the dental arch. Each of these classes of materials has certain advantages and disadvantages. 3

4. PURPOSE OF IMPRESSION MATERIALS Impression materials are used to make an accurate replica or mold of the hard and soft oral tissues. The impression is a negative reproduction of the tissues, and by filling the impression with dental stone or other model material, a positive cast is made that can be removed after the model material has set It is used to make models, cast, die or as duplicating materials The accuracy, detail, and quality of this final replica are of greatest importance 4

5. Impression material : any substance or combination of substances used for making an impression or negative reproduction Impression tray : A receptacle into which suitable impression material is placed to make a negative likeness A device that is used to carry, confine, and control impression material while making an impression Impression : a negative likeness or copy in reverse of the surface of an object; an imprint of the teeth and adjacent structures for use in dentistry GPT-9 5

6. Die : the positive reproduction of the form of a prepared tooth in any suitable substance Model : a facsimile used for display purposes; a miniature representation of something Cast : a life-size likeness of some desired form; it is formed from a material poured into a matrix or impression or from a CAM printed replica; Dental Cast, Diagnostic Cast, Preliminary cast, Refractory cast, Remount cast GPT-9 6

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9. Chronology of events in the history of impression materials 1756 : Phillip Pfaff made first impression using Wax 1782 : William Rae made cast using plaster of paris 1848 : Guttapercha was introduced as impression material ( colburn /Blake) 1844 : Plaster of paris introduced for impression of teeth (Dunning) 1857 : Charles stent develops Impression compound 1915 : Plastic-plaster impression technique (Rupert Hall) 1925 : Reversible hydrocolloid impression materials introduced 1930s : ZoE as impression material (Ward and Kelly) 1941 : Irreversible hydrocolloids introduced 1953 : Polysulfide Impression material introduced 1960s : Polyether was introduced in Germany 1970s : silicones were introduced 9 Starcke EN.A historical review of complete denture impression materials. Journal of the American Dental Association. 01 Nov 1975;91(5):1037-1041

10. 10 DESIRABLE QUALITIES A pleasant odor , taste, and acceptable color Absence of toxic or irritant constituents Adequate shelf life for requirements of storage and distribution Economically commensurate with the results obtained Easy to use with the minimum of equipment Setting characteristics that meet clinical requirements Satisfactory consistency and texture Readily wets oral tissues

11. 11 Elastic properties that allow easy removal Adequate strength to avoid breaking or tearing Dimensional stability over temperature and humidity ranges normally found in clinical and laboratory procedures for a period long enough to permit the production of a cast or die Compatibility with cast and die materials Accuracy in clinical use Readily disinfected without loss of accuracy No release of gas or other byproducts during the setting

12. classification

13. 13 According to their composition, mechanism of setting, mechanical properties

14. 14 According to tissue displacement

15. 15 According to uses in dentistry IMPRESSION MATERIALS USED IN EDENTULOUS PATIENTS (COMPLETE DENTURE) Impression plaster Impression compound IMPRESSION MATERIALS USED IN DENTULOUS MOUTH Alginates rubber base impression materials

16. INELASTIC IMPRESSION MATERIALS 16

17. 17 Inelastic impression materials exhibit an insignificant amount of elastic deformation when they are subjected to bending or tensile stresses. they fracture without exhibiting any plastic deformation if the stress from applied pressure exceeds their tensile, shear, or compressive strength values. These materials include :- impression plaster impression compound ZOE impression paste inability to sustain a substantial amount of elastic deformation without fracture : use in dental impression making is limited

18. 18 IMPRESSION PLASTER Impression plaster is a β- calcium sulfate hemihydrate used at a water/powder ratio of approximately 0.5 to 0.6. Its fluidity makes it suitable for making impressions of soft tissues in the uncompressed state Because of its rigidity, the use of impression plaster has been suggested for making preliminary impressions or splinting transfer coping utilized to produce long-span implant-supported prostheses. Co mposition To make plaster a suitable impression material, the expansion and the setting times should be controlled Potassium sulfate : anti-setting expansion agent to reduce the setting expansion Borax : retarder - to balance the setting acceleration caused by the potassium Alizarin red : A pigment, make a clear distinction between the impression and the model after casting of the model.

19. 19 Manipulation A custom tray (1- to 1.5-mm spacer ) is preferred over stock tray as it is too fluid Preliminary impressions can be made with dental compound, and impression plaster can be used as the wash material The technique for inserting the impression into the mouth involves “puddling” the impression into place. With the remaining plaster in the tray, the tray is seated in a single movement. Then the tray is gently moved from side to side and anteroposteriorly When the impression involves an undercut area, it is necessary to fracture the impression to facilitate removal from the mouth. Long, narrow strips of wax can be adapted around the periphery of the impression (Beading) : clear indication of where the impression ends, preventing overtrimming and overextension The impression is then coated with a thin layer of separating medium and cast in fresh plaster

20. 20 Impression making associated with implants requires the use of square direct or tapered indirect transfer copings for the transfer of implant positions before fabrication of the prosthesis. For multiple-unit impressions, the direct technique may use splinted or nonsplinted implant transfer copings. The materials used to splint copings can be resin composite, plaster, or acrylic resin with or without reinforcement Disinfection of a plaster impression can be achieved with a 10-min soak in sodium hypochlorite solution

21. 21 Advantages: Good surface  detail Excellent dimensional  stability   Disadvantages: Cannot be added  to (add another layer) Properties  affected by operator  handling technique Taste  and roughness may cause the patient to  vomit Heat evolved during setting

22. 22 IMPRESSION COMPOUND ( Modeling plastic) thermoplastic material supplied in the form of sheets and sticks. There are two types of impression compounds available. Type I : lower-fusing material for making impressions in the oral cavity supplied in either sheet or stick form Type II : higher-fusing material, called tray compound, used for forming trays to be used in the oral cavity. An impression of soft tissue is obtained with tray compound It is then used as a tray to support a thin layer of a second impression material, Secondary impressions may also be made with a ZOE wash paste, hydrocolloid, impression plaster, or elastomer.

23. 23 Composition composed of a mixture of waxes, thermoplastic resins, filler, and a coloring agent. Shellac, stearic acid, and gutta-percha are added to improve plasticity and workability. The waxes or resins in the impression compound are the principal ingredients of the matrix. Filler is added to increase the viscosity at temperatures above that of the mouth and to increase the rigidity of the compound at room temperature.

24. 24 Manipulation Softening by heat :- The fusion temperature : Above this temperature, the material remains plastic while the impression is being made. Once the impression tray is seated, it should be held gently in position until it cools below the fusion temperature. The thermal conductivity is very low : allow an extended time to achieve thorough cooling and heating of the compound. A compound stick may be softened over a flame : When a direct flame is used, the compound should not be allowed to boil or ignite so that the constituents are volatilized. sheet should be immersed in warm water : to ensure uniform softening of the material. Prolonged immersion or overheating in the water bath is not indicated; the com- pound may become brittle and grainy

25. 25 ZINC OXIDE–EUGENOL (ZOE) IMPRESSION PASTE The reaction between zinc oxide and eugenol yields a relatively hard Applications in dentistry :- Impression material for edentulous mouths Surgical dressing Bite registration paste Temporary filling material Root canal filling material Cementing medium Temporary relining material for dentures.

26. 26 Composition ZOE impression pastes are dispensed as two separate pastes. Oil of cloves(70% to 85% eugenol) produces less of a burning sensation The addition of rosin facilitates the speed of the reaction and yields a smoother product. Canada balsam and Peru balsam are used to increase flow and improve mixing properties

27. 27 setting reaction When the two pastes are mixed, the phenol –OH of the eugenol acts as a weak acid and undergoes an acid-base reaction with zinc hydroxide to form zinc eugenolate Acetic acid or zinc acetate : accelerate the reaction. Disadvantages : stinging or burning sensation caused by the eugenol as it leaches out and contacts soft tissues. Orthoethoxybenzoic acid (EBA) is a valuable substitute for eugenol in this regard

28. 28 Manipulation Mixing of the two pastes is accomplished on an oil-impervious paper or glass mixing slab. The proper proportion of the two pastes is generally obtained by squeezing two strips of paste of the same length A flexible stainless steel spatula is typically used for the mixing procedure. The two strips of contrasting colors are combined with the first stroke of the spatula, and the mixing is continued for approximately 1 min These materials are classified as Type I : hard paste - final set occur within 10 min Type II : soft paste - final set occur within 15 min setting time can be shorten by : Adding a small amount of zinc acetate or additional accelerator drop of water in the paste before mixing extending the mixing time. Prolonging the setting time. : cool spatula and mixing slab

29. HYDROCOLLOID IMPRESSION MATERIALS 29

30. 30 A colloid is a substance that is microscopically dispersed uniformly throughout another substance If the dispersion phase of a colloidal system is water, it is called a hydrocolloid CHARACTERISTICS OF HYDROCOLLOIDS Impression materials During impression making, the material sets from a flowable state to a solid state. This change of state associated with hydrocolloids is called the sol-gel transformation . A sol is a colloidal dispersion of very small particles in a continuous liquid medium an gel is a suspension that behaves like an elastic solid.

31. 31 AGAR (REVERSIBLE HYDROCOLLOID) The physical change of agar from sol to gel is induced by lowering the temperature. The gel liquefies to a sol when it is heated to liquefaction temperature (70 °C to 100 °C) When sol is cooled, it becomes a gel at a gelation temperature (between 37 °C and 50 °C). Composition : component function Composition (%) Agar Borate Sulfate Wax, diatomaceous earth, clay Thixotropic materials Water Thymol glycerin Brush-heap structure Strength Gypsum hardener Filler Thickener Reaction medium Bactericidal Plasticizer 13 − 17 0.2 − 0.5 1.0 − 2.0 0.5 − 1.0 0.3 − 0.5 Balance borax is a potent retarder for the setting of gypsum

32. 32 Making the Agar Impression This process requires a three-compartment conditioning unit The first step in impression making is to liquefy the hydrocolloid gel in the tube in boiling water( 100 °C) for a minimum of 10 minutes. Place it in a storage bath at 65 °C to retain the sol condition until it is needed The tempering time is 3 to 10 min, just sufficient to ensure that all the material has reached a lower temperature Gelation is accelerated by circulating cool water through the tray for 3 to 5 min. the tray must be held in the mouth until gelation has completed it is necessary to remove the impression with a snap and not to tease it out.

33. 33 ALGINATE (IRREVERSIBLE HYDROCOLLOID) The setting of alginate hydrocolloid is a process of cross- linking alginic acids with calcium ions Composition : component Function Weight (%) Potassium alginate Calcium sulfate Zinc oxide Potassium titanium fluoride Diatomaceous earth Sodium phosphate Soluble alginate Reactor Filler particles Gypsum hardener Filler particles Retarder 15 16 4 3 60 2

34. 34 Potassium alginate K2n _ Alginate Calcium alginate Can _ (Alginate)2 Calcium sulfate n Ca SO4 Potassium sulfate n K2SO4 Sodium phosphate 2 Na3PO4 Calcium sulfate 3 CaSO4 Calcium phosphate Ca3(PO4)2 Sodium sulfate 3 Na2SO4 Setting reaction Controlling Setting Time A slight modification in the W/P ratio or the mixing time The setting time is best regulated by the amount of retarder added during the manufacturing process. Fast-setting alginate : 1.5 to 3 min Normal-setting alginate (3 to 4.5 min)

35. 35 Making the Alginate Impression For impression to be retained on the tray, a perforated metal tray is preferred. If a plastic tray or a metal rim-lock tray is selected, a thin layer of alginate tray adhesive should be applied and allowed to dry completely The thickness of the alginate impression between the tray and the tissues should be at least 3 mm. Alginate impression should not be removed from the mouth for at least 3 min after gelation has occurred it is best to avoid torquing or twisting the impression in an effort to remove it

36. 36 PROPERTIES OF HYDROCOLLOID IMPRESSION MATERIALS syneresis : Once the hydrocolloid impression is removed from the mouth and exposed to air at room temperature, some shrinkage will occur. imbibition : if the impression is immersed in water, swelling occurs The storage medium, such as 2% potassium sulfate solution, or 100% relative humidity in a storage chamber to reduce the dimensional change of agar impressions Agar is among the most accurate of impression materials. Most alginate impressions are not capable of reproducing the finer details observed in impressions with other elastomeric impression materials.

37. 37 LAMINATE TECHNIQUE (ALGINATE-AGAR METHOD) combined agar-alginate technique : tray of chilled alginate that bonds to the agar expressed from a syringe. The alginate gels by a chemical reaction, whereas the agar gels by means of contact with the cool alginate rather than with the water circulating through the tray. Advantages : it is the agar, not the alginate, that contacts the prepared teeth, maximal detail is reproduced. equipment cost is lower and less preparation time is required. Disadvantages : the bond between the agar and alginate is not always sound the higher viscosity of the alginate material displaces the agar hydrocolloid during seating the dimensional inaccuracy of the alginate hydrocolloid limits its use to single units.

38. 38 DUPLICATING MATERIALS ANSI/ADA Specification No. 20 Both types of hydrocolloids are used in the dental laboratory to duplicate dental casts or models for the construction of prosthetic appliances Agar hydrocolloid is more popular in the lab because it can be used many times. with intermittent stirring, agar hydrocolloid can be kept in a liquid form for 1 or 2 weeks at a constant pouring temperature. The hydrocolloid type duplicating materials have the same composition as the impression materials but their water content is higher, the agar or alginate content is lower, which influences their compressive strength and percent permanent set.

39. ELASTOMERIC IMPRESSION MATERIALS 39

40. 40 Elastomers comprise a group of synthetic polymer-based impression materials that are chemically cross-linked when set and that can be stretched and yet rapidly recover to their original dimensions They are supplied in two components, a base paste and a catalyst paste that are mixed before making impressions Formulated in several consistencies, including extra low, low, medium, heavy, and putty (increasing order of filler content) there are three elastomers based on the backbone of polymer chains : Polysulfide Silicone (condensation and addition) Polyether.

41. 41 POLYSULFIDE Base paste : polysulfide polymer - multi- functional mercaptan (-SH) filler -lithopone or titanium dioxide Plasticizer - dibutyl phthalate Sulfur - approx 0.5% Accelerator paste (liquid) : Lead dioxide Filler Plasticizer Retarder - Oleic or stearic acid

42. 42 The reaction starts at the beginning of mixing and reaches its maximum rate soon after spatulation is complete hot and humid conditions will accelerate the setting of polysulfide impression

43. 43 CONDENSATION SILICONE supplied as a base paste and a low-viscosity liquid catalyst, a two-paste system, or a two-putty system. Base : α-ω- hydroxyl-terminated polydimethyl siloxane Filler – metal oxide/silica Color pigments Accelerator : Cross linking agent - Orthoethyl silicate Catalyst – stannous octoate

44. 44 Condensation polymerization of poly dimethyl siloxane with tetraethyl orthosilicate in the presence of stannous octoate (catalyst)

45. 45 ADDITION SILICONE This material is often called a polyvinyl siloxane (PVS) or vinyl polysiloxane Base : Polymethylhydrosiloxane divinylpolysiloxane . Fillers Accelerator : Divinylpolysiloxane Platinum salt Fillers Retarders

46. 46 No reaction by-products are formed as long as the correct proportions of divinylpolysiloxane and polymethyl- hydrosiloxane are used and there are no impurities The residual polymethylhydrosiloxane in the material can lead to a secondary reaction with each other or with moisture, to produce hydrogen gas

47. 47 These impression materials require a dry field for impression making. One of the disadvantages of the silicone impression materials is their inherent hydrophobic nature. A nonionic surfactant can be added to the paste in the manufacturing process to render a degree of hydrophilicity to the surface of the material. Sulfur contamination from natural latex gloves inhibits the setting of addition silicone Medium-body addition silicone, as a substitute for alginate impression material been formulated for making impressions for diagnostic purposes,. The advantage is the ability to make multiple, accurate diagnostic casts from one impression

48. 48 POLYETHER Base : Polyether polymer Filler - Colloidal silica Plasticizer – Glycolether or Phthalate Acceleratoe paste : Cross linking agent – Aromatic sulfonate ester Filler – colloidal silica Plasticizer - pthalate

49. 49 polyether Sulfonic ester Crosslinked rubber There are two types of polyether impression materials. The first is based on the ring-opening polymerization of aziridine rings The second type is based on an acid- catalyzed condensation polymerization of polyether prepolymer with alkoxysilane terminal groups

50. 50 Properties of Elastomeric Impression materials Hydrophobic : except Polyether Elastic properties : repeated pouring is possible High coefficient of thermal expansion Excellent reproduction of surface details Dimensional change and inaccuracies can occur : curing shrinkage Excellent tear strength Can be electroplated with cu/Ag Each elastomers has specific adhesive because they do not adhere to tray

51. 51 The working time : begins at the start of mixing and ends just before the elastic properties have developed The setting time : time that has elapsed from the beginning of mixing until the curing process has advanced sufficiently that the impression can be removed from the mouth with no distortion

52. 52 Craig’s Restorative Dental materials – Mosby; 13 edition

53. 53 Physical & Mechanical properties of Elastomeric materials Craig’s Restorative Dental materials – Mosby; 13 edition

54. 54 Type of material Advantages Disadvantages Polysulfide Long working time High tear resistance Margins easily seen Moderate cost Requires custom tray Stretching leads to distortion Compatible only with dental stone Stains clothing Obnoxious odor Pour within 1 hour Condensation silicone (putty wash) Clean and pleasant Good working time Margins easily seen High polymerization shrinkage Volatile by-product Low tear strength Hydrophobic Pour immediately Vinyl polysiloxane Clean and pleasant Margins easily seen Ideally elastic Pour repeatedly Stable: delay pour Hydrophobic No flow if sulcus is moist Low tear strength Polyether Least hydrophobic of all elastomers Margins easily seen, Good stability Delay pour, Shelf life: 2 y Stiff, high modulus, Bitter taste Need to block undercuts, Absorbs water Leaches components, High cost

55. 55 MAKING IMPRESSSIONS WITH ELASTOMERIC MATERIALS The use of elastomeric impression material to fabricate gypsum models, casts, and dies involves six major steps: preparing a tray managing tissue preparing the material making an impression removing the impression preparing stone casts and dies.

56. 56 The use of a custom tray is recommended to reduce the quantity of material required to make impressions : any dimensional changes attributed to the materials are minimized. A custom tray allows a uniform distribution of impression material The use of custom trays for polyether and addition silicone impressions is not critical, since these materials are stiffer and have less polymerization shrinkage than the polysulfide material. The stock tray should be rigid, thereby minimizing flexure of the tray during impression making. Disposable stock trays are also used to support the putty when the putty-wash technique is used for making impressions. Prior to making an impression, a uniform thickness of tray adhesive is applied on the tray surface, extending over its edge, and it is allowed to dry

57. 57 TISSUE MANAGEMENT The margins of tooth preparations for fixed prostheses often extend to or below the free margin of the gingiva. To ensure access for the tooth preparation and for making the impression, it is necessary to displace the gingival tissues, control gingival hemorrhage , and control sulcular fluids. The most popular methods of gingival displacement is the use of gingival retraction cord. An electrosurgical unit or a soft tissue laser can also be used.

58. 58 MANIPULATION OF IMPRESSION MATERIALS elastomeric impression materials are supplied for three modes of mixing : hand mixing, static mixing, and dynamic mechanical mixing Hand Mixing : If one of the components is in liquid form, such as the catalyst for condensation silicones, a length of the base is dispensed from the tube onto a graduated mixing pad and drops of the liquid catalyst corresponding to the length of the base are added. The two-putty systems available for condensation and addition silicone are dispensed by volume using an equal number of scoops of each material. The best mixing technique is to knead the material with one’s fingers until a uniform color is obtained.

59. 59 Static mixing : This technique transforms two fluid materials into a homogeneous mixture without mechanical mixing. The device used to accomplish this mixing is a gun for compressing materials in a two-cylinder cartridge which contains the base and catalyst separately, as well as a mixing tip When the trigger is pulled, the plunger is driven forward so that the base and catalyst pastes are forced from the cartridge into the mixing tip The pastes pass through the bore and exit the nozzle as a uniform mixed paste. Four different sizes of mixing tips : the more viscous the material, the larger is the mixing tip that should be used.

60. 60 Syringe tips can be fit to the nozzle to deliver the mixed paste directly to the prepared teeth. These syringes fit the cartridge, as the mixing tips. The base and catalyst are first injected into the respective barrel. A plunger then forces the material through a smaller static mixing tip. It can deliver the light body material directly onto the abutments Dynamic mechanical mixing : The motor-driving mechanism forces the material into the mixing tip and makes the impeller (insert) inside the tip rotate. The function of the impeller is only to mix the material.

61. 61 MAKING AN IMPRESSION Multiple-Mix Technique Monophase Technique Putty-Wash Technique

62. 62 Multiple-Mix Technique A syringe material (light body) and a tray material (heavy body) are used in this technique The lighter material is injected from the filled syringe or directly from a static mixing gun within and around the tooth preparation. The filled tray with heavy body is then inserted in the mouth and seated over the syringe material, which has been extruded on hard and/or soft tissue. The tray material will force the syringe material to adapt to the prepared tissues.

63. 63 Monophase Technique or single-viscosity technique Medium-body polyether and addition silicone are often used only one mixture is made, and part of the material is placed in the tray, and another portion is placed in the syringe for injection in the cavity preparation, prepared teeth, or soft tissue. The success of this technique depends on the pseudo- plastic properties of the materials.

64. 64 Putty-Wash Technique This method was originally developed for condensation silicone. The thick putty material is placed in a stock tray and a preliminary impression is made which serves as custom made tray Space for the light-body “wash” material is provided either by cutting away some of the “tray” putty or by using a thin polyethylene sheet as a spacer between the putty and the prepared teeth during preliminary impression making A mixture of the thin-consistency wash material is placed into the putty impression and on the preparation; then the tray is reseated in the mouth to make the final impression. An alternative approach is to inject the wash material around the preparation and then immediately seat the tray with freshly mixed putty over the wash material.

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66. 66 REMOVAL OF THE IMPRESSION The impression should not be removed until the curing has progressed sufficiently One method for determining the time of removal is to inject some of the syringe material onto a space that is not in the field of operation When a multiple mix technique is used, test both the syringe and the tray materials. Typically, the impression should be ready for removal within at least 10 minutes from the time of mixing The first step is to break the physical adhesion between the tissue and the impression The second step stretches the impression enough to pass under the height of contour of the hard tissue to remove the impression

67. 67 PREPARATION OF STONE CASTS AND DIES The hydrophobic characteristics of silicone impression materials make them suitable for pouring of epoxy resin to produce dies pouring with gypsum products challenging, as it increases the potential of forming voids in gypsum dies and casts. surfactant sprays( debubblizers ) : improve the surface wettability of the silicone impression material for the stone slurry (thin layer is applied) A dilute solution of soap is also an effective surfactant An alternative to the surfactant is to select a hydrophilized addition silicone

68. 68 Common failures occurring with the use of Elastomeric Impression Materials

69. 69 DISINFECTION Condensation silicones, addition silicones, and polysulfides can be disinfected with all EPA-registered disinfectants without the loss of surface quality or accuracy if the disinfection time is short. The impressions should be immersed for the time specified for each disinfectant. After disinfection, the impression should be removed, rinsed, and poured with the gypsum product as soon as possible. long immersion time may cause the surfactant in the hydrophilic polyvinyl siloxane to leach out and render the impression less hydrophilic. The current protocol for disinfecting hydrocolloid impressions recommended by the CDCP is to use household bleach (1:10 dilution), iodophors, or synthetic phenols as disinfectants. Can be either sprayed or immersed (not more than 10 min)

70. 70 Material Method Recommended Disinfectant comments Alginate Agar Polysulfide & silicone Polyether ZOE impression paste Impression compound Use only disinfectant for a short-term exposure time ( < 10 min for alginate) Immersion Use disinfectant only for a short time ( < 10 min) Immersion preferred; spraying can be used for bite registrations Chlorine compounds or iodophors Glutaraldehydes , chlorine compounds, iodophors, phenolics Chlorine compounds or iodophors Glutaraldehydes or iodophors Iodophors or chlorine compounds (2% Glutaraldehyde) Short-term glutaraldehyde has been shown to be acceptable Do not immerse in alkaline glutaraldehyde Disinfectants requiring more than 30-min exposure times are not recommended. ADA recommends any of the disinfectant classes; Not compatible with chlorine compounds! Phenolic spray can be used.

71. RECENT ADVANCES 71

72. 72 Digitial Impressions After the tooth preparation is complete and the tissues are retracted to visualize the tooth margins, the tooth is dried and readied for scanning scanning systems : use an oxide powder on the tooth to remove optical highlights from the surface of the preparation and to enhance the scan quality. Scanners use either a series of static images or a stream of video images to capture the geometry of the tooth preparation. Laser utilization allows scanning of a variety of different surface types and colors without the need for a contrast agent (powder). These scans are registered together to form a single 3-D model

73. 73

74. 74 Advantages of Digital Impressions Provide a clean and streamlined impression method Simplify impression making without the complexity of the many materials required for conventional impressions with an elastomeric material Reduces the environmental impact of disposing the materials required for conventional impressions Digital scanning takes less time than conventional impression, which increases the efficiency and productivity of dental office. The on screen image can be checked for margin visibility, preparation from and interocclusal clearance. Adjustment and isolated scanning is possible. Results are instantly visible and can be enlarged Scanning takes 3 to 4 minutes, no material restriction, no risk of error due to distortion Craig’s Restorative Dental Materials – Mosby 13 th edition p362-364

75. 75 Digital impressions are advantageous in Patients who are gaggers and cannot tolerate impression material in mouth for several minutes Presence of tori or undercut which make removal of a traditional impression difficult or impossible without causing the patient discomfort/ or tearing the margins No need for infection control No concern about the compatibility of specific material with specific disinfectants A Begum. R Ahmed. Digital Impressions. City Dental College Journal, 2012  

76. 76 The tear strength of some alginates has been improved, and some have been formulated so that the powder is dustless, thus reducing the health hazard as a result of patient inhalation of dust during the dispensing process Polyether and silicone impression materials have been modified so that the working time, viscosity, and flexibility of the polyethers have been improved Fluoride or silico fluorides can be added to Alginate A colour pH change to indicate different stages in manipulation (violet during spatulation, pink when loaded and white on setting) Chlorhexidine & triethanolamine modified Alginates also have been introduced R G Craig. Review of Dental Impression Materials.Advances in Dental Research.1988 Doubleday B.Impression Materials.British journal of orthodontics.May1988;25(2):8-10

77. 77 vinyl polyether siloxane (VPES) newer elastomer that has been developed is called as vinyl polyether siloxane (VPES) combining features of both addition silicone and polyether This material has to be used only in a one step multiple mix technique It is available in viscosities :- medium ( Monophase ) : Transfer (implantology), Crown and bridge, Inlay/ onlay , veneer Heavy Light : Precision impressions Advantages Excellent flowability Remarkable hydrophilicity Optimized elastomeric properties Easy and fast, true-to-detail fabrication of casts RM Shetty, GR Bhandar. Vinyl Polysiloxane Ether: A Breakthrough in Elastomeric Impression Material

78. REVIEW OF LITERATURE 78

79. 79 A systematic review was conduct to evaluate the evidence of possible benefits and accuracy of digital impression techniques vs. conventional impression techniques Digital impression accuracy is at the same level as conventional impression methods in fabrication of crowns and short fixed dental prostheses (FDPs). For fabrication of implant-supported crowns and FDPs, digital impression accuracy is clinically acceptable. In full-arch impressions, conventional impression methods resulted in better accuracy compared to digital impressions. Ahlholm P, Sipilä K, Vallittu P, Jakonen M, Kotiranta U. Digital versus conventional Impressions in Fixed Prosthodontics : A Review.J Prosthodont . 2018 Jan;27(1):35-41

80. 80 The different impression materials which can be used for an ocular impressions are Irreversible Hydrocolloids, Opthalmic Alginates, tissue conditioners, polyvinyl siloxane and dental impression waxes like Korecta wax no.4 and Iowa wax Mix Alginate with excess water, fill in a disposable plastic syringe, eyelids are drawn gently apart and impression material is introduced at inner side of plpebral opening Tissue conditioner is applied on a trimmed stock acrylic resin prosthesis and inserted and kept for 20 minutes. After removing excess material prosthesis is worn for 24-48 hours and if esthetics and adaptation is acceptable, it is relined with heat cure resin A thin layer of korecta wax no 4 is added on intaglio surface and borders of already existing prosthesis in order to reline it, dip it into 123 F water bath for five minutes and then place and check for the adaptation MF Mathews. The ocular impression: A review of the literature and presentation of an alternate technique .journal of Prosthodontics Implant Esthetic & Reconstructive Dentistry. Aug 2004;9(4):210-216

81. 81 conclusion

82. 82 REFERENCES Phillips’ science of Dental Materials. Twelfth Edition.p181-196 Craig’s Restorative Dental Materials – Mosby 13 th edition p362-364 A Begum. R Ahmed. Digital Impressions. City Dental College Journal, 2012 R G Craig. Review of Dental Impression Materials.Advances in Dental Research.1988 Doubleday B.Impression Materials.British journal of orthodontics.May1988;25(2):8-10 RM Shetty, GR Bhandar. Vinyl Polysiloxane Ether: A Breakthrough in Elastomeric Impression Material Ahlholm P, Sipilä K, Vallittu P, Jakonen M, Kotiranta U. Digital versus conventional Impressions in Fixed Prosthodontics : A Review.J Prosthodont . 2018 Jan;27(1):35-41