Impression materials

impression materials in dentistry specially those used in Prosthodontics.
Impression compound
Zinc oxide Eugenol impression paste
Alginate
Agar-agar
Elastomeric impression compounds like Polysulfides, Condensation silicones, Addition silicones(PVS), Polyether
detailed description with properties, mixing time, working time, setting time ,physical and properties etc

1. IMPRESSION MATERIALS Presented by- Dr Sakshi II Yr PG Dptmnt of Prosthodontics 1

2. WHAT IS AN IMPRESSION??? negative replica of the hard and soft oral tissues Register or reproduce the form and relationship of the teeth and oral tissues 2

3. Purpose of Impression M aterials Impression –negative reproduction of tissues Positive cast made by filling the impression with dental stone or other model material 3

4. Desirable qualities of Impression Materials A pleasant odour , taste and acceptable color Absence of toxic or irritant constituents Adequate shelf life for requirements of storage and distribution 4

5. Desirable qualities contd … Easy to use with minimum of equipment Setting characteristics which meet clinical requirements Low enough viscosity to adapt to the oral tissues,yet be viscous enough to be contained in the impression tray 5

6. Desirable qualities contd … Should have adequate wettability of the oral tissues Set impression should show adequate elastic recovery with no permanent deformation upon removal from mouth 6

7. Desirable qualities contd …. Dimensionally stable after setting over temperature and humidity ranges normally found in clinical and lab procedures until the pouring of the cast Compatible with cast and die materials 7

8. Desirable qualities contd …. Biocompatible ,non-toxic Must be of color and opacity that allows the dentist to evaluate the impression Readily disinfected without significant loss of accuracy or loss of mechanical properties 8

9. Desirable qualities contd …. Materials ,associated processing time and equipment –cost effective No release of gas or other by-products during setting of the impression materials 9

10. 10

11. CLASSIFICATON: Classified on the basis of setting and elasticity. Mode of setting rigid Elastic Set by chemical reaction(irreversible) Impression plaster, zinc oxide eugenol. Alginate, polysulfide, polyether, silicone, Set by temperature change(reversible) Compound, waxes Agar hydrocolloid. 11

12. 12

13. Impression Materials Non-elastic Elastic Aqueous Hydrocolloids Non-aqueous Elastomers Polysulfide Silicones Polyether Condensation Addition Agar (reversible) Alginate (irreversible) Plaster Compound ZnO - Eugenol Waxes O’Brien, Dental Materials & their Selection 1997 13

14. NON-ELASTIC IMPRESSION MATERIALS 14

15. Impression Plaster used as mucostatic impression material for making final impressions for edentulous patients Doesn’t compress and displace tissues during seating of tray due to its fluidity 15

16. Impression plaster contd … Applicable to patients with displaceable soft tissues that should be recorded in a passive state 16

17. Composition ß-calcium sulphate hemihydrate Reacts with water to form calcium sulphate dihydrate W/P ratio– 0.5-0.6 Expansion and setting times controlled by incorporating compounds designed to mediate handling properties 17

18. Potassium sulphate added as an anti-setting expansion agent Borax(retarder)- added to the powder to balance the setting acceleration caused by Pot. Sulphate and to bring the setting time under control 18

19. Alzarin red-to make clear distinction between the impression and model 19

20. Custom tray constructed using 1-1.5mm spacer with acrylic resin or shellac Impression plaster can be used as wash material Techinque - “ Puddling ” the impression into place 20

21. With remaining plaster in tray, the tray is seated in single movement Then tray is gently moved from side-to side and antero-posteriorly to take advantage of fluidity of material 21

22. Hemihydrate particles absorb moisture from the surface of the oral tissues allowing intimate contact between impression material and the tissues 22

23. Plaster impression material –very brittle and fractures easily When undercut is involved, fracture the impression to facilitate removal from mouth Fragments reconstructed to form completed impression 23

24. Beading of the impression done Coated with separating medium and cast in fresh plaster Disinfection- achieved with a 10 min soak in sod hypochlorite solution 24

25. Not used regularly due to mechanical limitations Used frequently as occlusal registration material 25

26. Zinc-oxide Eugenol Impression Paste Composition-2 separate pastes dispensed in tubes One tube contains zinc oxide and vegetable or mineral oil Other tube contains eugenol and rosin 26

27. Composition Components Percentage Tube no 1 (base) Zinc-oxide 87 Fixed vegetable or mineral oil 13 Tube no 2 (accelerator) Oil of cloves or eugenol 12 Gum or polymerised rosin 50 Filler(silica type) 20 Lanolin 3 Resinous balsam 10 Accelerator solution(CaCl 2 ) and color 5 27

28. Setting reaction of ZOE Ionic in nature Requires ionic medium in which it can proceed at any desired rate 1 st reaction-hydrolysis of zinc oxide to its hydroxide form 28

29. Zinc Oxide Eugenol When the 2 pastes are mixed,the phenol –OH of the eugenol acts as a weak acid and undergoes an acid-base reaction with zinc hydroxide Forms a salt- zinc eugenolate 29

30. ZOE contd … Two further coordinate bonds are formed by donation of pairs of electrons from methoxy oxygen to zinc 30

31. ZOE contd … Disadvantage- Stinging or burning sensation caused by eugenol Orthoethoxybenzoic acid -substitute 31

32. Manipulation Mixed on oil impervious paper or glass mixing slab Proper proportion of two pastes obtained by squeezing 2 strips of paste of the same length,one from each tube ,onto the mixing slab 32

33. Manipulation of ZOE contd … Flexible stainless steel spatula used for mixing 2 strips of contrasting colors combined with the first stroke of the spatula ,mixing is continued for approx 1 min, until a uniform color achieved 33

34. Types of ZOE Classified as Hard paste(type I) soft paste(type II) Final set for type I paste-10 min type II paste-15min Actual time shorter when setting occurs in mouth 34

35. Shorten the setting time – by adding small amount of Zinc acetate or additional accelerator or a drop of water in the paste before mixing or by extending mixing time Prolonging the setting time- cool spatula and mixing slab 35

36. Paste of thick consistency –compresses the tissues Thin,fluid material results in little or no compression Advantage of heavier consistency – increased strength 36

37. Dimensional stability Negligible shrinkage(less than 0.1%) may occur during hardening No significant dimensional change Impression can be preserved indefinitely without change in shape provided the tray material is dimensionally stable 37

38. Disinfection 2 % alkaline glutaraldehyde solution Immersed in solution for required time,rinsed and poured immediately 38

39. Applications of ZOE Final impression of edentulous ridges As a wash impression with other impression As an interocclusal registration material As a temporary liner material for dentures As a surgical dressing 39

40. Impression compound Also called “ modelling plastic” Thermoplastic material Supplied in the form of cakes(red) and sticks (green, gray or red) 40

41. Composition Mixture of –waxes(principal ingredient) thermoplastic resins filler(increase viscosity and rigidity) coloring agent Shellac, stearic acid and gutta-percha added to improve plasticity and workability 41

42. 42

43. Type I (Lower fusing material) Cakes- as an impression material for completely edentulous patients, the material is softened by heat, inserted into the tray and placed against the tissues before it cools to a rigid mass 43

44. Sticks- as a border molding material for the custom tray ,the material is used before making the final impression 44

45. Type II( Higher fusing material) Used as an adaptation material which requires more viscous properties Used for making primary impression of the soft tissues and then used a tray to support a thin layer of a second impression material such as ZnOE paste, hydrocolloids or nonaqueous elastomers 45

46. Manipulation Setting mechanism– reversible physical process Softening by heat – prerequisite Preheated and used warm ( ~45 C) Then cooled to the intraoral temperature(37 C) at which it is fairly rigid 46

47. Once the impression tray is seated , it should be held gently(passively) in position until the impression cools below the fusion temperature 47

48. Softened by heat over flame(green stick) or in a temperature controlled water bath When direct flame is used, the material should be moved over the flame in such a manner that it will not be allowed to boil or ignite so that the constituents are volatilized 48

49. Prolonged immersion or overheating in water bath makes the compound brittle or grainy due to leaching of low molecular weight ingredients 49

50. Dimensional stability- allow thorough cooling of the impression before removal from the mouth and to construct the cast or die as soon as possible after the impression has been obtained(at least within the hour) Disinfection – 2% alkaline glutaraldehyde solution 50

51. Thermal Properties Thermal conductivity-low thermal conductivity Significance During softening of the material, the outside will soften first and inside last, so to ensure uniform softening, the material should be kept immersed for a long time in water bath The layer adjacent to tissues will remain soft . Thus it is important to cool the compound thoroughly before removing the impression 51

52. b)Coefficient of Thermal Expansion- high COTE (0.3% acceptable) c)Glass Transition Temperature The temperature at which the material loses its hardness or brittleness on heating or forms a rigid mass upon cooling Approx 39°C 52

53. d)Fusion temperature– corresponds to a definite reduction in plasticity of the material during cooling Above this temperature, the material remains plastic while the impression is being made Approx 43.5°C 53

54. Significance of Fusion temp and Glass transition temp Above Fusion temp,the fatty acids are liquid and lubricate the softened material to form a smooth plastic mass while the impression is being obtained. Thus all impressions with compound should be made above this temperature 54

55. Once the impression tray is seated,it should be held firmly in position until first fusion temperature and later the glass transition temperature is reached Thus, impression is made above the fusion temperature and removed after it cools down to its glass transition temperature 55

56. Important considerations for proper use of impression compound Low thermal conductivity- adequate time needed to attain thorough heating and cooling Incorporation of water(wet kneading)- excessive flow of the material at mouth temp. producing distortion as the impression is removed from mouth 56

57. Tray used for impression must be strong and rigid enough to support the material and to avoid distortion of the impression Relatively high viscosity limits its ability to record fine details Cast should be poured as soon as possible to minimise distortion due to relaxation of the compound 57

58. Safely disinfected by immersion in sod hypochlorite, iodophors or phenolic glutaraldehydes 58

59. Elastomeric Impression Materials 59

60. AQUEOUS HYDROCOLLOIDS 60

61. Agar Reversible hydrocollloid Physical change of agar from sol to gel induced by lowering temperature Gel liquefies to sol when heated to a temperature known as liquefaction temperature(70 C-100 C) 61

62. When sol is cooled, it becomes gel at a point known as the gelation temperature( btw 37 C and 50 C) Thus called reversible hydrocolloid 62

63. Gelation temp- critical for impression making If too high,heat from the sol may injure the oral tissues If too low,below oral temperature,impossible to make impression because the sol will not convert to a gel 63

64. Polysaccharide- extracted from certain types of seaweed Water major constituent Supplied as gel Available in tray and syringe consistencies Tubes used to fill water cooled trays and cartridges used with syringes 64

65. Composition Agar Gelling agent Borax Improves strength Potassium sulphate Gypsum hardener Alkyl benzoates Preservatives Water Reaction medium(>80%) Coloring agents Flavouring agents 65

66. Fillers such as diatomaceous earth,wax , clay,silica,rubber and similar inert powders– used to control strength , viscosity and rigidity Thymol and glycerine added as bactericidal agent and plasticiser 66

67. Making the Agar impression Process requires a 3 compartment conditioning unit for the agar tray material Allows liquefaction,storage and tempering Syringe material used only in liquefaction and storage compartments 67

68. Liquefy the hydrocolloid gel in the tube in boiling water at 100 C for minimum 10 min tube then placed in a storage bath at 65°C to retain the sol condition until needed impression tray filled with hydrocolloid sol from the tube taken from storage bath , gauze pad placed over the top of the tray material Tray placed in water filled tempering compartment(at abt 45°C) 68

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70. Just before tempering completed,syringe material taken directly from storage compartment and applied to the prepared teeth Note--- tempering time-3-10 min if >10 min,partial gelation occurs syringe material doesn’t require tempering bcoz maintained in fluid state to enhance adaptation to tissues 70

71. Syringe material first applied to the base of the preparation,then remainder of the prepared tooth is covered Tip of the syringe is held close to the tooth and it remains embedded below the surface of the syringe material to prevent entrapment of air bubbles 71

72. Water soaked outer layer of hydrocollloid loaded tray and the gauze covering the tray impression material are removed to ensure firm bonding to the syringe hydrocolloid Tray immediately brought into position,seated with light pressure and held with a very light force 72

73. Gelation accelerated by circulating cool (18-21°C)through tray for 3-5 min During gelation process, tray must be held in mouth until gelation has proceeded to a point at which gel strength is sufficient to resist deformation or fracture Tray removed with a snap 73

74. Distortion during gelation Some contraction occurs due to physical change (sol gel) If held rigidly in the tray,shrink towards the center of its mass,thus creating larger dies Rapid cooling may cause stress concentration near the tray 74

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76. Dimensional stability Storage medium- 2% potassium sulfate solution or 100% relative humidity 76

77. Compatibility with Gypsum Contains borax- retarder for setting of gypsum products Deficiency of gypsum setting can be overcome by--Immersing agar impression in a solution containing a gypsum accelerator(2% pot sulfate solution) prior to pouring of the impression By incorporating gypsum surface hardener in the material such as sulfate 77

78. Disinfection House hold bleach or iodophors Accuracy Most accurate of impression materials 78

79. Alginate Irreversible hydrocolloid Most widely used material in dentistry Developed as a substitute for agar 79

80. Advantages Ease of manipulation No need of expensive equipments Relatively low cost Comfort to patients High viscosity Ability to displace tissues Disadvantages 80

81. Potassium or sodium alginate dissolves in water and reacts with calcium ions Calcium sulphate dihydrate A reactor ,reacts with potassium alginate to form a dihydrate insoluble alginate gel Zinc oxide Filler particles, affects properties and setting time Potassium titanium fluoride Accelerator ,counteracts the inhibiting effect of the hydrocolloid on the setting of stone,ensures good quality surface of the cast Diatomaceous earth Filler particles, controls the consistency of the mix and the flexibility of the set alginate Trisodium phosphate Retarder,controls the settting time to produce either regular or fast set alginates Coloring agents Flavoring agents Composition 81

82. Modified alginates Dustless alginates Include polyethylene glycol or polypropylene glycol on the alginate powder to agglomerate the particles Color indicators added to reveal the stage of setting reaction 82

83. Modified alginates Two paste alginate materials One paste contains sol of alginate,fillers,retarders and other ingredients like glycols and dextrose Other paste contains gypsum dihydrate , fillers, retarder , glycerol or glycol, gypsum surface modifier and some silicone oil 83

84. Gelation process Typical sol-gel reaction Soluble alginate reacts with calcium ions from calcium sulphate and forms insoluble calcium alginate Production of calcium alginate- rapid,doesn’t allow sufficient working time Retarder trisodium phosphate added to extend working time 84

85. Setting reaction of alginate (a) K 2n -Alginate+ n CaSO 4 nK 2 SO 4 + Ca n -alginate (b)2Na 3 PO 4 + 3CaSO 4 Ca 3 (PO 4 ) 2 + 3Na 2 SO 4 85

86. 86

87. Controlling Setting Time Ideal W/P ratio- 20 ml water/8gms of powder 40 ml water/16gms of powder Powder should be weighed not measured Approx 2.5:1 Slight modification in W/P ratio affects 2 important properties--- tear strength elasticity 87

88. Thus setting time best regulated by amount of retarder added during manufacturing Fast-set alginate- 1.5-3min Regular set alginate- 3-4.5 min Can also be influenced by altering the temperature of water 88

89. Cool water in hot weather Precool mixing bowl and spatula Tap water-contains certain levels of metallic ions( Ca,Mg ) Tap water with a high hardness may accelerate setting time 89

90. Preparation of Alginate Impression Materials Measured powder added slowly to premeasured water already poured into clean rubber bowl Powder incorporated into water by carefully mixing with a metallic spatula flexible enough to adapt well to the wall of the mixing bowl 90

91. Avoid incorporating excessive air into the mix Vigorous figure of 8 stropping motion Mixing time- 45sec to 1 min Result should be a smooth creamy mixture that doesn’t drip off the spatula when raised from bowl 91

92. Mechanical mixing devices Include rotating mixing bowl , mechanical mixer with time-control unit, a vaccum mixer for water/powder mixing Advantages- convenience, speed and reduction of human error 92

93. Making the alginate impression Perforated metal tray preferred Thickness of alginate impression between the tray and the tissues should be at least 3mm Compressive strength doubles during first 4 min after gelation,but doesn’t increase appreciably thereafter Improve elasticity over time which minimizes distortion of the material during impression removal(undercut areas) 93

94. Compressive strength of an Alginate gel as a function of Gelation time Time from Gelation (min) Compressive strength( KPa ) 330 4 770 8 810 12 710 16 740 94

95. Alginate impression should not be removed from the mouth for at least 3 min after gelation has occurred Tear strength increased when the impression is removed along a vertical path with a snap Speed of removal- between rapid movement and a slower rate 95

96. STRENGTH Manufacturer’s directions should be followed Any deviation from instructions can have adverse effects on the gel strength SHELF LIFE 2 factors affecting shelf life- storage temperature and moisture contamination 96

97. Dimensional stability Syneresis - loss of water when exposed to air at room temperature associated with shrinkage Imbibition - swelling of the impression if immersed in water 97

98. How can distortion be minimised ?? Poured immediately after making impression If pouring delayed, then rinsed in tap water,disinfected wrapped in a surgical paper towel saturated with water and placed in a sealed plastic bag or humidor 98

99. Compatibility with Gypsum Poatssium titanium fluoride-surface hardener or gypsum hardener Solubility-1.3gm per 100 ml of water at 20°C Fluoride on surface of alginate will form fine calcium fluoride precipitates with the calcium from the gypsum Fine calcium fluoride particles become nuclei that accelerate the setting of gypsum 99

100. Rough stone surface will result if excess rinsing water collected on the surface of the impression at the time of pouring the stone mixture A dried gel results in its adherence to the surface of the cast which results in tearing upon removal 100

101. Surface of impression should be shiny but with no visible water film or droplets at the time of pouring Stone cast or die should be kept in contact with the impression for a minimum of 30 min, preferably for 60 min before the impression is separated from the cast 101

102. Disinfection Household bleach(1-10 dilution) iodophors or synthetic phenols After rinsing,disinfectants sprayed on exposed surface Impression immediately wrapped in a disinfectant-soaked paper towel and placed in a sealed plastic bag for 10 min 102

103. Wrapped impression removed from the bag, unwrapped, rinsed and shaken to remove excess water Then poured with stone of choice Accuracy Not capable of reproducing the finer details compared with other impression materials 103

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105. Laminate technique(Alginate -Agar method) Modification of agar procedure Agar in tray replaced with a mix of chilled alginate that bonds to the agar expressed from a syringe Alginate gels by chemical reaction while agar gels by means of contact with the cool alginate rather than water circulating through the tray 105

106. Agar contacts the prepared teeth ,maximum detail reproduced Equipment cost low, less preparation time needed Main disadvantages- bond btw agar and alginate not always sound higher viscosity of alginate displaces agar during seating dimensional inaccuracy of alginate limits its use to single units 106

107. NON-AQUEOUS ELASTOMERS 107

108. Elastomeric impression materials Comprise a group of synthetic polymer-based impression materials that are chemically cross-linked when set and can be stretched Rapidly recover to their original dimensions 108

109. Based on the backbone of polymer chains 109

110. Elastomers Supplied in 2 components: base paste catalyst paste Formulated in several consistencies in increasing content of filler Extra light body Light body Medium or regular body Heavy body Putty(extra heavy) 110

111. Extra low and putty available only for condensation and addition silicones Polysulfide provided only in light body and heavy body No heavy body for condensation silicone 111

112. Different colored pastes dispensed either through a spiral mixing tip or in equal lengths on a mixing pad Setting occurs through a combination of chain-lengthening polymerization and chemical cross-linking by either a condensation or addition reaction 112

113. 3 types of systems available to mix the catalyst and base : hand mixing, static automixing , dynamic mechanical mixing To record soft tissues for edentulous patients under minimum compression: polysulfide or addition silicones (free flowing, minimum viscosity) 113

114. If moderate compression required: medium-viscosity polysulfides , addition silicones, polyethers Undergo shrinkage upon polymerisation Condensation type silicones undergo additional contraction 114

115. Polysulfides and condensation silicones : highest dimensional changes during setting Addition silicones and polyethers : lesser dimensional changes 115

116. Polysulfides First synthetic elastomeric impression material introduced in 1950 2 paste system Available in low, medium and high consistencies Made up of a base and accelerator/reactor Brands- COE-FLEX,PERMALASTIC,NEOPLEX etc 116

117. Composition The Base Polysulfide polymer(- SH,mercaptan gp ) Principal ingredient Titanium oxide and Zinc Fillers Sulphate ,copper carbonate or silica Strengthener Dibutyl phthalate Plasticizer (confers viscosity to base) The Accelerator Lead dioxide, hydrated copper oxide or organic peroxide Reactor Sulfur Promoter, accelerates the reaction Oleic acid or Stearic acid Retarder, controls setting reaction 117

118. Working time: 5-7 min( longest among elastomers ) Setting time: 8-12 min Pouring the cast: impression must be poured within 30 min to 1 hr 118

119. Each paste supplied in a dispensing tube with approx sized bore diameters at the tip Equal lengths of paste extruded from each tube to provide the correct ratio of polymer to cross-linking agent 119

120. Reaction starts at the beginning of mixing and reaches its maximum rate soon after spatulation is complete Resilient network started to form During final set, a material of adequate elasticity and strength is formed that can be removed past undercuts 120

121. Polymerisation results in chain lengthening and cross –linking with an increase in molecular weight Setting indicated by change of the color of the paste to dark- brown or gray- brown Color- presence of lead oxide 121

122. 122 Polymerisation of ploysulfide impression material SH groups interact with oxygen released from lead dioxide Completion of the condensation reaction results in water as a by-product Pendant –SH for cross-linking and terminal for chain lengthening

123. Hot and humid conditions accelerate the setting of polysulfide impression material Reaction slightly exothermic and yields water as a by-product 123

124. Good flexibility High tear strength Hydrophobic Messy, stains clothes and has an offensive odor Uses—impression for crown and bridge edentulous impressions 124

125. Condensation silicone Followed in 1955 Supplied as two-paste system or base-paste and a low viscosity liquid catalyst or a two-putty system Putty used as tray material in conjunction with a low-viscosity silicone Referred to as the Putty-wash technique Brands- Speedex 125

126. Composition The Base paste - -hydroxyl-terminated polydimethyl siloxane High molecular weight polymer Silica or calcium carbonate Fillers The Liquid Accelerator Tin octoate Metal organic ester Orthoalkyl silicate Oil-based diluents Thickening agents Increase viscosity 126

127. Condensation silicone Working time: 3 min Setting time: 6-8 min Impression must be poured as soon as possible within first 30 min 127

128. Curing involves a reaction of tri- and tetra-functional alkyl silicates in the presence of stannous octoate as a catalyst Sets by cross-linking between terminal groups of the silicone polymers and the alkyl silicate to form a 3-D network 128

129. 129 Condensation polymerisation of alpha-omega hydroxy –terminated poly( dimethyl siloxane ) with tetraethyl orthosilicate in the presence of stannous octoate (catalyst) This reaction results in the release of ethanol molecules

130. By-product: Ethyl alcohol Subsequent evaporation accounts for much of the contraction that takes place in the setting impression Extra-heavy or putty consistency developed to counteract the large polymerisation shrinkage 130

131. Addition silicone Commonly referred to as Polyvinyl siloxanes (PVS) Supplied as low, medium, high and very high consistencies Based on silicone prepolymers that carry vinyl and hydrogen side groups which can polymerize by addition polymerization Brand name- Aquasil 131

132. Composition The Base Polymethylhydrosiloxane Low molecular weight polymer Fillers The Accelerator Divinyl polymethyl siloxane Other siloxane pre-polymers Platinum salt Catalyst Reatrder Controls working and setting times 132

133. Working time: 2-4.5 min Setting time: 3-7 min Cast can be poured upto 1 week after making the impression 133

134. Reaction activated by a platinum salt catalyst ( chloroplatinic acid) without the release of by-products In presence of impurites or moisture, secondary reaction takes place between the residual hydrides and moisture leading to evolution of hydrogen gas 134

135. 135 Hydrogen atoms along the backbone str of PVS chain move to the vinyl group during addition polymerisation (top) Final str after platinum salt has initiated the addition polymerisation reaction (bottom) The zigzag line at the other end of divinylpolysiloxane represents repeating units of dimethylsiloxane with a vinyl terminal

136. This can cause minute gaseous voids in the gypsum casts and reduce the effectiveness of cross-linking polymer structure Automatic mixing systems simplified their manipulation, reduced voids in impressions, reduced the amount of material wasted and reduced the sensitivity of their mixing technique 136

137. 137

138. Advantages Most elastic of currently available materials Virtually negligible distortion upon removal from undercuts Exceptional accuracy in reproducing anatomic details Dimensional stability allows pouring long after impression making Excellent occlusal record registration material 138

139. Disadvantages Inherent hydrophobic nature Non-ionic surfactant wetting agent added to silicone paste rendering the surface of the impression more hydrophillic and called hydrophilized addition silicone 139

140. Sulfur contamination from natural latex gloves inhibits the setting of addition silicone Touching the tooth with latex gloves before seating the impression can inhibit the setting of critical surface next to tooth 140

141. Polyether Supplied as two-paste system in low, medium and high consistencies 2 types: 1. based on ring-opening polymerization of aziridine rings which are at the end of branched polyether molecules 2. based on an acid-catalyzed condensation polymerization of polyether prepolymer with alkoxysilane terminal groups Brands- IMPREGUM 141

142. 1 st type Main chain probably a copolymer of ethylene oxide and tetrahydrofuran Cross-linking and setting promoted by an initiator and an aromatic sulfonate ester R alkyl group Produces cross-linking by cationic polymerisation via the imine end groups Supplied as 2 pastes: base and accelerator 142

143. Composition The Base Polyether polymer Colloidal silica Filler Glycol ether or pthalate Plasticizer The Accelerator Alkyl aromatic sulfonate Initiator Filler Plasticizer 143

144. Working time: 2.5 min Setting time: 4.5 min Poured upto 1 week of storage 144

145. 145 Initiator ,aromatic sulfonate ester dissociates and forms alkyl cations that bind the nitrogen atoms of the azridine ring terminals of the prepolymer ( top,left ) The arrows indicate binding between the cations (R) with nitrogen atoms This action opens up the ring,and the reacted pre-polymer (center) now has 2 ethylene imine terminals(-NR-CH 2 -C + H 2 ) which can react with nitrogen atoms of adjacent unreacted prepolymers .(R2 Aziridine ring Chain propagation polymerization reaction yields a larger molecule(right) which continues growing by binding with aziridine rings of additional unreacted prepolymers Polymerisation reaction terminates when the growing chain combines with a counter ion

146. 2 nd type Based on an acid-catalyzed condensation polymerization of polyether prepolymer with alkoxysilane terminal groups Mechanism similar to condensation silicones Material often called hybrid Behave very much like the 1 st type due to ether linkages 146

147. High degree of wettability Inherent hydrophillic nature Relative stiffness Excellent material for good duplication of fine details and rigid support for pick-up copings 147

148. Making impressions with elastomeric materials Fabrication of gypsum models ,casts and dies involves 6 major steps: Preparing a tray Managing tissue Preparing the material Making the impression Removing the impression Preparing stone casts and dies 148

149. 1.Impression trays Custom tray recommended to reduce the quantity of material required In case of severe undercuts, custom tray avoided Prior to impression making, uniform thickness of tray adhesive applied 149

150. 2.Tissue management Displace the gingival tissues, control gingival haemorrhage and control sulcular fluids to ensure access for the tooth preparation and making impression Gingival retraction cord- most commonly used 150

151. 3.Manipulation of impression materials Supplied for 3 modes: hand mixing, static mixing and dynamic mechanical mixing 151

152. Hand mixing Dispense the same length of materials onto a mixing pad or glass slab Catalyst paste first collected on stainless steel spatula and then spread over base paste Mixture is then spread over the mixing pad Mass is then scraped up with the spatula blade and spread uniformly back and forth on the mixing pad 152

153. Process continued until the mixed paste is uniform in color with no streaks of the base or catalyst appearing in the mixture 2 putty systems(condensation and addition silicone) dispensed by volume using equal number of scoops of each material Knead the material with fingers until a uniform color is obtained 153

154. Static mixing Transforms 2 fluid(or paste-like) materials into a homogenous mixture without mechanical mixing Device used- gun for compressing materials into a 2-cylinder cartridge, which contains the base and catalyst separately, as well as mixing tip 154

155. Mixing tip is made of helical mixer elements in a cylindrical housing Mixer elements are series of alternating right and left –turn 180°helixes positioned so that leading edge of one element is perpendicular to the trailing edge of the next Length of each material is the same as the inner diameter of the cylinderical housing 155

156. Dynamic mechanical mixing Device uses motor to drive parallel plungers,forcing the materials into a mixing tip and out into an impression tray or syringe Motor driven impeller mixes the materials as they are extruded through the tip Materials supplied in collapsible plastic bags housed in cartridge Polyether and addition silicone 156

157. 4.Making an impression 3 techniques: Multiple-mix technique Monophase technique Putty wash technique 157

158. Multiple-mix technique Syringe material(light body)and tray material(heavy body) Lighter material injected within or around the tooth preparation Filled tray then inserted in the mouth and seated over the syringe material Tray material force the syringe material to adapt to the prepared tissues 158

159. Monophase technique Medium body polyether and addition silicone Only one mixture is made and a part of the material is placed in the tray and another portion in syringe for injection in the prepared tissues Success depends on pseudoplastic (shear thinning) property of material 159

160. Putty – wash technique Originally developed for condensation silicone to minimize the effect of associated dimensional changes Thick putty material placed in stock tray and a primary impression made Space for light-body “wash” material provided Mixture of thin consistency wash material placed into putty impression and preparation 160

161. 5.Removal of the impression Shouldn’t be removed until curing progressed sufficiently to provide adequate elasticity ,so distortion doesn’t occur Typically impression should be ready for removal within at least 10 min from time of mixing,allowing 6-8 min for impression to remain in mouth 161

162. Mechanics of removing impression– separation at the impression/tissue interface and stretching of the impression 1 st step to break the physical adhesion between the tissue and the impression Polyether requires extra effort 2 nd step stretches the impression enough to pass under the height of contour of hard tissue to remove impression 162

163. 6.Preparation of stone casts and dies Silicones– hydrophobic Surfactant sprays- debubblizers improve surface wettability of silicone impression material for stone slurry Dilute solution of soap also acts as surfactant Polyether n polysulfide- don’t require surfactant 163

164. Properties of elastomeric impression materials Working and setting time Reproduction of oral structure detail Rheological properties Elasticity and visco -elasticity Tear strength Dimensional stability Disinfection Wettability and hydrophillization Biocompatibility 164

165. 1. Working and setting time Mean working time(min) Mean setting time(min) Impression material 23°C 37°C 23°C 37°C POLYSULFIDE 6.0 4.3 16.0 12.5 CONDENSATION SILICONE 3.3 2.5 11.9 8.9 ADDITION SILICONE 3.1 1.8 8.9 5.9 POLYETHER 3.3 2.3 9.0 8.3 165

166. 2.Reproduction of detail Record detail to the finest degree When stone poured on the surface, finest details not always reproduced 166

167. 3.Rheological properties Viscosity and flow behaviour depends upon - ease of mixing -air entrapment during mixing -tendency of trapped air to escape before the impression is made All elastomers exhibit shear-thinning characteristics before setting 167

168. 2 categories of shear thinning phenomena— Pseudoplasticity Thixotropy Pseudoplastic material- displays decreasing viscosity with increasing shear stress and recovers its viscosity immediately upon a decrease in shear stress 168

169. Thixotropic material- doesn’t flow until sufficient surface energy in the form of an impact force or vibration force is applied to overcome the yield stress of material Extreme shear thinning- material retains an immobile state at rest but flows freely under stress 169

170. Significance of shear thinning Exhibited by addition silicone and polyether impression materials Enable the clinician to use a monophase impression making technique to capture details needed for fixed prostheses 170

171. 4. Elasticity and viscoselasticity Explained well by Maxwell- Voigt model Relative amount of permanent deformation Addition silicone<polyether<polysulfide Recovery of elastic deformation following strain is less rapid for the polysulfide material than for other three types of impression materials 171

172. MAXWELL-VOIGT MODEL 172 A -Maxwell-Voigt model in a stress-free state B -during loading,only S1 spring contracts in response to load C -when loading continues,the pistons in dashpot D1 and D2 move proportionally to the duration of loading . S2 spring contracts alongwith dashpot D2. No change in S1 D -the moment the load is released,S1 spring recovers instantly, whereas rest of the elements remain unchanged. S2 should also recover instantly but retarded by the sluggishness of dashpot D2 E - as time passes, S2 spring recovers and extends dashpot D2 slowly near to its original position. Dashpot D1 remains unchanged

173. Polyvinyl siloxane - exhibit most elastic recovery Distortion on removal from undercuts is virtually non-existent If material is in advanced stage of elasticity, and compressed excessively during seating of impression,distortion can occur when the material elastically rebounds 173

174. MOE in increasing order Polysulfide<condensation silicone<addition silicone< polyether 174

175. 5.Tear strength Low viscosity materials used in interproximal and subgingival areas Measures the resistance of an elastomeric material to fracture when subjected to a tensile force acting perpendicular to a surface flaw Tear strength in increasing order Silicones< polyethers < polysulfides 175

176. Influenced by consistency and manner of removing the materials 176

177. 6. Dimensional stability Dimensional accuracy v/s dimensional stability 6 major sources of dimensional change Polymerisation shrinkage Loss of condensation reaction by-product Thermal contraction from oral temp to room temp Absorption of water or disinfectant over a period of time Incomplete recovery of deformation because of plastic deformation Incomplete recovery of deformation because of viscoelastic behaviour 177

178. Absorption of water or fluids- negatively affects polyether impression Simultaneous leaching of water soluble plasticizer Stored in dry(relative humidity<50%),cool environment to maintain its accuracy Should never be left for protracted periods in disinfecting solutions 178

179. 7.Disinfection Polysulfides and silicones-- Glutaraldehydes , chlorine compounds , iodophors,phenolics Polyether- chlorine compounds or iodophors 179

180. 8.Wettability and hydrophilization Silicones-most hydrophobic Polyethers-hydrophillic Spray surfactant on hydrophobic impressions prior to pouring with gypsum Non-ionic surfactant added during manufacturing 180

181. Hydrophyllized PVS- ether group – hydrophillic and oriented towards the surface when the surfactant migrates by diffusion to the surface region Depends on the contact angle made by water droplets with the impression surface and surface energy 181

182. As soon as hydrophillized PVS encounters oral fluids during impression making, begins to release surfactant to its surroundings Adequate amount of surfactant trapped on the surface of set PVS impressions ,thus providing hydrophilicity for pouring of gypsum dies 182

183. 9.Biocompatibility Tests covered in ISO 10993-5,Biological Evaluation of Medical Devices:Tests for in-vitro cytotoxicity Polysulfide – lowest cell death count Polyether –highest cell toxicity scores 183

184. Elastomer induced biocomaptibility problem– fragment of impression material trapped in patient’s gingival sulcus Causes severe gingival imflammation Can also occur in 2 nd stage implant surgery Contact dermatitis from Polyether in dentists or dental technicians 184

185. 10. Shelf life Don’t deteriorate appreciably in tube or container before the expiration date when stored in dry , cool environment Clear liquid expressed along material-plasticizer segregation Indicates manufacturing error or excessive temperature extremes during storage 185

186. Effects of mishandling Failure to produce an accurate epoxy or gypsum die or cast 186

187. Common failures occurring with use of elastomeric impression materials 187 Type of failure Causes Rough or uneven impression surface Incomplete polymerization,improper ratio or mixing of components,presence of oil or plaque on teeth Surface agents (latex for PVS) inhibit polymerization Too rapid polymerization from high humidity or temp Excessively high accelerator /base ratio( cond silicone) Bubbles Air incorporated during mixing Irregularly shaped voids Moisture debris on teeth surfaces Rough or chalky stone casts Inadequate cleaning of impression Excess water not blown off impression Excess wetting agent left on impression Premature removal of cast,improper w/p ratio of stone Failure to delay pour(PVS) that doesn’t contain Palladium salt for 20 min

188. Distortion Resin tray not aged sufficiently , still undergoing polymerisation shrinkage Lack of adhesion of elastomer to tray Excessive bulk of material Lack of mechanical retention to the impression tray Excessive bulk of material Insufficient relief for the reline material Development of elastic properties in material before tray is fully seated Continued pressure against impression material that developed elastic properties Movement of tray during polymerization Premature/improper removal from mouth Delayed pouring of polysulfide or condensation silicone impression 188

189. Comparative properties of elastomeric impression materials Property Polysulfide Condensation silicone Addition silicone Polyether WT(min) 4-7 2.5-4 2-4 3 ST(min) 7-10 6-8 4-6.5 6 Tear strength(N/m) 2500-7000 2300- 2600 1500-4300 1800-4800 Percent contraction(at 24h) 0.40-0.45 0.38-0.60 0.14-0.17 0.19-0.24 Contact angle(°) 82 98 98/53 49 Hydrogen gas evolution N N Y N Automatic mixing N N Y Y Custom tray Y N N N Unpleasant odour Y N N N Multiple casts N N Y Y Stiffness 3 2 2 1 Distortion on removal 1 2 4 3 189

190. s 190 THANK YOU