7/19/2006 1 Dental Impression Materials Silicone Reversible hydrocolloids Irreversible hydrocolloids Purpose of Impression Materials • To make an accurate replica of the oral tissues • To give a negative reproduction of the tissues • To be filled with model material to make a positive cast Positive cast Desirable properties 1. A pleasant odor, taste, and esthetic color 2. No toxic or irritant constituents 3. Adequate shelf life 4. Economically commensurate with the results obtained 5. Easy to use with the minimum of equipment Desirable properties (con’t) 6. Setting characteristics that meet clinical requirements 7. Satisfactory consistency and texture 8. Readily wets oral tissues 9. Elastic properties with no permanent deformation after strain 10. Adequate strength so that it will not break or tear on removal from the mouth
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Dental Impression Materials
Silicone Reversible hydrocolloids
Irreversible hydrocolloids
Purpose of Impression Materials
• To make an accurate replica of the oral tissues
• To give a negative reproduction of the tissues
• To be filled with model material to make a positive cast
Positive cast
Desirable properties1. A pleasant odor, taste, and
esthetic color2. No toxic or irritant constituents3. Adequate shelf life 4. Economically commensurate with
the results obtained5. Easy to use with the minimum of
equipment
Desirable properties (con’t)6. Setting characteristics that meet clinical
requirements7. Satisfactory consistency and texture8. Readily wets oral tissues9. Elastic properties with no permanent
deformation after strain10. Adequate strength so that it will not
break or tear on removal from the mouth
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Desirable properties (con’t)11. Dimensional stability over temperature
and humidity ranges met in clinic and lab12. Compatibility with cast and die materials13. Accuracy in clinical use14. Readily disinfected without loss of
accuracy15. No release of gas during the setting of the
impression or cast and die materials
“ No impression material fulfills all the requirements”
� the selection of the material best suited for a particular clinical situation and technique rests with the dentist.
• Accuracy - exellent impression of highaccuracy and fine detail
• Require careful control and moreexpensive appartus than alginate
• Dimensional unstable on standing:model should be made as soon aspossible
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• Agar lose moisture and shrink in air• Should be poured immediatly• Store in 100%relative humidity not
more than 1 hour.• After initial setting of model material
should be store in humidor
Impression with reversiblehydrocolloid Viscoelastic Properties
• The stress-strain relationship of the hydrocolloid impression materials changes as the rate of loading changes.
Clinical significance• To reduce the amount of
permanent deformationattributed to dashpot D1, the impression should not be removed by a teasing or weaving method, but it should be removed suddenly in a direction as nearly parallel as possible to the long axes of the teeth.
Permanent Deformation• Deformation should be < 1.5%
following a 10% total compression on the material and a 30 second recovery.
• It is important to lower the amount of compressive loading on the material in undercuts; therefore, (again!) one should remove the material as fast as possible and have more thickness in areas of severe undercuts.
Irreversible Hydrocolloid (Alginate)
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Overview
• Substance is alginic acid, which was extracted from seaweed.
• Potassium salt form of alginic acid (Potassium alginate) react with calcium sulfate dihydrate and water to form a calcium alginic gel.
• Potassium titanium fluoride (3%)– Accelerator for the setting of
the stone• Zinc Oxide
– Filler
• Sodium phosphate (2%)– Retartder - reacts
with calcium ions to slow the working time
• Glycols – Decrease dust
formation• Quaternary ammonium
salts – Disinfectants
• Pigments, taste
Gelation Process(Sol) (Gel)
Soluble Alg + CaSO4 Insoluble Alg(rapid reaction)
Sodium or Potassium alginate
Add water-soluble salt e.g. sodium phosphate to react with CaSO4 --> prolong the working time.
Calcium alginate
Gelation Process (con’t)Calcium sulfate
Potassium alginateTrisodium phosphate
+ Water
2Na3PO4 + 3 CaSO4 --> Ca3(PO4)2 + 3 Na2SO4
K2nAlg + nCaSO4 --> nK2SO4 + CanAlg
Na3PO4 used up
Gel Structure
• The final structure Sodium alginate cross-
linked with calcium ions.
Cross-linking of Sodium alginate
Gelation Time• Once gelation starts, the impression
material must not be disturbed because the growing fibrils will fracture and the impression would be significantly weakened.– Fast setting = 1.5 -3 mins.– Normal setting = 3 – 4.5 mins
• Best regulated by the amount of retarder added during manufacturing.
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Gelation Time (con’t)• Water Temperature
– 1 min. of reduction in gelation time for each 10°C of temp. decrease
Strength• Gel strength doubled during the first
4-min period.• Water amount
– Weakened, less elastic• Spatulation
– Insufficient spatulation --> failure of the ingredients to dissolve sufficiently
• Overmixing– Breaks the CaAlg gel network --> decrease strength
Viscoelasticity• Strain-rate dependent• Avoid torquing or twisting when
remove the impression.
Accuracy
• Not as good as elastomeric impression materials– Reproduction of fine detail– Dimension stability– Roughness of the impression surface
• Can be used for Removeable Partial Denture (RPD)
Dimensional Stability• Syneresis, evaporation,
imbibition• The impression should be
exposed to air for as short a time as possible.
• Immediately constructing the stone cast
air water
Dimensional Stability
• 100% relative humidity is the best storage environment to preserve the normal water content of the impression.
Percentage of change in water content by weight
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Compatibility with Gypsum• The surface of the gypsum cast may be too
soft for waxing procedures.– Immerse the impression in the accelerator for
gypsum product– Incorporate a plaster hardener or accelerator in
the impression material (fluoride)
Disinfection• Certain disinfectants my result in gypsum casts
that have a lower surface hardness or diminished surface detail
• Current protocol– Impression --> thoroughly rinsed --> spray
disinfectant --> wrap in a disinfectant-soaked paper towel --> place in a sealed plastic bag for 10 minutes
– Iodophor, Sodium hypochlorite, Glutaraldehyde
Shelf Life• Storage temperature and moisture
contamination• Materials stored for 1 month at 65°C
are unsuitable to use, either failing to set or setting too rapidly.
• Should not stock more than one year’s supply.
Effects of Mishandling• Examples of common causes for
failures encountered with hydrocolloid impression materials– Distortion– Tearing– Rough stone model
Distortion
• Impression not poured immediately.• Movement of tray during gelation• Premature removal form mouth• Improper removal from mouth• Tray held in mouth too long (only with
• Inadequate cleaning of impression• Excess water left in impression• Premature removal of model• Leaving model in impression too long• Improper manipulation of stone