Chapter 3a/b Impression materials/Gypsum

  1. A model/cast/replica  is a __________ of the original
    positive reproduction
  2. Dental impressions are a ________ of an object
    negative reproduction
  3. What are some properties of the ideal materials
    • Fluid enough to flow around
    • Once in the mouth, they must harden within a few minutes to a semirigid state
    • Must be non-toxic or irritating and not have an offensive taste or odor.
    • Be compatible with all model and die materials.
    • Not breakdown with disinfection.
    • Good shelf life without degradation of properties
  4. Properties of stock trays
    • adequate for many applications.
    • Usually are plastic or metal.
    • Come in a limited number of sizes
  5. Custom tray is often used for
    precise fitting restorations like long span bridges, implant cases, dentures
  6. What are the steps to making a custom tray
    • 1. Use stock tray to make model
    • 2. Use preliminary model to make custom tray
  7. What are some examples of impression material systems
    • powder mixed with water
    • paste sysem
    • 2 tubs
    • tub and tube
    • softened or melted by heating
  8. What are the three types of impression materials
    • Inelastic
    • Aqueous elastomeric
    • Nonaqueous elastomeric impression
  9. What are the kinds of Inelastic impression materials
    • Plaster
    • Wax and Impression Compound
    • Zinc oxide-eugenol (ZOE)
  10. What are the properties of Impression plaster
    • At one time used for edentulous impressions. Now used to mount casts on the articulator
    • Very accurate (little dimensional change when it sets up), and very fast setting
  11. Study Model
    —positive image of mouth used for treatment planning, patient education, or information during treatment
  12. Cast
    —a more accurate replica used to fabricate a restoration or an appliance
  13. Desirable Properties of gypsum
    • —Accuracy—
    • Dimensional Stability—
    • Ability to reproduce fine detail
    • —Strength and resistance to abrasion
    • —Compatibility with the impression material
    • —Color—
    • Biological safety – must be biologically compatible—
    • Ease of use—
    • Cost
  14. Dies are
    —A positive likeness of a single tooth that is removable from the rest of the cast
  15. What are dies used for
    —Used to fabricate indirect restorations (crowns, bridges, inlays, etc.)
  16. Dies are poured in
    —high strength or improved stone, or die stone
  17. Gypsum is made from
    gypsum rock which is a mineral that is mined out the ground
  18. Chemically gypsum is
    Calcium Sulfate Dihydrate
  19. To create dental products, gypsum rock is heated in a process called
    Calcination that removes water and produces various powders
  20. Various powder created from calcination
    Calcium Sulfate Hemihydrate
  21. What are the Types of Dental Gypsum Products
    • Plaster
    • Stone
    • High-Strength Stone or Improved Stone (die stone)
  22. —The first gypsum product used in dentistry
    Model plaster
  23. What are the properties of model plaster
    • Weakest of all gypsum products
    • —Primarily used for study models and when mounting cases on an articulator
    • Usually white in color, features small irregular particles that require a great deal of water to rehydrate and turn back into solid, dihydrate form
    • Shortest working time
  24. To form model plaster Gypsum rock is ground into fine powder and then calcined (heated) in open kettle which shatters the crystal and forms
    betahemihydrate or Type II dental stone
  25. Articulators are
    Instrument used to replicate jaw function when fabricating an indirect restoration
  26. To form stone Gypsum rock is ground into fine powder and then calcined in closed pot under steam pressure to form
    Alpha hemihydrate or Type III stone or Hydrocal
  27. What are the properties of stones
    • —Usually yellow in color- but can be other colors
    • Particles are denser and more regular in shape and   requires less water than plaster producing models that are harder and stronger than plaster
    • Widely used in dentistry to make partial and full denture casts, bleach splints, night guards, orthodontic appliances and many other uses
  28. High Strength/Improved Stone/Die stone is Made by calcining the gypsum under pressure in a closed pot in a calcium chloride solution to form
    —Type IV Stone
  29. What are the properties of die stone
    • Very regular shaped
    • —dense particles leads to hardest and strongest of all dental stones
    • —Used in producing die models   for fabrication of crowns, bridges, inlays, onlays, and implant cases
  30. Type V stone
    —newer, higher compressive strength but also higher setting expansion
  31. Calcium Sulfate Hemihydrate + 1(H2O)
    • Calcium Sulfate Dihydrate + heat
    • By the process of hydration
  32. Water Powder Ratio
    defined as the proportion of water to powder to make a workable mix of a particular gypsum product
  33. Water Powder Ratio is dependent on
    the type of stone
  34. What is the water/powder ratio for plaster
    50 ml./100 g (about .5)
  35. What is the water/powder ratio for dental stone
    30 ml/100 g (about .3)
  36. What is the water/powder ratio for die stone
    20 ml/100 g (about .2)
  37. What is the Initial Set Time or Working Time
    Time from the start of the mix until mass is semi-hard and can no longer be manipulated
  38. The Initial Set Time or Working Time is also called
    —Loss of Gloss
  39. The Initial set time and Final setting time is shortest with ______ and longest with ________
    • plaster
    • die stone
  40. Final Setting Time
    —Time from beginning of the mix until cast is set and can be separated from the impression
  41. For increased setting time (slower setting) one would
    • Decrease the mixing time
    • Add more water (creates weaker model as well)
    • Add a chemical retarder – borax
  42. —For Decreased Setting Time (Faster setting) one would
    • Increase the mixing time (mix longer)
    • Use less water (creates stronger model)
    • Add a chemical accelerator – adding dihydrate granules
  43. As gypsum hemihydrate particles mix with water and become dihydrate particles, they form small centers of set stone called
    —Nuclei of Crystallization
  44. All gypsum _______ on returning to the dihydrate form
  45. ______expands the most, ______ the least
    • Plaster
    • die stone
  46. Strength is dependent on the _______ of the set material
  47. Wet Strength
    Strength right after final set. Still contains water
  48. Dry Strength
    Strength after 24 hours when all of the extra water has evaporated. Much higher than wet strength
  49. What are the properties of wax
    • probably the first material used.
    • It is thermoplastic
    • More commonly used for bite registrations and as adjunctive materials in the laboratory for the fabrication of crowns, bridges and other restorations. Also used to extend borders of an impression tray
  50. What is a thermoplastic material
    reversibly softens on heating and hardens on cooling. Purely a physical change-no mixing is necessary. Heat in H2O bath.
  51. What are the properties of impression compound
    • Also thermoplastic
    • Available in sheets and sticks
    • Used to fabricate custom trays for full dentures and a variety of other “accessory” functions rather than as impression material. Can be used for full denture impressions and taking impressions for denture “borders”
  52. What are the properties of Zinc Oxide Eugenol (ZOE):
    • Rarely used today as an impression material (replaced by other materials).Requires a custom tray in “wash impression” technique
    • Chemical reaction between zinc oxide and the eugenol requires mixing of two pastes on a pad (introduction of equal length strips
  53. What are the two Aqueous elastomeric impression materials
    • Alginate
    • Agar
  54. What are the properties of a hydrocolloid
    • Changes from a sol state to a gel state
    • Major component of hydrocolloids is water
  55. What is a sol state
    a solution of one material dissolved in another. In alginate the sol state is the viscous liquid when the material has been mixed but not yet set up
  56. What is the gel state
    • 1. a solid carbohydrate polymer network
    • 2.) water trapped in very small pockets of the material. In alginate, the gel state is when the material has set.
  57. What are properties of Alginate Hydrocolloid
    • Irreversible hydrocolloid- won’t re-liqueify upon heating
    • Limited accuracy
    • Most widely used impression material
    • Poor dimensional stability
  58. Alginate Hydrocolloid sets via
    chemical reaction
  59. Syneresis
    water evaporation
  60. Imbibition
    Taking water in
  61. Alginate + Calcium sulfate->
    Calcium Alginate
  62. What is added to alginate to slow the reaction
    Trisodium phosphate – retarder
  63. What are the properties of Agar Hydrocolloid
    • typically just called “hydrocolloid
    • Reversible hydrocolloid – will re-liquefy if reheated
    • Very accurate material – but very technique sensitive
    • Extremely limited dimensional stability- must pour immediately
    • Sets via a physical reaction
  64. What impression material uses this unit?Image Upload 1
    Agar hydrocolloid
  65. What are the four classes of nonaqueous elastomeric impression materials
    • 1.) Polysulfides 
    • 2.) Condensation Silicone  
    • 3.) Polyethers
    • 4.) Addition Silicones
  66. What are the properties of Polysulfides: (aka rubber base)
    • Remains dimensionally stable longer than agar hydrocolloid but not much and still needs to be poured quickly
    • More tear resistant than agar hydrocolloid
    • Horrible odor and difficult clean-up limits usage today
  67. What are the properties of Condensation Silicones
    • Supplied as base paste + accelerator
    • Ethanol is the byproduct of the condensation reaction
    • No longer popular
    • shorter work times
  68. What are the properties of polyethers
    • Very accurate, widely used material
    • Fastest setting of all elastomers
    • Expensive
    • Great dimensional stability
  69. What are the properties of addition silicones
    • Known as Polyvinylsiloxanes
    • very popular today, especially for crowns, bridges and implants
    • Very accurate
    • Very dimensionally stable
    • Very widely used material
    • Sets via an addition reaction with no by product
  70. What are the different impression concepts
    • Putty
    • Heavy
    • Light body
    • Medium body
  71. What is putty used for
    used to ‘customize’ a stock tray
  72. What is a heavy body used for
    used to fill most of a custom tray, then add light body
  73. What is light body used for
    (syringe) to precisely capture detail (margin) of prepared teeth. Could also be “extra light” body
  74. What is the medium used for
    often just used alone in a tray
  75. Chemically how much water is needed to wet gypsum
    18.6ml of water
Card Set
Chapter 3a/b Impression materials/Gypsum
Chapter 3a/b Impression materials/Gypsum