impression materials

Floating Topic
impression
1. to record accurately the dimensions of oral tissues & their spatial relationship
2. gives negative reproduction of the tissues
  1. positive reproduction = model/cast
3. desired properties
  1. ease of manipulation & cost
  2. adequate flow & wetting properties
  3. appropriate setting time
  4. sufficient mechanical strength
    1. resist tearing on removal
  5. good dimensional accuracy
    1. stable over time
  6. taste/odor acceptable to pt
  7. Safe, not toxiz/iiritating
  8. no significant degradation after disinfection
  9. compatible with all die & cast materials
  10. good shelf life
Alginate (Irreversible hydrocolloid)
1. most widely used aqueous imp mat
  1. Advantages
    1. ease of mixing & manipulating
    2. minimum equipment needed
    3. flexibility of set material
    4. accuracy, if properly handled
    5. low cost
  2. Disadvantages
    1. low tear strength
    2. do not transfer as much surface detail to gypsum as agar/elastomeric imp
2. Composition
  1. Sodium alginate +CaSO4 . H2O --> Calcium alginate + Na + SO4 + H2O
    1. Paste --> gel
  2. manufacturer add sodium phosphate (retarder)
    1. to control setting time
  3. irreversible hydrocolloids
3. Properties
  1. Mixing & setting time
    1. proper mix
      1. should be smooth, creamy, free-grain consistency in less than 1min
      2. setting time
      3. normal
      4. >2min
      5. workable for 2min after start of mix
      6. fast
      7. 1-2min
      8. workable for 1 1/4 min
      9. increase water temp
      10. shorter working & setting time
      11. powder : water
      12. thinner mix
      13. increase setting time
      14. Usual range of setting time normal-setting alginate
      15. 2 1/2 - 5min
  2. elastic recovery
    1. 96-98% elastic recovery
      1. 2-4% permanent deformation
      2. ANSI-ADA specification requires at lease 95%elastic recovery(no more than 5% permanent deformation
    2. alginate is flexible but not perfectly elastic
  3. flexibility
    1. amount of water & powder influence the flexibility
      1. thicker mix
      2. lower flexibility
  4. strength
    1. strength increase if thicker mix used
      1. but flow become inadequate
    2. tear & compressive strength increase at removal if time of removal is delayed
  5. dimensional change
    1. loss of accuracy with increased time of storage
    2. set alginate is a hydrocolloid with large quantity of water
      1. water evaporates if stored open air
      2. imp shrinks
      3. if placed in water, it absorbs water
      4. expand
    3. Syneresis
      1. formation of exudate on the surface of gel
      2. continual shrinkage of the gel
    4. should be poured promptly
    5. when stored in 100% relative humidity
      1. some can be stored up to 5days
  6. reproduction of detail
    1. must be able to record the detail of oral tissues & transferred to model/die
  7. disinfection
    1. alginate may be disinfected by immersion without clinically significant dimensional change
4. Dispensing the alginate
  1. measure water acc to manufacturer's recommendation
  2. water temp should be room temp (22-23*C)
Agar (Reversible hydrocolloid)
1. the first successful aqueous imp mat used in dentistry
2. supplied in either
  1. Gel (in a tube)
    1. used with water-cooled imp tray
  2. a number of cylinders (in glass jar)
    1. used with syringe
3. when heat in boiling water (100*C)
  1. Agar gel convert to sol
  2. become gelagain by cooling to 43.3*C
  3. Sol = colloidal system, in dispersed phase is solid, continuous phase is liquid
4. Once gel convert to sol
  1. remain fluid for full day (stored at 65*C)
5. Gel has liquefaction temp different from its solidification temp
  1. termed HYSTERESIS
6. Reversible hydrocolloid
  1. gel to sol is reversible with heat
7. Highly accurate at time of removal from mouth
  1. BUT shrink when stored in 100% humidity
Addision silicone
1. Composition & setting reaction
  1. elastomeric imp mat
    1. a.k.a vinyl polysiloxane (VPS)
  2. supplied as 2paste / 2-putty system
    1. low-molecular weight silicone(terminal vinyl grp) ,reinforcing filler, chloroplatinic acid catalyst
    2. low-molecular-weight silicone (with silane hydrogens) & reinforcing filler
  3. no volatile byproduct (eg water/ethanol) formed from the reaction
    1. minimal dimensional change occur during polymerization
  4. contains surfactant
    1. hydroplilic
      1. better wetting to oral tissues surface
2. consistencies
  1. low-, medium-, high-, or very high-
  2. single-consistency (monophase) or can be used as both low- & high-viscosity
3. Properties
  1. dimensional change in 24Hrs
    1. very low (-0.1%)
  2. elastic recovery at time of removal
    1. 99.8% (permanent deformation of 0.2%)
      1. highest of all imp mat
  3. percent flow values is low
  4. short working time
  5. low to moderate flexibility
    1. removal from undercut may be a little difficult
4. Manipulation
  1. putty should not be mixed with latex rubber gloves
    1. will retard/prevent setting
      1. by poisoning the platinum catalyst
    2. vinyl gloves/bare hand acceptable
Polyether
1. hydrophilic & good wetting properties
  1. BUT
    1. poor taste, limited dimensional stability over time
2. composition & setting reaction
  1. supplied as
    1. base & catalyst system
  2. low-molecular-weight polyether (ethylene imine terminal) (BASE)
    1. react with
      1. aromatic sulfonic acid ester catalyst
3. Properties
  1. medium viscosity
  2. elastic recovery is slightly less than addition silicones
  3. Dimensional change higher than addition silicones
    1. absorbs water
      1. should not be stored in water, should be washed & dried of water & disinfectants
  4. aromatic sulfonic acid ester catalyst
    1. cause skin irritation
      1. avoid direct contact
      2. thorough mixing before place inside pt's mouth
4. Manipulation
  1. 2 mixing systems
    1. a cartridge with a mixing gun
    2. a sausage with a dynamic mechanical mixer
  2. should use adhesive on tray
  3. should not stored in water, in direct sunlight & prepare model/die promptly
  4. Remove
    1. unreacted polyether
      1. with organic solvents(acetone/chloroform) or soap& water
    2. set elastomer
      1. with chloroform/ chlorinated solvent (trichloroethylene)
  5. good shelf life
    1. usable after 2 years storage in room temp
Condensation silicone
1. not commonly used in clinic
  1. but used as an accurate duplicating material in dental lab
2. Supplied as
  1. base & accelerator (catalyst)
3. added reinforcing agents
  1. silica
    1. to give proper consistency to the paste
    2. give stiffness to set elastomer
4. low value of permanent deformation & flow
  1. high shrinkage 24hours after set
5. setting reaction sensitive to moisture & heat
  1. increase in either will cause
    1. shorter setting time & working time
6. putty-wash system
  1. increase accuracy
    1. because
      1. putty -lower dimensional change than wash mat
      2. dimensional change in putty will not significantly affect accuracy
      3. because it happens before final impression taken
      4. Dimensional change for thin wash material is small
Polysulfide
1. a.k.a rubber base
  1. 2paste system
    1. low, medium or high viscosity
2. base mat
  1. LMW polymer (contains reactive mercaptan grp), & reinforcing agents
3. catalyst
  1. lead dioxide or copper hydroxide
4. permanent deformation slightly higher than hydrocolloid mat
rigid imp mat
1. imp compound
  1. resin, waxes, organic acids,fillers, coloring agents
    1. sticks/cones
      1. heated before use
2. Zinc oxide eugenol
  1. initial set
    1. 3-5min
  2. final set
    1. less than 10min
  3. not strong
    1. must be supported with imp tray