master impressions Flashcards
what are the non-elastic impression materials?
zinc oxide eugenol
compounds
waxes
what are the synthetic elastomer impression materials?
silicones
ployethers
polysulfindes
what are the hydrocolloid impression materials?
reversible
irreversible
what is the purpose tray spacing
in special trays for master impression
prevents over seating and distortion
allows an even distribution of material
why do you take a master impression?
to produce a master case
what is a master cast
cast taken with custom/special tray made on primary cast
poured in 100% dental stone, more accurate than primary cast
treat with care to avoid risk of abrasion
duplicates can be made for use at a later stage
when do you take a master impression?
you have a finalised denture design on surveyed mounted casts
after all tooth prep and rest. carried out according to tx plan
verify sufficient clearance for denture base and components
-additional occlusal registration stage needed if casts cant hand articulate
how do you take a master impression?
special/custom tray made on primary cast
tray spaced appropriately for selected impression material
disinfect tray before use
how do you try in the upper special tray?
stand behind px and rotate into mouth
how do you try in a lower special tray?
stand in front of px and rotate tray into mouth
what should be done if the tray is over-extended?
trim back the peripheries with a bur
what should be done if the tray is under-exteneded?
add to peripheries using greenstick compound or wax
how is tearing and distortion of the impression material prevented?
large interdental spaces beneath contact points should be blocked out in the mouth using soft ribbon wax or other material prior to impression
what impression materials can be used?
polyvinylsiloxane
polyether
irreversible hydrocolloid
impression compounds
describe polyvinylsiloxane
extrude
light, medium, heavy body available
medium most common
apply appropriate adhesive to tray surface and periphery
describe polyether
impregum
automatic mixing
sets rigidly
-dont use for sever undercuts
good flow and accuracy, dimensionally stable
apply appropriate adhesive to tray fit surface and periphery
describe irreversible hydrocolloid
alginate well mixed good if undercuts present careful with air blows in key areas stone casts should be poured immediately appropriate adhesive to tray surface and periphery
describe impression compounds
non elastic, set rigidly not used for dull arch impressions in dentate px useful in free end saddle areas red- heat in bowl of hot water green- bunsen
when are materials combined?
free end saddle areas can be difficult to capture accurately
impression compound for initial, wash of flowable eg alginate/silicone for final
what are the stages of master impression taking?
try in and trim if needed
modify with greenstick if needed and fit surface for free end saddles
apply adhesive and use alginate/polyether/PVS for impression
disinfect place in sealed bag and write instructions to tech
how is the impression inspected?
examine for defects
surface should exhibit clear detail
no part should be detached from the tray
any excess unsupported bulk of material removed w/ sharp instrument to prevent distortion
how should you reflect on the impression?
are there any errors?
can you accept it?
do you need to repeat it?
how do you write a lab prescription for cobalt chrom?
pour model in improved stone
construct chrome framework per design
-clear, signed
how do you write a lab prescription for acrylic?
pour model in 100% dental stone
construct record blocks
-shellac base upper
-wire strengthened lower
