master impressions

Question 1

what are the non-elastic impression materials?

Answer 1

zinc oxide eugenol
compounds
waxes

Question 2

what are the synthetic elastomer impression materials?

Answer 2

silicones
ployethers
polysulfindes

Question 3

what are the hydrocolloid impression materials?

Answer 3

reversible

irreversible

Question 4

what is the purpose tray spacing

Answer 4

in special trays for master impression
prevents over seating and distortion
allows an even distribution of material

Question 5

why do you take a master impression?

Answer 5

to produce a master case

Question 6

what is a master cast

Answer 6

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

Question 7

when do you take a master impression?

Answer 7

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

Question 8

how do you take a master impression?

Answer 8

special/custom tray made on primary cast
tray spaced appropriately for selected impression material
disinfect tray before use

Question 9

how do you try in the upper special tray?

Answer 9

stand behind px and rotate into mouth

Question 10

how do you try in a lower special tray?

Answer 10

stand in front of px and rotate tray into mouth

Question 11

what should be done if the tray is over-extended?

Answer 11

trim back the peripheries with a bur

Question 12

what should be done if the tray is under-exteneded?

Answer 12

add to peripheries using greenstick compound or wax

Question 13

how is tearing and distortion of the impression material prevented?

Answer 13

large interdental spaces beneath contact points should be blocked out in the mouth using soft ribbon wax or other material prior to impression

Question 14

what impression materials can be used?

Answer 14

polyvinylsiloxane
polyether
irreversible hydrocolloid
impression compounds

Question 15

describe polyvinylsiloxane

Answer 15

extrude
light, medium, heavy body available
medium most common
apply appropriate adhesive to tray surface and periphery

Question 16

describe polyether

Answer 16

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

Question 17

describe irreversible hydrocolloid

Answer 17

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

Question 18

describe impression compounds

Answer 18

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

Question 19

when are materials combined?

Answer 19

free end saddle areas can be difficult to capture accurately

impression compound for initial, wash of flowable eg alginate/silicone for final

Question 20

what are the stages of master impression taking?

Answer 20

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

Question 21

how is the impression inspected?

Answer 21

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

Question 22

how should you reflect on the impression?

Answer 22

are there any errors?
can you accept it?
do you need to repeat it?

Question 23

how do you write a lab prescription for cobalt chrom?

Answer 23

pour model in improved stone
construct chrome framework per design
-clear, signed

Question 24

how do you write a lab prescription for acrylic?

Answer 24

pour model in 100% dental stone
construct record blocks
-shellac base upper
-wire strengthened lower