impression materials

Question 1

what is the function of an IM?

Answer 1

to produce an accurate replica of the surface and shape of hard and soft tissues

Question 2

what is dental stone used for?

Answer 2

to fill the impression and produce a positive replica which represents the position, shape, size and orientation of each tooth

Question 3

what is a dental cast used for?

Answer 3

evaluating dentition when orthodontic, occlusal or other problems involved, in lab fabrication of rest. prostheses etc.

Question 4

what can IM capture?

Answer 4

single tooth
whole dentition
edentulous mouth

Question 5

how is a good quality/accurate impression taken?

Answer 5

IM with right properties

clinician with good technique

Question 6

what is mucostatic?

Answer 6

fluid materials that displace the soft tissues slightly ie. give an impression of the undisplaced mucosa
eg. zinc oxide eugenol, low viscosity alginates)

Question 7

what is mucocompressive?

Answer 7

viscous materials that record an impression of the mucosa under load ie. give impression of displaced soft tissues
eg impression compound, high viscosity alginates

Question 8

what is elastic?

Answer 8

material set=solid
by using force removed from mouth-bulges 
elastic strain during removal
IM temporarily changed shape
after removal elastic removal
final form matches closely but not 100%

Question 9

what is viscoelastic?

Answer 9

not fully elastic

-some deformity

Question 10

how should you remove an impression tray?

Answer 10

short, sharp pulling force
load time less
=less permanent strain

Question 11

give 2 examples of hydrocolloids

Answer 11

agar- no longer used
alginate
-irreversible

Question 12

give 3 examples of elastomers

Answer 12

polysulphides
polyethers
silicones

Question 13

describe prep of IM

Answer 13

mixing
chemical reaction
heat
material flows
-setting reaction
-as IM flows into tray it begins setting reaction, when inserted it surrounds hard/soft tissues and setting continues

Question 14

how is accuracy achieved?

Answer 14

flow
-surface detail recorded
setting changes
-no dimensional change
removal
-no effect on oral tissues
-no dimensional change
-complete elastic recovery
storage
-no dimensional change over time eg moisture
decontamination
-dimensions/surfaces unaltered
compatible w/ cast material

Question 15

which properties affect accuracy?

Answer 15

viscosity
-determines flow over surface so detail
setting mechanism
-cause dimensional contraction
thermal expansion coefficient
-low -mouth to room temp
hydrophobic/hydrophilic
-surface contact
elasticity
tear strength
-withstand tear on removal

Question 16

what are the ideal properties for px comfort?

Answer 16

non-toxic, non-irritant
acceptable taste and smell
short setting time
removal w/o damage to oral tissues

Question 17

what are the ideal properties for operator convenience?

Answer 17

quick, simple technique
convenient working/setting times
must be able to be decontaminated

Question 18

what are the (2) non-elastic IM

Answer 18

impression compound

impression paste

Question 19

what is a hydrocolloid?

Answer 19

colloid= 2 phase system of fine particles of 1 phase dispersed in another phase
dispersing medium=water

Question 20

what are the (2) states of hydrocolloid?

Answer 20

sol
-viscous liquid state
gel
-jelly like consistency
-agglomeration of particles
-entangled framework of solid particles enclosing liquid by capillary forces

Question 21

what are the components of alginate?

Answer 21

salt of alginic acid
calcium sulphate
trisodium phosphate
filler
modifiers, flavours, chemical indicators

Question 22

what is the purpose of trisodium phosphate?

Answer 22

preferentially reacts with ca ions released by ca sulphate
only when all used ca ions released reacts with na alginate
delaying formation of alginate gel
used to adjust working time

Question 23

what does the ca alginate formed consist of?

Answer 23

long cross-linked molecules surrounding undissolved particles

Question 24

describe how to use alginate correctly

Answer 24

correct powder/liquid ratio
water 18-24 c
perforated tray and adhesive
remove with sharp pull

Question 25

why should you wait for a further minute after apparent set?

Answer 25

crosslinking continues after apparent set

Question 26

describe alginates px comfort factors

Answer 26

non-toxic
non-irritant
acceptable taste, smell
setting time ok

Question 27

describe alginates operator conventience

Answer 27

easy to use
setting time ok
storage
-avoid moisture
-prepare casts asap to avoid dimensional change

Question 28

what is syneresis?

Answer 28

release of water

Question 29

what is imbition?

Answer 29

uptake of water

Question 30

describe alginates accuracy factors

Answer 30

flow- fine detail ok
setting changes ok
nearly elastic
poor tear strength
storage
-syneresis/imbition may occur