RPD LAB - PRIMARY IMPRESSIONS & POURING PRIMARY CASTS

Question 1

how many scoops of alginate and how much water for lower impressions

Answer 1

alginate - 3 level scoop

water - 3/4 of water

Question 2

how many scoops of alginate and how much water for upper impressions

Answer 2

alginate - 4 level scoops

water - 1 cup

Question 3

what does cold water do to alginate

Answer 3

slows down setting time

Question 4

what does warm water do to alginate

Answer 4

decrease the setting time

Question 5

how should you place the impression

Answer 5

place it front first and push back to prevent air blows

Question 6

how much water and how much dental stone for each impression

Answer 6

25/30ml

100g dental stone

Question 7

what much of a crust should we have

Answer 7

5mm

Question 8

why do we saturate

Answer 8

it softens the particles of the dental stone which will cut down the spatulation time

Question 9

what does an increased spatulation time do

Answer 9

increased spatulation time increases the set of the mix

Question 10

how do we spatulate

Answer 10

place the spatula against the side of the bowl and rotate the bowl - we want a homogenous, thick creamy mix

Question 11

what are the steps for the impression in the study cast preparation

Answer 11

1. sift in the dental stone into the water and leave to saturate
2. spatulate the mix
3. fill impression from posterior aspect -
4. invert impression
5. agitate impression
6. gradually refill impression
7. clean off excess
8. add additional material to create retention
9. score lightly into surface for extra retention

Question 12

why do we fill the impression from the posterior and vibrate rather than just filling from the front?

Answer 12

so we do not trap air into the occlusal surfaces or incisor edges

Question 13

why do we invert the impression and vibrate the mix back out

Answer 13

to ensure the material is down into the incisal edges and occlusal surface of all the teeth

Question 14

why do we agitate the impression

Answer 14

so any left over material will fill the occlusal surfaces

Question 15

what should be the height of the retentions

Answer 15

1cm

Question 16

why do we score into the surface and when

Answer 16

we score to add extra retention and we do it once the material has set so it looks matte

Question 17

how much water and dental stone for impression base

Answer 17

30ml water
110g dental stone
50/50 dental stone and dental plaster

Question 18

what are the steps for the impression base

Answer 18

1. agitate mix into impressions
2. use material to create base
3. invert impression into base
4. ensure the handle is horizontal
5. remove excess and bring base material to meet the impression
6. ensure we remove the material in the lingual aspect of the lower
7. remove excess
8. remove material overlapping impression
9. trim cast
10. rinse impressions
11. smooth sharp edges
12. smooth study cast with sand paper

Question 19

how do we trim the cast?

Answer 19

we want the occlusal plane to be horizontal and we want to check that the back of the base is 90 degrees to midline
we want the occlusal plane to be 1cm deep in its deepest part

Question 20

what are alginates based on

Answer 20

alginic acid

Question 21

why do we use the snap removal technique for alginates

Answer 21

to obtain an elastic response

it ensures that the time for which the material is under compression is as short as possible

Question 22

why do we want the time the material is under compression to be shorter

Answer 22

the longer the material is under compression the higher the amount of permanent deformation

Question 23

why must the impression be rinsed after removal from the patient’s mouth

Answer 23

to remove any saliva as this will interfere with the setting of the gypsum model.

Question 24

why should surface water be removed prior to pouring the model

Answer 24

residual water will dilute the model material and result in a soft surface which is easily damaged.