RPD LAB - BLOCKING OUT

Question 1

Why is blocking out done

Answer 1

to prevent a part of the removable prosthesis entering an area it should not enter
a rigid enter should not and cannot enter undercut areas

Question 2

How do we block out

Answer 2

add wax to the undercut

duplicate the cast with the wax so we now have no undercuts

Question 3

What happens if you don’t block out

Answer 3

the rigid acrylic denture base cannot be fitted

Question 4

When can blocking out be done

Answer 4

on the master cast before it is duplicated in the refractory material

block out undercuts with dental plaster on surveyor with chisel tooth, trial and processing is undertaken on this master cast (no duplication) - ONLY FOR PMMA

Question 5

Where do you only block out

Answer 5

in relation to path of insertion and removal

between high and low survey lines where a connector is being placed

from undercut gauge mark to low survey line where a clasp is being place

Question 6

What is done when the blocking out is done on the master cast before it is duplicated in the refractory material

Answer 6

the master cast block out the undercuts to the path of insertion and removal using wax and chisel on the surveyor

this will be duplicated in the refractory material for a CoCr framework

duplicated again to give a stone working cast

the PMMA can be processed on the working cast and delivered on the master cast

Question 7

What is the advantage of blocking out on master cast before duplicating

Answer 7

accuracy of the prosthesis which can be checked on the master cast both in laboratory and at chairside

the master cast is not destroyed/broken at processing stage

Question 8

What is the disadvantage of blocking out on a master cast before duplicating

Answer 8

another duplication and another error
requires more stages and time
not always possible to fit onto master cast which is rigid compared to oral mucosa

Question 9

What is the disadvantage of blocking out with plaster on master cast and not duplicating

Answer 9

the master cast cannot be in tack after processing

teeth may break off the master cast which usually are abutment teeth and the ones important when fitting RPD at chair side

Question 10

What is the advantage of blocking out with plaster on master cast and not duplicating

Answer 10

requires less stages and time

however there is less technique and skill in blocking out plaster

Question 11

How do you prepare the upper casts

Answer 11

block out in areas that RPD requires the undercut to be blocked
pin dams kept back from GM
post dams is the posterior seal but in front of soft palate

Question 12

What are duplicating materials

Answer 12

agar (reversible hydrocolloid)

condensed cure silicone

Question 13

What is the advantage of agar as duplicating material

Answer 13

it is reusable up to 10 times and keeps its elasticity - more environmentally friendly

Question 14

What is the disadvantage of agar as the duplicating material

Answer 14

like alginate impressions it suffers from imbibition (water absorption) and syneresis (contraction)

short shelf life

Question 15

What is the advantage of using condensed cured silicone

Answer 15

very accurate and great shelf life

Question 16

What is the disadvantage of using condensed cured silicone

Answer 16

very expensive compared to agar

Question 17

What does a gel machine to

Answer 17

melts gel at 90 degrees

brings down to 45 degrees for a pourable state

Question 18

Describe the process of agar duplication

Answer 18

soak casts in water 
surface dry the cast
place cast in duplicating flask
pour in agar
allow to set 
remove cast and pour new cast/refractory cast from mould

Question 19

What is a silicone mould used for

Answer 19

used for pouring refractory cast to wax up the cobalt chrome base
used for pouring second stone cast for working cast - for record block, trial denture and processing PMMA

Question 20

What is the cobalt chrome procedure

Answer 20

master cast blocked out
duplicate for refractory cast
wax up design and sprue on refractory
invest waxed up design heat and cast alloy framework for the RPD
CoCr base is fitted to the second cast/working stone cast and tooth trial and processing can take place o this duplicated cast