Blocking out Flashcards
Why is blocking out done
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
How do we block out
add wax to the undercut
duplicate the cast with the wax so we now have no undercuts
What happens if you don’t block out
the rigid acrylic denture base cannot be fitted
When can blocking out be done
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
Where do you only block out
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
What is done when the blocking out is done on the master cast before it is duplicated in the refractory material
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
What is the advantage of blocking out on master cast before duplicating
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
What is the disadvantage of blocking out on a master cast before duplicating
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
What is the disadvantage of blocking out with plaster on master cast and not duplicating
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
What is the advantage of blocking out with plaster on master cast and not duplicating
requires less stages and time
however there is less technique and skill in blocking out plaster
How do you prepare the upper casts
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
What are duplicating materials
agar (reversible hydrocolloid)
condensed cure silicone
What is the advantage of agar as duplicating material
it is reusable up to 10 times and keeps its elasticity - more environmentally friendly
What is the disadvantage of agar as the duplicating material
like alginate impressions it suffers from imbibition (water absorption) and syneresis (contraction)
short shelf life
What is the advantage of using condensed cured silicone
very accurate and great shelf life
