Finishing/flasking RPD's

Question 1

What is the name for the technique used for processing a denture?

Answer 1

The lost-wax technique

Question 2

What is the ‘lost wax’ technique? (4 points)

Answer 2

• A metal dental flask is used in the processing of the PMMA denture
• A two-part closed mould is made, enclosing the wax denture using dental plaster
• The two-part mould is opened and the wax is evacuated from the mould
• The space left by the removed wax is filled using PMMA

Question 3

What is PMMA?

Answer 3

Polymethyl methacrylate - acrylic

Question 4

What is meant by the term ‘flasking’?

Answer 4

• The wax denture is sealed onto the duplicate cast and embedded in the dental plaster into the deeper half of the denture flasks. The wax palate is left exposed. On setting, the plaster mould is coated with a separating medium (sodium silicate) and the mould is completed with a 50/50 plaster/stone mix
• Following the setting of the completed mould, the dental flask is immersed in hot water for 10 mins to soften the wax inside. The flask is separated and the wax evacuated with boiling water

Question 5

What is the hooded method of flasking?

Answer 5

The artificial teeth and clasps held in the investing plaster following flasking

Question 6

What happens when filling the mould - ‘packing’?

Answer 6

• The mould prior to ‘packing’ the PMMA. The plaster surfaces and cast are coated with a ‘mould seal’ to prevent plaster adhering or penetrating the denture. When the mixed PMMA has reached a dough stage it is packed into the mould with special attention being paid to packing the mould around the teeth. The mould is closed under pressure to expel excess material which on re-opening is removed

Question 7

Give a summary of the ‘flasking’ and ‘packing’ stage of producing a denture? (6 points)

Answer 7

• On setting, the mould is placed in hot water for 10 minutes to melt the wax inside the flask and to assist the opening of the mould
• On opening the flask, the wax is evacuated with boiling water and the mould is cleaned with a detergent to remove any wax residue
• The 2 halves of the mould are coated with a mould seal and PMMA is packed into the mould at the appropriate ‘dough’ stage
• A trial of the packing stage is conducted to ensure the mould is completely filled
• The flask is opened and excess is removed
• The 2 halves are closed and placed into a spring compress and processed

Question 8

What is the powder in PMMA?

Answer 8

A polymer

Question 9

What is the liquid in PMMA?

Answer 9

• A monomer

Question 10

Why do you need accurate mixing of PMMA (10mls to 24g)?

Answer 10

• To prevent granular porosity (from too much powder)

Question 11

Why is PMMA processes under pressure?

Answer 11

To prevent contraction porosity

Question 12

Why is heating when mixing PMMA controlled?

Answer 12

To prevent gaseous porosity

Question 13

Why is PMMA bench cooled?

Answer 13

• To avoid internal stresses

- If put in cold water will increase internal stresses and cause the PMMA to crack

Question 14

How is the denture removed from the plaster?

Answer 14

The plaster is removed from the mould and the plaster mould removed form the denture and cast

• Need to try to separate without breaking the acrylic

Question 15

How can we used cold cure to produce a denture?

Answer 15

• Wax trial on cobalt-chrome - use lab putty with an activator and put over wax
• Remove the putty and boil off the wax
• Stick the teeth to the putty
• Can then pour acrylic in
• Can cause problems with air blows
• Use cols or luke warm water
• It is hard enough but softer than heat cure
• Acrylic teeth are only half processed so they are softer
• Acrylic doesn’t look as natural

Question 16

How is the occlusion checked following the process of making the denture?

Answer 16

• The split cast system (where can put casts back onto the articulator)
• The duplicate model with denture is secured onto the original mounting with plaster bandage
• The occlusion is checked and refined as required against the opposing dentition
• The incisal post at rest on the incisal table indicates the occlusal vertical dimension

Question 17

What should happen following occlusal refinement of the denture?

Answer 17

• The duplicate model is removed form the denture by sawing the model from the denture
• The denture is finished and polished
• Following polishing, the denture can be fitted to the definitive cast

Question 18

When looking at the final denture, what should you check? (7 points)

Answer 18

• The denture is fitted to the master cast
• The anterior flange should have a thin edge to blend with the surrounding soft tissue
• The denture is finished above the survey line
• The post dam and the palatal periphery is thinned to blend with the palate
• The occlusal vertical dimension must be the same as the original registration
• There should be no sharp edges
• The denture should be polished only on the non-impression surface