Finishing of partial dentures

Question 1

what is flasking?

Answer 1

the process of investing a cast with a wax denture within a flask to create a two-sectional mold that is used to form the acrylic resin denture base

Question 2

outline the process of flasking

Answer 2

• the bottom half of the metal flask is filled with dental plaster to secure the denture in place
• once set, the top half is filled with dental plaster and the flask is closed
• the flask is immersed in hot water for 10 mins to soften the wax
• the mould is opened and the wax is evacuated with boiling water (not burned out)
• the space left by the wax is filled using PMMA
• it is hard to eliminate undercuts so they are blocked out with dental plaster during the first stage

Question 3

how is the denture invested when flasking?

Answer 3

• wax denture sealed onto a duplicate cast and embedded in dental plaster in the bottom half
• 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

Question 4

what is the hooded method of flasking?

Answer 4

when the artificaial teeth and clasps are held in the investing plaster following flasking

Question 5

how is the mould packed with PMMA?

Answer 5

• 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
• a trial of the packing stage is conducted to ensure the mould is completely filled
• the flask is opened and excess material is removed
• the two halves are closed and placed into a spring compress and processed

Question 6

what are the conditions needed for correctly mixed PMMA? what can happen if these are not met?

Answer 6

• accurate mixing of powder (polymer) and liquid (monomer) 10ml to 24g - lack of monomer liquid = granular porosity
• process under pressure - low pressure = contraction porosity
• controlled heating - premature increase in temperature = gaseous porosity
• bench cooling to room temperature - cooling too quickly = internal stress

Question 7

how does porosity affect PMMA?

Answer 7

affects aesthetics and strength of PMMA

Question 8

how do you remove the denture from the plaster?

Answer 8

• weak points are sawed into the plaster to help removal
• then the plaster mould is removed from the denture and cast

Question 9

what is the alternate method of adding PMMA to dentures?

Answer 9

• the partial denture with teeth is set in wax on a duplicate cast
• a putty impression of the denture where the PMMA is required is taken
• the lost wax technique is used to remove the wax
• a hole is made in the impression and cold cured acrylic resin is poured in
• the putty impression is removed and the acrylic replaces the wax

Question 10

what are the disadvantages of cold cured acrylic resin?

Answer 10

• not as hard as heat cured acrylic
• prone to air pockets
• teeth are only half processed (softer) so need to be heat cured to harden

Question 11

how do you check the occlusion after processing the PMMA denture?

Answer 11

split cast system
* the duplicate cast with the denture is secured to the original mounting with plaster bondage
* the occlusion is checked and refined against the opposing dentition
* the incisal pin resting on the incisal table indicates the occlusal vertical height (OVD)

Question 12

what happens after the occlusion is checked?

Answer 12

• the duplicate cast is removed from the denture by sawing the model from the denture
• the denture is trimmed with a handpiece and bur
• the denture is finished and polished
• the denture can be fitted to the definitive cast
• adjust clasps to the shape of the cast - close adaptation needed

Question 13

what is the final checklist before trying the finished denture on the patient?

Answer 13

1. the denture should be fitted to the definitive (master) cast
2. the anterior flange (gum line) should have a thin edge to blend with the surrounding soft tissue
3. the denture is finished above the survey lines
4. the post dam and palatal periphery in thinned to blend with the palate
5. the OVD must be the same as the original registration
6. there should be no sharp edges
7. denture should be polished only on the non-impression surface