PARTIAL DENT - Dental Polymers

Question 1

what are the standard 2001 ISO requirements of an ideal denture base polymer? (9)

Answer 1

• bond to denture teeth
• biocompatible
• high polish
• translucent
• colour stability
• no porosity
• low residual monomer
• low solubulity
• > 65MPa of flexural strength
  2 GaP of flexural modulus

Question 2

what are desirable properties which aren’t part of the standard requirements? (8)

Answer 2

• resilient
• tough
• high impact strength
• dimensional stability
• repairable
• good thermal conductor
• radio-opaque
• cheap

Question 3

define resilience

Answer 3

the amount of energy absorbed by a material up until the point of permanent deformation

Question 4

give 3 denture materials used historically

Answer 4

porcelain - made clicking sound
vulcanite - high cross-linked rubber
bakelite - poor mechanical properties but was first mouldable plastic

Question 5

what is PPMA?

Answer 5

polymethyl methacrylate

Question 6

describe the chemistry of PMMA

Answer 6

addition polymerisation

Question 7

what are the setting reactions of PMMA

Answer 7

addition polymerisation
- activation - heat, light or chemical
- initiation - free radical connects with a monomer
- propagation
- termination

Question 8

describe the initiation phase.

Answer 8

• denture base resin contains benzoyl peroxide
• heat above 60 degree
• molecules yields 2 free radicals

Question 9

describe the propagation phase.

Answer 9

• free radicals break double bond in methyl methacrylate
• multiple MMA bond together

= as a result get PMMA

Question 10

give 10 advantages and 5 disadvantages of PMMA

Answer 10

advantages
- aesthetics
- repairable
- polishable
- high glass transition temperature
- cheap
- easy to make
- bonds to denture tooth
- light
- low water sorption
- rigid

disadvantages
- 21% shrinkage
- porosity during processing
- poor impact strength
- poor thermal conductor
- prone to fracture

Question 11

what is glass transition temperature?

Answer 11

the temperature above which an amorphous polymer becomes soft

• the point of a glass to rubber transition

Question 12

what temperature must a denture have a higher glass transition temperature?

Answer 12

• liquids in the mouth can be up to 70 degrees
• must be higher than 70 to prevent going soft

Question 13

what is a factor affecting glass transition temperature?

Answer 13

molecular structure
- methyl has higher GTT than ethyl, propyl, butyl

Question 14

how is the shrinkage of PMMA tackled? what does the shrinkage reduce to?

Answer 14

prepolymerize the base resin

shrinkage reduces to 6% and 0.5% linear shrinkage

Question 15

what are the two components which need to be mixed to create the heat cured acrylic resin?

Answer 15

powder and a liquid

Question 16

describe the powder component of acrylic resin
- what is it
- the initiator
- what is may also contain

Answer 16

• granules of PMMA
• initiator = benzoyl peroxide
• may contain dyes, opacifiers, plasticisers and fibres

Question 17

describe the liquid component of acrylic resin.
- what is it?
- the initiator
- what else is may contain.

Answer 17

• monomers of MMA
• inhibitor = hydroquinone
• may contain cross linking agents

Question 18

describe how the PMMA changes in sensation as it sets (4)

Answer 18

sandy - initial melting of beads

sticky/stringy - thickened entanglements

dough - gelation - this is when it needs to be applied into the mould

rubbery - monomer penetrates to the core of beads, plasticising them - too late, it has set

Question 19

how are acrylic dentures manufactured?

Answer 19

• wax pattern
• flasking
• boiling
• dough moulding
• boiling - activate polymerisation
• deflasking

Question 20

what are the 3 types of porosity?

Answer 20

gaseous
contraction
granular

Question 21

describe gaseous porosity

Answer 21

when MMA is boiling during polymerisation
- MMA boiling point = 100.3 degree

• due to reaction being exothermic, temperature of 170 degrees can be reached
• deepest parts of the denture hardens

Question 22

how do you prevent gaseous porosity?

Answer 22

heat MMA very slowly so it fully hardens before its boiling point it reached

Question 23

describe contraction porosity

Answer 23

due to 6% shrinkage and because theres not enough material or pressure

Question 24

how is contraction porosity prevented?

Answer 24

add extra PMMA dough before heating

mould is kept under high pressure to prevent bubbles growing in size

Question 25

describe granular porosity

Answer 25

due to dough drying out before processing

Question 26

how many dentures fracture within the first 3 years?

Answer 26

68%

Question 27

which denture is most likely to break the most and why?

Answer 27

upper partial denture
- more weak areas where small saddles are connected to major connector

Question 28

where do 50% of all complete denture fractures occur?

Answer 28

in the midline

Question 29

fractures to the midline can occur due to stress concentration and increased flexing. give anatomical structures which contribute to each.

Answer 29

stress concentration
- deep or sharp fraenal notches
- diastema

increased flexing
- worn teeth
- ill fitting denture
- hard bony structure in midline
- palatine tori

Question 30

how can you increase the strength of dentures?

Answer 30

bars - little evidence

add butadiene styrene co-polymer
- known as high impact acrylic
- best solution yet
- increases toughness but reduces modulus of elasticity

Question 31

what are the properties of plastic acrylic teeth?

Answer 31

• chemically bind to denture
• can be adjusted
• not cause wear to opposing tooth
• aesthetic colour match
• resilience
• may stain over time
• will wear under high force occlusion

Question 32

what are the three ways how you can cure?

Answer 32

chemical
heat
light

Question 33

describe chemical curing

Answer 33

initiator = benzoyl peroxide
accelerator = tertiary amine
inhibitor = hydroquinone

Question 34

describe light curing

Answer 34

photo-initiators - camphorquinone

or blue light

Question 35

give one other alternative materials to PMMA

Answer 35

polyamide (nylon)
- more flexible and bouncy than brittle, rigid PMMA