Long Term Soft Lining Materials

Question 1

How long are long term soft lining materials supposed to last

Answer 1

At least 4 weeks to several months/years

Question 2

What patients do we tend to use long term soft lining materials for

Answer 2

Patients who can’t tolerate a rigid denture base material

Question 3

Describe the function of long term soft lining materials

Answer 3

• Distribute masticatory forces more evenly and absorb some of the energy
• Lining deforms elastically, then the energy is released as it returns to pre-deformed shape

Question 4

What are the 4 types of Long Term SLM

Answer 4

Heat Cured:
- Addition silicones
- Soft Acrylics
Room Temperature vulcanised cured:
- Condensation silicone
- Addition silicone

Question 5

What are the uses for long term soft lining materials

Answer 5

• Obturators (covers palatal defects) or other type of prostheses like maxillofacial ones

Question 6

What are the disadvantages of Long term soft lining materials

Answer 6

• Expensive
• Difficult to modify and polish
• More prone to fracture
• Reduction in denture SLM thickness increases its hardness (minimum ~3mm)

Question 7

What are the ideal properties of long term soft lining materials (just name as many as you can)

Answer 7

• Non-toxic and non-irritant
• Good adhesion to PMMA denture
• Should not support the growth of candida
• Permanently resilient
• Permanently compliant
• Low water absorption
• Not adversely by denture cleansers
• Easy to clean and not stained easily
• Sufficient mechanical strength and abrasion resistance
• wetted by saliva

Question 8

What does it mean for a material to be resilient

Answer 8

This is the ability of a material to absorb energy when deformed elastically and to return it when unloaded

Question 9

What is the setting reaction for heat cured silicones

Answer 9

• Addition reaction involving free radicals
• Peroxide initiator oxidises the CH3 on neighbouring siloxane chains to form cross links
• Silane acts as a cross linker
• the methacrylate groups react with the denture base to form a bond

Question 10

How are heat cured addition silicones dispensed

Answer 10

One paste systems

Question 11

How are room temperature vulcanised addition and condensation silicones dispensed

Answer 11

2 paste system - base paste and catalyst paste

RTV condensation silicones aren’t on the market anymore

Question 12

Describe the composition of RTV addition silicone pastes

Answer 12

Paste 1:
- Vinyl terminated poly dimethylsiloxane
- Pt based catalyst e.g. chloroplatinic acid
- Filler
Paste 2:
- Vinyl terminated poly dimethylsiloxane
- Hydrogen terminated poly dimethyl siloxane
- Filler

Question 13

What kinds of silicone SLM use glazes/polishes

Answer 13

RTV silicones

Question 14

What do glazes/silicones do to silicone SLMs

Answer 14

• Smooths and seals trimmed areas
• Not used on tissue contact surface
• Contain unfilled addition silicone, some with solvents

Question 15

What are the advantages of using RTV addition silicones compared to Condensation silicones

Answer 15

Claim to have better mechanical and adhesion to PMMA

Question 16

What are the advantages of using Heat-cured addition silicones compared to RTV silicones

Answer 16

These have the best adhesion to the denture and lower water uptake than RTVs

Question 17

What are the advantages of using Silicones (as opposed to acrylics) as SLMs

Answer 17

• Silicones are resilient
• Compliant
• Theses properties are not adversely affected in the oral environment

Question 18

What are the disadvantages of using Silicones (as opposed to acrylics) as SLMs

Answer 18

• Poor adhesion to PMMA
• Poor tear strength
• Not wetted by saliva (hydrophobic)
• Some support the growth of candida
• Single component hear cured materials require refrigeration (limited shelf life)

Question 19

How are soft acrylic SLMs usually dispensed

Answer 19

powder/liquid systems

Question 20

What is added to the acrylic SLMs to make them “soft”

Answer 20

• Higher methacrylate e.g. EMA/BMA

- A plasticiser to reduce the Tg to below mouth temperature

Question 21

What is the Glass transition temperature (Tg)

Answer 21

This is the temperature range where a polymer substrate changes from a rigid glassy material to a soft (not melted) material

Question 22

What is the powder phase of soft acrylics usually made up of

Answer 22

• PEMA

- OR BMA/EMA copolymer

Question 23

How does the Tg of PEMA compare to different BMA/EMA co polymers

Answer 23

PEMA’s Tg is higher than BMA/EMA copolymers

Higher BMA:EMA ratios = lower Tg

Question 24

Describe the composition of the liquid phase of soft acrylics and give examples where you can boi

Answer 24

• Higher methacrylate monomer e.g. butyl, hexyl
• Cross linking agent e.g. ethylene glycol dimethacrylate
• Plasticiser, usually a phthalate e.g. butyl phthalyl butyl glycollate
  OR a citrate e.g. acetyl tribute citrate

Question 25

How do the monomers polymerise in the setting reaction of soft acrylics

Answer 25

Free radical addition polymerisation on heating

Question 26

What are the advantages of using soft acrylics as SLMs (as opposed to silicones)

Answer 26

- Bond well to PMMA - Good tear resistance - Wetted by saliva - Initial compliance is good - Can be polished if chilled

Question 27

What are the disadvantages of using soft acrylics as SLMs (as opposed to silicones)

Answer 27

- Hardening due to loss of plasticiser, potential toxicity concern - High water absorption - Less resilient than the silicones - Permanent deformation can occur

Question 28

What compound are light cured SLMs (these are usually acrylics) usually based on

Answer 28

Urethane dimethacrylate

Question 29

What do light cured SLMs require to take a mould with and name a disadvantage

Answer 29

- Special moulds | - They do not bond well