Long Term Soft Lining Materials Flashcards
How long are long term soft lining materials supposed to last
At least 4 weeks to several months/years
What patients do we tend to use long term soft lining materials for
Patients who can’t tolerate a rigid denture base material
Describe the function of long term soft lining materials
- 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
What are the 4 types of Long Term SLM
Heat Cured: - Addition silicones - Soft Acrylics Room Temperature vulcanised cured: - Condensation silicone - Addition silicone
What are the uses for long term soft lining materials
- Obturators (covers palatal defects) or other type of prostheses like maxillofacial ones
What are the disadvantages of Long term soft lining materials
- Expensive
- Difficult to modify and polish
- More prone to fracture
- Reduction in denture SLM thickness increases its hardness (minimum ~3mm)
What are the ideal properties of long term soft lining materials (just name as many as you can)
- 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
What does it mean for a material to be resilient
This is the ability of a material to absorb energy when deformed elastically and to return it when unloaded
What is the setting reaction for heat cured silicones
- 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
How are heat cured addition silicones dispensed
One paste systems
How are room temperature vulcanised addition and condensation silicones dispensed
2 paste system - base paste and catalyst paste
RTV condensation silicones aren’t on the market anymore
Describe the composition of RTV addition silicone pastes
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
What kinds of silicone SLM use glazes/polishes
RTV silicones
What do glazes/silicones do to silicone SLMs
- Smooths and seals trimmed areas
- Not used on tissue contact surface
- Contain unfilled addition silicone, some with solvents
What are the advantages of using RTV addition silicones compared to Condensation silicones
Claim to have better mechanical and adhesion to PMMA
What are the advantages of using Heat-cured addition silicones compared to RTV silicones
These have the best adhesion to the denture and lower water uptake than RTVs
What are the advantages of using Silicones (as opposed to acrylics) as SLMs
- Silicones are resilient
- Compliant
- Theses properties are not adversely affected in the oral environment
What are the disadvantages of using Silicones (as opposed to acrylics) as SLMs
- 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)
How are soft acrylic SLMs usually dispensed
powder/liquid systems
What is added to the acrylic SLMs to make them “soft”
- Higher methacrylate e.g. EMA/BMA
- A plasticiser to reduce the Tg to below mouth temperature
What is the Glass transition temperature (Tg)
This is the temperature range where a polymer substrate changes from a rigid glassy material to a soft (not melted) material
What is the powder phase of soft acrylics usually made up of
- PEMA
- OR BMA/EMA copolymer
How does the Tg of PEMA compare to different BMA/EMA co polymers
PEMA’s Tg is higher than BMA/EMA copolymers
Higher BMA:EMA ratios = lower Tg
Describe the composition of the liquid phase of soft acrylics and give examples where you can boi
- 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
How do the monomers polymerise in the setting reaction of soft acrylics
Free radical addition polymerisation on heating
What are the advantages of using soft acrylics as SLMs (as opposed to silicones)
- Bond well to PMMA
- Good tear resistance
- Wetted by saliva
- Initial compliance is good
- Can be polished if chilled
What are the disadvantages of using soft acrylics as SLMs (as opposed to silicones)
- Hardening due to loss of plasticiser, potential toxicity concern
- High water absorption
- Less resilient than the silicones
- Permanent deformation can occur
What compound are light cured SLMs (these are usually acrylics) usually based on
Urethane dimethacrylate
What do light cured SLMs require to take a mould with and name a disadvantage
- Special moulds
- They do not bond well
