D2 - hydrocolloid Impression Materials and Disinfection and Alternatives to Impressions Flashcards
what are the requirements for impression materials?
- surface detail reproduction
- dimensional accuracy and stability
- no changes when exposed to disinfectants
- no interaction with model/die materials
- non toxic
- easily manipulated
- palatable taste
- realistic working/setting time
Name for when moist surfaces repel?
Hydrophobicity
What does a high viscosity do to impression material?
Less detail but can displace soft tissues
What does a low viscosity do to impression material?
More detail but tray won’t contain
what are the two classifications of impression materials?
- non elastic
- elastic
Give two example of a non elastic impression material
- Zinc Oxide and Eugenol pastes
- impression plaster
Give two examples of elastic impression materials
- hydrocolloids
- synthetic elastomers
Name a reversible hydrocolloid
agar
Name a irreversible hydrocolloid
alginate
what two forms do hydrocolloids exist in and what are their characteristic?
sol - fluid, low viscosity, random chain arrangement
gel - increases viscosity, parallel chain arrangement, elastic
How can a sol change into a gel?
By a temperature or chemical reaction
How can a gel change into a sol?
temperature
What is agar supplied in gel with?
- borax (to give more body)
- potassium sulphate (to counteract retardant effect of borax upon dental plaster (gypsum))
What does agar require?
Water bath conditioning. In clinical use it requires water cooled trays
What are the advantages of agar? (2)
- take up moisture
- re use
What are the disadvantages of agar? (5)
- special equipment
- cost
- cross infection concerns of reuse
- low tear strength
- dimensionally unstable due to water content
What is the name for the loss of water from squeezed chains?
Syneresis
What is the name for uptake of water? It causes swelling due to chain separation
Imbibition
What can agar be used for clinically?
partial dentures
crowns
bridges
How can agar be used in the laboratory?
duplicating material
what are the properties of alginate?
- low viscosity
- adequate working time
- rapid set
- elastic
- must use no less than 3-5mm thick to prevent distortion on removal
- susceptible to syneresis and imbibition
how is alginate supplied?
powder mixed with water
What is a variant to alginate which offers marginally better detail reproduction and tear resistance? It has poor dimensional stability and is supplied as two pastes
Modified alginate with silicone polymers
Give 2 alternatives to impressions
CAD CAM
intraoral scanner
what is immersio disinfection?
- solution freshly made
- follow manufactures instructions
- time immersion
- rinse with water
How do you make up Actichlor tablets?
- 10 tabs in 1000ml water. Each tablet weighs 1.7g
- effervescent chlorine releasing tablets
- 1000ppm= 1% available chlorine
- rinse
- immerse for 2 mins
