Hydrocolloids Flashcards
state the two types of trays
Stock trays: for patients with avg mouth sizes, they are perforated
Special trays: non-perforated, unique to the patient’s mouth
one of the requirements for dental impressions is surface detail reproduction which depends on the viscosity of the dental material. state the advantages and disadvantages of high and low viscosity.
- Viscosity of the material changes over time.
- High viscosity: Fewer details captured but can displace the soft tissues, useful for denture extensions but not for flabby ridges
- Low viscosity: more detail, difficult for trays to contain the material
explain the link between hydrophobicity and surface detail reproduction
- moist surfaces repel impression material
- the dryer the better
state other requirements for dental materials
- got to have dimensional accuracy and stability
- non-toxic
- palpable taste
- no reaction with disinfectant, die or model
- realistic setting time
what are impression materials classified into:
elastic and non-elastic. elastic materials include hydrocolloids and synthetic elastomers which are used in DDH.
state the different types of synthetic elastomers
- polyethers
- silicone rubbers
- polysulphides
what are the two types of hydrocolloids?
- revesible (agar)
- irreversible (alginate)
hydrocolloids exist in both sol and gel forms. describe their properties
sol - fluid, low viscosity (inserted into the patient)
gel - increased viscosity (removed from patient)
explain how agar and alginate exist in both sol and gel form
- agar can be heated up to go into gel form, and when cooled down it goes into sol form
- alginate can only go from sol form to gel form by being heated up.
what is agar
complex polysaccharide from seaweed
how is agar supplied
supplied in gel form mixed with:
- borax: to give more body
- potassium sulphate: to counterattack retardant effect of borax upon dental plaster (gypsum)
explain the water bath conditioning of agar
- 100C: gel to sol form
- 65C: holding temp
- 45C: clinical usage
state the advantages and disadvantages of agar
+ take up moisture
+ reusable
- cost
- special equipment needed
- risk of cross-contamination when reused
- low tear strength
- dimensionally unstable due to water content
how is agar used in clinics and labs?
clinical usage: bridges, crowns and dentures
lab: duplicate material
how is alginate supplied?
powder mixed w water
what is the composition of alginate?
- 11-17%: CaSO42H2O - the source of calcium for cross-linking
- 1-3%: Na3PO4 - controls working time
- 65-75% - inert filler to provide body for manipulation
- reaction indicator used in some to indicate that the material has set
how does sodium phosphate control setting time
alginate forms cross-links, and to do this, it requires calcium. sodium phosphate also reacts with calcium, therefore when competing for calcium, alginate is left w less calcium so takes longer to form cross-links, meaning setting time slows down.
state the properties of alginate
- elastic
- rapid set
- adequate working time
- low viscosity
whose responsibility is it to disinfect the impressions and why do the impressions need to be brushed and rinsed before disinfections
- responsibility of the dentist
- brushing and rinsing the impression removes debris which is essential and disinfectant material only works when it has surface contact with the submerged impression.
what are the common requirements of impression disinfection
- solution freshly made up
- follow manufacturers instructions
- time immersion
- rinse w water before and after
how does DDH disinfect their impressions
- ACTICHLOR tablets (10 tabs per 1000ml h2o)
- remove debris
- immerse for 2mins
- rinse
what are some of the alternatives to impressions
- CAD CAM
- Intraoral scanning
