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