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M2 Dental material science > Hydrocolloids > Flashcards

Hydrocolloids Flashcards

1
Q

state the two types of trays

A

Stock trays: for patients with avg mouth sizes, they are perforated

Special trays: non-perforated, unique to the patient’s mouth

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2
Q

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.

A
  • 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
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3
Q

explain the link between hydrophobicity and surface detail reproduction

A
  • moist surfaces repel impression material

- the dryer the better

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4
Q

state other requirements for dental materials

A
  • got to have dimensional accuracy and stability
  • non-toxic
  • palpable taste
  • no reaction with disinfectant, die or model
  • realistic setting time
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5
Q

what are impression materials classified into:

A

elastic and non-elastic. elastic materials include hydrocolloids and synthetic elastomers which are used in DDH.

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6
Q

state the different types of synthetic elastomers

A
  • polyethers
  • silicone rubbers
  • polysulphides
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7
Q

what are the two types of hydrocolloids?

A
  • revesible (agar)

- irreversible (alginate)

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8
Q

hydrocolloids exist in both sol and gel forms. describe their properties

A

sol - fluid, low viscosity (inserted into the patient)

gel - increased viscosity (removed from patient)

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9
Q

explain how agar and alginate exist in both sol and gel form

A
  • 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.
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10
Q

what is agar

A

complex polysaccharide from seaweed

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11
Q

how is agar supplied

A

supplied in gel form mixed with:

  • borax: to give more body
  • potassium sulphate: to counterattack retardant effect of borax upon dental plaster (gypsum)
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12
Q

explain the water bath conditioning of agar

A
  • 100C: gel to sol form
  • 65C: holding temp
  • 45C: clinical usage
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13
Q

state the advantages and disadvantages of agar

A

+ take up moisture
+ reusable

  • cost
  • special equipment needed
  • risk of cross-contamination when reused
  • low tear strength
  • dimensionally unstable due to water content
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14
Q

how is agar used in clinics and labs?

A

clinical usage: bridges, crowns and dentures

lab: duplicate material

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15
Q

how is alginate supplied?

A

powder mixed w water

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16
Q

what is the composition of alginate?

A
  • 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
17
Q

how does sodium phosphate control setting time

A

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.

18
Q

state the properties of alginate

A
  • elastic
  • rapid set
  • adequate working time
  • low viscosity
19
Q

whose responsibility is it to disinfect the impressions and why do the impressions need to be brushed and rinsed before disinfections

A
  • 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.
20
Q

what are the common requirements of impression disinfection

A
  • solution freshly made up
  • follow manufacturers instructions
  • time immersion
  • rinse w water before and after
21
Q

how does DDH disinfect their impressions

A
  • ACTICHLOR tablets (10 tabs per 1000ml h2o)
  • remove debris
  • immerse for 2mins
  • rinse
22
Q

what are some of the alternatives to impressions

A
  • CAD CAM

- Intraoral scanning

M2 Dental material science (3 decks)

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