dental materials

Question 1

list pros of impression material

Answer 1

1. ease of manipulation
2. appropriate setting time
3. stable dimensional accuracy
4. no degradation of properties w disinfection

Question 2

list 4 classes of impression materials

Answer 2

1. alginate [irreversible hydrocolloid]
   - one of the most widely used
2. addition silicones
3. polyether
4. polysulfide

Question 3

describe alginate setting characteristic

Answer 3

irreversible chemical reaction of soluble alginate and calcium sulphate = calcium alginate gel

addition of trisodium phosphate provides working time
- sodium phs will react w calcium sulphate first
- then remaining calcium sulphate will react w sodium alginate and water = forms the calcium alginate gel

setting time = 1.5-3mins

Question 4

describe synersis and imbibition of alginate

Answer 4

synersis
- water lost by evaporation [squeezed out from between polyssacharide chains]
- causes impression shrinkage = inaccuracy

imbibition
- excess water = separation of polysaccharide chains
- causes impression to swell = also inaccurate

Question 5

describe polysuphides and their setting characteristics

Answer 5

TWO tube paste system
1. base paste
2. catalyst paste

light-reg-heavy body pastes available
very good tear resistance

setting = condensation polymerisation reaction
- chain extension and cross linkage

Question 6

contrast heavy body polysuphides from light body

Answer 6

• heavy body = more accurate because they exhibit less setting and thermal contraction + better dimensional stability
• however their high viscosity = cannot capture fine details like light bodied range

Question 7

briefly describe polyethers

Answer 7

[need gun]

two pastes [base/catalyst] in two connected tubes - mixed through syringe tip

cataylst paste - reaction initiator
base paste - paste forming oils/ fillers

Question 8

briefly describe addition silicones

Answer 8

[need gun]

two paste system in in two connected tubes
- both contain liquid silicone prepolymer + filler
- one contains catalyst

light body onto tooth as surface wash, and heavy body over to support wash

Question 9

describe GIC composition

Answer 9

fluoride containing cement powder
- aluminosilicate [either calcium or strontium]
- finer powders –> luting cements
- coarser powders –> restorative materials [provide better translucency]

acidic polymer
- polyalkenoic acid

Question 10

describe GIC setting reaction

Answer 10

1. dissolution
   - glass surface attacked by polyacid - forms diffusion based union between two
   - 20-30% glass decomposed
   - calcium/strontium, aluminum and fluoride ions released = cement sol
2. precipitation of salts
   - calcium, strontium and aluminium bind –> polyanions at carboxylate groups
   - initial setting = Ca ions released –> cross linkage w alkenoic acid chains
   - maturation over next 24 hrs as less mobile aluminium ions become bound in the matrix = more rigid cross linking
3. hydration of salts
   - water hydrates salts to facilitate cross link setting
   - too much water = dilutes metal ions = lacks strength
   - too little water = cracking of material
   - i.e water loss/ contamination = weak cement

Question 11

adv / diasadv GIC

Answer 11

advantages
- fluoride content = makes GIC plaque resistant
- biocompatibility [calcium, strontium, fluoride all able to migrate in and out of material - aids remin]
- fluoride release aids remin [can last up to 7 yrs]
- ease of handling

disadvantages
- disintegrates quickly with xerostemia
- susceptible to brittle fracture
- poor resistance to acid attack
- inferior mechanical properties to resin
- 24 hr setting time - needs coating agent