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2c. BDS2 CDS DMS > 83. Impression Materials > Flashcards

83. Impression Materials Flashcards

1
Q

L83: What is an impression?

A

A negative replica/ reproduction of tissues

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2
Q

L83: What is the function of an impression material?

A

To produce an accurate replica of the surface and shape of hard and soft oral tissues

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3
Q

L83: What material is commonly used in dentistry to create positive replicas?

A

Dental stone (gypsum) to produce stone cast

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4
Q

L83: To obtain a good impression, the impression material must have the right properties but what else is important?

A

Clinician’s technique

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5
Q

L83: What are the two main clinical properties an impression material can be?

A
  • Mucostatic;

- Mucocompressive.

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6
Q

L83: What does mucostatic mean?

A

A material that flows over tissues to represent them in a state of rest

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7
Q

L83: What is an example of a mucostatic impression material?

A
  • Zinc oxide eugenol;

- Low viscosity alginates.

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8
Q

L83: What does mucocompressive mean?

A

A material that obtains an impression when the mucosa is subject to compression

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9
Q

L83: What is an example of a mucocompressive impression material?

A
  • Impression compound;

- High viscosity alginates/ elastomers.

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10
Q

L83: What are the two main material properties an impression material can be?

A
  • Elastic (not truly);

- Non- elastic (not used anymore).

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11
Q

L83: How do impression materials display elastic behaviour?

A
  • Rigid after setting reaction;
  • Undergo strain upon removal over bulbosity of tooth;
  • Snap back to same shape after.
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12
Q

L83: Why should you wait (time of Tf-Tl), after taking an impression, before pouring a cast?

A

Tf represents the permanent strain on the impression material, Tf-Tl is the period of elastic recovery after maximum strain as been exerted

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13
Q

L83: Why are impression materials not considered truly elastic?

A

Because of permanent strain (slight change in shape) to the material, no full recovery

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14
Q

L83: Why is it better to apply and remove loads quickly from impression materials?

A

If load time is less, there is less overall permanent strain (i.e. lower deformation)

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15
Q

L83: What ‘elastic’ materials are used in dentistry?

A
  • Hydrocolloids;

- Elastomers.

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16
Q

L83: What hydrocolloid material is used in the dental school, is it reversible?

A

Alginate, irreversible

17
Q

L83: What is the process summary of preparation of an impression material?

A
  • Mixing of two materials (2 pastes/ paste + liquid/ powder + liquid etc.);
  • Chemical reaction;
  • Heat;
  • Material flows/ takes up space.
18
Q

L83: What is the process summary of setting of an impression material?

A
  • Chemical;
  • Polymerisation;
  • Cooling;
  • Formation of solid replica.
19
Q

L83: What should be considered when storing a taken impression material?

A
  • Depends on material;

- Before dimensional changes become significant).

20
Q

L83: What are the three main ideal properties an impression material should custom for?

A
  • Accuracy;
  • Patient factors;
  • Operator factors.
21
Q

L83: What are the ideal properties of an impression material with regards to maintaining accuracy?

A
  • Surface detail recorded;
  • No dimensional change on setting/ removal/ decontamination or storage;
  • Complete elastic recovery;
  • Compatible with cast material.
22
Q

L83: What material properties affect accuracy and how?

A
  • Viscosity (ability to flow over surfaces and record detail);
  • Setting mechanism (does this lead to dimensional contraction?);
  • Thermal expansion coefficient (ideally low, to cope with mouth to room temperature change);
  • Hydrophobic/ hydrophilic (surface contact, effect of saliva);
  • Elasticity (elastic behaviour);
  • Tear strength (to withstand removal, especially from undercuts).
23
Q

L83: What are the ideal properties of an impression material with regards to maintaining patient comfort?

A
  • Non toxic, non irritant;
  • Acceptable taste and smell;
  • Short setting time;
  • Removable without damage to oral tissues.
24
Q

L83: What are the ideal properties of an impression material with regards to maintaining operator convenience?

A
  • Quick, simple technique;
  • Convenient working/ setting times;
  • Ability to be decontaminated;
  • Cost (a consideration, not a property).
25
Q

L83: Hydrocolloids are elastic impression materials, what do they consist of?

A

A two phase system of fine particles (1-200nm) dispersed in another phase (water - hydro)

26
Q

L83: Give an example of an irreversible hydrocolloid

A

Alginate

27
Q

L83: What component of an alginate reacts with Ca ions?

A

Salt of alginic acid (e.g. sodium alginate)

28
Q

L83: What component of an alginate provides Ca ions?

A

Calcium sulphate

29
Q

L83: What component of an alginate delays gel formation?

A

Trisodium phosphate

30
Q

L83: What component of an alginate provides cohesion and strength?

A

Filler

31
Q

L83: What is the setting reaction of an alginate?

A

2 NanAlg + n CaSO4 > nNa2SO4 + CanAlg

32
Q

L83: What part of an alginate setting reaction delays the setting of an alginate?

A

Na reacting with a trisodium phosphate before Ca

Ca ions react with PO4 ions, then remaining Ca reacts with Na to form CaAlg

33
Q

L83: What type of chemical bonds are formed in an alginate setting reaction?

A

Crosslinking (Ca with carboxyl group of alginate)

34
Q

L83: Do alginates set quicker on soft or hard tissues and why?

A

Soft - warmer

35
Q

L83: Why is it sometimes beneficial to wait a little longer after alginate has ‘set’ in the mouth?

A

Crosslinking continues after apparent set, further crosslinking will lead to greater elastic properties

36
Q

L83: With regards to storage of an impression, alginates can both release and absorb water, what does syneresis mean?

A

Release of water

37
Q

L83: With regards to storage of an impression, alginates can both release and absorb water, what does imbibition mean?

A

Uptake of water

2c. BDS2 CDS DMS (12 decks)

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