JVD 2013 #2 - FOUNDATIONS - Dental Impression Materials

Question 1

According to FOUNDATIONS - Dental Impression Materials, Rachel Perry,

Which impression materials are inelastic?

Answer 1

Impression compound

impression plaster

ZOE

Question 2

According to FOUNDATIONS - Dental Impression Materials, Rachel Perry,

Which impression materials are elastic, but not elastomeric?

Answer 2

agar hydrocolloid

alginate hydrocolloid

Question 3

According to FOUNDATIONS - Dental Impression Materials, Rachel Perry,

Which impression materials are elastomeric?

Answer 3

polysulfide

condentsation silicone

polyether

addition silicone (PVS/VPS)

Question 4

According to FOUNDATIONS - Dental Impression Materials, Rachel Perry,

to what level of detail does the ADA specify elastomeric impressions must record?

Which material is the most effective in this regard?

Answer 4

25 um

polyvinyl siloxane

Question 5

According to FOUNDATIONS - Dental Impression Materials, Rachel Perry,

What level of detail do heavy body materials have to record?

Answer 5

75 um

Question 6

According to FOUNDATIONS - Dental Impression Materials, Rachel Perry,

What is the limiting factor with regards to accuracy of replication?

Answer 6

ability of gypsum materials to reproduce detail in the impression

Question 7

According to FOUNDATIONS - Dental Impression Materials, Rachel Perry,

What is elastic recovery?

Answer 7

ability of a material to return to it’s original shape after removal (i.e. from an undercut)

Question 8

According to FOUNDATIONS - Dental Impression Materials, Rachel Perry,

What is dimensional stabilty and what materials possess the best?

Answer 8

ability of a material to maintain accurate dimensions over time

PVS - has no byproduct when setting, so maintains shape best.

Question 9

According to FOUNDATIONS - Dental Impression Materials, Rachel Perry,

What processes affect the dimensional stability of alginates over time?

Answer 9

syneresis (evaporation of water)

imbibition (absorption of water)

Question 10

According to FOUNDATIONS - Dental Impression Materials, Rachel Perry,

How long should alginate be wrapped in a moist towel before pouring models?

Answer 10

<1 hour, especially older materials

(some newer materials can be poured up to 100 hours after taking impression)

Question 11

According to FOUNDATIONS - Dental Impression Materials, Rachel Perry,

What is a thixotropic material?

Answer 11

viscous under normal conditions but become less viscous allowing better flow when stressed or agitated (or pushed through a syringe).

Question 12

According to FOUNDATIONS - Dental Impression Materials, Rachel Perry,

What material is the most flexible?

Answer 12

Alginate

Question 13

According to FOUNDATIONS - Dental Impression Materials, Rachel Perry,

Which of these two materials displays better wettability?

Answer 13

material A

When they talk about wetting angles, they mean the angle at the external interface between the droplet and the surface.

wetting angle of 0 would be perfect wetting, and angle of 180 would be no wetting (material would be a perfect sphere sitting on the surface and a single tiny point of contact)

Question 14

According to FOUNDATIONS - Dental Impression Materials, Rachel Perry,

What is the main problem with agar hydrocolloids?

Answer 14

need special cooled impression trays to allow the material to set

Question 15

According to FOUNDATIONS - Dental Impression Materials, Rachel Perry,

What is the chemical reaction in the setting of alginate?

Answer 15

A sodium or potassium alginate salt will dissolve in water,

which then reacts with calcium sulfate present in the powder

to form insoluble calcium alginate that is the set impression material

Question 16

According to FOUNDATIONS - Dental Impression Materials, Rachel Perry,

Why is sodium phosphate added to alginate materials?

Answer 16

•sodium phosphate as a retarder to preferentially react with soluble calcium ions. slows reaction down because calcium alginate isn’t formed until the sodium phosphate is exhausted.

Question 17

According to FOUNDATIONS - Dental Impression Materials, Rachel Perry,

What is the difference between type I and type II allginate?

Answer 17

Type I - fast set - form a gel in 1-2 minutes

Type II regular set - form a gel in 3-4 minutes

Question 18

According to FOUNDATIONS - Dental Impression Materials, Rachel Perry,

What are disadvantages of polyether materials?

Answer 18

very rigid,

short working time

absorb water from the atmosphere, need to pour within 30 mins

Question 19

According to FOUNDATIONS - Dental Impression Materials, Rachel Perry,

what is the difference between addition silicomnes (like PVS) and condensation silicones?

Answer 19

condensation silicones give off ethyl alcohol while setting, contributes to shrinkage

addition silicones give off no by-product, and this contribute to dimensional stability

Question 20

According to FOUNDATIONS - Dental Impression Materials, Rachel Perry,

What is the chemical reaction in addition silicone like PVS?

Answer 20

reaction is between terminal hydrogen and vinyl groups in the presence of a platinum catalyst.

Although hydrogen is produced, this is scavenged by the platinum.

No byproduct given off after setting.

Question 21

According to FOUNDATIONS - Dental Impression Materials, Rachel Perry,

what is the elastic recovery of PVS?

Answer 21

99.8%

Question 22

According to FOUNDATIONS - Dental Impression Materials, Rachel Perry,

What is a major disadvantage of PVS material?

Answer 22

It is hydrophobic, making it difficult to take impressions and pour stone

Question 23

According to FOUNDATIONS - Dental Impression Materials, Rachel Perry,

What is the advantage and disadvantage of light wash PVS materials?

Answer 23

•The lower the viscosity, the higher the surface detail reproduction, but the greater the polymerization shrinkage.