Dental Materials Sciences

Question 1

Mention restorative materials

Answer 1

Metals/Alloys→ Amalgam, Cobalt Chromium, Titanium, Gold & Stainless Steel.
Composites → Composed of both low & high viscosity resins.
Glass Ionomer Cements
Compomers
Porcelain

Question 2

Mention impression materials

Answer 2

-Alginate (Hydrocolloid)
-Polyethers & Silicones (Elastomers)

Question 3

What is negative & positive replica?

Answer 3

Impression – negative replica
Dental stone (gypsum) \ study cast - positive replica

Question 4

What is Mechanical properties?

Answer 4

A force that applied to a material may cause:
- Stretch/compress
- Deform (Change shape) - this may be temporary or permanent
- Fracture (failure)

Question 5

Mention types of forces than may applied to material

Answer 5

Compressive
Tensile
Shear

Question 6

What is stress in mechanical properties?

Answer 6

Stress is a physical quantity that describes forces present during deformation
Stress (Pa (Pascals)) = Force (F=mg)/ Unit Area

Question 7

What is the unit of measurement for stress?

Answer 7

Pascals (Pa)

Question 8

What is Strain?

Answer 8

Change in length / Original length

Question 9

What is {Strain - Stress Curves} ?

Answer 9

Used to identify how a material performs under pressure

Question 10

FS stands for?

Answer 10

Refers to the point at which the material fractures.

Question 11

PL stands for?

Answer 11

Refers to the limit at which after stress is removed, the material can return to its original shape.

Question 12

What kind of forces are the teeth typical exposed to?

Answer 12

• Compressive forces - via biting
• Abrasive & frictional forces - via Grinding & chewing

Question 13

What is Elastic (Young’s) Modulus?

Answer 13

• YM = Stress/strain
• a measure of the ability of a material to withstand changes in length when under lengthwise tension or compression
• Effectively how rigid/stiff a material is

Question 14

Fracture

Answer 14

Large force causing catastrophic destruction of material’s structure

Question 15

Hardness

Answer 15

Ability of surface to resist indentation

Question 16

Abrasion

Answer 16

Material surface removal due to grinding

Question 17

Abrasion Resistance

Answer 17

Ability to withstand surface layers being removed

Question 18

Fatigue

Answer 18

Repetitive ‘small’ stresses causing material fracture

Question 19

Creep

Answer 19

Gradual dimensional change due to repetitive small forces

Question 20

Deformation

Answer 20

Applied stress cause permanent change in materials dimension when greater than the elastic limit of material

Question 21

De-bond

Answer 21

Applied forces sufficient to break material-tooth bond

Question 22

Impact

Answer 22

Large, sudden force causing a fracture

Question 23

Chemical properties

Answer 23

• Setting mechanism.
• Setting time.
• Corrosive Potential.

Question 24

Physical properties

Answer 24

• Viscosity.
• Thermal Conductivity & Expansion.
• Density
• Radiodensity.

Question 25

What is Amalgam?

Answer 25

An alloy of:
- Mercury (Liquid component)
- Silver, Tin, Copper & Other trace metals (Powder component)

Question 26

What are the compositions of Amalgam?

Answer 26

Powder (50% by weight)
Silver/Tin- Copper- Zinc- mercury (Hg)

Liquid (50% by weight)
mercury (Hg)

Question 27

Wha are the functions of constituent parts of Amalgam?

Answer 27

Silver/Tin - Forms Intermetallic compound
Copper - Increases strength/hardness
Zinc - Acts as a scavenger of oxygen - oxidises to ensure clean castings
Hg (Powder) - Found in ‘pre-amalgamated alloys’ - causes faster reaction
Hg (Liquid) - Triple distilled (Pure), reacts with the other powder metals

Question 28

Particles type of Amalgam

Answer 28

Lathe cut:
- Coarse, medium or fine
- Formed by filling ingots

Spherical/Spheroidal
- Range of particles sizes
- Formed by spraying molten metal into inert atmosphere

Question 29

Amalgam setting reaction

Answer 29

Silver/Tin + Mercury → Silver/tin + Silver/mercury + Tin/Mercury

Question 30

Gamma phases

Answer 30

Gamma - Has good strength & corrosion resistance (Unreacted particles)

Gamma-1 - Has good corrosion resistance (Forms amalgam matrix)

Gamma-2 - Has poor corrosion resistance (Forms amalgam matrix)

Question 31

Zinc & Expansion

Answer 31

Reactions with saliva and forms Zinc Oxide and bubbles of Hydrogen within the amalgam.

Question 32

What H2 bubbles that formed in Zinc reaction can do?

Answer 32

• Cause pressure build up → Leading to expansion
• Downward pressure → Causing pulpal pain
• Upward pressure → Restoration sits above occlusal surface

Question 33

What are the advantages of Spherical Particles of Amalgam?

Answer 33

Less Hg required
Higher tensile strength
Higher early compressive strength
Less sensitive to condensation
Easier to carve