Intro

Question 1

What are the materials used in impressions?

Answer 1

Impression compound
Impression paste
Hydrocolloids (alginate)
Elastomers (polysulphides, polyethers, silicones)

Question 2

What is stainless steel possibly used for?

Answer 2

Denture base
Orthodontic appliances

Question 3

What are the types of force?

Answer 3

Compressive
Tensile
Shear

Question 4

What are the mechanical properties of enamel?

Answer 4

Rigid
Brittle
Strong
Hard

Question 5

What is a typical biting force?

Answer 5

500 to 700N
(50-70kg)

Question 6

What forces are present during grinding/chewing?

Answer 6

Frictional forces -abrasion

Question 7

What forces are present when biting?

Answer 7

Compressive

Question 8

What forces can remove material from enamel?

Answer 8

shear force

Question 9

What is the cavity design for amalgam?

Answer 9

Flared out towards bottom (undercut) to allow secure position and retention (mechanical)

Question 10

What is the cavity design for composite resin?

Answer 10

Minimal and retention by adhesive

Question 11

What do stress and strain mean?

Answer 11

stress = force per unit area
strain = deformation of material when under stress (change in length/original length)

Question 12

What is young’s modulus?

Answer 12

the resistance of a material to elastic deformation under an applied force or stress.
It is defined as stress/strain

Question 13

What are the values of young’s modulus in enamel, dentine and composite?

Answer 13

Enamel: 65 GPa
Dentin: 20 GPa
Composites: 15 GPa

Question 14

What does a high YM mean?

Answer 14

high stiffness/rigidity

Question 15

What do fracture, hardness, abrasion resistance, and fatigue mean?

Answer 15

Fracture -large forces cause destruction of materials substance

Hardness - ability of surface to resist indentation

Abrasion resistance - ability to withstand surface layers being removed due to grinding (loss of material surface layers, roughened surface)

Fatigue - repetitive small stresses causing material fracture

Question 16

What is creep?

Answer 16

The gradual dimensional change or deformation that occurs in a material under the influence of mechanical stress applied over a long time period.

Question 17

What is permanent deformation?

Answer 17

Applying stress to a material that is larger than the elastic limit resulting in permanent deformation

Question 18

What is de bonding?

Answer 18

the separation or detachment of a bonded dental material or appliance from the tooth surface to which it was initially adhered.

To remove orthodontic appliances, shear force is applied to separate bracket/bonding material from tooth surface

Question 19

What is ductility, brittleness and tear strength?

Answer 19

Ductility - The ability of a material to deform plastically (permanently) without fracturing when subjected to a tensile stress.

Brittleness - The opposite of ductility. Brittleness is the tendency of a material to fracture or shatter with little preceding plastic deformation.

Tear strength - The ability of a material to resist the propagation of a crack or tear after initial fracture has occurred.

Question 20

How is the oral environment varied?

Answer 20

Saliva
Temperature variations
Ph variations
Oral bacteria

Question 21

What are the chemical properties?

Answer 21

• setting mechanism (setting to hard state -light cure, heat cure etc)
• corrosive potential (degradation in oral environment - release of ions)
• biocompatibility (does it release toxic materials)
• adhesion
• solubility/ degradtion (resistance to dissolution and breakdown, especially for biomaterials exposed to oral fluids.)

Question 22

What are the physical properties of materials?

Answer 22

Viscosity (resistance of a fluid material to flow when a force is applied)
Thermal conductivity (ability of a material to transfer heat through its mass.)
Thermal expansion (fraction or percentage increase in dimension that a solid material undergoes when heated)
Density (The mass per unit volume of a material)