Alloys for Cast Metal Restorations

Question 1

what are cast metal restorations

Answer 1

crown and bridges and porcelain fused to metal

Question 2

how can you describe a porcelain fused to metal crown

Answer 2

cast metal coping onto which is fired a ceramic veneer

Question 3

porcelain has good aesthetics but what may be the downside

Answer 3

microcracks tend to form at the fitting surface which means its prone to mechanical failure

Question 4

what forces is porcelain unable to withstand

Answer 4

biting forces

Question 5

what is meant by compressive strength

Answer 5

stress to cause fracture

Question 6

what is elastic modulus definition

Answer 6

the stress required to change shape

Question 7

what is the definition of brittleness (ductility)

Answer 7

dimensional change experienced before fracture

Question 8

what is hardness definition

Answer 8

resistance of surface to indentation or abrasion

Question 9

what three properties can be ascertained by the stress/ strain curve

Answer 9

strength (compressive and tensile)
brittleness
elastic modulus

Question 10

what cannot be ascertained from a stress/ strain curve

Answer 10

hardness

Question 11

what are the properties of porcelain

Answer 11

ductile
hard
strong
rigid
brittle

Question 12

what are the properties of an alloy

Answer 12

hard
strong
rigid
ductile

Question 13

what is the definition of rigid

Answer 13

large stress required to cause strain

Question 14

what is the definition of hard

Answer 14

surface withstands abrasion/ indentation well

Question 15

what is the definition of strong

Answer 15

high compressive strength but low tensile strength which means a tendency to form surface defects

Question 16

what is the definition of brittle

Answer 16

low fracture toughness

Question 17

what is the definition of brittle

Answer 17

low fracture toughness

Question 18

what is the definition of brittle

Answer 18

low fracture toughness

Question 19

what bonds the alloy structure to the porcelain

Answer 19

metal oxide

Question 20

what does the bonding of a metal oxide to porcelain help to eliminate

Answer 20

defects/ cracks on the porcelain surface

Question 21

what does the alloy act as in a PFM

Answer 21

acts as support and limits the strain that porcelain experiences

Question 22

what are the 5 required properties of alloys used in PFM

Answer 22

forms good bond with porcelain
thermal expansion coefficient similar to porcelain
avoid discolouring of porcelain
mechanical strength
melting recrystallisation temp of alloy

Question 23

why should the thermal expansion coefficient of the metal be similar to that of porcelain

Answer 23

it avoids setting up stresses during fusing of porcelain to alloy

Question 24

what metals need to be avoided to reduce chances of discolouration of porcelain

Answer 24

Ag in AgPd can produce green discolouration
Copper is not used in high gold alloy

Question 25

what are the five alloys that are suitable for PFM

Answer 25

high gold alloys low gold alloy silver palladium (AgPd) Nickle chromium (NiCr) cobalt chromium

Question 26

what alloy is more difficult to achieve good bonding to porcelain with

Answer 26

Nickel chromium

Question 27

what must be the melting/ recrystallisation temperature of the alloy be in relation to porcelain

Answer 27

must be higher than the fusion temperature of porcelain otherwise creep may occur

Question 28

what is the definition of creep

Answer 28

gradual increase in strain experienced under prolonged application of stress

Question 29

when does creep occur

Answer 29

when the material temperature is more than half its MPt

Question 30

what metals are included in high gold alloys

Answer 30

gold (80%) platinum and palladium silver

Question 31

why is there no copper in high gold alloys

Answer 31

so no green hue is imparted to the porcelain

Question 32

what are two disadvantages of high gold alloys

Answer 32

melting range may be too low young's modulus too low

Question 33

what are the metals included in low gold alloys

Answer 33

gold (50%) palladium silver

Question 34

what are two advantages of using low gold alloys over high gold alloys

Answer 34

increased melting temperature better mechanical properties

Question 35

what metals are incorporated into silver-palladium alloys

Answer 35

palladium silver tin

Question 36

what percentage of nickel and chromium are in nickel chromium alloys

Answer 36

nickel 70% chromium 30%

Question 37

give one advantage and one disadvantage of nickel chromium alloys

Answer 37

high melting point and young - advantage low-ish bond strength and high casting shrinkage - disadvantage

Question 38

what alloys are the best for casting

Answer 38

high gold and low gold

Question 39

what alloys have excellent properties for creep

Answer 39

low gold silver palladium nickel chromium

Question 40

what alloys have the best elastic modulus

Answer 40

nickel chromium cobalt chromium

Question 41

what alloys have the best bonding properties

Answer 41

high gold low gold silver palladium

Question 42

what alloy has the best biocompatibility

Answer 42

high gold

Question 43

what alloy has the worst biocompatibility

Answer 43

nickel chromium

Question 44

where is it best to have failure to occur in PFM

Answer 44

the porcelain

Question 45

what is the stressed skin effect of porcelain-metal bonds

Answer 45

slight differences in thermal contraction coefficiencies leads to compressive forces that aid bonding