CSAR 14.1 + 14.2 - Dental ceramics for indirect restorations

Question 1

Define ceramic material.

Answer 1

Non-metallic inorganic material
Formed by ionic bond
Crystalline

Question 2

Define glass material.

Answer 2

No regular structure - amorphous

Metamorphous material

Question 3

What are the mechanical properties of glasses and ceramics?

Answer 3

Hard
Strong in compression, weak in tension
Notch sensitive
Brittle

Question 4

What are the 2 main ingredients of dental porcelain?

Where are they found and what are they like in temperatures?

Answer 4

Feldspar - naturally forming glass - flows at elevated temps

Silica - naturally forming glass - stable at high temps - good translucency, like enamel

Question 5

What material isn’t used in dental porcelain anymore?

Answer 5

Kaolin - lacks translucency and adds orange colour

Question 6

How are the dental porcelain materials made into frit powder for the lab?

Answer 6

Feldspar and silica heated to a high temperature to created a molten mass
Mass quenched in water, causing it to shatter
Then ground into fine powder to create frit powder

Question 7

How is the frit powder of dental porcelain made into a crown?

Answer 7

Frit forms paste with water which can be moulded well
Can be died with gypsum materials
Sublayer - dentine - more opaque and yellow
Layer on top - enamel
Translucent incisal edge

Paste compacted/blotted to remove water to reduce shrinkage up to 30% during firing

Put in oven to cause sintering of the material, allowing glass particles to flow and join, which is cohesive and stronger

Needs to be slow cooled as ceramics are insulators, so stress concentrations can form causing fractures

Question 8

If there are exterior cracks on porcelain crown, how can they be filled?

Answer 8

Glazed and fired at high temperature

Question 9

What is the most common failure of anterior porcelain crowns?

Answer 9

Fitting surface cracks

Question 10

What are advantages of the PFM layers, 1. porcelain exterior layer and 2. alloy sub-layer?

Answer 10

1. Aesthetics, chemical resistance

2. High stiffness, strength and reduced tensile stresses forming in ceramic

Question 11

How is the metallic sub-layer hidden in PFM crowns?

Answer 11

Opaque porcelain layers

Question 12

What are the 3 ways of bonding the porcelain and metal alloy layers of PFM crown?

Answer 12

Thermal contraction - matching thermal coefficients
Chemical bonding - ceramic reacts with the oxide layer on alloy - good interdigitation
Mechanical interlocking - roughen alloy and alloy ceramic to flow in undercuts during firing

Question 13

What are 3 alloys that are commonly used for PFM crowns?

Answer 13

Gold
Pd/Ag (60:30)
Ni/Cr (80:15)

Question 14

Why are gold alloys not favourable for PFMC?

Answer 14

If not copper free -> stain ceramic blue/green
Stiffness problem –> therefore use low gold alloys for greater stiffness
Can deform during firing causing loss of fit

Question 15

What are the + and - of Pd/Ag alloys for PFMC?

Answer 15

\+ stiffer than gold alloys
\+ good bonding, stable thin oxide layer 
\+ high melting point, therefore do not deform 
\+ cheaper than gold 
- susceptible to casting faults

Question 16

What are the + and - of Ni/Cr alloys for PFMC?

Answer 16

+ very high melting point, therefore do not deform
+ very high stiffness
+ can be used for long spans in the mouth
- bonding is weak, oxide layer is too thick
- casting difficulties, shrinkage difficult to compensate

Question 17

What materials are used as ceramic cores? where and why?

Answer 17

Alumina (Al2O3) - only for anterior crowns for palatal portion, much stronger than porcelain
Ceramic core - strong, tough and aesthetics

Question 18

Why is alumina added to porcelain crowns?

Answer 18

Improves flexural strength and better at stopping cracks than silica

Question 19

What is the downfall of using alumina?

Answer 19

It is opaque, therefore poor aesthetics

Fine for core, however need to make a veneer

Question 20

What is the difference in amount of alumina added to porcelain in 1. aluminous porcelain and 2. sintered alumina?

Answer 20

1. 40-50%

2. 85%

Question 21

What is the technique of slip casting?

Answer 21

Building up layers on a refractory die

Question 22

How is sintered alumina crown placed on to a tooth?

Answer 22

Needs rough fitting surface

Dual cure resin for satisfactory polymerisation - limited light transmission through ceramic

Question 23

Name 3 sintered materials and their strengths?

Answer 23

Sintered alumina - 400-500MPa
Sintered spinel (spinel is complicated Mg salt) - 350MPa
Sintered zirconia - 700MPa

Question 24

Which has better aesthetics sintered spinel or sintered zirconia?

Answer 24

Sintered spinel

Question 25

What is the disadvantage when a crown material is sintered?

Answer 25

Inaccurate fit

Question 26

Once a block is milled in crown production, it is sintered, how much shrinkage needs to be compensated and by what process can this be done?

Answer 26

20-25% and added on by CADCAM

Question 27

What are the properties of zirconia?

Answer 27

White ceramic
Polycrystalline - when cooled
Strong
High flexural strength

Question 28

What are the different crystal structures of zirconia?

Answer 28

Monoclinic zirconia

Stabilised tetragonal zirconia - Yttrium oxide trapped some as tetragonal (which takes up more volume)

Question 29

What are the disadvantages of zirconia?

Answer 29

Needs veneer layer and delamination is a problem

Really hard to remove once placed in the mouth