Ceramics: Indirect Restorations

Question 1

Give 3 indications for porcelain, laminate and veneers.

Answer 1

Thin facings to correct:

1. Colour
2. Shape
3. Position

Question 2

Why are ceramic inlays less commonly used nowadays?

Answer 2

As they do not cover cusps, fail more often, and direct composite is less invasive and used more commonly now.

Question 3

What is an indication for ceramic onlays?

Answer 3

1. Replacing cusps
2. Providing cuspal coverage

Question 4

What are 2 indications for glass ceramic crowns and bridges?

Answer 4

1. Anterior crowns that extend to pre molars
2. Anterior bridges.
3. Glass ceramic crowns and bridges are frequently used for anterior dentition.

Question 5

What materials are used for high strength core-veneer crowns and bridges?

Answer 5

Aluminia and zirconia
^polycrystalline materials

Question 6

What are 2 indications for high strength core-veneer crowns and bridges?

Answer 6

1. Anterior AND posterior crowns as more durable.
2. Posterior 3-unit bridges.

Question 7

What 3 things must you consider when using monolithic high strength crowns and bridges?

Answer 7

1. Optimal properties
2. Fabrication routes
3. Adhesive capabilities

Question 8

What are 2 indications for monolithic high strength crowns and bridges?

Answer 8

1. All crowns
2. 3-4 unit posterior bridges

Question 9

What are 2 indications for porcelain fused to metal crowns (MCC/PFM) and bridges?

Answer 9

1. All crowns
2. Anterior and posterior bridges

Question 10

• What 2 common materials are used for glass ceramic crowns and bridges?

Answer 10

1. Leucite reinforced
2. Lithium disilicate reinforced.

Both look good aesthetically for anterior crowns and bridges.

Question 11

What 2 common materials are used for high strength core-veneer crowns and bridges?

Answer 11

1. Alumina core
2. Zirconia core
3. Both veneered with dental porcelain.

Core must be strong as they are used for posterior teeth with high mechanical demands.

Question 12

What common material is used for monolithic high strength crowns and bridges?

Answer 12

Zirconia for posterior crowns and bridges

Question 13

What common material is used for porcelain fused to metal crowns (MCC/PFM) and bridges?

Answer 13

1. Precious and non-precious alloys
2. With dental porcelain.

Question 14

Why are dental porcelains not compositionally ‘true porcelain’?

Answer 14

they lack kaolin

Question 15

Give 5 types of use for dental porcelains in contemporary dentistry.

Answer 15

1. Porcelain laminate veneers
2. Inlays
3. Onlays
4. Aesthetic veneering layer on high strength ceramic or metal cores
5. Fabrication of porcelain denture teeth for high end patients.

Question 16

What is meant by the fusing temperature of a dental porcelain?

Answer 16

The fusing temperature is where individual particles begin to coalesce with each other during sintering.

Question 17

Give 2 other ways dental porcelains can be classified.

Answer 17

1. Composition
2. Crystalline content

Question 18

How are restorations with dental porcelain made?

Answer 18

look on notes

Question 19

What is meant by the green state in dental porcelains?

Answer 19

1. The material will be structurally able to hold itself.
2. But must be handled very carefully as it has no real/significant mechanical properties at this stage.

Question 20

What happens if you heat too quickly in the firing stage of dental porcelain application?

Answer 20

The steam can cause cracks in the porcelain mass.

Question 21

What is meant by the bisque state in dental porcelains?

Answer 21

A state where the shape holds well, and it is used to have a preliminary look at whether the restoration being made has the correct shape and is functionally sound.

Question 22

What can excessive heating during the sintering phase lead to?

Answer 22

1. Pyroplastic flow
2. Restoration will lose its form and will not be able to hold itself.
3. Surface will also look highly glazed.

Question 23

Why does slow cooling minimise the generation of thermal residue stresses that may cause fracture?

Answer 23

As the bulk of the porcelain cool at a different rate to the external surfaces.

Question 24

What is the final stage in dental porcelain application?

Answer 24

1. Thin film of flass that fuses at low temperatures.
2. Then fire the crown for the glaze to adapt to the surface, filling any surface porosity without associated changes to the dimensions of the restoration.

Question 25

Why is glazing of dental porcelains required?

Answer 25

1. To generate a smooth surface by filliung in surface pores. 2. This in turn reduces biofilm formation. 3. Rough surfaces will attract dental plaque, pick up stains and be abrasive to the opposing tooth structure.

Question 26

What is another term for metal ceramic restorations?

Answer 26

1. MCC 2. PFM (Porcelain fused metal crowns) application of dental porcelain to metal core

Question 27

What is the most likely cause of failure for metal ceramic restorations?

Answer 27

Breakdown of the metal and ceramic interfacce leading to chipping and delamination

Question 28

What is the interfacial bond between the metal and ceramic in metal ceramic restorations dependent on?

Answer 28

mechanical retention - roughended using burs chemical boning - ceramic sintering coefficient thermal expansion - ceramin anf underlying metal

Question 29

How is the correct coefficient of thermal expansion between the ceramic and metal chosen?

Answer 29

The metal and porcelain are chosen to have a SLIGHT mismatch in the coefficients of thermal expansion Because the metal and ceramic are tightly bonded, the ceramic "holds back" the metal's contraction. This puts the ceramic into compressive stress This is beneficial as ceramic normally fails under tensile loading, so this increases the energy required for a crack to propagate thereby strengthening the ceramic itself.

Question 30

How is the correct alloy chosen for the metal ceramic restorations?

Answer 30

The melting temperature of the alloy must be significantly higher than the sintering temperature of the chosen porcelain.

Question 31

What are the 4 different alloys used in metal ceramic restorations?

Answer 31

high gold alloys gold-palladium alloys palladium-silver alloys nickel/colbalt chromium

Question 32

What is zirconia composed of chemically?

Answer 32

Zirconium oxide.

Question 33

What are the 3 main structural phases of zirconia restorations?

Answer 33

1. Cubic 2. Tetragonal 3. Monoclinic.

Question 34

- What is the strength of zirconia dependant on?

Answer 34

The dominant phased of the unit cell of the crystal

Question 35

What are the different strengths of the 3 main structural phases of zirconia restorations?

Answer 35

1. Monoclinic - lower strength, occupies a larger volume. 2. Tetragonal - highest strength, reduction in volume. 3. Cubic - moderate physical properties, smallest volume.

Question 36

What is the optimal structural form of zirconia restorations?

Answer 36

Tetragonal form - only forms at 1170 degrees.

Question 37

Why must we modify zirconia for use in dentistry?

Answer 37

1. As zirconia is its fully crystallised form is too hard to machine efficiently. During cooling from sintering a transformation from the tetragonal structural phase to the monoclinic structural phase occurs between 670-1070 degrees which is undesirable. This is undesirable as it is associated with an expansion of 3-4%, generating stress and would crack the object. **This is why we cannot use pure zirconia.** 1. So we machine in green state, sinter at high temperatures and use a stabilising element with zirconia.

Question 38

What stabilising element is used with zirconia what does it allow, and what are its properties?

Answer 38

1. Yttrium oxide 2. Maintenance of the tetragonal form at oral temperature 3. High strength, fracture toughness, high hardness, excellent chemicla resistance.