DMS-ceramics

Question 1

What is Feldspar?

Answer 1

This is a component found in dental porcelain that binds the parts together.

Question 2

How do we make a feldspathic ceramic?

Answer 2

We sinter it.

Question 3

How do we make sintered ceramics?

Answer 3

After building the feldspathic ceramic up into a restoration we heat it again (at 1140-1500*C) to form Leucite.

This causes fusion of ceramic into a single mass (from a dry powdery mass to a solid powdered block)

The material also contracts by 20%.

Question 4

Discuss some of the good properties of of a feldspathic ceramic?

Answer 4

Aesthetic- It has a stable colour, has a smooth surface (so less staining) & is translucent.

Chemical- it is unaffected by the pH in the mouth and good biocompatabilty.

Thermal properties- Similar to the tooth substance

Mechanical- it has high hardness and compressive strength.

Question 5

Discuss some of the negative properties of a feldspathic ceramic?

Answer 5

There is a 20% shrinkage during firing.

It has very low tensile strength

Very low flexural strength

Very low fracture toughness

Static fatigue (decrease in strength due to the wet environment)- causing slow crack growth.

Surface micro-cracks form easily, making the material more prone to fracture.

Question 6

Where do we use conventional feldspathic cements?

Answer 6

Only in anterior teeth as the mechanical problems mean they can only be used in low stress areas.

Question 7

How can we improve the strength of ceramics?

Answer 7

• milling
• Metal cores

Question 8

Explain milling ceramics

Answer 8

This is when we get a block of ceramic and carve the crown out of it.

The heat treatments make it stronger than a non-milled material.

Question 9

What is stress coping?

Answer 9

When we use a strong metal core and attach porcelain to it.

Question 10

Compare alumina and Zirconia cores.

Answer 10

An alumina core contains max 50% alumina.

adv- it has double the flex strength of porcelain

disadv-It is opaque and no tstrong enough for posterior tooth use.

We use yttria stabilised zirconia which contains 1% ytrria to reduce the chance of cracking on cooling. This is the most popular material.

Adv- yttria closes up any cracks through slight material expansion

Disadv- There is the pottenital for the vennering porcleain to debond from the core.