DMS- porcelain

Question 1

Discuss the disadvantages of using porcelain restorations?

Answer 1

porcelain cannot withstand large biting forces as microcracks form at the fitting surface.

Question 2

How is a porcelain metal restoration formed?

Answer 2

A metal oxide layer is produced on the alloy surface when the alloy and porcelain are heated in a furnace. This layer bonds the porcelain to the metal alloy.

Question 3

How does the metal alloy provide mechanical support to porcelain?

Answer 3

The metal alloy changes shape slightly when exposed to the stress.

This limits the strain that porcelain is exposed to.

Question 4

Discuss how the thermal expansion co-efficent influences our choice of alloy and porcelain?

Answer 4

You want an alloy with a slightly higher themal expansion co-efficient. (Ideal difference of 0.5ppm/ºC)

This is so that in the cooling stage, the alloy is slightly compressing the porcelain.

Question 5

Discuss how the recyrstalisation temperature influences the choice of alloy and porcelain?

Answer 5

You want a higher recrystalisation temperature of the alloy than porcelain to prevent creep (permanent deformation of the metal)

Question 6

Which alloy do we choose in the GDH and discuss the adv and disadv

Answer 6

Cobalt chromium.

Adv- It has a HIgh melting point

High tensile strength

High rigidity

Disadv- Casting shrinkage occurs.

Question 7

Why do we never use nickel chromium alloys?

Answer 7

Due to the biocompatability concerns. (allergies)

Question 8

Discuss the 3 ways that the porcelain and metal alloy bond

Answer 8

Mechanically- The surface irregularities on the alloy and the porcelain interlock.

Stressed skin effect- the alloy contracting slightly more puts compressive forces on the porcelain to help bonding.

Chemical- The oxides in the metal oxid coating of the alloy migrate with the oxides within the porcelain itself.