Alloys for cast metal restorations + Crowns

Question 1

What are the stages in crown prep?

Answer 1

Occlusal/ incisal reduction
Separation
Buccal reduction
Palatal or lingual reduction
Shoulder & chamfer reduction
Occlusion check/ adjustment
Polishing the preparation

Question 2

What is the occlusal reduction and buccal shoulder measurement for posterior MCC?

Answer 2

Occlusal - approx 2.5mm
Buccal shoulder - 1.5mm shoulder placed approx. 1mm into gingival sulcus

Question 3

What is the occlusal reduction and buccal shoulder measurement for posterior ACC?

Answer 3

Occlusal - approx. 1.5-2mm
Buccal shoulder - 1.5-2.5mm buccal shoulder approx. 1mm into gingival sulcus.

Question 4

Why can porcelain not withstand large biting forces?

Answer 4

MICROCRACKS - this is what makes porcelain fail.
They form during production and make them likely to crack further under large forces

Question 5

What are the characteristics of porcelain?

Answer 5

Rigid
Hard
Strong
Low tensile strength - leads to fracture at low stress (BRITTLE)

Question 6

How does porcelain-fused alloys improve the mechanical properties of porcelain?

Answer 6

Changes from porcelain being brittle to being DUCTILE. Also increases strength, rigidity and hardness.

Question 7

How does porcelain bond to metal alloy?

Answer 7

Via metal oxide.
Bonding of metal oxide helps eliminate defects/ cracks on porcelain surface

Question 8

What are the required properties of metal alloys that bond to porcelain?

Answer 8

Form good bond to porcelain
Similar thermal expansion co-efficient to porcelain
Must avoid discolouration of porcelain
Melting temp. must be higher than porcelain to prevent creep

MECHANICAL PROPERTIES
Must bond well to metal oxide
Surface hardness
Elastic modulus - want it to be HIGH (RIGID) to support porcelain.

Question 9

What are some modes of failure in PMA (porcelain-metal-alloys)?

Answer 9

Oxide layer fracturing
Oxide layer detaching from alloy
Oxide layer detaching from porcelain