Mechanical Considerations

Question 1

In metal-ceramic crowns, what keeps the 2 materials bonded to each other?

Answer 1

1. Chemical bonding (52%)
2. Compressive forces (26%)
3. Mechanical bonding (22%)
4. Van der Waals forces

Question 2

How does chemical bonding between metal & ceramic work?

Answer 2

1. Small proportion of base metal(oxidiser) in alloy
2. Atoms on surface bind oxygen => form oxide layer on surface
3. When fired in air, oxides bind to similar oxides (SiO2 & other metal oxides) in the ceramic
4. Oxygen atoms from both sources combine => secure ceramic to alloy surface

Question 3

How do the compressive forces between metal and ceramic work to keep them tgt?

Answer 3

• metal has slightly higher COTE
• metal cools then contracts slightly more => pull & compresses porcelain/ceramic in

Question 4

How does mechanical bonding between metal and ceramic work to keep them tgt?

Answer 4

• occurs as ceramic flows into the microscopic spaces in the surface of the metal
• must eliminate irregularities & increase surface area
• via air abrasion

Question 5

How do van der waals forces between metal and ceramic work to keep them tgt?

Answer 5

• VDW forces shorten distance between metal & porcelain => allow chemical bonding
• initiate chemical bond on microscopically rough surface

Question 6

Why is feather edge margin design not recommended?

Answer 6

Does not provide sufficient room for bulk of material

Question 7

Retention can be increased if the restoration is roughened by air-abrading the fitting surface. Why?

Answer 7

Prolly cuz it creates space for micro-mechanical retention

Question 8

Modes of fracture are..

Answer 8

1. Shear (lateral)
2. Tension (pulling)
3. Torsion (twisting)

Question 9

Name the luting agents in decreasing resistance:

Answer 9

Adhesive resin>GIC>ZOE>zinc-phosphate

Question 10

Cannot have too little or too much base metal(oxidiser) when bonding metal to ceramic cuz:

Answer 10

1. Insufficient base metal(oxidiser)
   - deficient oxide layer, no binding w ceramic
2. Excessive
   - thick oxide layer
   - oxides NOT anchored within bulk of alloy => mechanically weak oxide layer

Question 11

Rank the abrasiveness of the crown materials in increasing order:

Answer 11

Metal>Porcelain/Ceramic

Metal: least abrasive if polished well
Porcelain: abrasive, if glazed surface is compromised & roughened
Ceramic: equally abrasive, if roughened

but exposure to low pH carbonated beverage will increase wear rate of enamel when opposed by all unglazed, glazed & polished porcelain

Question 12

List out Goodacre’s principles:

Answer 12

1. Total occlusal convergence: 10-20 degrees
2. Minimum inciso-cervical length (incisors & PMs): 3mm
3. Minimum occlusal-cervical length (Ms): 4mm