In metal-ceramic crowns, what keeps the 2 materials bonded to each other?
- Chemical bonding (52%)
- Compressive forces (26%)
- Mechanical bonding (22%)
- Van der Waals forces
How does chemical bonding between metal & ceramic work?
- Small proportion of base metal(oxidiser) in alloy
- Atoms on surface bind oxygen => form oxide layer on surface
- When fired in air, oxides bind to similar oxides (SiO2 & other metal oxides) in the ceramic
- Oxygen atoms from both sources combine => secure ceramic to alloy surface
How do the compressive forces between metal and ceramic work to keep them tgt?
- metal has slightly higher COTE
- metal cools then contracts slightly more => pull & compresses porcelain/ceramic in
How does mechanical bonding between metal and ceramic work to keep them tgt?
- occurs as ceramic flows into the microscopic spaces in the surface of the metal
- must eliminate irregularities & increase surface area
- via air abrasion
How do van der waals forces between metal and ceramic work to keep them tgt?
- VDW forces shorten distance between metal & porcelain => allow chemical bonding
- initiate chemical bond on microscopically rough surface
Why is feather edge margin design not recommended?
Does not provide sufficient room for bulk of material
Retention can be increased if the restoration is roughened by air-abrading the fitting surface. Why?
Prolly cuz it creates space for micro-mechanical retention
Modes of fracture are..
- Shear (lateral)
- Tension (pulling)
- Torsion (twisting)
Name the luting agents in decreasing resistance:
Adhesive resin>GIC>ZOE>zinc-phosphate
Cannot have too little or too much base metal(oxidiser) when bonding metal to ceramic cuz:
- Insufficient base metal(oxidiser)
- deficient oxide layer, no binding w ceramic - Excessive
- thick oxide layer
- oxides NOT anchored within bulk of alloy => mechanically weak oxide layer
Rank the abrasiveness of the crown materials in increasing order:
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
List out Goodacre’s principles:
- Total occlusal convergence: 10-20 degrees
- Minimum inciso-cervical length (incisors & PMs): 3mm
- Minimum occlusal-cervical length (Ms): 4mm
