Posterior Metal Ceramic Crowns

Question 0

What are the advantages of having the lingual cusps covered in metal in a patient that bruxes?

Answer 0

1) Reduce depth of reduction and conserve tooth tissue
2) Provide greater structural durability and eliminates the possibility of porcelien fracture in function
3) Functional cusps in metal causes less wear of opposing teeth than porcelein

Question 1

What factors neeed to be considered when cutting a PBM prep?

Answer 1

1) Aesthetic requirements - which surfaces to be veneered with porcelein and which are visible in the arch
2) Conservation of tissue
3) Structural durability
- need a sifficient thickness of metal and porcelein
4) Marginal adaptation
- metal is better than porcelein
5) Junction between porcelein and metal

Question 2

What are the four kinds of margins availible for the buccal margin finish?

Answer 2

1) Flat radial shoulder 1.0 to 1.3mm wide with rounded internal axio-gingival line angle. Pocelein forms a butt joint at the finish.
2) A heavy chamfer 1.0mm deep with porcelein built to the margin
3) Heavy chamfer 1.0mm with a metal collar 0.5mm wide
4) A bevelled (45 degree) shoulder 1.0mm wide with a metal collar at the buccal margin and a rounded axio-gingival line angle

Question 3

What are the addvantages and disadvantages of finishing the buccal margin with a flat radial shoulder with a roudned internal line angle?

Answer 3

Advantage:
Most aesthetic margin and is the margin of choice for anterior PBM crowns with good structural durability and satisfactory marginal adaptation.
Disadvantage:
Least conservative

Question 4

What are the advantages and disadvantages of using a heavy cchamfer with porcelein built to the finish for the buccal margin?

Answer 4

Advantages:
More conservative than shoulders, hence less potential for pulp damage
Moderately aesthetic. However, inferior colour matching forr lighter shades as there is less thickness for porcelein
Disadvantage:
Porcelein at margin less than 90 degrees and hence more fragile

Question 5

What are the ddisadvantages of finishing the buccal margin with a heavy chamfer with a metal collar?

Answer 5

Advantages:
more conservative than shoulder design.
Good structural durability and good marginal adaptation of acute angle off gold to tooth
Disadvantages:
Gingival display of gold may not be asthetically acceptable

Question 6

When are each of the four types of buccal finish margins indicated?

Answer 6

1) Flat radial shoulder:
When gingival third of crown highly visible and good aesthetics essential
2) Heavy chamfer with porcelein built to margin:
GIngival third only just visible
Long clinical crown e.g. margins on root surface
3) Heavy chamfer with gingival collar:
As above but if the gingival third not visible
4) Bevelled shoulder with metal collar:
High stress situations
Short worn clinical crowns when retention is crucial

Question 7

What are the occlusal reduction depths for a posterior PBM?

Answer 7

Metal veneered with porcelein: 2mm
Metal alone:
- functional cusp: 1.5mm
- non-functional cusp: 1.0mm

Question 8

What are the buccal reduction depths for a posterior PBM?

Answer 8

Occlusal 1/3rd: 1.5mm
Gingical 1/3:
1) Shoulder: 1.0 to 1.3mm
2) Heavy chamfer: 1.0mm
3) Bevelled shoulder: 1.0 to 1.3mm

Question 9

What are some general indications for a anterior or posterior full crown?

Answer 9

When restoration of natural tooth appearance and colour is essential
When more coonserative restorations are unsuitable and lack adequate strength and durability

Question 10

What are some specific indications for PBM crowns??

Answer 10

1) High stress situations - bruxism, epilepsy
2) Atypical preparations - mutilated, fractured teeth, deep overbite
3) FIxed bridge retainers

Question 11

What are some potential problems with PBMs?

Answer 11

1) Heavier redduction with higher potential for pulpal damage especially in younger patients.
2) Greater wear on opposing teeth than FGC
3) More ddifficult to contour occlusal surface
4) Limited interoccclusal clearance and heavy bite may contraindicate heavy reduction needed for full occlusal porcelein
5) Cost

Question 12

What types of metals are used in PBMs?

Answer 12

1) High noble alloys: High gold alloys (minimum 40%) with platinum, palladium
2) Noble alloys: Platinum alloys (25%) with palladium and silver.
3) Base metals
High noble alloys preferrred for superior casting properties, marginal fit, colour and optimum thicjkness of oxide layer for bonding to porcelein