PBM and PBZ Flashcards
What are the indications of PBM’s and PBZ’s?
- Natural tooth appearance required
- Maximum protection for broken down tooth or heavily restored tooth
- More conservative restorations lack sufficient durability
- High stress situations (e.g. bruxing)
How can PBM’s and PBZ’s be used (what is their clinical application)?
- Anterior/posterior teeth single crown
- Full coverage for cracked or fractured tooth
- Reshaping abutment teeth for RPD
- Retainers for fixed bridgework
- Splinting periodontally weakened teeth
What is the thickness of the metal coping in PBM for base and noble metals?
Base: 0.2mm
Noble: 0.3-0.5mm
What is the minimum and optimum thickness of porcelain for PBM?
Minimum: 0.7mm
Optimal: 1.0mm
What is the function of opaque porcelain, body dentine and enamel porcelain for PBM?
Opaque: Mask metal and plays role in metal ceramic bond
Body: Colour
Enamel: Translucence
What properties make gold content alloys preferable for PBM?
-Superior casting, colour, thickness of oxide layer for bonding to porcelain
What forces bond the metal to porcelain in PBM?
Micromechanical: Use of air abrasion on clean, uncontaminated metal surface
Compressive force: Thermal co-efficient of expansion for metal greater than porcelain (squashes metal into porcelain)
Molecular: Van der Waals molecular forces of attraction
Chemical: Metal oxides dissolve in softened glass phase of opaque porcelain at high temp
What are the laboratory steps in manufacturing a PBM?
- Construct die
- Wax up and cast metal coping + oxidise surface at high temp
- Apply opaque porcelain
- Build up dentine and enamel porcelain
What is the fusing temperature of dental ceramics? What is the resultant shrinkage?
980 degrees Celsius in vacuum
Shrinkage: 20%
What is the purpose of fusing porcelain in a vacuum?
Reduces air bubbles which reduce translucency
What are some shortcomings of PBM?
- Destructive preparation (potential pulp trauma especially in young teeth and unsuitable in very short crowns without lengthening)
- Require adequate labial reduction to provide sufficient space for porcelain and avoid overcontouring
- Expensive $1200 +
- Aesthetics inferior to full ceramic crown due to reflecatance from opaque layer of porcelain
- Porcelain brittle and may fracture: need correct prep design, copings, lab techniques
- Increased wear of opposing natural teeth or gold restorations
What can increase fracture risk for porcelain?
- Subsurface porosity due to increased thickness
- Insufficient extension of metal coping to support porcelain
What types of buccal margins are possible for PBM anterior/posterior? What are the advantages + disadvantages of each one? When are they indicated?
Ceramic radial shoulder 1.0-1.3mm (90 degree inner angle)
(+) Most aesthetic margin thus always use for labial anterior PBM
(+) Good structural durability
(+) satisfactory marginal adaptation
(-) Least conservative of tooth structure
Indicated: Gingival third of crown highly visible and good aesthetics essential (e.g. premolars and first molars)
Ceramic heavy chamfer 1.0mm (shoulder but with curved inner angle)
(+) More conservative than shoulder
(+) Moderately aesthetic
(-) Less thickness means inferior colour matching for lighter shades
(-) Porcelain at margin less than 90 degrees thus thinner and for fragile than shoulder
Indicated:
-Gingival third only moderately visible; colour matching not critical
-Small anterior teeth
-Long clinical crown (e.g. recession means margin finishes on root)
Heavy chamfer (1.0mm) with gold collar (0.5mm high) (heavy chamfer but with gold sticking out/visible on external surface)
(+) more conservative than shoulder and less potential for pulp damage
(+) good structural durability
(+) good marginal adaptation of acute angle of gold to tooth
(-) Gingival gold display may not be aesthetically acceptable
Indications: as with heavy chamfer but when gingival third not visible
45 degree bevelled shoulder with gold collar (shoulder with gold colour and 45 degree bevel added on external surface) (1mm wide, gold 0.8-1mm high)
(+) Excellent structural durability
(+) Good marginal adaptation of acute 45 degree bevel of gold
(-) Display of gold may not be acceptable aesthetically
(-) Shoulder design less conservative than heavy chamfer
Indicated:
-High stress requiring high structural durability e.g. bruxism and gingival 3rd not visible
-Short worn clinical crown (e.g. lower molar) to optimise retention by increasing length of axial wall
What are some advantages of porcelain bonded to zirconia crowns?
- No metal framework=better aesthetics and good for metal alllergies
- Zirconium dioxide stiff and does not bend easily
- More economical?
What are the stages for a PBZ crown?
- Prepare tooth, take impression, temp crown
- Pour model and create die
- Scan with Piccolo scanner
- Design coping shape with graphics design program (I.e. Zirconia coping component; Procera brand))
- Send via internet to manufacturing site which manufactures coping
- Coping sent back to dental lab and ceramic build up is added (brand Noble Rondo)
