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Flashcards in PBM and PBZ Deck (20):

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
-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) (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)
(+) 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
-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)


What is the thickness of the zirconia coping and veneering ceramic in PBZ crowns?

Zirconia: 0.3-0.8mm (usually 0.6mm)
Veneering ceramic: 1.0mm


For PBM's when should you consider covering the occlusal surface with metal only in posterior PBM's and why?

-Short occlusal or worn clinical crown and/or bruxing patient

-Reduces depth of reduction and thus conserve tissue
-Greater structural durability: reduced chance of porcelain fracture
-Metal causes less wear of opposing tooth than porcelain


What are the factors that have to be changed when converting an anterior PBM prep to a PBZ?

-Use Technik 856 bur rather than 847
-Convert all shoulders to heavy chamfers
-Increase margins around the entire tooth to 1.3mm (from 1mm labial (usual measurment used in sim lab); 0.5mm proximal and 0.5mm palatal
-Remove any sharp corners or deep troughs and valleys
-If palatal concavity is done to 0.5mm (as usual in sim lab for metal only) then increase this to 1.0-1.5mm


What are the factors that have to be changed when converting posterior PBM to PBZ?

-Use Technik 856 bur rather than 847
-Convert all shoulders to heavy chamfers
-Increase margins around the entire tooth to 1.3mm (usually buccal 1.0-1.3mm; proximal 0.5mm; palatal 0.5mm)
-Remove any sharp corners or deep troughs and valleys
-Increase occlusal reduction of non-functional cusp from 1.0mm to 1.5-2.0mm if initially done for metal only


What is the problem with PBZ crowns?

-Poor bonding between porcelain and zirconia means porcelain normally falls off after a while
-Zirconia is extremely hard thus trying to remove it from tooth is difficult-->hard to retreat