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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 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
-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