Essay 57 - Partially veneered metal-acrylic crown technology

partially veneered metal-acrylic crown technology sections

• why are they used?

• Opaque porcelain application

• Porce4lain margin fabrication

Opaque porcelain application

• Opaque porcelain is first applied to mask metal, to give restoration basic shade and initiate porcelain-metal bond

• small amount of opaque powder mixed with distilled water or specially formulated liquid, forms thin wash which is applied with brush

• coping dried and fired under vacuum to specific temp.

• Second application of opaque porcelain (should mask metal)

• Powder + liquid mixed to creamy consistency and applied to coping with brush in vibrating motion

• Opaquer layer applied AS THIN AS POSSIBLE!

• Coping gently vibrated to condense porcelain, excess water removed with tissue

• Second layer of opaquer fired using same firing cycle. Opaque layer of porcelain should be approx 0-3mm thick

porcelain margin formulation

• formed using direct lift technique

• mark shoulder finish line on die using red pencil

• seal porous surface of gypsum die by brushing on sealing material

• Lubricant applied to facial shoulder of sealed die with brush

• Investment liquid hardens as wet porcelain mix dries on die, makes for easy removal without fracturing margin

• After firing, residual silica particles as as inclusions in porcelain, weakening it thus it’s more prone to fractures

• Add initial increment of shoulder porcelain to facial shoulder (extend approx 2-3mm into coping)

• Condense porcelain and blot dry with tissue

• Carve porcelain with large spoon excavator or small cleoid to produce slight bevel or under contour (this produces space for narrow layer of dentin porcelain over shoulder porcelain)

• lightly smooth shoulder porcelain at margin with no.10 sable condensing brush

• Carefully tease coping from die

• Inspect inside of casting for specks of porcelain and remove them. This can be ground out after firing but it is more easily seen and removed in the prior state

• Place coping in sagger tray

• Thoroughly dry porcelain in front of furnace. Fire under vacuum at temp of approx 30DC higher than corresponding dentin+enamel porcelains

• When initial increment of shoulder porcelain is inspected on die after firing, small opening may be apparent at facial margin. More shoulder porcelain can be added to discrepancy with crown seated on die. Vibrate into opening with small vibrating sable brush

• Some ceramists prefer to add very small amount of shoulder porcelain of runny consistency to gingival aspect of fired margin to correct discrepancy

• Place coping back into die, apply alternative firm seating pressure and vibrating die. Be sure metal margin on lingual of casting seats, completely

• If not, removed coping from die and scape away some newly added “correction porcelain”. Condense + smooth porcelain

• Use same firing cycle for correction bake as used in initial application

• When margins satisfactory, continue to build up of dentin+enamel

• Mix dentin porcelain to creamy consistency with distilled water or manufacturer recommended liquid. Then apply over opaque with sable brush or small spatula, start at gingivofacial of coping, which is seated on working cast

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