Metal Ceramic - Dental Ceramic-Metal
Posted by John Doe at Dental Assistant on February 4, 2012.
Categories: Dental Materials
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Metal Ceramic Problems
Problem 1
The ceramic of a metal-ceramic crown placed in the mouth fractured from the metallic substructure. What factors might have produced such a failure, and how can it be avoided?
Solution a
Surface contamination of the alloy before placing the ceramic may be a causative factor. Impurities on the metal surface, such as organic powder from the grinding stones or grease and oils from fingers, may prevent a good wetting of the ceramic, and air bubbles will be present at the ceramic-metal interface. To avoid this problem, you should use vitrified grinding stones, protect the metal surface from polishing debris, and not touch the metal with the fingers.
Solution b
Underfired opaquer may be a factor. When the opaque ceramic has not been brought up to its fusing point, a complete fusion into the surface of the metal has not been achieved. Proper firing technique with the opaque material will eliminate this trouble.
Solution c
Another cause for the fracture may be improper metallic thickness. A uniform metal thickness is very important to prevent failures in the metal-ceramic bond. A minimum thickness of 0.4 mm is allowable. Thinner metal substructure will not protect the ceramic from fracture.
Solution d
The reuse of alloys may cause a fracture in the substructure. When sprue buttons are used to cast a new substructure, tin, or indium may be decreased or eliminated, and a very weak bonding with the porcelain is the result. The use of fresh alloys for casting the substructure is ideal, but a combination of a 50% (75% is better) fresh alloy and 25% to 50% used alloy may be used without detrimental effects on the metal-ceramic bond.
Problem 2
When a ceramic-metal restoration was removed from the oven, cracks in the ceramic were observed. What factors may have caused this failure, and how can it be avoided?
Solution a
Improper selection of ceramic and alloy will cause such cracks. Manufacturers often produce ceramic with special characteristics to match the thermal properties of a particular alloy. When another metal is tried with the same ceramic, a mismatch in the thermal expansion may be sufficient to cause cracking. Only the alloy and the ceramic suggested by the manufacturer should be used.
Solution b
Another causative factor may be overglazed or overtired ceramic. An overglazed or overtired ceramic no longer matches the alloy properly. Irregularities on ceramic surfaces should be finished before glazing to avoid overglazing.
Solution c
When the ceramic-metal restoration is allowed to cool in the furnace after the ceramic has been baked, cracks in the ceramic material will be produced. The ceramic should never be cooled in the furnace because slow cooling may change some physical properties of the ceramic, creating a mismatch with the alloy.
Solution d
When hot ceramic is touched with a cold instrument, a thermal shock can produce cracks.
Problem 3
When a metal-ceramic fixed appliance was completed, the shades appeared too gray. What could be the cause of such a dark ceramic, and how can it be avoided?
Solution a
When the coating of opaque ceramic is too thin or incomplete, the transparency of the body ceramic will allow the gray coping to show through. The opaque bake should be examined for gray areas and reopaqued if any are present. Two thin coats of opaque ceramic with separate firings are often recommended.
Solution b
When opaque ceramic has been fired to maturity at the opaque bake, by the time a third or fourth bake is made, the ceramic may have become too glazed and lost some of its opacifying qualities, thereby allowing the metal to reflect through the opaque layer, creating a gray shade. The manufacturer's suggested technique to bake the opaque should be carefully followed.
Solution c
When a crucible contaminated by an alloy containing base metals is used, a dark shade may be obtained. To avoid this problem, do not use a crucible that has been used to cast any other alloy. Only clean crucibles without ceramic liners or fluxes should be employed to cast the alloys for metal-ceramic restorations.
Solution d
Nonprecious alloys may contaminate the oven. Another source of oven contamination is the formation of volatilized impurities when the ceramic furnace has been used often for degassing and soldering operations. When the contamination accumulates in the oven, ceramic fired there will be dark. To avoid this problem you should purge the ceramic oven frequently.
Problem 4
A metal-ceramic restoration was cemented in the mouth. After insertion, flaking or chipping of the ceramic was observed. What may have been the cause of this failure?
Solution
The main problem with ceramic is it does not withstand much bending without fracture. When fired on a thin or flexible substructure, deformation of the metal under stress may deform the ceramic beyond its limit, and flakes or chips in the ceramic material are produced. The alloy selected to prepare ceramic-metal restorations or appliances should be built with enough bulk and have a high enough rigidity to withstand masticatory stresses without excessive deformation.
Problem 5
A crown preparation was made on a small lower anterior tooth that allowed little space for a metal coping. A Ni-Cr alloy was selected for the coping because its stiffness was reported to be twice that of the noble alloys. Because of the higher stiffness, the thickness of the coping was halved. In service the ceramic fractured. Why?
Solution
It is a mistake to halve the thickness in response to a higher modulus. In the deflection of the coping, the thickness of the coping is much more important than the modulus. If inadequate space is available for the coping and the desired color of a normally thick coping cannot be achieved without overcontouring the ceramic, an all-ceramic crown should be considered.
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