for the ceramometal restorations,the type of finish line is:
a- chamfer***
b- beveled shoulder.
Preparation of metal-ceramic restorations:
The realization of reinforcement and screeds for CCM techniques is very close to that of only metallic constructions.
A significant difference exists in the re-use of alloys.
As has already been said, during melting and casting, some elements of the alloy can be lost, especially those involved in the formation of oxides.
These elements are very important for the formation of the metal-ceramic bond with precious alloys.
That is why, at each casting of these alloys, at least half of the alloy must be new.
The surface treatments of screeds are also important.
These are sandblasting of the surface (Al2O3 between 25 and 50μm) and the formation of oxides (either in air or under partial vacuum).
With some alloys containing Pd, this cooking not only allows to create this outer layer of oxides, but also of internal oxides which penetrate into the metal from its surface and significantly increase its roughness which leads to an improvement of metal-ceramic adhesion.
Some non-precious alloys lead to the formation of an oxide layer too thick which weakens the bond.
With these alloys the screeds undergo a heat treatment and are then sanded to remove excess oxides.
The first layer of ceramic is very important since it must hide the metal.
Special opaque ceramics must be used. After application and firing of this layer, the dentin ceramic which contains less opaque oxides and fluorescent pigments and oxides is applied and sintered.
Finally, translucent enamel ceramics are applied and sintered.
Preparation of metal-ceramic restorations:
The realization of reinforcement and screeds for CCM techniques is very close to that of only metallic constructions.
A significant difference exists in the re-use of alloys.
As has already been said, during melting and casting, some elements of the alloy can be lost, especially those involved in the formation of oxides.
These elements are very important for the formation of the metal-ceramic bond with precious alloys.
That is why, at each casting of these alloys, at least half of the alloy must be new.
The surface treatments of screeds are also important.
These are sandblasting of the surface (Al2O3 between 25 and 50μm) and the formation of oxides (either in air or under partial vacuum).
With some alloys containing Pd, this cooking not only allows to create this outer layer of oxides, but also of internal oxides which penetrate into the metal from its surface and significantly increase its roughness which leads to an improvement of metal-ceramic adhesion.
Some non-precious alloys lead to the formation of an oxide layer too thick which weakens the bond.
With these alloys the screeds undergo a heat treatment and are then sanded to remove excess oxides.
The first layer of ceramic is very important since it must hide the metal.
Special opaque ceramics must be used. After application and firing of this layer, the dentin ceramic which contains less opaque oxides and fluorescent pigments and oxides is applied and sintered.
Finally, translucent enamel ceramics are applied and sintered.
Qualities required for a metal-ceramic system:
- High melting temperature of the alloy (> 1100 ° C). The temperature must be higher (> 100 ° C) than the sintering temperature of ceramics.
- Sintering temperature of the lowered ceramic. This temperature must be lower than that of ceramics used for ceramic-ceramic restorations ("low-melting" ceramics). In this way, no deformation of the metal screeds can occur.
- The ceramic must wet the alloy when it is applied in the form of a paste, so as to avoid bubbles at the interface and to allow medium-distance interactions (interatomic bonds and hydrogen bridges). Good adhesion between the alloy and the ceramic is essential and is obtained thanks to the interactions of the ceramic with the surface metal oxides and the roughness of this surface.
- Compatibility of the coefficients of thermal expansion of the ceramic and the alloy so as to avoid the fracture of the ceramic during the elaboration.
- Rigidity and strength of the reinforcement avoiding constraints in ceramics (especially at the level of bridge spans).
- High resistance to deformation at high temperature. The screeds and reinforcements are relatively thin and no deformation should occur at the time of elaboration of the ceramic suprastructure. Such deformations affect the insertion and adaptation of restorations.
- High casting accuracy of alloys, even at high temperatures.
- Design of the shape of the appropriate reinforcement:
+ Minimum thickness of the alloy.
+ Minimum thickness for ceramics.
Evaluation of the metal-ceramic adhesion:
Many tests have been used to measure the adhesion of a ceramic to an alloy: tests in shear, in flexion. However, the data obtained using various tests are not comparable.
In 3-point bending tests, the ceramic is sintered on a rectangular metal blade. The test piece (ceramic face down) is placed on two knives and a force is applied in the middle of the upper face until fracture of the ceramic.
It is considered that a correct adhesion is obtained from 25 MPa. However with many systems, values of 40 to 60 MPa can be achieved.
A ceramic-metal bond can be broken at 3 different levels.
Evaluation of the metal-ceramic adhesion:
Many tests have been used to measure the adhesion of a ceramic to an alloy: tests in shear, in flexion. However, the data obtained using various tests are not comparable.
In 3-point bending tests, the ceramic is sintered on a rectangular metal blade. The test piece (ceramic face down) is placed on two knives and a force is applied in the middle of the upper face until fracture of the ceramic.
It is considered that a correct adhesion is obtained from 25 MPa. However with many systems, values of 40 to 60 MPa can be achieved.
A ceramic-metal bond can be broken at 3 different levels.
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