Metal-ceramic systems in dental restoration
Porcelain restorations have extremely good aesthetics, but the material must usually be reinforced with a metal core to provide adequate strength and durability. This metal core must be surface pre-treated to bond well to the porcelain - leading to the development of complex 'metal-ceramic' systems for use in dentistry.
Metal-Ceramic Systems: The effect of individual elements, within various alloy types, on ceramic bonding, biocompatibility and spectrophotometric colourimetry are all being investigated at Sheffield.
For some years it has been known that the development of a metal oxide layer on the metal is vital to the metal-porcelain bonding process. Research at Sheffield, however, has recently shown that a number of other changes are also occurring in the metal surface during the pre-treatment regimes that have been developed in refining this process.
Typically alloys used as cores for porcelain restorations include Pd/Ag and Ni/Cr. These are commonly pre-treated using variants on the following scheme:
- devesting and Al2O3 blasted;
- ground smooth and Al2O3 blasted;
- oxidation firing;
- firing cycle for opaque porcelain application.
Above right: Graph showing the bulk and surface composition of a Pd/Ag alloy after being treated to bond to porcelain. The difference in composition largely reflects changes occurring during this treatment.
Studies at Sheffield have followed the changes in the metal surface chemistry consequent on each stage of preparation using two forms of analytical spectroscopy. X-ray fluorescence spectrometry (XRF) determines the bulk composition of the alloy, and X-ray photoelectron spectroscopy (XPS) gives the composition in the surface of the alloy that bonds to the porcelain.
This technique has shown that the surface composition of the metal-ceramic alloys used is dramatically different from their bulk composition. Notably, the oxidation preparation stage brings high concentrations of trace elements in the alloy to the surface.
Other studies have demonstrated that in addition to these changes, the aluminium-oxide blasting stage of preparation of the alloy surface contaminates the alloy surface with alumina particles.
Right: Scanning Electron Micrograph showing the treated surface of a metal alloy, prepared for bonding to porcelain. The surface shows deep scoring as a result of blasting with alumina powder, and some particles of alumina remain embedded in the surface (arrow, centre field).
Researchers engaged in this work are Dr. Tony Johnson, Prof. Richard van Noort and Dr Christopher Stokes.