Porcelain Bonding
Please find Part 1 by clicking here and learn more about the basics
of bonding as used in cosmetic dentistry and explained
in the 'Procedures' section.
If you want to learn what happens with the tooth during the preparation steps for dental bonding, go
here. Reading all these references will give you a more complete picture about
the bonding process for porcelain veneers to tooth structure.
Understanding these principles will help you understand what a cosmetic dentist does when he designs a smile that lasts for many years.
To bond a porcelain veneer to tooth structure, a variety of preparation techniques are being used in succession.
This includes the surface treatment with acids, particle abrasion, various adhesives, and chemical couplers such as silane.
Hydrofluoric acid is commonly used at concentrations of 4 to 10% when etching the inside (intaglio) surfaces of porcelain veneers.
Etching time have a great impact on the shear bond strength of porcelain restorations. Dental porcelain consists of an amorphous
glassy phase or a matrix and a crystalline phase. HF selectively dissolves the glassy matrix, resulting in a microscopically porous,
high-energy, and micro-retentive surface.
While all feldspathic porcelains contain silica dioxide, aluminum oxide, and potassium oxide, the relative amount of each can vary
considerably depending on the their ratios and other components. Alumina content, crystalline microstructure, size and the concentration
have an influence on the etching time. Many dental porcelains contain leucite crystals, which affect optical,, thermal, and physical properties.
The producer of IPS Empress, which contains a concentration of leucite of 35 to 40%, recommends an etching time 60 seconds with 5% HF.
Longer etching times or higher concentrations of HF with IPS Empress may actually result in over-etching, which weakens and
degrades the porcelain surface. On the other hand, the manufacturer of Authentic recommends to lightly sandblast the internal surface
of porcelain restorations with aluminum oxide particles of 50 microns and to leave 5 to 12% HF for 90 seconds to five minutes.
The two lithium disilicate-based ceramics have entirely different specifications: that should be etched for only 20 seconds with 5% HF.
The etching times for veneers that consist of powder/liquid or ?stackable? porcelains is recommended to be between 90 and
150 seconds with 9.5% HF.
Prior to the treatment of the porcelain surface with the silane coupling agent, the etched inside surface of the porcelain
has to be evaluated for white residues that are signs of over-etching the porcelain. These residues, which consist of porcelain salts
and crystalline debris should be considered contaminants and be removed. This is best done with a rinse with ethanol and ultrasonication
for several minutes. The idea is to have a clean and chemically reactive porcelain surface for the next step ? the application of silane.
Silanes are a class of organic molecules that contain one or more silicon atoms. The one silane that is used for the surface treatment
of porcelain veneers and restorations is 3-methacryl-oxypropyl-trimethoxy-silane. It is a long molecule that has a methacrylate group
at one end, which can co-polymerize with adhesives and dental resins. At the other end, it has the silicon group, which forms chemical
bonds with the porcelain surface. It is therefore a valuable coupling agent in dentistry that links resin-based with inorganic materials,
such as porcelains.
What happens when silane is used as a coupling agent?
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Silane must be hydrolyzed to be able to interact chemically with the surfaces of porcelain veneers. Acetic acid is commonly used to
hydrolyze silane by reacting with the three methoxy groups (located at the bottom of the molecules on this image).
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The hydroxyl groups at the lower end of the silane molecule can now react directly with the corresponding hydroxyl groups
at the surface o fthe porcelain. They first form a hydrogen bonding, then water dissepates and a condensation polymerization takes place.
Covalent bonds are formed (see image below).
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Here the silane molecules have covalently bonded to the porcelain and the adjacent silane molecules. Therefore, a polymer network ensues.
At the top of the silane molecules are methacrylate groups that react with methacrylate groups of adhesive resins and composite materials.
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Do you want a "bonding" experience?
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Click
here to see the entire bonding experience in sequence.
(This
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