May 2008, Volume 4, Issue 5
Published by AEGIS Communications
Question: What is the best way to repair a chipped porcelain restoration?
The ideal restorative procedure is to replace the missing ceramic with composite resin that is similar in color and translucency, which is not difficult. The challenge lies in getting the restorative material to stay attached under functional load and thermal cycling. The success of the procedure depends upon creating a high-energy ceramic reparative surface (the exposed chipped ceramic) and a chemical/mechanical link to the restorative composite resin. This can be accomplished by first isolating the area with a rubber dam and then cleaning/preparing the surface with a micro-etcher (20-µm aluminum oxide under 35 psi). The next step is to etch the ceramic surface with 5% to 9% hydrofluoric acid. Knowing the exact type of ceramic is helpful because different porcelains (feldspathic and pressed) have specific etching times and concentrations. If the specific ceramic is unknown then a general rule of 9% for 60 to 90 seconds should suffice. Then rinse thoroughly with water.
Creating a chemical bonding surface for the restorative composite resin is the next step. This is completed by applying a minimal amount of a pre-hydrolyzed silane. The silane will create chemical receptor sites for the methacrylate present in the composite resin. After the silane application (no more than two coats), the ceramic surface should still have a "frosty" appearance as it did after etching. If the surface is "shiny" then the silane is too thick and should be removed by sandblasting and re-applied in a lesser amount. Warm-air drying of the silane has been shown to increase the bond strength of the composite to the porcelain, as any residual water will decrease the condensation polymerization reaction that chemically bonds the silane to the ceramic. Using a warm-air dryer for 60 seconds removes the remaining water.
The last step is to apply a bonding adhesive which should be light-polymerized before application of the restorative composite resin.
The obvious answer is to repair a chipped veneer with composite. Depending on the size, location, and color of the veneer, this may or may not work—sometimes small incisal chips can be satisfactorily contoured and polished,
Unfortunately, any repair with composite usually ends up being only a short-term fix to the problem. Because composite is not the same material as porcelain, it is hard to get an esthetic blend. Also, bond strength is an issue; to achieve enough micromechanical retention to hold the composite you have to bevel away a lot more porcelain than just the fractured area. Likewise, on a porcelain-fused-to-metal restoration, if the porcelain is broken down to the metal, it is hard to get the proper opaquing and the surface area for bonding is again an issue.
The only predictable method to repair chipped porcelain veneers is to replace the veneer. With an ErCr:YSGG laser, we can often remove the veneer without altering the preparation, so we can order a new veneer to be made on the original laboratory model and appoint the patient, remove the broken veneer with the laser, and cement the replacement. CAD/CAM technology would also allow a one-appointment replacement.
There are a couple of options here, dictated by the severity of the fracture. In a Class 4 fracture or something similar, I would typically lightly roughen up the porcelain with a fine diamond to bevel it. I would typically make the finish line slightly corrugated so that when we blend the composite into the porcelain, if there is a slight difference in color it will tend to blend in better.
Next, I would use an intraoral sandblaster or air abrader to blast it with 50-mm aluminous oxide. That will clean off any loose particles of porcelain, plaque, or any other contamination on the surface. My goal is to very lightly break the glaze of the porcelain slightly beyond the finish line placed on the chipped porcelain area, because if you attempt to attach to glazed porcelain it does not work very well, even with silane.
I use the two-bottle materials, which have been shown to have shelf-life of years. It is not a time-expensive procedure to mix the two components, so it just does not make sense to risk the bond with a material that has no shelf life left.
After the silane is mixed, painted on, and dried, the solvent needs to be evaporated; this step is very important. Then I use an unfilled resin, and because we are attaching to porcelain, which is a very hydrophobic material, we do not want something that has HEMA in it, or any other hydrophilic monomer, so I use a HEMA-free unfilled adhesive. The adhesive is then blown thin and light-cured. The composite is built up directly on top of that, some sculpting is done, and then it is light polymerized in one, two, or three increments.
Here is where we start to come into problems with porcelain. It is pretty clear that the longer it sets, the better the bond. So I try to sculpt it as close as possible, polymerize it, and if I possibly can, not touch it with a bur that day. That is not always realistic, but I do try to have the patient come back for another visit, do the bur work and adjust occlusion and protrusion. At the very least I make sure I wait at least 10 minutes to let the bond mature.
I think where people run into problems is using old silane that has lost its shelf life, using hydrophilic monomers and adhesives that probably will not work on a hydrophobic material like porcelain, and finishing too quickly, disrupting the bond that is not very mature at that point. Those are the three key points in the process.
It is different for a catastrophic failure of the porcelain. In cases of a fixed bridge on top of implants or an old case on top of natural teeth where a patient fractures the porcelain on a tooth down to the metal framework, I would look at prepping the metal and making a crown for it. If it is a single crown that has popped all the way down to the metal or zirconia framework, to me, you might as well just re-prepare the tooth. Clearly there is a problem with the bond, so trying to reattach to that might be a little bit of an exercise in futility.
Gregg A. Helvey, DDS, MAGD
Robert A. Lowe, DDS
Charlotte, North Carolina