The 5 most common mistakes when fixing a porcelain fracture.

by Nelson Gendusa, DDS
Director of Research

C&B-Metabond® can help you offer amazing services. A patient walks in with a horrible ceramic fracture smack in the middle of a 6 unit bridge ... the kind that once would have required removing the prosthesis and sending it to the lab for stripping and refiring. An hour later (before her parking meter has even expired!), she walks out with an excellent esthetic repair.

Of course, to get the most out of 4-META-based adhesives, you have to use them correctly. Based on my conversations with dentists who phone with technical questions, I put together this list of the most common oversights when using adhesives to repair ceramometal fractures.

1.) Failing to identify the underlying cause.
Assuming the lab properly prepared the metal and the ceramists knew what they were doing, your intraoral repair will never recreate the strength of the original ceramometal bond. Today’s super-adhesives can work wonders ... but there still are limits. So if the porcelain fracture was caused by a traumatic habit (chewing ice-cubes, for example) ... occlusal pathology ... or a fundamental structural flaw in bridge design, your repair may be short-lived.

Even though you conscientiously looked for an underlying cause before repairing the restoration, if the fracture recurs ... look harder. Some things to consider ...
• Long spans with pontics that bounce under occlusion
• Thin copings that flex at the margin ... causing fracture at the facial margin.
• Evidence of clenching and bruxing (Worn occlusal surfaces, severe abfraction lesions, TMJ complications.)

2.) Failing to properly prepare the porcelain
Tapering the fractured ceramic surface with a diamond accomplishes several things. It removes loose or unsupported fragments. It increases the surface area available for bonding (the greater the area, the stronger the bond). And it improves esthetics by allowing a gradual transition from the composite shade to the porcelain shade. If you’re not using Etch-Free bonding liner, you must etch the porcelain with hydrofluoric acid (NOT phosphoric), rinse, dry and apply a silane coupler. If you’re using Etch-Free and C&B-Metabond you can skip the acid and silane.




Using a diamond, taper the fractured porcelain. This will remove any weakened ceramic areas, increase the surface area for bonding, and soften the visual line between composite and porcelain.


3.) Failing to air-abrade the metal
Got a call from an irate dentist who had used C&B Metabond to rebuild an old acrylic facing with composite. He swore (and I don’t mean figuratively. He was literally swearing!) that he had done everything according to our directions, yet his composite facing had come off after two weeks. It had happened twice. Even though he seemed absolutely certain he had followed the instructions.

If it LOOKS rough, you’re doing it wrong. In the case on the left, the dentist abraded the metal using a diamond,leaving a surface that appeared rough. In fact, the dull matte surface of the blasted crown on right is 3 times rougher. In adhesive repairs, it's the microabrasion (not the retention beads, or scratches) that makes the difference.


We started from STEP 1.

“How did you roughen the metal?”

“Didn’t roughen it at all. Didn’t have to. The acrylic retention beads were there.”

BINGO!
Over the past few years I’ve talked to a number of dentists who share this doctor’s confusion concerning the nature of micro-roughened metal. Many are using abrasive stones. One thought he was roughening the metal with a Prophy Jet® ... another was using pumice in a prophy cup. If these dentists could have seen a photo-micrograph of the surfaces after they had finished “roughening” them, they would have instantly understood the difference.

A micro-roughened surface doesn’t look rough to the naked eye. It has a dull matte finish. In fact, if you can actually see the roughening ... you probably haven’t prepared the metal very well.

There is only one good way to prepare metal for C&B-Metabond adhesion, and that is to air-abrade the surface. A distant second choice is a diamond bur. But you can expect a more permanent bond and about 3-times the bond strength if you air-abrade. When you use it in the mouth, several seconds of blasting is generally all that’s necessary. Be sure to protect the soft tissue and adjacent teeth with a rubber dam, use HVE suction, and cover the patient’s eyes. Failure to blast exposed metal is probably the single most common mistake in repairing porcelain fractures. And this is probably the second most common error.

4.) Applying the composite before the adhesive has fully set.
Light-activated composites shrink as they cure. As they shrink, they are drawn toward the curing light and away from the metal substrate. If you start curing the overlying composite before your adhesive liner has completely polymerized, the shrinking composite may actually tear the unset adhesive off the metal.

C&B-Metabond takes a long time to polymerize. (In fact, several researchers have speculated that its extraordinary bond strengths are somehow related to its slow polymerization.) The less powder you add when you mix C&B-Metabond, the longer it takes to set. So after applying the adhesive, visit another patient while the adhesive cures. And don’t apply the composite until you can hear an explorer scrape over the surface.

5.) Implying to the patient that the composite repair is permanent.
In many cases you can’t determine what caused the popoff. Furthermore, you often don’t know what type of alloy you’re bonding to. (Remember, adhesive bond strength can vary dramatically depending on the nobility of the metal.)

C&B-Metabond forms strong bonds to a wide range of alloys, so it’s an excellent choice when you’re bonding to a mystery metal. Nevertheless, even C&B-Metabond shows a significantly stronger bond to non-precious alloy than to a high-noble alloy. So if there’s any possibility that the metal is precious, apply MTL-V primer before the C&B-Metabond to improve long-term bond. And it’s always prudent to explain to the patient that adhesive repairs are fast and economical, but they don’t come with a warranty.


Douglas S, et al. Shear bond of adhesives to 3 metals. Jour Dent Res, 75: Spec, Abstr #1250, p174, Mrch 96
Have a technical question best answered by a fellow Dentist? E-mail your questions directly to our Director of Clinical Research, Dr. Nelson Gendusa, DDS. (Click here) for more information.