Indirect Opportunities
With the advances in strength and esthetic value in glass ceramics and zirconia, and with the advent of monolithic design applications, all-ceramic restorations have clearly become the primary indicated restorative material.
Clark says, “We find that patients tend to prefer restorations that are tooth colored and that do not have metal substructures, which can potentially show disagreeable dark or opaque margins. There is also the advantage of eliminating alloy, which can potentially cause soft-tissue irritation due to sensitivity to metal.”
This tendency, he explains, is reflected in the decline in requests for metal-based PFMs and the continual increase in all-ceramic restorations that he’s seeing at his company. “Currently our product mix demonstrates this point, as we are roughly averaging a 76% all-ceramic ratio to a 24% conventional crown and bridge.” This mix can be further broken down among monolithic full-contour zirconia (52%), porcelain to zirconia (9%), lithium disilicate (15%), PFMs (21%), and all-metal crowns (3%).
Michael DiTolla, DDS, director of clinical education and research for Glidewell Laboratories, says the most common restoration fabricated in his Newport Beach, California, laboratory is the single-unit posterior crown. “Posterior crowns make up almost about 85% of our restorations, and nearly 75% of those crowns are being fabricated from BruxZir (zirconia oxide), because it is the strongest tooth-colored material that we have ever seen,” he explains. “It’s not as strong as cast gold, but it is stronger than anything else we have, including PFM.” He notes that most dentists opt for solid zirconia even though it is not as esthetic as IPS e.max (lithium disilicate), for example, because they put a premium on longevity and strength when it comes to a single-unit posterior crown since it isn’t readily visible. Nonetheless, he adds, 14% of his company’s dentists are prescribing IPS e.max for posterior crowns, although it is less conservative (requiring more tooth reduction) than zirconia, and it is contraindicated in patients who are bruxers.
For single-tooth restorations, Kois favors monolithic lithium disilicate. “It’s my first choice because it’s a glass ceramic, so it can be used as a bonded ceramic for adhesively retained restorations, and it is strong enough to be used as a single crown for cohesively retained restorations.”
Because of the esthetics of lithium disilicate, Lee Culp, CDT, prefers it when indicated for single restorations on teeth, saying “You can get good esthetics with anything if properly trained, but with lithium disilicate it’s easier, as it is a very translucent material.” However, his main consideration is achieving long-term predictable results, so he selects the most appropriate material, either lithium disilicate or zirconia. “In my laboratory, I am generally 99% all ceramic, meaning lithium disilicate or zirconia. Between them, you can pretty much do anything, and they can be matched to all materials, especially each other,” says Culp.
Sometimes other issues come into play when choosing an indirect material. “When we move into implants, we have to rethink a little bit,” Culp explains. “But as soon as we go into a bridge or full-mouth restorations on implants, the choice is zirconia. If we are going to bridge in the posterior, we immediately lose lithium disilicate as a choice, and now we have to move into the zirconia if we want to keep it all ceramic. Or we have to go back into metal ceramics in order to bridge those.”
In his practice, Kois uses zirconia for smaller fixed partial dentures but he still uses metal substructures for longer-span bridges and implant abutment connections, in nonesthetic applications, or anywhere strength issues are a concern. When there are esthetic and functional risks, “I typically block out the metal with monolithic ceramic materials as a coping cemented to the metal abutment, then fabricate a monolithic porcelain restoration, which is then cemented to that custom abutment.”
There are fewer and fewer PFMs now, Margeas notes, “because they are more expensive, and the esthetics of restorations with new materials are getting just as good and cheaper.” However, when layering is needed, he turns to an old standby. “Even today, my favorite material for porcelain veneers, if done properly, is a feldspathic type porcelain. If you’re bonding to enamel, feldspathic porcelain is still as strong as any of the new materials, because layering weakens monolithic indirect restorations—both zirconia and lithium disilicate.”
And then there’s gold. Glazer says he still uses gold for second or third molars because, “It’s still the finest, longest lasting restoration material we have.” To those who object to the esthetics, he says, “If they’re that close, they’re not looking at your teeth.”
DiTolla agrees, but is increasingly acceding to the requests of his patients—including his own mother—to replace gold with more esthetic materials he considers inferior. “I tried to talk her out of it. I told her this is the best restoration in dentistry, and I’m not taking it out just to put something weaker in. But she wore me down, so I took it out. And her smile changed, so I’m now okay with it, as long as patients know that it won’t be as strong.”
Making the Move to a New Product
Often the primary determining factor in clinicians’ choice of dental material is their familiarity with it—having an understanding of its properties and how it can be used for a given indication in a clinical situation.
Glazer explains, “You need to understand the limitations and the value of a given material. Do the criteria of the patient fit the functionality of the material? Are the benefits valid for the indications of the patient, including the desire for restorations that are esthetic? For example, if the patient has tremendous masticatory force, will the material be able to withstand that, without fracturing or wearing excessively?”
Despite advances in products, dentists may be reluctant to do more than briefly consider those presented as the latest and greatest if they are already satisfied with the performance of a trusted material with which they are familiar and have no problems. “Before I switch to a new product, I need to make sure it’s going to be better than the one I’m using,” Margeas notes. “The only reason would be that it’s easier, faster, and better.”
DiTolla agrees that dentists are not quick to jump ship to a new product; however, when a product is clearly better, it’s a different situation. This, he says, was the case with IPS e.max. “Dentists knew with IPS e.max that this was a different crown. It looked good, it needed less reduction, and dentists fell in love with it.” As for zirconia, he says, “It wasn’t as good-looking as IPS e.max, but it had this unbelievable strength. Patients wouldn’t really object to it, especially when compared to cast gold, even though it really didn’t look like a natural tooth.”
Lithium Disilicate or Zirconia?
Selecting between zirconia and lithium disilicate prompts significant discussion regarding their respective strengths and weaknesses. For example, Clark notes a general trend in dentistry toward the placement of monolithic full-contour zirconia in posterior applications and the use of layered lithium disilicate or porcelain-layered zirconia in the anterior region, where layering and translucency play a significant role. For large-span bridgework (greater than a 3-unit bridge), he says, “We usually see a combination of full-contour zirconia combined with layered zirconia (porcelain to zirconia) as we move into the esthetic anterior portion of the arch.”
“We have to consider the margin preparation and the amount of occlusal reduction that has been provided,” Clark says. “Zirconia is much more forgiving due to its strength and thus requires less tooth reduction (feather edge margins and minimal posterior occlusal reduction of 0.8 mm are acceptable), whereas lithium disilicate requires a chamfer or rounded shoulder margin prep of roughly 1.0 mm and at least 1.5 mm for posterior occlusal reduction.”
In addition, large-span bridgework can now be CAD/CAM milled, Clark explains. “We now have the capability to predictably CAD/CAM mill zirconia up to a 14-unit bridge very precisely, designing the bridge with monolithic full contour in the posterior and then transitioning to layered zirconia in the anterior. We do this routinely with conventional and implant applications.”
These same trends, he says, tend to transfer over to fixed implant applications as well, with the ability to design either cement-retained implant crowns to custom-milled abutments or screw-retained (one-piece) implant restorations.
