Guidelines regarding bonding or cementing zirconia and lithium disilicate can be found from a variety of sources.1,2 “We refer to Dr. Christensen's cement protocol. If it is a retentive tooth preparation (about 4 mm of axial wall length from gingival margin to tooth preparation occlusal surface), you can choose a conventional resin-modified glass ionomer. Restorations with questionable retentive tooth preparations (crowns less than 4 mm of axial wall length from gingival margin to occlusal surface of the prep or onlays and inlays; highly tapered) should be bonded with a resin cement,” says Clark.
DiTolla says, “For an IPS e.max crown on a posterior molar, we recommend 1.5 mm of occlusal reduction on the tooth to end up with a crown that is thick enough to withstand the biting forces. If you compare that to solid zirconia, we only need a reduction on 0.6 mm in the posterior; we prefer 1 mm, but we can work with as little as 0.6. That’s less tooth structure that is being removed than you need for an IPS e.max crown, so you end up being able to be a little more conservative with a solid zirconia crown.”
However, he adds, for anterior teeth, it’s different, so 44% of the anterior crowns fabricated at Glidewell have been IPS e.max crowns. “Because the forces are about half as strong on anterior teeth, it is not as critical that you have the strongest material available. In fact, we do not see a high instance of IPS e.max crowns fracturing in the anterior regardless whether they are cemented or bonded into place.”
Many experts emphasize the importance of understanding a material’s strengths and weaknesses by keeping up-to-date on material testing and literature. Although this can be very informative, Kois notes a concern about the focus on evidence-based data.
“Evidence-based data can be very misleading because it often cites the failure, but not the reason for the failure, and then the failure may be inadvertently blamed on the material itself,” he explains. “For example, if a restoration needs to be replaced due to recurrent caries because the decay is apical to the margin, the reason is caries susceptibility, not the restoration. However, if the decay is inside the restoration, the reason is cement fatigue and washout of the luting agent, which may be due to the flexure of the restoration relative to the tooth. This reason may be more related to occlusal management of the patient, or the fatigue characteristics of the luting agent and not the material at all.”
According to Kois, direct composites for conservative treatment have demonstrated a very high level of success, but, he continues, “When treating more advanced disease in high-risk people, you’ll have more failures. This last point is important because many dentists are misled again by the reasons for the failure. The evidence is hard to understand if you don’t know what you’re looking for. The most significant concern is that the physical properties and composition of materials are not necessarily good predictors of clinical performance.”
Clinicians can make the best decisions by carefully assessing and evaluating the available evidence base for any material they consider using.
Bioactives: Materials of the Future?
“Recently, interest has increased in incorporating various calcium compounds (silicates, phosphates, aluminates) to help rebuild damaged dental tissues,” Fundingsland notes. “This is an interesting approach that certainly deserves more clinical study.”
Kois and Janyavula have their reservations. Janyavula says at this time it’s “too soon to tell” if bioactives will fulfill the hopes of many, but he considers it “a step in the right direction.” Kois points out, “Many restorative materials require protocols that these newer bioactive materials will not meet. The clinician must also be aware that what may be best for the tooth may not be what is best for the restoration—especially for luting agents. The practitioner must consider the factors involving the interfaces that are being managed.”
On the other hand, Glazer has no doubt that bioactive materials are the future. “First there were glass ionomers and other restoratives that released fluoride—which was wonderful. New bioactives not only release fluoride, they release calcium and phosphate, which are necessary not only to restore but also to repair the tooth, replacing lost elements and helping to stimulate dentin growth.”
The development of dental restoration products has not reached its peak, and a discussion of best practices and indications will continue as the industry adapts to the next levels of material technology. For clinicians, this will mean having a go-to approach for evaluating products, both new and time-tested. Understanding the advantages and limitations of each class of materials can, and perhaps should, start with the literature and evidence base, as long as one critically considers reported outcomes. Manufacturer information, the experiences of colleagues, and the assessment of thought leaders also play important roles in the decision-making process of many practitioners. Armed with the best information possible about the properties and use of a given material, clinicians can feel confident selecting it for appropriate cases to reach the ultimate goals—optimal clinical outcomes and satisfied patients.
1. Christensen GJ. All-ceramic restorations: simplify your technique. Clinicians Report. 2012;5(7).
2. Tysowsky GW. Guidelines for predictably preparing and cementing all-ceramic restorations. Continuum. 2010;23(3):3-7.
Ceramic Restoration Removal
The good and bad news about bonded zirconia and lithium disilicate restorations is that they don’t come off easily. This presents a significant challenge to dentists when it is necessary to remove them.
There are crucial differences between these “sister materials” when it comes to cleaning before final seating and bonding, as well as ultimate removal. As DiTolla puts it, “Cement if you can, bond if you must.” Margeas also prefers cement: “In 25 years, I’ve never had a porcelain veneer bonded to enamel come off. Some have chipped, stained, or fractured—but they never debond.”
Both of these materials are hard to cut through, even with the correct burs, explains DiTolla. “If I am cutting a zirconia or IPS e.max crown that has been cemented, usually I will cut up the buccal surface, the occlusal surface, and then go to a crown remover. Then I can split the crown in half, so the half comes off and the other half stays on. Then I cut that half in half, put a crown remover between it and twist it. If the crown was cemented, you are typically done at this point.”
Bonding is more difficult. According to Margeas, if you have a bonded restoration, you have just one choice: “You have to grind until you reach tooth structure because you cannot separate or pry it off.” This, he adds, is not a reason to avoid bonding restorations, but “you’d better make sure they’re done well.”
Another challenge is the negative impact of saliva contamination on the restoration, typically during try-in. “When a zirconia crown becomes contaminated with saliva, it is almost impossible to bond or cement.” Therefore, says DiTolla, the dentist should always assume a zirconia crown is contaminated every time it goes into the mouth, and therefore take measures to remove the contamination either by sandblasting the zirconia crown for 15 seconds or using a special solvent as directed, followed by a zirconia primer.
Other ceramic crowns—including IPS e.max—can be cleaned with phosphoric acid, which DiTolla warns is even worse than saliva for zirconia. “What gets confusing for dentists is that if you take that same phosphoric acid that you use to clean out your IPS e.max crown inside a zirconia crown, nothing is going to bond to that crown.”
According to DiTolla, sandblasting won’t harm the zirconia—Glidewell sandblasts all zirconia crowns to clean them up. But it cannot be used on lithium disilicate, because it will weaken the crown.
“Here we have these two materials that are almost like sister products. One is a little weaker but prettier, and it turns out you have to approach them totally differently for how you have to treat treat the inside of the crown.”