Selecting the Right Restorative Materials
Practical tips for choosing and using today’s advanced restorative products
With so many advanced new materials on the market for both direct and indirect restorations, the evaluation and selection process has become a practical challenge. For restorative dentists, the days of relatively easy drilling and filling posterior restorations with amalgam are largely over, mainly due to patient demand for tooth-colored composite resin fillings for esthetic reasons, as well as the trend toward conservative dentistry. Yet many dentists have found these composite materials time and technique sensitive, with varying handling properties, and numerous steps—including layering and light curing.
When it comes to indirect restorations fabricated chairside or at the laboratories, material selection isn’t exactly easy. While many dentists would gladly stick with gold for posterior crowns, that time-tested material is not only expensive, but patients balk at its esthetics, preferring crowns made of new ceramic materials. Dentists are also being lured away from the porcelain-fused-to-metal (PFM) standard for multi-unit restorations by instead using milled monolithic ceramics—which can be cut back and layered in the anterior—for single crowns, bridges, and dentures, both traditional and implant-supported.
To help clear up the confusion and simplify selection, Inside Dentistry asked practitioners and other insiders to share their personal knowledge and preferences regarding the newer direct and indirect restorative materials, their clinical indications, and the most successful techniques.
Weighing the Options
As Sridhar Janyavula, BDS, MS, sees it, the choice of material boils down to both patient-related and tooth-related factors. A clinical research dentist at DENTSPLY Caulk, Janyavula considers patient history (eg, allergic reactions, dietary habits), treatment history, caries risk, prognosis of the restoration, gingival and surrounding tooth health, and ability to isolate. He also takes into account issues related to the tooth itself, its location, and the cause of the problem.
“You also have to consider occlusion, whether the patient has any parafunctional biting habit,” Janyavula explains. “Then too, shade is a factor. Most people want to have white teeth, but over time, teeth do get darker, so we must come to clear idea of the shade that will fit well.”
Similarly, John C. Kois, DMD, MSD, founder and director of the Kois Center in Seattle, Washington, identifies four main issues that determine the material to be used for a restoration. Patient diagnosis/risk assessment is most important and is described in more detail below. The other three factors include evaluation of the patient’s remaining tooth structure; patient-related issues such as finances, time, and insurance coverage; and the properties of the materials themselves.
The patient diagnosis/risk assessment enables the clinician to directly address the underlying issues and/or patient objective. It involves determining to what extent the following factors must be accommodated:
· Biofilm-mediated—ie, caries or periodontal disease
· Environmental/chemical-mediated—ie, acid erosion due to either extrinsic or intrinsic causes—or abrasion due to a foreign substance
· Occlusion, which creates loss of tooth structure due to friction
· Developmental disturbance of the tooth
· Esthetics
An evaluation of these issues, Kois explains, determines whether the approach will be direct or indirect and what materials will be used. “In my practice, I tend to use direct composites when more conservative options are warranted. However, I move toward indirect restorations when there’s more extensive loss of tooth structure or when cuspal protection is much more critical.”
However, factors can sometimes be at odds with one another, such as function versus esthetics, notes R. Scott Clark, president of Dental Arts Laboratories. The primary issues in choosing a material are usually determined by evaluating function versus esthetic risk with each individual case. In addition, clinicians need to factor in the limitation of the product span—ie, single versus 3-unit versus long-span bridgework—and preparation requirements.
The complexity of material choice doesn’t end there. Jon Fundingsland, professional relations manager of 3M ESPE Dental, notes, “Choice of direct versus indirect techniques depends on the extent of the repair needed—can the remaining dentition be supported by bonding or is an indirect restoration needed? Is the tooth cracked? In some restorations, either direct or indirect techniques could be employed. Then the choice is up to the clinician.”
The Direct Approach
When considering the alternatives for direct restorations, Fundingsland explains that handling preferences and shade and opacity options have driven clinicians towards using a universal restorative for anterior restorations, saving a dedicated bulk-fill material that can be placed in 4 mm to 5 mm increments for posterior locations. “Nano-filled composites maintain the strength and wear properties of the hybrid category—which has weak long-term polish retention—but are able, due to nanotechnology, to display considerably improved retention of initial polish. This category of restorative was quickly adopted by the marketplace, and today is the most popular restorative.”
“Universals are the hottest adhesives in dentistry now that nearly all manufacturers incorporated 10-methacryloyloxydecyl dihydrogen phosphate (MDP); that ingredient was the holy grail of adhesion,” observes Robert C. Margeas, DDS, who is adjunct professor, department of operative dentistry at the University of Iowa College of Dentistry and maintains a private practice in Des Moines, Iowa.
Janyavula also admits to being a fan of universal bonding agents, because they enable the dentist to use a single product for a variety of clinical situations. “I can choose what technique to use for the same product, which depends on the patient, restoration, and clinical situation.” He especially prefers using a bulk-fill flowable to layering smaller amounts of material, which he says have a greater tendency to create voids. “The advantage of flowable composites is they adapt well and self-level; the dentist just squirts it in and it flows into the nooks and crannies into the deepest part of the proximal box,” he says. “I like these not just because they save time, but I feel confident that it has adapted well to the cavity.”
Kois uses both both hybrids and bulk-fill composites, typically using the latter for posterior teeth. “The manufacturers have expedited the outcome without any significant clinical compromises, and I haven’t experienced any problems once I understood their handling characteristics,” he explains.
For his practice, Howard Glazer, DDS, FAGD, of Fort Lee, New Jersey, favors flowable restoratives, particularly with “little moving targets” such as children, when “you want to get in and out as quickly as possible.” However, a caveat with bulk fill, he says, is making sure the curing light is physically able to cure to the full depth of the cavity preparation, beyond the highest cusp, which otherwise will prevent the light from penetrating beyond that point. Glazer, who recalls the silver alloy days when light curing and creating a tight contact were not issues, says verifying the amount of material inserted can be a problem when it is just squirted into the cavity. “Without measuring whether it is in fact 4 mm, a doctor could squirt in 5 mm or 6 mm of material, which could lead to problems when photocuring.”
As for cement, Margeas says, “If you asked me if I could only use one for everything, it would be resin-modified glass ionomer. You place it in the crown, have the patient bite, and 2 minutes later you clean it off.”
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.
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.”
Restoration Failures
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.”
Conclusion
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.
References
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.”
