Inside Dental Technology
Material developers shape the future direction of the industry.
The paradigm shift away from conventional, handcrafted restorations to partially or fully machine-driven restorative solutions has refocused and refueled the efforts of material researchers to find new, innovative materials geared for automated manufacturing processes. Whether reformulating and tweaking existing dental materials, or adapting materials used in other industries to dentistry, dental material manufacturers are on a quest to discover the next generation of machinable, indirect restorative materials that can provide the biocompatibility, strength, esthetics, and wear kindness of natural teeth. Yet the new material must not be so technique-sensitive that dental laboratory technologists encounter difficulties handling and machining them, risking potential failure in the oral environment.
It is a delicate balance that originates in the manufacturer’s research and development (R&D) laboratory, with researchers tinkering with different formulations and combinations and then refining the formulation to meet the criteria necessary for a dental material to survive the rigors of the oral environment. The new material is then strenuously tested based on International Organization for Standardization (ISO) standard testing procedures, and the material’s in vitro strength, material toughness, wear properties, and stability is analyzed using manufactured in-house testing protocols. Often, the new material is sent to dental universities for clinical professors and researchers to evaluate and test, and then is finally sent to carefully chosen dental laboratories and operatories around the country for beta testing before release to the general dental population.
However, few R&D laboratory-testing procedures can precisely replicate what will happen in a patient’s mouth and predict the clinical realities and long-term performance of the material. And for those beta testing the materials, their production protocols are held to strict manufacturer standards that often get thrown out the window once the new material hits the pavement. “Even though the formulation may work well on a small scale in the research laboratory, it may not work well on a large scale,” says Russell Giordano, DDS, assistant professor and director of biomaterials in the Department of Restorative Dentistry/Biomaterials at the Boston University Goldman School of Dental Medicine. “The biggest problem we all face in dentistry is that all of these materials coming out on the market are introduced ahead of rigorous clinical testing.”
It is only when the material reaches the masses of dentists and laboratories and is placed in the mouth, Giordano explains, that the real “clinical testing” of the new materials begins. It is a scenario that has played out each time a new generation of material is introduced to the market, from porcelain-fused-to-metal in the 1950s to pressable materials in the 1980s and zirconia in the 2000s.
The lack of sufficient clinical testing prior to a new material’s market introduction is a cause for concern among dental professionals. “I’ve been in the profession for 30 years and it has not changed,” notes Ed McLaren, DMD, MDC, CDT, who is the director of the UCLA Center for Esthetic Dentistry. “But I don’t see any other way around it. The cost for clinical trials is out of reach for materials manufacturers, so dental practitioners, laboratory technicians, and our patients end up being the clinical trials for these new materials. It’s then we discover problems the hard way if there are any.”
The sheer number of new-generation materials being thrust onto the market in the past few years has exacerbated the situation, he says, making it increasingly difficult for clinicians and laboratory technologists to stay abreast of new material science developments, indications, and handling protocols. Each tweak to an existing material, whether it’s to increase translucency or reduce the crystalline size, changes the way that material is handled in the laboratory and the dental clinic. The ultimate success of these new indirect materials and the definitive protocols for handling them may not be discovered for months or even years as anecdotal information trickles in from the field, long-term clinical studies begin, and focused small-scale clinical trials are conducted by organizations such as The Dental Advisor and large, controlled, practice-based comparative clinical studies are undertaken by organizations like TRAC Research.
In the meantime, another generation of new materials has been introduced by the industry and marketed heavily to the dental profession⎯challenging dental laboratories to meet client demand. Laboratory owners only have a few choices. They must react by finding a source for the new product, or purchase the equipment necessary to manufacture the material and struggle through the learning curve, or cautiously wait before adding that material to their product line. Unfortunately, if they wait until all the clinical and technical kinks have been worked out, they risk not cashing in before the material becomes commoditized. “The new materials coming on the market today are very different than those we’ve worked with for decades,” says Chris Brown, BSSE, owner of Apex Dental Milling. “They require handling protocols that are a paradigm shift for most laboratories. And the speed at which as these new materials are being introduced to the market, accepted by general dentists and widely prescribed, challenges a laboratory’s ability to stay relevant.”
Staying relevant in the industry and making the shift from traditionally crafted, metal-based products to all-ceramic products and moving from hand-layered to machinable monolithic solutions means that laboratory owners must find a business strategy that will keep them competitive. “On the whole, it appears the industry is rapidly changing to all-ceramic and from layered to monolithic whether that material is zirconia, lithium disilicate, or composite resins,” notes Sabiha Bunek, DDS, editor-in-chief of The Dental Advisor. “Monolithic seems to have resolved earlier issues that laboratories and clinicians were experiencing with layered restorations,” adds John Powers, PhD, senior vice president and senior editor of The Dental Advisor.
Debates from the Field
Staying ahead in the materials game requires that laboratory owners keep abreast of reports in the clinical literature, study the results of clinical trials, and listen to the findings of key opinion leaders in the industry, McLaren says. Even though some of these materials have been on the market for several years, there are still issues surrounding them that are being studied and uncovered.
One of the discussions among clinical experts currently is the generally accepted practice of conventionally cementing unsupported glass-ceramic restorations such as IPS e.max. “I don’t necessarily agree that you should use conventional glass-ionomer cements,” McLaren emphasizes. “The problem is that they are much more soluble than resin-based cements. If, over time, the cement solubilizes and fails, exposing the glass ceramic to saliva, the restoration can undergo hydrolytic degradation which weakens the material.”
McLaren recommends dentists bond full-contour, glass-ceramic crowns. Not only does it prevent the potential degradation of the cement and weakening of the glass ceramic, but it also allows clinicians to preserve tooth structure with a conservative preparation.
Bunek recommends bonding for zirconia-based restorations where the preparation is non-retentive. “The big controversy among opinion leaders is whether you need to use a zirconia primer every time,” Powers adds. “What we have found is that if the preparation is retentive, then using a traditional cementation technique is fine. Otherwise, you definitely need to prime and bond the restoration.”
Giordano says new research is showing that bonding zirconia to natural tooth structure almost doubles the load and increases the resistance to failure, though he admits no one is quite certain why this is happening. If the dentist is using glass-ionomer cement, then Giordano suggests that laboratories sandblast the interior of the restoration with a small-particle size alumina at low pressure to reduce the risk of the zirconia separating from the tooth. “Even though there are some studies that suggest sandblasting may degrade the strength of the zirconia, it does not degrade the bond strength. There is absolutely no bond between the cement and the zirconia unless there is some mechanical retention.”
Another major area of some concern among key opinion leaders and one that has not been fully studied is the potential for full-contour zirconia restorations to wear opposing dentition. John Burgess, DDS, assistant dean for clinical research at the University of Alabama, has completed a laboratory test of three commercially available zirconia materials from Glidewell, 3M ESPE, and Ivoclar Vivadent for wear against human enamel using standard wear machines. “What we found after 200,000 and 400,000 cycles in our wear machines (7 to 9 years of clinical wear) is that highly polished zirconia produces the least enamel wear; in fact, significantly less than some commonly used feldspathic porcelains,” Burgess explains. “Polished-then-glazed zirconia wears significantly more than polished-only zirconia. It seems as though the glazing material produces most of the enamel wear as far as enamel wear against zirconia is concerned, with even polished-then-glazed zirconia exhibiting aggressive enamel wear.” In general, the roughness of the zirconia surface influences wear with a rougher surface producing increased enamel wear (See chart on Zirconia Versus Tooth Enamel).
A recent status report on a practice-based controlled clinical study at one year shows that all materials and their opposing dentition exhibit small wear facets less than or equal to 2-mm areas on some but not all patients' restorations. However, statistically, wear facets opposing glazed BruxZir crowns were more numerous and larger at this point in time. It is not known whether this trend will persist until next year at this time when the 2-year recall is completed and the statistical analyses are compiled. This study performed by TRAC Research—under the leadership of Rella Christensen PhD, co-founder, and director for 27 years of the former Clinical Research Associates (CRA) and currently lead scientist at TRAC Research—includes 22 dentists from 13 states who are experienced CEREC© users and instructors. Each dentist placed posterior full-crown restorations on molar teeth. The restorations were fabricated from BruxZir full-contour zirconia, IPS e.maxCAD full-contour milled lithium disilicate, and PressCeram veneer ceramic pressed over milled zirconia substructures (study control). Both the crowns and their opposing dentition are being monitored carefully using scanning electron microscope images of all the test crowns and their opposing dentition. In this study, a wide variety of materials are represented in the opposing dentitions, such as enamel, silver amalgam, composite resin, gold alloy castings, and a variety of ceramic materials. “So far, at one year, wear patterns of the monolithic materials have mimicked natural dentitions. This means they create small wear facets on the full variety of opposing dentition materials and enamel, and opposing dentition of all types creates small wear facets on the zirconia and lithium disilicate. These patterns are similar to opposing arches composed of enamel only, which routinely produce small facets on each other. But in the BruxZir cases, it appears the wear facets are more numerous and larger,” Christensen observes.
In this study, the full-contour zirconia restorations were all lightly polished and glazed by Glidewell Laboratories. Christensen cautions that, “Not all zirconia is the same. The results could differ with zirconia from different sources. We need to specify the brands and sources that we study and use because zirconia from different sources can be very different. For example, different particle sizes can change the clinical performance of the zirconia and how it wears opposing dentition. For the specific materials and brands we are studying, the results at one year are very promising, particularly in light of the typical problems we have seen with the veneer ceramic applied over the zirconia substructure in our study control material.” (See sidebar: “One-Year Controlled Clinical Study Status Report: BruxZir and IPS e.maxCAD.”)
On the laboratory side, Brown, who operates a milling center in Ann Arbor, Michigan, is seeing an interesting phenomenon in terms of fractures in milled, full-contour zirconia bridges⎯an issue that he says has also been reported by others. “While this phenomenon is not common, it is surprising such a strong material would have this issue at all,” he notes. “We have a good feel for what to expect with zirconia as a substructure material. Additional research may be needed for using this material in some of the more complex multi-unit, full-contour cases.”
On the Horizon
McLaren believes that the rapid shift to the monolithic approach for posterior restorations will overtake production for the bulk of the single-unit crowns prescribed in the United States. “All of the R&D money is being spent on creating higher- or multi-translucent millable materials and multi-layered, multi-chroma blocks that can handle the 70% to 90% of the work out there,” he explains. “I think the industry will eventually move to milling 99% of full-contour restorations.” However, he notes that until the cost of the equipment and milling materials comes down in price, pressing full-contour restorations will continue as a strong option.
Some manufacturers recently introduced monobloc-millable and pressable full-contour materials for the anterior. Vident introduced the first multi-shaded milling block last year, the VITABLOCS RealLife, which is a feldspar ceramic, enamel-layered-over-dentin design block for milling monolithic anterior restorations. Glidewell and others have tweaked zirconia block formulations to produce a more translucent solution for the anterior, and later this spring Ivoclar Vivadent will be launching its IPS e.max Multi, a 400-MPa multi-shaded pressable lithium-disilicate press ingot for fabricating monolithic anterior and posterior crowns as well as veneers.
The push to develop monolithic-millable materials for the anterior has spurred development of more translucent zirconia milling materials in shaded gradations and different opacities to aid in the esthetics of the final product. Tosoh, the leading zirconia powder manufacturer headquartered in Japan, created a new zirconia powder exclusively geared for the dental market to meet the demand for a more translucent framework material and, more recently, for full-contour restorations. Trademarked Zpex, the material is the company’s highest translucent powder for the dental industry and is being used by milling block manufacturers to create zirconia-milling pucks that, once milled and sintered, result in visually more translucent frameworks. Jay Thomas, Tosoh product manager, explains that the higher translucency is achieved by adjusting some compositional items as well as certain powder physical characteristics.
“In a new product introduction like Zpex, Tosoh addresses translucency improvement, high bending strength, and hydrothermal aging properties when sintered properly,” he says. Tosoh also makes a base yellow zirconia powder that is designed to mix with the company’s equivalent white standard grades to achieve shaded gradations of zirconia. “Our goal is to one day be able to offer dental manufacturers zirconia powders that match all the shades in the VITA shade guide,” he elaborates.
Glidewell, recipient of Tosoh’s 2010 Best Product Innovation Award in the Advanced Ceramic category, has increased the translucency of its BruxZir zirconia milling material by chemically and mechanically reprocessing the base zirconia powder. “By breaking the powder particles down to a 15-nanometer size and using a colloidal process, we can produce high-translucency, high-density milling blocks that show no loss in strength,” explains Robin Carden, vice president for research and development at Glidewell. “The higher translucency also allows BruxZir to be used in the anterior.”
Other manufacturers such as 3M ESPE, Zahn, DENTSPLY, Sirona, Wieland, and Sagemax have also joined the ranks of manufacturers offering a higher translucency of zirconia milling materials.
But not all research dollars are being relegated to developing new monolithic glass-ceramic and higher-translucency zirconia materials. According to Giordano, composite resins with fine nanoparticle sizes also show promise, and may overtake ceramics at some point in time because of their energy-absorption properties. However, manufacturers have to find a way to increase the stiffness value or elastic modulus of the material and get it closer to that of ceramics. Otherwise, the material can actually move under stress and break the bond between the crown and tooth structure. There are also problems with wear of the material itself. The other issue that needs to be overcome, he believes, is the ability to color the composite once milled. “3M ESPE has been putting a lot of energy into creating composite materials that have very fine nanoparticle sizes which resulted in their Lava™ Ultimate milling block,” Giordano says. “It possesses higher strength properties, better wear, and is more color stable. The dentist can also repair it in the mouth using a direct composite material.”
Lee Culp, chief technology officer at DTI (Dental Technologies Inc.), is testing new composite materials being developed for the market that are in millable-puck format rather than single blocks, which is what he needs for his high-volume operation. “We are 85% digital in everything we do here,” Culp declares. “So if we can find a millable composite puck, then we plan to transition all our hand-layered composite products to digital processing.”
He is also testing a high-strength biomedical fiberglass material in machinable-puck format for milling implant overdenture bars. If this material works, Culp says it would dramatically reduce the cost of manufacturing hybrid restorations and not require the use of specialized milling equipment. This would allow laboratories to mill in-house rather than outsource. To automate DTI’s denture department, he is working with a biomedical plastics company outside the industry to produce a machinable acetyl-resin puck for manufacturing partial denture frameworks. “The most exciting aspect of producing dentures digitally is that by using industrially processed materials, you eliminate the porosity and residual monomers present in conventional hand-processed materials.”
New pressable products and indications are also on the horizon. Glidewell’s ceramics research facility, BioCeramics, which is located in Fort Lauderdale, Florida, has developed a new lithium-silicate material the company calls Obsidian. Carden explains that it is a single-silicate, 360-MPa compound that currently can be pressed over metal and will be developed into an optional millable ceramic for the anterior. Although Glidewell is the largest user of IPS e.max worldwide, Carden says that the company continues to press a majority of the restorations produced rather than mill them. “We can only get one crown out of a 20-millimeter milling block but get two to three crowns out of a small pressing pellet. By pressing the full-contour crowns rather than milling them, we increased our yields and eliminated the labor involved in touching up the margin areas which can chip during the milling process.”
Where Carden sees the real advantage for Obsidian is in the posterior region for bridges. The press-over-metal technique is less labor- and production-intensive than current protocols for milled materials that require machining both a zirconia framework and a ceramic crown that are then bonded together after the framework is sintered. “Obsidian brings to the laboratory the ability to integrate the material into existing pressing processes without any capital investment in equipment,” Carden says.
Pressable ceramics are also revolutionizing the custom implant-abutment market. To reduce the cost of producing custom implant abutments, Ivoclar Vivadent recently announced a new pressable indication for the IPS e.max Press material. The IPS e.max Press Abutment Solution will allow technicians to fabricate hybrid patient-specific implant abutments that are bonded to a titanium base in-house without the need to outsource production. According to the company, this new material indication will allow laboratories to manufacture premium custom implant abutments at a very favorable cost. (For more on Glidewell’s Obsidian and Ivoclar’s Abutment Solution and Multi, see the sidebar, “Here or Soon to Come.”)
Material developments will continue to evolve at lightning speed. Watch for a new millable metal in the soft “green” state that can be machined quickly and then fired to achieve final hardness. “Net shape” milling blocks that are manufactured closer to the shape of the final product may be coming soon to help reduce milling time and material waste. A self-polishing zirconia with a grain size and surface energy that, if adjusted in the mouth, will re-polish itself naturally is also on the horizon. As the level of innovation in material development continues, the quest to develop next-generation materials that outperform their predecessors can only benefit the profession and, ultimately, the patient.