The Race for the Crown

To better understand the question of "when," it may be helpful to explore the question of "why." More specifically, why are so many resources being poured into the research and development of a new fabrication technology when milling machines can produce beautiful, long-lasting crowns relatively efficiently? In short, 3D printing has the potential to produce restorations with even better esthetics and strength at even lower costs when compared with milling. "Dental laboratories around the world have been saturated with milling machines for several years," says William Song, chief marketing officer for Korean manufacturer AON. "Many notice that there are some limitations regarding geometries, the high maintenance costs necessary for the machines, and other factors that make 3D printing more appealing."

Esthetically, milling is limited because the internal color and translucency gradients that are built into discs and blocks are not case specific. Nesting becomes critical, and staining and glazing is often necessary, which poses its own challenges if the dentist needs to adjust the crown. Alternatively, what if a crown could be built from scratch with the color gradient customized for each case? Multi-material 3D printing could make that possible, and several companies already offer that capability; however, it is only for polymeric materials. "A natural tooth has a dentin core surrounded by enamel," says Daniel Bomze, director of medical solutions for Austrian manufacturer Lithoz. "We can reproduce this with multi-material 3D printing. We can start with a darker, more red/orange ceramic material, perhaps more opaque in the center, to mimic dentin. Then we can have a lighter, blueish shell to mimic the enamel. This is a game changer."

Regarding precision, 3D printers can produce certain geometries that milling machines simply cannot. "3D printing offers optimal mechanical adaptation between the tooth and the intaglio surface of the restoration, whereas milling does not always provide the necessary tool compensation," says Georgio Haddad, dental product manager for Massachusetts-based manufacturer Formlabs.

In addition, restorations can be printed as thin as 0.1 mm to 0.2 mm. "Printing veneers that are 0.2 mm or thinner allows the dentist to utilize minimal tooth preparation, and that is very difficult to accomplish with milling," says Maurizio Costabeber, CTO of Italian manufacturer DWS.

Although quality needs to remain the priority, the cost savings associated with 3D printing is significant. Unlike milling, there is no disposal of valuable scraps of material. Bomze estimates that 80% to 90% of printing materials go into the products being printed. "The cost per part is ridiculously attractive," Haddad says. "Regarding the resin consumed, the cost can be as low as $2 to $3 per printed crown." The hardware itself is largely affordable as well. There are several dental 3D printers available for less than $15,000, whereas the lowest-cost milling machines are approximately $30,000.^2,3

Efficiency is another important advantage. Unlike a milling machine, a 3D printer can manufacture dozens of crowns in one cycle, limited only by the size of the build plate. "You can print, for example, 100 veneers simultaneously, so for dental laboratories with high throughput, the overall processing time and manual work necessary are dramatically reduced," Bomze says.

Clearly, the potential benefits of 3D printing permanent restorations are significant. "We are at an inflection point, and this will be the next big thing," says Minh Tran, a dental technician at Essex Dental Laboratory in Windsor, Ontario, who beta tests various materials for manufacturers. "Post-processing has been a challenge because solvents tend to rip polymer chains apart and weaken the materials, but as formulations and protocols are developed that can maintain integrity and high strength over long periods of time, these materials will become widely used."

Which specific materials will provide practices and laboratories with all of these benefits? In recent years, zirconia and lithium disilicate have emerged as the dominant materials for indirect restorations,^4,5 and because dental professionals already have a certain level of trust and familiarity with them, manufacturers have fervently pursued printable versions of these ceramics. However, a new class of 3D printed composite resin materials is showing great promise. "The market seems to be divided into two parts: one very conservative, very large segment that is still demanding zirconia and one that is keen to use these new composites," Costabeber says.

Of course, with new fabrication methods come opportunities for new chemistries that can provide comparable characteristics in terms of strength and esthetics. "In these early stages of developing materials for 3D printed permanent crown and bridge restorations, the desired properties are based on materials that we know well and trust," says Jamie Stover, CDT, senior manager of dental laboratory applications for California-based manufacturer Carbon. "It is difficult to say what the most successful materials of the future will be for these indications, but it is possible that they will be polymer-based hybrid ceramic composites."

The market for 3D printed permanent restorations, even composite ones, has taken more time to develop than the market for other indications, in part because of the functional and esthetic demands for permanent restorations, but also because the US Food and Drug Administration (FDA) has more stringent requirements for Class II medical devices than for other 3D printed appliances. "The FDA does not mess around with products that are going in the mouth long term," says Walter Renne, DMD, vice president of clinical strategy for California-based manufacturer Desktop Health. "The whole system needs to be properly validated."

Maintaining a high level of precision after the necessary post-processing involved in 3D printing is a challenge that is more pronounced for zirconia but exists for composites as well. "What the industry has seen up to this point with 3D printed zirconia restorations is that achieving a highly accurate fit is challenging due to the shrinkage of the material," Stover says.

Despite the hurdles to approval, multiple composite materials for permanent restorations are now available on the US market, including VarseoSmile Crown plus (SprintRay), Permanent Crown (Formlabs), Flexcera^™ Smile Ultra+ (Desktop Health), and CROWNTEC (NextDent by 3D Systems). In addition, Irix^® Max and Irix^® Plus (DWS) are CE-certified in Europe and in the process of obtaining FDA 510(k) clearance in the United States, and FREEPRINT^® crown (DETAX) is expected to be cleared in both the United States and Europe later this year.

On June 29, 2020, VarseoSmile Crown plus, which was originally manufactured by German company BEGO, became the first of these materials to achieve FDA 510(k) clearance. Shortly thereafter, BEGO partnered with Formlabs to offer the material as part of the latter's workflow under the name Permanent Crown. "The 3D printing of permanent restorations has always been considered the end game of 3D printing," Haddad says. "It was what everyone was striving for, so being able to offer a solution to the market was a major breakthrough."

Haddad notes that although Permanent Crown should not be directly compared with zirconia or lithium disilicate, it had to meet the same requirements that any other material does to be indicated for permanent restorations. The testing included 2.4 million mechanical cycles and 12,000 thermocycles, simulating at least 10 years of use inside the mouth. In addition, abrasion, surface roughness, long-term cementation stability, and many other factors were also tested. "Achieving the necessary mechanical properties was a given, but the esthetic perception of the material proved to be more challenging," says Elisa Praderi, DDS, Formlabs' senior clinical protocols & KOL manager. "People frequently asked how it compared with other materials and how it functioned in the posterior and anterior regions, and we explained how the color, modulation, or optical perception of the material worked. This is a monolithic material because it is printed as a whole and does not include layering. Although it will not permit the passage of light, we can manipulate the design and minimum thicknesses to modulate that. What is nice is that you can add characterization with veneering composites to enhance the esthetics. In the posterior region, the mechanical properties are exceptional, and in the anterior region, we have seen preliminary results of users combining techniques to achieve very nice clinical outcomes in critical esthetic areas." Haddad adds that Permanent Crown is softer on opposing natural dentition than many other restorative materials and that the cost is roughly 10 times less per unit than glass-ceramics.

According to Renne, Flexcera Smile Ultra+ exhibits many of these same characteristics but is unique in that it is a proprietary oligomer—a polymer with long chain chemistry. "Most resins on the market are methacrylate-based esters that have been around in different forms since the 1950s," he says. "However, we went back to the drawing board to develop a new oligomer that is more resistant to moisture with high work fracture, and we loaded it with ceramic nanoparticles to impart beautiful esthetics and color stability. We are confident that it will not become brittle over time in the mouth." Renne notes that the esthetics of Flexcera Smile Ultra+ are on par with a high-quality lithium disilicate. "It is a beautiful material with a refractive index very similar to natural tooth structure," he says. "It is still a polymer, so it is not replacing glass-ceramics, but I utilize it frequently for permanent veneers in situations in which I otherwise would have used direct resin. I can print it to be 0.2-mm to 0.3-mm thick, which allows me to be very conservative and provide excellent esthetics with a precise digital design. I can also print a set of 10 veneers in only 15 minutes—a process that would take hours with subtractive manufacturing." After printing, Flexcera Smile Ultra+ can be characterized with resins, but Renne mentions that he usually only polishes it. "The material is intrinsically beautiful and naturally gets more translucent as it gets thinner," he says.

South Carolina-based manufacturer 3D Systems is developing a 3D printed permanent restoration material of its own but has also entered into a strategic partnership with Swiss company Saremco that includes validating CROWNTEC for the NextDent^® 5100 printer. "Saremco's material was first, and it was superior," says Stef Vanneste, vice president and general manager of dental at 3D Systems. "Innovation moves so quickly that one company cannot always be out in front. Companies challenge each other. We prioritize improving patient outcomes, so opening our platform to an innovative material like CROWNTEC was necessary." The material is available in five shades for maximal esthetics. Vanneste is hesitant to compare it to other materials but suggests that the technical specifications are comparable to those of zirconia. "In the end, we need to make sure that any new material does the job as well or even better than what is on the market already," he says.

Still on the horizon in the US market are DWS's Irix Plus and Irix Max, which are expected to receive FDA 510(k) clearance within the next few months. Independent studies comparing these materials to milled materials have indicated that they demonstrate comparable or higher fracture resistance.^6,7 DWS refers to its printing technology as "tilted stereolithography," which Costabeber explains produces a denser product. In addition, the software's Photoshade feature allows color gradients to be built into prints, eliminating the need for staining and glazing in some cases. "In Europe, the early adopters and academic entities are very excited about these hybrid composites," he says. "These materials seem to better mimic the natural teeth in terms of flexibility. If you think about durability, esthetics, and time factors, those three values currently are more oriented toward composites than zirconia."

German materials manufacturer DETAX says that it has performed in vivo studies spanning more than 10 years for FREEPRINT crown, which is currently validated for two Asiga printers as well as the Rapid Shape line, and more validations are pending. Markus Stratmann, divisional director 3D for DETAX, emphasizes that the material is particularly easy to work with. "It is a bit more liquid than other materials," he says. "The post-processing workflow entails blowing it with compressed air, placing it in an ultrasonic cleaner with isopropyl alcohol for two 1-minute cycles, letting it dry for 30 minutes, and then curing it. The material does not cure automatically in the vat, so it can be left there for weeks if desired. Furthermore, the support structures can be much thinner (0.1 mm to 0.15 mm) when compared with those of some other materials (0.25 mm)."

One company, Lithoz, has found greater success 3D printing lithium disilicate. "It has become our main focus because of the very nice esthetic and mechanical results," Bomze says. Lithoz employs what it refers to as lithography-based ceramic manufacturing technology, which allows for the use of the same powders and furnaces as injection molding or milling and, thus, produces ceramic parts with comparable mechanical properties and surface quality. Bomze notes that restorations can be printed with edges as thin as 0.1 mm and that multi-material printing capabilities facilitate exceptional esthetics. "Multilayer milling blanks provide, at best, two-dimensional color or translucency gradients," he says. "3D printing allows for much more esthetic, lifelike results."

Despite the excellent results achieved by composite and lithium disilicate materials, manufacturers agree that zirconia still has the most appeal because of how entrenched its millable form has become in dentistry. The first printable zirconia to hit the US dental market will likely be from AON, which obtained FDA 510(k) clearance for its INNI-CERA material that is printed on its ZIPRO printer. AON uses zirconia powder purchased from Europe and mixes it with a proprietary binder to create a liquid material, or zirconia slurry. In the United States, AON's products will be distributed exclusively by Henry Schein. "Zirconia is a very stable and safe material with properties that fit the needs of restorative dentistry well," Song says.

The most significant challenge to 3D printing permanent zirconia restorations has been the sintering process. Earlier, first-printed zirconia restorations took approximately 2 days to adequately sinter. For AON, the sintering process is also slightly different from other manufacturers because of the liquid nature of its material. "The sintering time is required to be a bit longer," Song says. "The heating rate needs to be 0.1 ºC per minute. This is very important in order to perfectly sinter a 3D printed crown without any cracks." AON has co-developed sintering schedules of 15 and 21 hours for its products with Israeli furnace manufacturer Shenpaz. "Although the first schedule that we developed was 2 days, we continued to test new ideas for reducing the sintering time," Song says. "We developed the schedules for conventional sintering furnaces, but we also plan to introduce a new AON furnace later this year."

DWS's 3D printable zirconia material, Irix Z, possesses a similar liquid form and uses the same tilted stereolithography technology and Photoshade software as the company's composites. The material is still in R&D, but Costabeber hopes to have it ready by the end of the year. Currently, he does not believe that a sintering cycle of less than 24 hours can produce the necessary fracture resistance. "We have very high expectations regarding quality," he says. "We could likely be delivering this material already if our quality standards were lower, but we really want to excel in terms of not only sintering cycles but also the absence of cracks, which is the most typical problem with 3D printing zirconia. We are working to increase the density of the material in order to shorten the debinding and sintering time required. We have tried three different iterations, and we are very close."

Lithoz has faced similar challenges with its LithaCon 3Y 210 zirconia, which has a formulation based on the standard commercial powders that are used to press milling blanks or to create ceramic injection molding materials. "The challenge is not enabling zirconia to be printable, but rather, it is yielding parts without defects such as cracks or pores in the end," Bomze says. "We need to be able to remove the polymeric binder after printing and sinter the material to full density. If your powder is too coarse, you cannot print the desired accuracy or layer height. If it is too fine—especially if the specific surface area is too large—then the viscosity of the material is so high that it becomes very difficult to recoat after each layer. All of this affects the binder itself, which must be matched to the specific ceramic powder." Lithoz's current thermal post-processing time is approximately 10 hours, but Bomze notes that they will continue working to shorten it further. "There will be a limit because pyrolysis is necessary in order to remove the binder, but I do not believe that we have reached that limit yet," he says.

Whether the material is composite, lithium disilicate, or zirconia, one critical factor in 3D printing—perhaps more so than milling—is following the instructions for use (IFU). Thus far, most of the aforementioned materials have been validated only for specific printers offered by the manufacturers that are selling them. "From our experience, no other printer on the market can handle our material and produce a sufficient result," Bomze says. "It is not only about material development but also hardware and software development. The result will only be good if all of those aspects are involved. Otherwise, users will always ask, ‘Why does this material you sold me not work?'"

According to Stratmann, the curing process presents an even more significant challenge than the printer in having an "open" material. "Finding the perfect settings for the printer is a challenge in itself," he says, "but the curing device is much more complicated because curing shrinks the material and also impacts biocompatibility."

Some anticipate systems becoming more open, but others are skeptical due to the nature of 3D printing. "A milling block has already been processed," Renne says. "When you 3D print an item, you are in charge of processing it: washing, cleaning, and curing. Materials are optimized to work with certain machines under very specific protocols."

Straying from the protocols can lead to irregularities that are even more critical to avoid for permanent restorations than for other products. "Undercuring can lead to leaching of residual monomers and cytotoxicity for the patient, but overcuring even slightly can cause the strength characteristics to drop off dramatically," Tran says. "That technique sensitivity is less important with a temporary restoration that is not expected to last more than 6 months."

In addition to avoiding undercuring and overcuring, there are times when re-curing or other material specific protocols may be necessary. "If the IFU for a 3D printed material for temporary or permanent fixed restorations states that after the final UV cure, re-curing is required if the surface is altered, then any adjustment or even polishing by a clinician during seating requires re-curing at the lab before final cementation can happen," Stover explains. "It's important that laboratories and clinicians understand and follow the IFUs so that the materials are safe for patients."

Even with the correct hardware, materials might not print as intended if the specific IFU provided by each manufacturer is not followed in full. These companies spend thousands of hours testing and validating processes to determine which ones will produce the most optimal characteristics from the materials. "I cannot stress enough how important it is to comply with the instructions," Praderi says. "The reason that we have protocols is to guarantee biocompatibility and mechanical properties at the end of the process and ensure patient safety, performance, and optimal clinical outcomes."

Despite the progress achieved in 3D printing final restorations, the limited number of options available on the market can be partly attributed to the fact that many manufacturers are still not convinced that the materials science is quite ready. "There is more work to be done in terms of both esthetics and strength," Stover says. "The industry-wide question for laboratories and clinicians to answer will be: what levels of esthetics and strength are acceptable enough to displace the current digital all-ceramic restorations? Currently, there are millable materials that have been in use for years that provide high strength, durability, and levels of esthetics that are accepted by clinicians and patients. This is the standard that has been established in the industry, and these properties will need to be achieved by 3D printed final restorations to notably disrupt the current materials and processes, which will take time but is very exciting."

Even if printable permanent restorative materials—current or future—produce optimal clinical outcomes, adoption could be slow. Some dentists still place amalgam restorations, and more than half still do not utilize digital impression scanning technology.⁸ Milling machines, which have been available to dentists since the 1980s,⁹ have been widely adopted by laboratories but are still in fewer than 20% of dental offices.⁸ "The dental market is a very traditional, conservative market," Bomze says.

So, what will the adoption curve for 3D printing permanent restorations look like? "We believe it will follow the adoption curve of intraoral scanners more so than chairside milling machines," Haddad says. "The market is being driven by the accessibility and adoption of intraoral scanners. Approximately 45% to 50% of US laboratories have 3D printers, whereas we estimate that only 5% to 7% of US clinics have them. That jump will happen very fast and very soon. Regarding the materials, we broke the barrier with Permanent Crown, and we have proven that definitive restorations can be 3D printed. It is very good to see that other players have followed because all of us together will drive more and more awareness about the capability of 3D printing definitive restorations that will create even more hype. We cannot deny that the adoption of these materials has been slow so far, but that was expected. Skepticism always exists surrounding innovative new materials and production technologies, and we understand that it is our duty to inspire users with confidence and justify why this is a good option for them and their patients. I am confident because we have already seen it many times with implants, zirconia, intraoral scanners, and many other materials and technologies that overcame initial skepticism. The future of dentistry is definitely in 3D printing."

Vanneste suggests that much of the burden rests on the manufacturers to meet the needs and demands of not only the early adopters but also the early majority and late majority. "There are two types of users: those who don't have experience with the technology, and those who believe that they know more about it than they do and assume that they can jump right in. Although both groups are unique in their perspectives, they both require training," he says. "The early adopters like innovation. For the early majority and the other people who eventually will try it, ease of use is key."

Collaboration among dentists and laboratories could be key as well because many laboratories already have extensive experience with 3D printing and will be able to serve as resources. "When talking about 3D printed final restorations replacing milled restorations in an impactful way, we are talking about a new manufacturing process and new materials disrupting and replacing a firmly established process and its materials," Stover says. "Laboratories will need to manage this change with clinicians just as they currently are with other technologies such as intraoral scanners and digital dentures, which requires strong communication centered on education. Clinicians are responsible for providing high-level patient care, so understandably, they may be reluctant to switch away from something that they trust, but regarding fixed restorations specifically, laboratories have change management blueprints that they created when transitioning from traditional PFM restorations to those made from materials such as zirconia and lithium disilicate, so this won't be all that different."

The first steps were developing the products and acquiring the regulatory approvals, and now, the next step involves long-term independent research. "Even with zirconia, we will need to start over in examining durability, wear of opposing dentition, color, translucence, and more," says Rella Christensen, PhD, of TRAC Research, the human studies section of Clinicians Report.

Tran notes that although one printed material has displayed even better wear characteristics than lithium disilicate during in vitro testing, the true measure will be in vivo testing. "Ceramics do not absorb moisture the same way that print resins do," he says.

Along with the ongoing development and validation of the materials themselves, ongoing hardware improvements will also continue to be impactful. "As long as we do not have really small machines," Bomze says, "this will not be a mass product in every dental office. However, for large manufacturing centers, lithography-based ceramic manufacturing technology is already the right solution today. Of course, we are constantly considering how to create a broader portfolio of solutions for all needs."

Factors related to patient insurance will likely have an effect on adoption as well. Renne notes that the printable hybrid ceramics will not be covered by insurance until the American Dental Association (ADA) adds "3D printing" to its guidelines on what constitutes a ceramic. "Until then, it is not coded as a ceramic, so insurance-driven offices will be slow to adopt it," he says. "Milling is included in that language. A milled material with polymers infiltrated with glass is considered a ceramic if it is more than 50% ceramic. Once 3D printing is added to that language, we will see better reimbursements and higher adoption rates."

According to the ADA, the porcelain/ceramic materials definition has been revised by the ADA Council on Dental Benefit Programs and will be included in the Current Dental Terminology (CDT) 2023 manual. In order to extend the definition to include materials to create prostheses via 3D printing, the 2022 definition of porcelain/ceramic, which "refers to pressed, fired, polished, or milled materials containing predominantly inorganic refractory compounds including porcelains, glasses, ceramics, and glass-ceramics," will be updated to remove the specified fabrication methods and simply state, "refers to materials containing predominantly inorganic refractory compounds including porcelains, glasses, ceramics, and glass-ceramics."

Additional indications for the hardware could make 3D printing more appealing to adopt as well. Several of these manufacturers are also developing 3D printable solutions for dental implants and bone augmentation, among other applications. "Our vision is to implement a new paradigm for all dental laboratories around the world to transform their current production processes from milling to 3D printing," Song says. "It is safer, more affordable, associated with reduced maintenance costs, and able to produce better results."