Flexibility for the Future

What those in dental technology call "resins" or "polymers" are an aggregate of multiple materials suspended in a resin matrix, so they are classified according to these intrinsic properties, rather than being classified based on their manufacturing methods. 3D printing and milling are simply methods to output these polymers efficiently.

 As new developments in millable materials have arisen, there has been renewed interest in varieties of PMMA, which are also considered polymers, albeit rather simple in composition. This resurgence of PMMA materials is mainly due to CAM milling capabilities. Ceramic-infiltrated resin is yet another exciting recent development that provides for an esthetic result for crowns, veneers, inlays/onlays, etc, both in a millable block form and in a new additive option by BEGO, the VarseoSmile Crown Plus, which was approved by the FDA just this June.

Not only are there resins for final restorations, but there are also resins suitable for implant bars and substructures that possess increased strength due to certain additives and specific matrix formulations. For example, TriLor (Bioloren S.r.l.) features fiberglass suspended in a composite resin, and is millable. PEEK is also milled, but PEKK can either be milled or pressed. These kinds of materials are known as "high-performance polymers," or techno-polymers, and they are used for implants and substructures because of their light weight, biocompatibility, ability to absorb forces, and high yield strengths. There are also acetal-based resins that are utilized for partial dentures and needed for semi-flexible connectors. Some are more rigid or more flexible and classified for use in partial dentures and nesbits (flippers).

The common thread that ties together all of the polymers and resins now in development is the pursuit of patient comfort. "All material research lately is focused on the patient," says Robert Kreyer, CDT, Director of Advanced Denture and Implants at MicroDental Laboratories in Livermore, California. "Years ago, some prosthetic materials were prone to breakage within the oral environment. Now we are trying to make prosthetics more accurate, durable, and comfortable." Technological advancements in dental materials have never been so keenly focused on providing benefits to both dental professionals and patients.

The materials currently available for additive production are designed specifically for the various technologies used by dental 3D printers. Common types of 3D printing for dentistry include fused deposition modeling (FDM), which is an older and more basic system with greater limitations; stereolithography (SLA), which offers highly detailed output; and selective laser sintering (SLS), which is known for using materials with excellent mechanical features. However, the latest advancements in 3D dental printing use digital light projection (DLP) technology, which cures an entire layer at a time; the resins used in this process are known as photopolymers. Currently SLA and DLP are the most widely used additive methods for dental restorations.¹

Matching the right material with the right 3D printer is incredibly important. While closed-system 3D printers are designed for use with a specific, manufacturer-validated material, open-system 3D printers allow the user to choose from a wide variety of material options. An open-system printer provides great flexibility, but selecting the correct print resins can be confusing and challenging. There are hundreds of resins on the market, each with their own specifications and calibrations. Laboratories that engage in additive manufacturing must also remember that having an open system does not necessarily mean just any resin will work. Printers have been tested, validated, and calibrated by their manufacturers for a finite number of resins, and the machines are FDA-cleared for use with only those validated resins. Using any resin in a machine for which it is not FDA-cleared could be dangerous for the patient (due to the material breaking down into harmful components or by compromising the restoration's strength) and a serious legal liability to the maker.

Most laboratories perform in-house testing to determine which materials they prefer for their most commonly printed indications. Often the technician will need to communicate closely with the manufacturer's customer support in order to calibrate the machine correctly for each different material. Sometimes the manufacturer's own material seems to work the best for one laboratory, while another laboratory may prefer another.

"We use mostly model resins in all printers because about 70% to 80% of the models sent to us are intraoral scan files," says Philip Reddington, RDT, MDTA, managing director of Beever Dental Technology Ltd, in Leeds, United Kingdom. "We usually use the manufacturer's own model resin. I'll try new things as they come out and see if they do better, but most often we stick to the one we like. It's hard to beat our tried-and-true favorites. We don't see materials as an opportunity to save money. We use what works the best."

Jessica Birrell, CDT, owner of Capture Dental Arts in West Jordan, Utah, admits that sticking with the manufacturer's resins is sometimes best.

"When we got our first printer, we wanted to test out all kinds of different materials," she says, "but we found that calibrating everything was a very tricky and time-consuming process, and at the end of the day, we weren't wowed by the results. After a lot of testing, we concluded that the printer makes the best product with the material made specifically for that machine and indication in many cases, though we have had some success more recently with open-source materials."

Different laboratories find different ways to profit from the technology enabled by print resins. For instance, Birrell has found it very profitable to bring surgical guides in-house by utilizing 3D print technology.

"Surgical guides have been great," she says. "We used to outsource to a big company with a long turnaround time. Then we started to use the software to enable us to design and print surgical guides and plan implant cases, which allows us to keep this work in house and collaborate better with the dentist. The printing shortened the time frame, making it very simple and profitable. It's been a game-changer for us."

Kreyer offers the example of occlusal splints as a way he has reaped the benefits of 3D printing through increased efficiency and reduced material waste. "In the past, most splints were hard and required gypsum master casts, duplicate casts, wax design, investing, processing, etc, all of which require more time and capital for labor and production materials," he says. "The time difference between that process and 3D printing these splints is significant; a process that used to take up to 90 minutes in labor plus the variables in materials have been reduced to about 30 minutes. It saves us an hour of labor time, requires far less material, and allows us to control the variables while we save the acquired design data for future use."

Additive manufacturing capabilities can also enable a laboratory to expand into digital dentures. Automating the process has helped make dentures a more attractive option to laboratories, especially considering the growing demand for removables by an aging population. Laboratories within this market need to decide whether to print or mill the denture base and whether to attach printed, milled, or pre-made denture teeth. There are many varieties available, in every level of esthetics and budget, so it is up to the patient, laboratory, and dentist to decide what fits the patient's criteria.

"In dentures, there is trade space between beauty, strength, and cost based on patient preferences," says Mike Clark, DDS, with a private practice in Yakima, Washington. "For a variety of reasons, we print 85% of our denture cases. We use our workhorse printer and characterize the printed denture so that is ‘pretty enough'—it looks great to the patient, even if our peers in dentistry may be critical of the end result lacking complete esthetics. It really depends on the patient's priorities, though, not your peers'."

Marko Tadros, DMD, a prosthodontist in Atlanta, Georgia, prefers to offer a 3D printed option that can deliver same-day dentures for patients. "It's a better product than conventional," Tadros says, "and having the multiple color options for the base is nice. We use that to print the base, but we like to mill the teeth because the multilayer PMMA offers better color and esthetic properties."

Though 3D printing provides some undeniable benefits, being able to recognize the specific limitations of the materials and machines is another important part of the learning curve. Birrell now knows that her smaller desktop printers are not quite suited to printing full-arch cases, especially considering the challenges she first faced when trying to print 1- to 3-unit bridges.

"Smaller printers have a much smaller print bed or tray, so when you want to print a slightly larger restoration, you have to print them on an angle, which can lead to distortion of the material during the printing process that you don't see with, say, pieces that you've outsourced to be printed on a much larger printer," Birrell says. "Those high-end printers will build on a flat bed/print plate, which eliminates the distortion. They've also got what they call a ‘dead zone' or ‘air-inhibition layer' that allows the machine to do one long, continuous print, never detaching from the product it's building. Now we know it's better for us to outsource our full-arch cases for now."

Beyond how the material behaves during the printing process, technicians need to be aware that post-processing should not be overlooked or used as an opportunity to save some money. "When printing is done, that's when the work starts," Reddington says. Proper, thorough clean-up is one such step. "It's not hard but some people try to skimp on it," Reddington continues. "People think that printed restorations just pop out finished. Nobody talks about cleaning resins off the curing plate, cleaning the models, using the right light boxes with the proper wavelength, etc, because it sounds like a hassle, so it can be a big shock when they realize all the steps that are needed. But it is not hard. Just clean properly and cure properly in order to get the right end result. I actually have one person on my staff who is completely dedicated to the post-processing; it is too important to risk overlooking it."

Another important but often-overlooked component is the light box or curing box. "It is so important to get a good curing light," Birrell says. "Sometimes technicians get ‘creative' with curing boxes, trying to use a cheaper version, making the mistake of thinking they're all the same. But using the wrong light box on the material can make the printed restoration brittle, shrink, or even change color. Buy the box that is packaged with the printer; they have been calibrated to work best together, without the risk of ruining your resins."

Once post-processing is complete and the finished product is delivered to the patient, there are yet more material advantages to consider, and some of these are available only through additive manufacturing. For example, GC America Inc. is preparing to launch a printable resin for temporaries that is methacrylate-free, making it an option for patients who are allergic to MMA. There are also two new products specifically known for their unique bioreactive qualities: Dentsply Sirona's Lucitone Digital Print 3D Denture Resin and Keystone Industries' KeySplint Soft. These are among the first resins with body-temperature-activated attributes, becoming stronger or softer, respectively, when placed in the mouth. The KeySplint Soft, in particular, offers both structure and flexibility.

"My patients can actually feel the difference between this and the usual night guard material, and they prefer the KeySplint Soft," Tadros says.

All the advances in additive production cannot erase the strides made in the milling space over the past decade, especially with polymer materials. CAM milling has given new life to familiar materials, while at the same time new polymers, composites, and hybrids are in development that promise to surpass the strength and esthetics of both conventional and millable options currently on the market.

PMMA, the acrylic standard material for dentures, has been "made over" in recent years as a pre-polymerized millable puck that is available with multiple layers of shading, which vastly improves its esthetics.

"These multilayered PMMAs really are amazing," Reddington says. "For anything the patient is going to wear for more than a few weeks, we want to give them something that will both look good and last a long time, and we know we'll get that by milling. Plus, we don't have to use any stains or glaze on the multilayer PMMA—just polish it."

These inherent esthetics have a practical benefit to the laboratory as well. By removing the need to characterize monochromatic PMMA teeth, the laboratory can gain back some of the time it needs for milling, which typically takes longer than printing.

While many improvements have been made to PMMA itself, another peripheral advancement has made it even more attractive to the laboratory. Though not specific to any particular material, improved design software has enabled these multishaded PMMAs to be used to their utmost advantage.

"The differentiator is software for planning and design that allows ‘shell geometry' and takes CAM into consideration," Kreyer says. "The layers and shades are factored into the design in order to mill properly, show the right colors for the best esthetics, etc."

Though PMMA is a classic, another millable material option has developed its own following in recent years—high-performance polymers (HPPs). Reddington is one such aficionado.

"I've done over 300 cases with polymer arches over the last 6 years and only had about three failures, so I'm really happy with that," he says. "These materials, used in the right instance, are really the gold standard."

Strength is only one benefit of HPPs. Their ability to absorb shock is especially important in their use in implant bars and full arches.

"The amount of force that goes through a zirconia arch to an implant is reduced by about 60% when using a polymer frame," Reddington says. "They put much less stress on the bone, so if you can use these materials, you need to go for it."

For other indications, Reddington has found both advantages and disadvantages with certain HPPs, as well as ways to harness their strengths while mitigating their weaknesses.

"In the past, when I have used fiber-reinforced HPPs, the preparations sometimes snapped off," he says. "I like using acrylic wraps with those, making them super strong. An acrylic wrap is never going to break with this kind of HPP under it."

The key to working with any new materials, he says, is about finding their limitations.

"I've had failures, too, even following the manufacturers guidelines, like with distal extensions, abutment gaps, etc," Reddington says. "Now I've got my own set of criteria about how far I'll push these materials. You've got to bear in mind that most HPPs have been used in medical applications—like for implants, bone replacement, etc—for over 20 years, long before they were used in the dental field, so we know these things work." He emphasizes that technicians should educate themselves enough to use HPPs correctly, and then they will have very few problems, if any.

Getting a little help from your friends and colleagues isn't a bad idea, either, he says.

"I had a tough time milling the PEKK at first and couldn't seem to get the settings right," Reddington admits. "So I reached out to another technician, one who I knew was using this material and the same machine, and asked. It can all be done on a dry mill; the trick is slowing down the mill speed enough and using the right tools. Then it all came so easily. You just need to understand the materials really well…or know someone else who does," he adds.

Though both 3D printing and milling have a wide variety of material options available, their materials are not created equal. In some cases, the materials and processes available through either additive manufacturing or milling will render it more beneficial for a particular indication. For example, Tadros notes that he finds it much more advantageous to print temporary dentures because the embrasures are better, and it is not necessary to recontour them in the post-processing stage.

"It's impossible to mill embrasures perfectly," Tadros says, "so the technician needs to grind and polish them into the restoration after the CAM milling. I would rather print the temporary because it takes less time and labor overall."

However, bondability is one specific advantage that milled polymers have over printed materials at this time.

Tadros notes, "The problem with 3D printed temporary materials/temporary crowns is the bonding. We don't know how to bond 3D materials well yet." This issue will have to be addressed by the manufacturers before printed crowns will be widely adopted.

Overall, even the greatest fans of 3D printing concede there is still space for milling and its materials based on the technology currently available. This is doubtless why so many laboratories still rely heavily on their milling capabilities, rather than transitioning fully to additive manufacturing.

"It's good to have milling options for cases with minimal inter-residual space and implant prosthetics with minimal prosthetic space due to implant abutments, attachments, bars, or secondary structures that compromise tooth and acrylic-resin space," Kreyer says.

Reddington agrees that having both systems works really well.

"I never liked the idea of limiting yourself to only one treatment modality," Reddington says. "If you use only one, you're always going to try to force your work to fit around that capability. Like the saying goes—if you've only got a hammer, everything looks like a nail; and that's not the approach we want to take toward our patients or our clients."

The wide variety of polymers and resins currently on the market is likely only the start of the material innovation that is coming to dental laboratories. In fact, some of the developments that technicians are eagerly awaiting are on the horizon and could even be seen within the next few years. Not surprisingly, the material advances that dental professionals most want to see are primarily in the additive space, which is fast gaining prominence.

"I'd love for there to be a printer for pure polymer, no fillers or ceramics," Reddington says. "Pure PEKK would be amazing. There are a few options out now, but their strength and geometries are not quite there yet because of the filament size. We tested one of the first PEEK/PEKK printers a few years ago, so it's out there in development; it's just taking a while to fine-tune the process. That's what I'm looking forward to in new materials—print polymers that have all the same properties as the millable ones."

"The next leap for us will be getting materials to print fixed crown and bridge restorations," Clark says. "That will revolutionize everything." Birrell sees the opportunity in this space too, adding that she looks forward to testing BEGO's new printable permanent crown material.

Multicolored print materials including ceramics, zirconia, and metal are on the wish lists of quite a few technicians, as well as the possibility to print with more than one type of material at a time.

"I'm hoping for multigradient shading in print and increased strength for all-on-X cases, removable partial dentures, long-term temporaries, etc," Tadros says. "So far they tend to break and are brittle without support. For our RPDs, we currently attach a metal framework to the printed restoration, but it would be nice to have it all print at once. I understand that printable zirconia is in the works, too, but manufacturers still need to control the shrinkage rate."

Kreyer cites a few interesting developments already in the works, including research at the University at Buffalo into delivering certain drugs to patients through prosthetics. Another is the idea of storing all of a case's information and data, such as design files, material specifications, etc, within a chip that is embedded in the patient's denture.

"That way if a patient breaks their denture anywhere in the world, they can have any dentist scan it and print a new one," Kreyer says.

As he looks even further into the future of dental material technology, Kreyer believes print resins will be the basis for bringing nanotechnology into dentistry. "I'd like denture bases that form to the arch," Kreyer says, "so the base would conform tighter to the patient, and the teeth would self-occlude using nano-bots."

All of these material and process advancements, all the wish lists for future capabilities, are ultimately for the benefit of the patient.

"When you compare current processes to those of the past, you can see we've made those processes more efficient, and we've improved the materials for the patient," Kreyer says. All the research and development, all the laboratories testing their own approaches to new technology—it all comes down to the patient. The ultimate goal is to make everything more comfortable and durable for them. No matter how the restorations are made, the patient is the most important part of the process. Thanks to the adaptability, variety, and endless innovation of polymers and resins, there will always be a place for them in patients' mouths and within the laboratory.

1.Polanco S. Building a better product: Printable materials lay the groundwork for quality and efficiency. Inside Dental Technology. 2020 Jan;11(1). https://www.aegisdentalnetwork.com/idt/2020/01/building-a-better-product. Accessed on July 10, 2020.