Always Evolving

"In the past 2 years, interest in digital dentures has exploded," says George Cowburn, co-owner of Perfit Dental Solutions, a dental milling center in Kelowna, British Columbia. "How long it takes to occupy the majority of the market remains to be seen, but it likely will not be very long. Crown-and-bridge laboratories have learned that we cannot sit on the fringes; when the technology is ready, it is time to jump. Digital denture technology is now ready to be employed broadly."

The first thing many dental professionals might not realize about digital dentures is how easy they can be to implement. While some innovators and early adopters are successfully utilizing extremely advanced workflows in laboratories and clinics, easy paths to entry exist as well.

"It is not necessary to purchase an expensive piece of equipment to do digital dentures," says Andrew Johnson, DDS, MDS, CDT, FACP, Director of Prosthetic Dentistry at Precident Center for Dental Medicine in Rogers, Arkansas. "Anyone can make a digital denture with the basic materials and equipment that every traditional dental office already has."

Indeed, various degrees of both analog methods and outsourcing can be utilized as part of a process that also includes digital elements. Even experienced users can vary their workflows depending on each case.

"One of my favorite aspects is the flexibility of the workflow," says Wendy Auclair Clark, DDS, MS, a professor at the University of North Carolina, Chapel Hill, in the Department of Prosthodontics. "Sometimes I do three visits. Sometimes I do two visits and go straight from the scan to printing. Sometimes I still do five visits, take a wax rim, and scan that. It is really nice to have those options and still maintain consistency from step to step."

Cowburn stresses the hybrid analog/digital option in his outreach to other dental professionals. He has created reverse-engineering software to take a scan of a wax-up, an existing denture, or denture with a reline impression, and extract teeth automatically, give them roots, and provide the user with a production file to fabricate the case in whatever fashion they choose.

"It is essentially an intelligent Xerox machine for dentures," Cowburn says. "The goal is to move people into the digital world without the need for them to climb the steep learning curve that is CAD."

Of course, more experienced users can operate completely digitally. Both milling and 3D printing can be used for fabrication. Both monolithic and bonded options are available.

"Prosthodontists to a certain extent dictate the workflow," says Robert Kreyer, CDT, co-founder of the independent consulting firm Dentgnostix. "We discuss their objectives and design a specific workflow for each case. Every case has its own set of variables."

One of the primary considerations when planning treatment is to mill or print. Printing is significantly more efficient; not only is it sometimes twice as fast and the materials typically cheaper, but certain 3D printers' build platforms make it possible to produce up to 12 denture bases simultaneously, while a mill can fabricate only one at a time.

"Milling 12 denture bases can take 36 to 48 hours, but you can print 12 in 2 hours," Kreyer says.

Milling boasts its own advantages, of course. The materials are more clinically proven. Monolithic blocks in particular, with both the base and the teeth pre-bonded, are very popular.

"When you can mill the teeth and the base from the same puck, the prosthetic is stronger because the bonding process is eliminated," Kreyer says.

Alexander Bendayan, DDS, CAGS, FICD, says he has tried informal strength tests with milled dentures and they have proven to be as strong or stronger than conventional dentures.

"Patients are human," says Bendayan, Assistant Dean for Digital Dentistry Development and Clinical Training at Boston University's Henry M. Goldman School of Dental Medicine. "They drop their dentures in the sink or on the floor. Conventional materials were so brittle that they would often break, but we are seeing far fewer fractures with milled dentures. Doing fewer denture repairs has saved me considerable amounts of time."

Clark took part in a project that involved delivering both a conventionally processed denture and a monolithic milled denture to some patients, asking them to wear both. When the patients returned, Clark observed plaque, tartar, and stains on the conventional dentures, while the milled prosthetics were clean.

"The density of the milled material seems to resist tartar buildup and stain, though I would like to follow up with long-term research," Clark says.

Questions and issues persist with printed dentures, conversely. Clark uses 3D printing for anything provisional, such as an immediate denture she intends to remake. However, she still prefers milling for permanent dentures.

"In some cases, there are zero problems and the patients are really happy with printed dentures," Clark says. "However, we have seen a handful of clinical problems with printed dentures, even within the first few months after delivery. In one case, the teeth fractured out of the denture base. In another, the tuberosity area of the flange fractured off the denture. In another, the teeth debonded. Those were all made with different printers, different materials, and different technicians, which indicates that it is not just one problem; the technology is simply not yet consistent."

The consensus, however, seems to be that printing will catch up and eventually pull ahead as the predominant fabrication method.

"Printing is in its infancy," Cowburn says. "There are very few approved materials. There are a lot of open questions about longevity and other long-term aspects. However, it is inevitable that printed dentures will take over the industry as the technology progresses and matures, and we are able to rival the esthetics and function of the milled world."

Because milling technology and materials are so far along, some advantages of digital dentures are indisputable at this point. Perhaps most significant is the fit that can be attained with the precision of CAD/CAM.

"Clinically, the most terrifying moment used to be going from the try-in to the final," Clark says. "So many times, the final prosthetic was not the exact same as the try-in—whether because of processing error, wax distortion, damage during shipping, or various other possibilities. The digital workflow has really improved my ability to discuss cases with technicians, send screen captures, and go back and forth. The fit of a monolithic milled denture is very predictable. Even in very resorbed arches and cases that would typically be very challenging, the fit of that milled base to the tissue is just so nice. I had not seen anything like that in analog dentures."

Johnson has seen a decrease of more than 50% in patients returning for adjustments.

"With the old technology, the final denture would not be a perfect reflection of the impression due to issues from the polymerization of the acrylic to the movement of the teeth," Johnson says. "Now, we have out-of-the-box fits that make it much simpler to adjust, deliver, insert, and send the patient home."

Patients notice the fit of digital dentures—especially patients who have had dentures previously. Kreyer tested conventional dentures versus digital dentures in university test cases and says the patients overwhelmingly preferred digital for fit.

"They say it feels more like their natural teeth," Kreyer says. "Digitally, we can replicate the natural anatomy and contour of a patient's palate, thus making the digital denture thinner and less bulky than an analog denture. The patients notice the feeling they get from a digital denture that is lighter in weight while having improved retention in their mouth."

The impact of this improved fit on the dental practice in particular is significant. Clark estimates that her insertion appointments, which once required a full hour of hands-on work, now take only 15 to 20 minutes.

"I put the dentures in and they almost always fit perfectly," she says, "so beyond just decreasing the number of appointments with the digital workflow, my chairtime for those appointments decreases as well."

One advantage that does not depend on whether the denture is milled or printed is reproducibility.

"The digital component adds huge value because of the ability to acquire and retain all this data," Kreyer says.

Even the strongest dentures can break or get misplaced. When that happens with a conventional denture, the entire process needs to be repeated from the start. With a digital denture, it is as simple as opening the design file and re-milling or re-printing the prosthetic.

"Being able to remake a prosthesis without seeing the patient has probably had more of an impact on my practice than the efficiency of the initial process has," Clark says.

Johnson notes that the design file becomes just as important for digital dentures as the final product, as it likely will be used to produce duplicates in the future.

"We can reproduce the product as many times as we want," he says, "which is a far cry from starting over every single time when a denture is lost."

Even proactively, laboratories can provide two identical dentures from the start.

"We can offer an ‘embarrassment denture' that comes with the final restoration as a value add," says Steve Edmonds, CDT, President/Owner of Edmonds Dental Prosthetics, which has three locations in Missouri and Arkansas.

Bendayan says patients are more likely to return more frequently when the replacement process is easier.

"When patients are charged a large amount of money, they like to wear the dentures for more than 10 years, and it wears down and changes over time, causing changes to the bone and the tissue," Bendayan says. "Now, I am able to offer this recall system whereby the patient returns in 2 years and I only need to make an impression with the existing denture, scan it, and have my laboratory duplicate the same setup."

This level of precise reproducibility is an advantage that conventional processes simply cannot offer.

"In the analog world, too many steps involve variables that compound to result in a different product," Cowburn says. "Providing a backup or a spare had been unheard of in denture technology, but, to be frank, it can eliminate the No. 1 paranoia or fear of every denture patient: the social stigma of losing or breaking their dentures. That is the greatest benefit of digital dentures."

For all the advantages digital dentures offer, the most persistent questions and concerns about them involve esthetics.

"Unless you do a lot of modification on the teeth with composite resins—expending time, materials, and labor—great esthetics are hard to achieve," Kreyer says. "We will overcome this soon; the last hurdle in digital dentures is to really achieve a higher level of esthetics."

Anecdotally, the consensus seems to be that denture patients have slightly lower esthetic standards than crown-and-bridge patients.

"The patient who already can chew food is more inclined to look for an esthetic result," Bendayan says. "Of course, all patients are different, but denture patients usually are looking more for function."

The denture base has been the primary challenge esthetically. Removables technicians have used stock denture teeth for many years, but their craftsmanship shines through in the characterization of the gingiva. And while stock denture teeth can be bonded into a digitally fabricated base, printed denture teeth have posed another challenge.

"A tooth is not one color," Bendayan says. "The base needs to appear natural as well. With our existing 3D printing workflows, we need to layer composite to make the prosthetic look natural."

Help is on the way, though, as groundbreaking new materials are expected to be launched this month for both denture bases and teeth. Kreyer and Edmonds both say they have tested new printing materials that will more closely match the esthetics of conventionally processed dentures than anything else on the market. Those materials are expected to be introduced at the IDT International Digital Denture Symposium on September 27-28 in Atlanta, Georgia (idds.cdeworld.com).

"We may have a worthy competitor to our traditional denture," Edmonds says.

As new materials and workflows continue to be introduced, their impact will depend on effective implementation, and lessons learned from the crown-and-bridge CAD/CAM revolution could prove helpful. While digital dentures remain in the innovator/early adopter phase of Everett Rogers' technology adoption lifecycle, the crown-and-bridge segment of the industry is further ahead, at least on the laboratory side, as more than 60% of US laboratories owned or leased a CAD/CAM system in 2018.¹ The introduction of zirconia led to a wave of offshoring and commoditization of crowns and bridges, but the tide seems to be turning now as CAD/CAM continues to progress toward near-universal acceptance.²

"With the digital crown revolution," Edmonds says, "we did not know what to expect and quickly realized that our production process and workflow would have to change almost overnight to keep up with the demand. Dentures will be a little slower to completely transition into the digital world due to the case complexities and compromised situations that denture technicians face daily. You can choose to blend digital processes into the traditional work area or separate them out to better streamline your workflow—which is our approach. There also will always be cases that require a skilled technician's hand to fabricate, while more basic restorations can be produced with an automated digital workflow."

The complexities of denture cases often revolve around the unpredictability of the edentulous arch.

"Dentures are more of a challenge because we are working with a very unsecure base of soft, squishy gingiva, but there are a lot of parallels and insights that we can gain from crown-and-bridge," Cowburn says.

The importance of effective collaboration between dentist and laboratory is one insight that many have carried over from the crown-and-bridge side: Precise information and an open line of communication are more important than ever.

"When a laboratory has done great work for many years, it is easy to forget that computers will not act subjectively based on a long-term relationship—at least until artificial intelligence advances further," Johnson says. "Computers act objectively every time, without bias. Dentists who have relied on laboratories to gloss over their mistakes will now see those mistakes front and center every time."

Indeed, Edmonds was well-prepared to explain the importance of immaculate data to his dentists.

"I fear some of their excitement about digital dentures is due to their perception that it will be easier for them now since it is a digital process," Edmonds says. "That could not be further from the truth. Laboratories will need accurate impressions that capture every anatomical feature in the oral cavity in order for this to work."

The importance of authentic materials that have been well-researched is another lesson learned. Bendayan cautions against using 3D printing materials that have not been certified.

"There are so many materials, implant brands, and systems available," Bendayan says. "Years ago, laboratories always provided dental alloy certificate stickers with PFM crowns to document the materials, but the introduction of zirconia in the market coincided with financial forces that led some laboratories to use materials purchased from other countries with no regulations. I have learned not to work with laboratories that do not use authentic parts or materials that have been scientifically proven to be safe for the patient."

In some cases, CAD/CAM may even require slight changes to the way restorations are designed and fabricated.

"We know now that we need to think more like the machines that will be fabricating the restorations, the same way beveled-shoulder margins for lingual-collar PFMs went out of style and now everything is a deep chamfer or a rounded shoulder, because that is how the milling burs pass through and are best tolerated by the material," Johnson says. "It is not always as simple as replacing analog steps directly with digital alternatives. We originally were scanning the dies that were poured up in gypsum and then converting that to a digital model to produce a wax-up that was hand-modified and then invested and cast or pressed. Now that has evolved into a process with a totally digital, model-less workflow. The same is happening and will continue to happen with dentures."

Some developments on the crown-and-bridge side have directly helped with digital dentures. For example, Cowburn notes that the emphasis on fast turnaround times that has helped bring crown-and-bridge work back from overseas likely will be an immediate factor for dentures. He also notes that much of the same equipment can be used, and the prices for that equipment have decreased dramatically. Additionally, CAD software in general is so advanced now that denture modules are already very strong.

"A lot of the kinks with digital technology were worked out with crown and bridge, especially with digital articulators and some of the accuracy issues, so we will not see those problems with dentures," Clark says.

The lessons have not only been passed on to removables laboratories; many crown-and-bridge laboratories have become full-service and in some cases are actively helping to advance removable prosthetics as a whole.

"Crown-and-bridge laboratories look at their fees much differently; they typically charge per unit, so a 6- to 8-unit restoration could cost $600 to $800," Kreyer says. "The average denture in the past cost only about $400 to $500 despite having 14 teeth. Crown-and-bridge laboratories, however, are already getting $600 to $700 for a denture, if not more. They are changing the perception of the value of the product."

Clark says there will always be a need for economy pricing, but that the removables segment may avoid the mass commoditization phase.

"I am seeing so much beautiful work with people characterizing these layered printed or milled dentures, including ceramists who have converted to removables, which is changing the way we perceive acrylic resins," she says. "It is so exciting that this passion for truly esthetic, functional dentures is returning. There may be a small race to the bottom, but there is a concurrent race to the top, and that is so important and so wonderful."

Several forces are driving that race to the top. While manufacturers are pushing the development of hardware, software, and materials, they are receiving significant input from technicians, dentists, and denturists regarding what works well and what areas for improvement still remain.

"There are three aspects of innovation: machines, materials, and techniques," Kreyer says. "For removable prosthetics, the machines evolved first, and then the materials needed to be developed; the prosthodontists and technicians are driving the development of techniques. It is all changing much more rapidly than people realize. What will be introduced this month at the IDT International Digital Denture Symposium will just totally blow people away. It will push removable prosthodontics forward into the next decade."

Indeed, the near future may bring digital denture solutions that today might seem unfathomable to some. Improved print resins could be just the start.

"While it is an exciting time for the removables world with new digital opportunities," Edmonds says, "we feel the best solution is still on the horizon. I am sure 2020 will have its share of new offerings and opportunities to take removable dentures to the next level with fit and esthetics. The real winner in this race is the patient, who will unknowingly receive the most state-of-the-art precise-fitting, digitally designed, esthetically pleasing removable prosthetic available."