Digital Options Expanding for Dentures

A majority of dental laboratories today directly or indirectly utilize digital workflows in the fabrication of fixed prosthodontics. Many have their own 3D scanners and CAD software, even milling technology to design and produce substructures or full-contour restorations. Those that don’t own this equipment often outsource to other laboratories, milling centers, or production houses to access the digital processes for some portion of the casework.

While today’s CAD/CAM solutions support a very broad range of fixed prosthodontic restorations, options for removable prosthodontics have been somewhat limited. The issue has not been with CAD software. The challenge has been producing the try-ins and final dentures from those CAD files. That is changing. As usual, change brings on a varied set of improvements, compromises, choices, and opportunities.

Digital Denture Workflow

The workflow for digital-based dentures is not surprising to anyone familiar with digital crown-and-bridge workflows. An impression and appropriate measurements are taken by the dentist and sent to the laboratory. The laboratory scans the impressions prior to computer-aided design (CAD). The dentures are designed according to the prescription, measurements, and impressions. Depending on the specific workflow, a try-in or prototype may be produced. If, after seating, adjustments are necessary for the try-in, they are made accordingly and the final denture is fabricated or finished from the adjusted try-in.

Current digital denture production options are broken down into two categories. The first combines a small amount of laboratory work with companies that act as denture sales, marketing, and production partners. AvaDent Digital Denture Solutions and Heraeus Kulzer (Pala Digital Dentures) currently offer products for this model. The second category involves companies such as Amann Girrbach and Ivoclar Vivadent/Wieland Dental that provide a standalone denture solution for which the laboratory is responsible for all aspects of the process.

Partner Company Workflow

Digital denture partner companies AvaDent and Heraeus Kulzer market their products directly to dentists and rely on laboratories only to scan and deliver the final cases.

Good impressions are still a critical part of the digital denture process. The AvaDent and Heraeus Kulzer workflows include their own specialized trays for denture impressions (Figure 1). The trays are available in different sizes and may have removable sections. They both offer methods to register or note the patient’s vertical dimension, centric relation, and lip position. Depending on the system, there may also be options to take Fox plane and other measurements. Use of these trays can speed up the impression process as fabrication of custom trays is not necessary.

Once the clinician has completed the impression-taking process and requisite measurements, the case is forwarded on to the laboratory for scanning.

The partner company may have an optimized scanning process using a particular brand of scanner. However, many open-architecture STL format scanners are capable of capturing the necessary information from the impressions that can then be used by the partner company.

The process usually starts by scanning the bite relation impressions (Figure 2), followed by the individual arch impressions. Contrary to most crown-and-bridge scanning requirements, the laboratory isn’t required to perform the bite relationship alignment to the individual arch scans. Once complete, the scan files are sent to the partner company for denture design.

AvaDent or Heraeus Kulzer performs the setup and case design based on the scans received, the submitted prescription, and any other supporting documentation or measurements. Once case design is complete, an electronic case proposal may be generated for review and acceptance. After acceptance, the case is released for production by the manufacturing partner.

The Heraeus Kulzer workflow produces a printed prototype denture that can be tried in the mouth. The printed prototype is often recommended for the first few digital denture cases but then becomes optional. The AvaDent process includes a milled denture acrylic base and denture teeth held in place with sticky wax (Figure 3). If any adjustments are made to the try-ins, the laboratory may have to re-scan the adjusted try-ins or prototype before final production can take place—or, in the case of Avadent’s dentures, simply finalize any minor adjustments made at the try-in and bond the teeth to the base (Figure 4 and Figure 5) and polish. Heraeus delivers the final denture to the laboratory after receiving feedback and any additional scans, if necessary, from the laboratory and dentist.

Standalone Laboratory Workflow

Amann Girrbach and Ivoclar Vivadent offer a complete scan, design, and production solution for the laboratory. Amann Girrbach’s solution is available today. Ivoclar Vivadent’s is slated for release in 2017. In both cases, the laboratory is responsible for handling all aspects of sales, marketing, and production. It is likely that laboratories are required to purchase special CAD/CAM software modules or equipment and use proprietary materials following the solution provider-approved processes.

Amann Girrbach takes an approach very similar to the traditional impression and model processes. Once the models have been poured and the vertical dimension and centric relation set from the bite rim, the models and the bite rim are scanned in the company’s scanner, similar to the process shown in Figure 6. The Ivoclar Vivadent/Wieland Dental method is slightly different, producing its own custom patient-specific impression trays and the appliances to determine and hold the impressions or models in proper VDO and CR.

Both solution providers have a preferred/required scanner to collect and enter the necessary digital information for the case. Once scans are complete, alignment of the data must be performed by the laboratory and appropriate software. Models and bite rims in a scanner can be exceedingly tall. Current-generation scanners or special software modules may be required for the scan alignment process.

3Shape, Dental Wings, and exocad all offer CAD denture modules with their software. Most of the standalone-solution providers are distributing their own branded versions of one of these programs, customized to their production process.

As is common with crown-and-bridge CAD software, the denture CAD software or module guides the user through the entire process, requiring the technician to identify all of the common elements and landmarks required for denture design (Figure 7). The solution provider often includes an assortment of denture tooth libraries with its software.

Be aware that generic versions of these CAD programs are not necessarily guaranteed to include a readily available manufacturing process to actually produce the designed removable prosthetic. Also be advised that some of the production processes of standalone solution providers may be protected by patents and might not be available without a manufacturing or license agreement.

Once case design is complete, the Amann Girrbach system has a fixture to mill the denture base out of a wax/acrylic material and another fixture to mill the basal surface of the denture teeth. The base and teeth are assembled to form a patient try-in. Once fit has been validated, the try-in is flasked, boiled out, packed or injected, and cured using traditional denture processing methods.

Ivoclar Vivadent/Wieland Dental also has a mill for milling the denture base of the try-in. Ivoclar Vivadent’s plans are to give the laboratory a choice of materials for milling for the try-in. A wax try-in can be milled and finished using conventional denture methods or a denture acrylic disk can be milled which then can be become the final denture after try-in and finishing. In both cases a tooth positioning jig can be milled to assist in tooth placement in the base. Manual adjustments to the basal surfaces of the denture teeth may be necessary prior to bonding to the denture bases.

The case workflows above describe the processes to create dentures for two edentulous arches (Figure 8). In cases in which vital dentition is present, the impression and scanning workflow is typically modified accordingly. The duplication of existing dentures is also possible with many of the systems described above.

On the Horizon

What is next for digital dentures? AvaDent and Heraeus Kulzer are already starting to set up arrangements for laboratories to take on a greater portion of the processes described herein. Unlike monolithic zirconia restorations, the digital denture manufacturing process still requires a substantial amount of hands-on time. At this point it’s not possible to eliminate that time, but it can be distributed to local laboratories where it makes sense.

Companies are working on specialized discs that contain both gingiva- and tooth-colored material. Others are working on or have developed biocompatible materials that can be 3D printed.

Companies are also working on special fixtures, milling strategies, and processes that may allow laboratories to mill basal surfaces of denture teeth in such a way to avoid patents already in place.

Finally, when software and machines are used to produce these devices, we can be assured of a certain degree of consistency, reliability, and control. We’ve already seen this in fixed prosthetics with monolithic materials. But we’ve also seen a compromise between esthetics and productivity. In the completely digital realm, we may never achieve the esthetics made possible by traditional methods of denture fabrication. There will likely always be demand for both segments of the market: purely digital, and purely traditional. However, given the creativity of a new generation of digital dental technicians, it will be very interesting to see the direction this segment of the market takes. The most likely choice is in a direction that adopts the best of both worlds.

Acknowledgements

The author thanks Olson Dental Laboratory (Clinton Township, Michigan), Heraeus Kulzer (South Bend, Indiana), and Medit (Seoul, Korea) for providing some of the materials used in the photos for this article.

Chris Brown, BSEE, is Manager of Aclivi Consulting in Pinckney, Michigan.