How to Digitally Design a Flexible Partial Denture

VisiClear CAD is a crystalline resin that is translucent, but not transparent. A completely transparent denture material may show dark shadows or appear dirty (Figure 1) and cause patient dissatisfaction. The inherent translucence of the VisiClear material on the other hand allows the resin to absorb, transmit, and reflect light, delivering a chameleon effect that blends naturally in the oral environment (Figure 2).

There are two major CAD/CAM software programs that now provide modules for designing partial dentures. Either can be used successfully with VisiClear CAD, but there are some slight differences in design that users will encounter between them. The first software, exocad, is robust and reliant, allowing the technician to complete complex designs and integrate with many different scanners. The second, 3Shape, is an intuitive software that enables exceptional digital craftsmanship. The following compares the same case designed in both software programs to highlight some of the similarities and differences.

Both design programs have a color scale to show the level of undercut. The exocad software starts by showing the lightest undercut in blue, then moving to green, yellow, orange, and finally red, the most extreme (Figure 3). 3Shape shows transitions from yellow for the lightest undercut to deep red for the most extreme (Figure 4).

Similarly, during the design process, both software programs will initially block out all undercuts, leaving it to the discretion of the technician to expose the necessary undercut. The last 60% of the tip of the clasp can go into any amount of undercut except the red. The base of the clasp needs to be above the undercut.

The clasp design on an anterior or premolar tooth will start at 2.5 mm to 3.5 mm vertically and about 1 mm to 1.4 mm horizontally, tapering out to the tip of the clasp. On a posterior tooth the base will be 3.5 mm to 4 mm vertically and 1 mm to 1.4 mm vertically from the tooth. Making a clasp narrower than this can result in fattening the clasp, which can make it less esthetic and easier to catch food (Figure 5).

The saddle block-out needs to be thicker than typically designed for a metal frame, approximately 0.8 mm. When placing tissue stops, making them oversized (at least 3 mm by 3 mm) will make for a more stable frame during try-in and will keep the frame from warping during processing of the acrylic.

The major connector can be designed the same way as metal, with a few minor changes. Whenever possible, design the frame with an apron. The wider the major connector, the thinner it can be. With a major connector such as the one shown in Figure 6, the thickness can be 1 mm. If the major connector is not as wide (Figure 7), thicken it to 1.2 mm but never thicker than 1.4 mm. On a frame such as in Figure 8, create a bar, and build an apron around that bar to provide more thickness at the lingual vestibule. The bar will be 3 mm, and the apron will be 1.2 mm.  There is no need to have texture or stipple on the major connector; it just needs to be smooth with the contour of the rugea coming through.

The last item to add when designing a case is the finish line, which can easily be done using either software (Figure 9). The finish line for a VisiClear CAD frame will be a little bigger, with a bit of undercut compared to a metal frame. This guarantees the PMMA acrylic and resin will be locked together. Designing in exocad or 3Shape with VisiClear CAD will produce a high-quality framework that will be comfortable and durable for the patient (Figure 10).

Kris Schermerhorn, CDT, is the owner of Northern Virginia Dental Lab in Woodbridge, Virginia.

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