Digitally Designed RPD Framework

Embracing new technologies for higher quality and better results.

By Robert Kreyer, CDT

Throughout the history of removable prosthetics—and, more specifically, removable partial dentures (RPDs)—there has been an evolution of techniques and technology. Partial dentures were once created by bending metal wires and fastening framed ivory teeth to them. Pierre Fauchard, who is considered by many to be the father of modern dentistry, first described this technique in 1728. 1 Techniques were further developed in 1899 when Dr. William G.A. Bonwill described clasping abutments with individually contoured gold circumferential clasps that were then soldered to a main plate or major connector.1

The first commercially available instrument developed for use in surveying models of teeth was designed by Weinstein and Roth in 1921. From 1930 to 1950, many articles were written suggesting the need for analytical surveying of the master cast for the purpose of planning the mechanics of an RPD.1 New chromium cobalt alloys were developed to replace the use of gold partials, and along with the new alloys came techniques for duplication and casting.2 The refractory model has been used for more than 70 years to make partial denture frameworks. Techniques and materials have evolved this process—however, the variables involved with duplicate refractory casts remain the same.

Revolutionary Digital Design

Since 2005, the digital design revolution has forever changed dentistry and dental technology. The manufacturing processes used for creating RPD frameworks have evolved to the technique the author is presenting in this first of two consecutive articles on digitally designed RPD frameworks using CAD technology. The intent of this article is to show what can be done with the 3Shape CAD software (www.3shape.com), not the exact protocols or sequence because that is better discussed in a hands-on course environment.

To begin the digital process of CAD design, blockout, and waxing, the master cast (Figure 1) are scanned using a 3Shape D710 scanner. After the scan was complete and the digital prescription was prepared with the proper missing teeth and abutment teeth on the master cast, the virtual digital image appeared visible on the computer screen.

The virtual digital cast was now prepared for surveying and blocking out. The blue directional rod in the center of palate assists in determining the desired path of insertion and is the virtual surveying tool. The cast is then oriented at the proper tilt to the desired path of insertion and zeroed out. With a click of the mouse, the undercut blockout is completed at the time of the survey (Figure 2). Blocking out undesirable undercuts and the location of desired undercuts is completed in the next sequence of digital tasks (Figure 3).

On the right is the 3Shape undercut color-coded depth gauge (Figure 4) measuring 0.00 yellow, 0.25 gold (.010 undercut gauge), 0.50 orange (.020 undercut gauge), and 0.75 red (.030 undercut gauge). Retention grid design and relief are next in the digital sequence. The desired relief area is outlined in yellow as in the edentulous area replacing teeth Nos. 12 through 15. The icon is clicked, and the relief plus the selected retention grid are simultaneously designed and placed in their desired position. This completes both of the retention grid design and the placement of the relief. Next, internal finish lines are created from automatic placement of the pink relief pattern under the orange grid on the palatal aspect of the virtual model (Figure 5).

Major and minor connectors are outlined with a connected dark blue dotted line and a major connector pattern is selected (Figure 6). The buccal clasp arms are completed next by extending a digital dotted blue design line to the desired clasp contour and shape (Figure 7 and Figure 8). The desired clasp is selected, clicked, and completed. The clasp width, thickness, and length can be measured with a unique design tool for verification of proper dimensions of the digital RPD design.

The next sequence is the design of the right posterior external finish line by connecting the blue dotted digital design line from the posterior to anterior extension of the major connector (Figure 9 through Figure 12). The digital waxing of the retention grid, clasps, and major and minor connectors is now complete. Only the addition of posterior tissue stops remains. Tissue stops are easily added in the next sequence, and are represented here by a blue dot, which then becomes part of the digital RPD framework with a click of the mouse (Figure 13 through Figure 15).

Support bars are then added to the RPD digital framework at the anterior and posterior segments. The support bars (Figure 16 and Figure 17) are essential for spruing before 3D printing of the partial framework in wax for casting using the alloy of choice. The digitally designed, completed RPD framework is then raised from the cast, providing a visual check of the intaglio or tissue surface and tooth surface of clasps and rests (Figure 18 through Figure 20).

With the new 3Shape D800 series scanner, the hand-designed drawing is transferred to the digital model providing an automatic RPD framework design creation for the design technician (Figure 21 and Figure 22).

As dental technology evolves with the digital dentistry revolution, new technologies must be embraced that help to improve quality, consistency, and efficiency. The next article will cover the printing and casting process for the digitally designed RPD framework.