Product Specials




    Share:

    Inside Dental Technology

    March 2012, Volume 3, Issue 3
    Published by AEGIS Communications


    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.

    References

    1. Becker CM, Kaiser DA, Goldfogel MH. Evolution of removable partial denture design. J Prosthodont. 1994;3(3):158-166.

    2. Wulfes H. Precision Milling and Partial Denture Constructions: A Manual. 1st ed. Bremen, Germany: BEGO Bremer Goldschlägerei; 2004.

    About the Author

    Robert Kreyer, CDT
    Director of Removable Prosthodontics
    Microdental/DTI
    Dublin, California

    <


    Share this:

    Image Gallery

    Figure 1  The digital design of an RPD begins with scanning the master cast and performing virtual blockout and waxing with proper missing teeth and abutment teeth.

    Figure 1

    Figure 2  The virtual digital cast is oriented for the desired path of insertion.

    Figure 2

    Figure 3  Undesirable undercuts and the location of desired undercuts is then completed.

    Figure 3

    Figure 4  The color-coded depth gauge tools guide undercut depth.

    Figure 4

    Figure 5  With a click of the mouse, the proper relief pattern and retention grid design is selected.

    Figure 5

    Figure 6  Major and minor connectors are outlined on the screen and the connector pattern is selected.

    Figure 6

    Figure 7  Buccal clasp arms are designed to the desired contour and shape.

    Figure 7

    Figure 8  Buccal clasp arms are designed to the desired contour and shape.

    Figure 8

    Figure 9  The right posterior external finish line is designed by connecting the blue dotted digital design line from the posterior to anterior extension of the major connector.

    Figure 9

    Figure 10  The right posterior external finish line is designed by connecting the blue dotted digital design line from the posterior to anterior extension of the major connector.

    Figure 10

    Figure 11  The right posterior external finish line is designed by connecting the blue dotted digital design line from the posterior to anterior extension of the major connector.

    Figure 11

    Figure 12  The right posterior external finish line is designed by connecting the blue dotted digital design line from the posterior to anterior extension of the major connector.

    Figure 12

    Figure 13  Tissue stops are added to the digital design.

    Figure 13

    Figure 14  Tissue stops are added to the digital design.

    Figure 14

    Figure 15  Tissue stops are added to the digital design.

    Figure 15

    Figure 16  Support bars are added to the design for spruing in preparation for 3D printing of the design in wax and conventional casting.

    Figure 16

    Figure 17  Support bars are added to the design for spruing in preparation for 3D printing of the design in wax and conventional casting.

    Figure 17

    Figure 18  The digital RPD framework is virtually raised from the cast for visual checking of the intaglio or tissue surface and tooth surface of clasps and rests.

    Figure 18

    Figure 19  The digital RPD framework is virtually raised from the cast for visual checking of the intaglio or tissue surface and tooth surface of clasps and rests.

    Figure 19

    Figure 20  The digital RPD framework is virtually raised from the cast for visual checking of the intaglio or tissue surface and tooth surface of clasps and rests.

    Figure 20

    Figure 21  With new CAD features, the hand-designed drawing is transferred to the digital model for automatic creation of the framework design.

    Figure 21

    Figure 22  With new CAD features, the hand-designed drawing is transferred to the digital model for automatic creation of the framework design.

    Figure 22