Advanced Milling Strategies
Construction of teeth and a denture base utilizing an oversized milling strategy on a cast partial frame
The analog protocol for creating a cast partial denture involves articulating and setting teeth in wax with conventional methods. Most techniques utilized today involve milling the teeth but still setting them in wax for conventional processing. The author's digital process involves milling the teeth and base, and bonding them together—instead of mounting and arranging the teeth.
Image Acquisition, Model Preparation, and Order Setup
Figure 1 shows before the frames were incorporated into the digital mesh. An additional scan of the framework was completed on the model, which can be either poured in stone or 3D printed. Wax also was added to the underside of the frame in the edentulous areas to give the mesh a more solid appearance in the scan. The new file was then saved to include "framework on model" with the patient's name so it would quickly identify the file when the technician needed it (Figure 2).
Within the CAD software (3Shape Dental System, Shape), certain intricacies allow these cases to be either very easy to process and manufacture or very difficult and frustrating. For this case, the Medit Design application and the boolean tool were used to fuse the framework to the model and close any holes via the smooth tool and the cut and hole-fill tool. This strategy facilitates a much easier digital design. The author's attitude with CAD software is, "If you can keep it calm, it will be good to you."
Figure 3 shows a screenshot of the case. This is not a standard form, but it shows how the case was set up in order to demonstrate the workflow. This case was milled on a large production mill (PrograMill PM7, Ivoclar) in order to manufacture with oversized milling.
The key component to the design phase of this case was to create a strap or some kind of extension that could be clamped down later as part of the technique. This was accomplished in the outline step for the pink base. The framework was designed in order to provide some sort of stop for the acrylic to go down on, and then it could be clamped afterward so that it could be processed accurately. Figure 4 and Figure 5 show the connection portion between the two quadrants; that is how the outline should be handled in the design.
Milling and Processing
Normal protocol was followed for oversized milling, just as with any other case (Figure 6). The next step was the exciting part, as it was much easier than flasking and packing a denture.
The technician started with a stone model on which the denture could be processed. Whether an impression was intraorally scanned or taken via analog methods and poured will determine whether the technician needs to fabricate a duplicate stone model for this technique. The author utilized both: The poured analog became the processing model, and a 3D printed model was used for fitting (Figure 7 and Figure 8). If there is no stone copy of the digital model, one can be fabricated with a traditional hydrocolloid duplicating technique.
After everything was dry fitted, the model was prepared by being soaked for 15 to 20 minutes in water to remove any air bubbles, and then a separator solution was applied. After the model dried and was ready for processing, a small portion of material could be mixed—the author utilized a high-impact pourable acrylic denture base material (Lucitone HIPA, Dentsply Sirona) because the shade was a good match, it becomes runny like honey when it is ready to be used, and it presses quite easily if the material is at the right consistency.
After applying a small amount of the material under the frame, the technician pushed down on the model with the framework to disperse the material through the mesh retention. A small amount of acrylic was then applied, with care being taken to not overload the denture saddle areas; the technician then pressed that down onto the model with finger pressure. Using the clamp, a visual check was performed to ensure that it was fully seated, examining the strap portion across the pallet or the lingual. This also stops the saddles from overseating, and it provides a useful holder so that the material does make a mess.
Once the material had been carefully manipulated and any voids that may have appeared were filled, the case was placed in hot water in a pressure pot for 20 minutes (Figure 9).
The models and partials were removed from the pressure pot, and a normal protocol for trimming and polishing was carried out. The bite was checked to ensure that the process worked correctly, and the case was ready for delivery (Figure 10 through Figure 13).
The clinician and patient were both extremely pleased with the prosthesis. From the laboratory's perspective, a successful outcome was achieved very efficiently and predictably by utilizing this digital process.
About the Author
Dental Laboratory Manager
True North Denture & Implant Centre
Cochrane, Alberta, Canada
Disclosures: The authors had no disclosures to report.