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Processing SLM Overdentures
An efficient and predictable workflow that includes practical use of digital and conventional techniques
Alexander Wuensche, CDT
Treating the dental needs of the baby boomer generation presents an exciting opportunity and challenge for the dentist and technician alike. This is especially true in edentulous and partially edentulous situations. Many patients have active, demanding lives and consequently have high expectations regarding restoration functionality, ease of servicing, and, of course, the esthetics of the restorative outcomes.
More patients in this demographic are enhancing their understanding about the advantages of dental implants. Due to this development, implant-supported partials and dentures are an attractive option for treating these cases. Achieving a predictable, yet cost-effective, result is easier today with the use of CAD software, digital workflows, and alternative manufacturing processes such as selective laser melting (SLM) technology. In addition to various bar designs and attachments, many technicians are finding that SLM tertiary structures provide a secure connection for the overdenture to the bar, while providing a more rigid housing for the attachments. SLM tertiary structures can also provide excellent support for the acrylic and denture teeth. Managing these critical connections can offer a positive step toward higher patient satisfaction and better long-term outcomes.
Using modern digital workflows and a variety of techniques, laboratories can help patients achieve treatment goals that involve restoring function, providing long-term stability, and improving ease of serviceability, while being cost-effective and esthetic. In the following article, the author will show an efficient and predictable workflow that includes practical use of digital and conventional techniques, yet results in a highly desirable hybrid, implant-supported denture.
The SLM Process
For the design of certain types of restorations, the use of scanners and CAD software has become commonplace in supporting our daily work. Along with casting or milling, we also now have additive workflows such as 3D printing for creating models or wax patterns, or selective laser melting for producing metal frameworks.
SLM is an additive production process. In contrast to the subtractive methods such as milling or spark erosion, 3D-printed objects are built layer by layer, appearing to rise out of dust. One advantage of SLM is that a large number of parts can be fabricated in parallel, which means a short production time per unit and a reduction in cost per unit.
During SLM, the first powder layer is applied to the production platform and a laser beam outlines the border of each part to be produced. When the borders are solidified, the laser beam melts the inner material (called the hatch process, or hatching). For this process, specific strategies have been developed, which influence the speed and quality.
After the first layer is solidified, the production platform is lowered by the amount of the virtual thickness of the next slice to be produced and a new layer of powder is applied. Then the solidification starts over. The whole procedure is then repeated until all slices are completed.
When producing implant-supported metal frameworks using traditional methods, we all know the challenges of casting internal tertiary frames for overdentures. These require a perfect fit over a titanium implant bar in order to spread masticatory forces and deliver friction and/or carry attachments (Figure 1 and Figure 2). At the same time, we are looking for a good surface structure that can support the bond between the acrylic and tertiary frame. Casting in CrCo requires experience and knowledge and still takes time for processing the design, investing, casting, divesting, and fitting. If the fit is not acceptable, the whole procedure has to be started again.
These types of substructures can now be produced in a fully digital procedure. However, all preliminary work such as impression taking, verification jig, and wax setup has to be done using conventional methods. After the wax setup is complete and the decision is made for processing the overdenture, the case moves to the CAD/CAM department for scanning.
Production Steps for SLM-Printed Bars
Heretofore, the bar has to be screwed on and the attachments are positioned. These areas are blocked out and then scanned (Figure 3). Afterward, the wax setup is scanned on the same model like the previous bar. Scanning of the opposing arch is not needed because the wax setup was tried in, and we already have enough information about teeth position and vertical height. Having everything digitalized, the case is imported into the CAD construction software and the design process can begin.
First, the margin must be marked all around the lower bar margin (Figure 4). Then an offset coping needs to be designed to cover the bar surface. From this point, the uploaded wax setup can be used to help design proper retention for the teeth and acrylic (Figure 5 and Figure 6). An important factor is to make sure that enough coverage is on top of the attachments or attachment housings.
After the CAD design is finished, the restoration can be uploaded into the File Generator for sending the design as an STL file to start the SLM process.
The construction must be fitted on the titanium bar, which takes no more than 5 minutes. After fitting, the attachments are incorporated into the frame, either with acrylic or certain bonders used (Figure 10 through Figure 12).
The next step is to transfer the wax setup on the tertiary frame for a final try-in (Figure 13).
When all components are tried in and verified, the restoration can be finished as usual (Figure 14 through Figure 16). This digital workflow process can be used for cases involving Locator (Zest Anchors, zestanchors.com), Hader Bar® (Sterngold, sterngold.com), Preci Bar, ball attachment, MK1 Universal Dental Attachments (Advanced Implants, advancedimplants.net), and many more.
Using a digital workflow allows the dental laboratory to stay competitive and deliver a high-quality and economically priced product using an efficient, labor-saving process that can’t be duplicated using conventional casting methods.