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Quickly and accurately fabricating single crowns
Delivering modeless restorations using CAD/CAM technology
Advancements in CAD/CAM technology have helped dental laboratory technicians streamline many aspects of their day-to-day work. Not only does the technology aid in the creation of large, complicated, full-mouth restorations, but it also has revolutionized the production processes involved in producing everyday restorations such as the single crown, making them faster and easier to manufacture with unprecedented accuracy. On the clinical side, advancements in digital impression technology offer practitioners the ability to digitize an impression and email the scan file to the laboratory for creating a restoration without the need for a working model. The concept of fabricating and delivering a crown without a working model is a difficult one for some laboratory technicians and clinicians alike. However, the accuracy delivered by digital impression data along with the precision settings in the CAD software—including proximal and occlusal contact strengths, as well as internal space—eliminates the need for a physical model.
These technological advancements have led many laboratories to modify their business models and reorganize their production methods. One example is in the fabrication of the single crown. Single crowns, which account for the majority of cases for many laboratories, today can be designed digitally and milled instead of hand-waxed and cast or pressed. There are several ways a laboratory can approach the CAD design and CAM fabrication of a modeless monolithic zirconia restoration, and by keeping the production processes simple, accurate, and cost effective, the finished product can offer an accuracy and consistency unattainable using analog production methods.
What follows is an account of the author’s approach to using CAD/CAM technology to create modeless crowns, and how using diverse CAD/CAM technologies can improve the laboratory’s workflow processes.
Receiving the Case
When a clinician sends a digital impression to the laboratory, an email alerts the technician that a new case has arrived in-house. The technician logs into his or her digital design network. In this case, the author uses Sirona’s Connect Portal (www.SironaConnect.com). The technician reviews the impression data as well as the work ticket and downloads the digital impression. The data and work ticket can be printed if the technician wishes to keep paper records.
The ability to access case files as soon as they arrive allows the technician to view the impression data while the patient is still in the chair. This affords the opportunity to communicate any issues with the clinician before the patient leaves the dental office to ensure there is no missing data and that there is adequate tooth reduction for the prescribed restorative material (Figure 1 and Figure 2). Once reviewed, the technician officially accepts the digital impression, and a notification of acceptance is automatically sent to the clinician.
Prescription and Material Selection
For this case, the author’s laboratory fabricated a monolithic zirconia crown for tooth No. 18 using only digital methods. The author chose to use BruxZir® preshaded monolithic zirconia (www.glidewelldental.com) for its demonstrated strength (>1200 MPa)1 and esthetics, especially in posterior cases. Also, because it does not require layering, BruxZir was a very cost effective option, using up less technician time and materials.
The Design Process
The administration window of most CAD design software programs allows technicians to make any necessary changes to a case order (Figure 3). In this particular case, the software indicates that tooth No. 18 will be designed as a crown using the Biogeneric Individual mode, which means that the program’s proposal will try to match the anatomy of the adjacent teeth.
Once the material selection has been made, the virtual model will be set in axis (Figure 4). The clinician marked the margin; however, adjustments can be made if needed by clicking “Draw Margin.” Next, the insertion axis is defined (Figure 5). Technicians must be prudent to ensure that the insertion axis is accurate; otherwise there may be problems in calculating the restoration’s design.
Setting the correct parameters during the digital restorative process is important when CAD designing a modeless restoration. It especially helps to know the clinician’s preferences for contacts and occlusion. In most cases, an accurate digital impression and clearly defined parameters will result in minimal to no adjustments chairside. The parameters used for this case show that the choice to consider milling instrument geometry is turned off. This is because it will be milled on a 5-axis milling machine and not the related inLab MC XL 3-axis mill (www.sirona.com).
With the parameters set, the software program calculates a design. The technician then makes any adjustments to shape, anatomy, contacts, or occlusion as needed. The author believes that it is best to make minimal adjustments in order to ensure streamlined production and best accuracy of the final product. During the design process, different colors indicate different degrees of contact. Very dark blue or red indicates heavy occlusion, and light blue or white indicates light or negative occlusion (Figure 6 through Figure 9). In this particular case, the occlusion is taken to light blue to avoid the need for chairside adjustments (Figure 10).
Export Files in STL
Once the design is complete, the next step is to adjust the mill position (Figure 11). If the technician saves the CAD design file as an STL File (*.stl) (Figure 12), the design can then be transferred to nesting software, such as SUM3D (www.CAP-US.com), and sent to the third party, 5-axis milling machine, such as the Roland DWX-50, (www.rolanddga.com). In order to have the option of converting an inLab file to *.stl file, the author’s laboratory had to purchase an inLab open architecture license, which renews annually. The nesting software requires an annual fee as well. The author chose to use a third party 5-axis mill because of its stability, which provides more accurate internal fitting and detailed anatomy on restorations at .5 mm, a higher production rate, and the ability to mill more complex strategies if needed. In the case of the DWX-50, between 20 to 25 units can be milled at a time from one puck, and the machine can be left to mill overnight.
Once the design is transferred to the SUM3D software, the technician creates a new job, selecting zirconia as the material and the DWX-50 as the milling unit and hits “Save”. From there, the technician selects the new file and chooses the type of restoration, in this case, “anatomic crown.” The design will appear onscreen with sprues placed, but they can be adjusted if needed.
Next, the technician will import the machining parameters by choosing the type of restoration (crown) and the milling burs (diamond-coated or carbide). The author’s laboratory prefers to use diamond-coated milling burs because they offer superior longevity compared to carbide burs.
The next step involves choosing the best type of machining for milling the restoration. In this case, a fast substructure was chosen with the option to trim the connectors in order to save the technician time. When deciding which zirconia puck to use, the technician can either choose from a library of partially used pucks based on puck thickness (in this case, 12 mm), shade (in this case, 100), and available space, or create a new puck for a particular restoration. The software saves this information and shows on which pucks previous restorations were milled.
After the design’s inclination is reduced to save space, it is automatically positioned and placed in the puck by the software (Figure 13). The technician loads the puck into the milling machine, the file is completed, and the CNC file is sent to the Roland DWX-50 for milling (Figure 14).
Milling and Finishing
Once milled, the restoration is cut out of the zirconia puck using a high-speed carbide bur, the sprues are removed using a diamond-impregnated silicone wheel, and the restoration is cleaned in an ultrasonic cleaner (Figure 15), and is dried in a microwave.
The desired shade of this restoration is A1, with A3 at the gingival. The zirconia is milled from a pre-shaded puck for A1, so additional coloring before sintering is not necessary. To attain the shade A3 at the gingival, it could have been stained after sintering during the glazing cycle. However, to add depth to the central fossa, where the anatomy is shallow, and to lessen the stain needed to reach the A3 shade on the gingiva, coloring liquid (BruxZir® Coloring Liquid www.glidewelldental.com) is added. The restoration is placed in a drying cabinet (Figure 16) for an additional 15 minutes before being placed into a sintering bowl containing zirconia beads. The beads allow the restoration to move as it shrinks during the sintering process (Figure 17). The bowl is then put into the sintering oven (inFire HTC, www.sirona.com) (Figure 18). During the sintering process, the oven will reach a temperature of 1530 ̊ C, hold for two hours, and then slow cool. The total oven time is eight hours.
After sintering, the restoration is prepared for the final glaze (Figure 19). The occlusion of the restoration is polished using a white silicone wheel (Figure 20) and then lightly sandblasted with 50-micron aluminum oxide, cleaned in distilled water in the ultrasonic cleaner, and dried.
A thin layer of glaze paste (e.max® FLUO, www.ivoclarvivadent.com) is applied evenly to the restoration. Additional character stain is added to the central fossa (Figure 21) before sending the restoration through a nine-minute zirconia glaze cycle, which is a 450° C increase from 100° C to 900° C with no hold or slow cool. Once cooled, the crown is polished with Diashine fine polish (www.vhtechnologies.com) using a Robinson’s soft bristle brush #11 (www.buffalodental.com). As the final step, the restoration is cleaned, placed into a gel box, packaged into a sealed bag, and made ready for delivery (Figure 22).
Expanding technological choices in production processes combined with the ability to deliver a modeless restoration provides laboratories with an economical approach to manufacturing. By employing the inherent precision that CAM technology and CAD software offer as well as the technical expertise of a knowledgeable technologist, a laboratory can improve the production efficiencies of everyday bread-and-butter restorations, increase the quality and consistency of the final product, and positively impact the bottom line.
The author would like to thank Eric Klumb, DDS, for providing the case, and Jack Griffin Jr., DMD, (www.eurekasmile.com) for his continued support.
The author would also like to thank Dan Becker, CDT, owner of Becker Dental Laboratory in Herculaneum, Mo., and photographer for this article for his support, knowledge, and commitment to dental technology.
1. BruxZir® Restorations Deliver More Lifelike Results. BruxZir® Scientific Validation. Updated 2013. Accessed April 10, 2013. Available at: http://www.bruxzir.com/science-bruxzir-zirconia-dental-crown/.
About the Author
Theresa Parker, CDT
Ceramist and CAD/CAM designer
Becker Dental Laboratory