Issue: Volume: 23 Issue: 10 (October 2000)

Digital Dentistry




By Audrey Doyle

Patients visiting Dr. Carl Boscketti have a reason to smile. The co-owner of a small dental practice in Exeter, New Hampshire, Boscketti uses a new CAD/ CAM system that saves his patients the time and discomfort traditionally associated with repeat visits to the dentist's office for restorative dental work. It also frees up Boscketti's time so that he can see more patients on a daily basis. "This is one of the best things to come along for the dental profession in a long time," he enthuses. "My patients love it, and I love it. It's an integral part of my practice. I use it for 80 percent of the restorations I do."

The system to which Boscketti is referring is the Cerec 3, a modular, Windows NT-based CAD/CAM system from Sirona Dental Systems (Bensheim, Germany). Originally developed at the University of Zurich by professor Werner Mömann and Dr. Marco Brandistini, the Cerec methodology has evolved from creating single- and dual-surface inlays to one that can produce multisurface inlays, single onlays, veneers, and posterior and anterior crowns. And perhaps the most innovative aspect of the Cerec system is the fact that the restorations are digitally designed and built by the dentist in his or her office within a matter of minutes, rather than by hand at an off-site laboratory over the course of a few weeks. That means patients can have restorative work done on their teeth in just one appointment, rather than the usual multiple visits.
Dentists such as Dr. Carl Boscketti are using Sirona's Cerec 3 CAD/CAM application to more quickly and accurately model crowns and other restorative tooth fixtures.




Traditionally, dental restorations are created by hand at laboratories by skilled dental technicians. Although dentistry is a nontraditional market for CAD/CAM, the use of CAD/CAM tools and technologies in dentistry isn't limited just to the Cerec system. For instance, Atlantis Components (Cambridge, MA) has developed a method that uses 3D CAD software and computer numerically controlled (CNC) milling machines to create custom abutments, pieces of metal that connect a full crown to a fixture (the part that anchors the crown to the patient's jawbone). Another company, Align Technology (Sunnyvale, CA), has developed a procedure that combines 3D modeling and animation software and stereolithography machines to design and manufacture customized orthodontic appliances for adults. (For more information on these technologies, see "Making an Impression," pg. 52.)

Although Atlantis and Align offer their systems in the form of a service they provide to dentists, Sirona's Cerec system is actually used by the dental practitioner. "We're putting the CAD tools into the hands of the dentists," says Steve Sutton, director of Cerec support in North America. He estimates that thousands of dentists worldwide are using a version of the Cerec system.

The term "Cerec" is derived from ceramic reconstruction, or the replacement of missing tooth substance with ceramic material. The Cerec 3 system incorporates a mobile PC that's equipped with a flat-panel monitor; a proprietary, high-precision intraoral measuring camera connected to the PC via cable; and an Intel microprocessor-controlled image capture card. In addition to displaying the grayscale images generated by the camera, the proprietary card also processes the images in real time. The PC runs proprietary, Windows NT-based Cerec 3 CAD software that has been optimized for dental applications and does not require any special computer knowledge. The final component, the proprietary six-axis Cerec 3 milling unit, is a tabletop device that communicates with the camera via a wireless RS connection and features cylindrical and conical diamond burrs for speedy and precise milling. The SiroCAM 2, an intraoral color video camera, is available as an option.

Use of the system is relatively straightforward. Let's say a patient needs a crown placed on his or her tooth. After administering anesthesia to the patient, the dentist drills away any decay remaining on the tooth, and preps it by painting it with an imaging liquid and then covering it with a special opaque powder. The dentist then positions the small, handheld camera above the tooth and simply takes a picture of it. Within a few seconds the 3D image appears on the computer screen. "The imaging process replaces the impressions everybody hates in dentistry," says Boscketti. "That whole process of obtaining an impression of the tooth-putting the gunk in your mouth and then letting it sit there for a few minutes to harden-is eliminated."
With the Cerec system, dentists can digitally design and build tooth restorations within minutes right in their offices. (Images courtesy Sirona Dental Systems.)




Next, the dentist or dental assistant uses the CAD software to design the crown on the computer screen. Using a trackball and simple CAD commands, the operator defines the equators of the neighboring teeth as well as a baseline on the tooth that needs restoration. The 3D software then determines the size of the required crown based on the size, height, and orientation of the neighboring teeth. "The software is really easy to use. It's like using a big Etch-a-Sketch," notes Boscketti.

After the crown is designed, it is ready for milling, a fully automated process that takes from 10 to 15 minutes, depending on the crown's size and complexity. The Cerec 3 CAD software tells the operator what size ceramic block is required. After inserting the corresponding block into the milling unit, the operator clicks an icon on the screen to initiate the milling process. When the milling is complete, the crown is bonded into place using conventional ceramic bonding techniques and then polished.

According to Boscketti, the Cerec method offers numerous advantages over traditional techniques. Dentists usually administer novocaine to the patient, drill away the decay, take the impression, and fit the person with a replacement crown. The impression would then be sent to a lab, and a few weeks later the dentist would replace the temporary crown with a permanent one. "With the Cerec system, I can service my patient in just one visit that lasts a little over an hour," says Boscketti. "He doesn't have to worry about temporary crowns that crack or fall out."

The system also has other benefits. Traditionally, Boscketti says he would have to drill away the decay and build up the tooth again using core material-metal pins, or plastic which he would then have to drill down so that he would have something stable to attach the crown to. "All that drilling weakens your teeth," he points out.

But with the new system, Boscketti is milling the crown out of solid ceramic that works well with to day's strong bonding adhesives. There fore, he doesn't need to use pins to secure the crown in place, nor does he have to drill as much of the original tooth. "This not only saves the patient discomfort, but also it reduces the chance of fracturing neighboring teeth," Boscketti says.

Of course, there are a few limitations to the Cerec system. "At this point, it can't do bridges, or multiple-tooth restorations with a connection piece between the teeth," comments Sutton. The company, however, is planning to release a CAD package next year that will accomplish such a procedure.

Another limitation is the Cerec 3's $90,000 price tag, which includes "everything the dentist needs to begin using the system," Sutton says. Although Sutton admits that some small dental offices might find the price high, he claims that the return on investment is typically less than two years. Indeed, Boscketti notes that he achieved an ROI within a year after making his purchase.

According to Sutton, dentists and their patients aren't the only people excited about the Cerec technology. "Other software companies have approached us with proposals that could make the system work even faster and more efficiently," he says. "And because Cerec is NT-based, a platform that's fairly common in the industry, the Cerec PC could be used to also run different off-the-shelf technologies-commercial imaging programs or programs that can morph teeth to show the patient a before and after image before the work is done."

Even without these capabilities, the Cerec system is still an innovative use of CAD/CAM technology, and it shows how CAD tools are be coming easier to use by people in nontraditional markets.

"We took existing optical imaging, 3D CAD, and CNC milling technologies-which have been used in other areas for years-and simplified and miniaturized them for use by dentists so that they can design and build restorations themselves," Sutton concludes. "So now, dental visits no longer have to be feared or dreaded, or thought of as a long, drawn-out process. Instead, they can be pleasant, convenient experiences that let people receive the proper necessary dental treatment but in a less painful and more efficient way."
After the dentist digitally designs a crown using Cerec, a CNC milling machine automatically creates the new dental piece from a ceramic block.




Audrey Doyle, a contributing editor to Computer Graphics World, is a freelance writer and editor based in Boston. She can be reached at audreyd@mediaone.net.




In addition to Sirona, other companies are capitalizing on CAD/CAM technologies for dental applications. Atlantis Components, founded in 1996 by a Boston prosthodontist, uses CAD/CAM software and hardware to develop customized restorative implant components.

Usually, the best way to permanently replace a missing tooth is with a dental implant, which consists of an implant fixture, an abutment, and a crown. A hole is drilled in the patient's jawbone, and the implant fixture is placed in the hole. After the bone heals, or osseointegrates, around the implant, the abutment is mounted on the implant, mimicking the natural shape of a tooth emerging from the soft tissue. The crown, which replicates the contour and appearance of the visible portion of the replaced tooth, is then mounted on top of the abutment. The abutment is a crucial component of a dental implant because it determines the fit, form, and functionality of the total implant.

According to Atlantis, currently available off-the-shelf abutments present some limitations. First, they're cylindrical in shape and, therefore, bear little resemblance to the contours of the natural teeth they replace. Also, stock abutments are available only in standard sizes and shapes, requiring modification by the dentist or a dental lab.

As an alternative to stock abutments, dentists can order custom abutments from dental labs. With this process, after the bone osseointegrates, the dentist cuts open the soft tissue and mounts a temporary "healing abutment" on the fixture to provide the correct soft tissue emergence. Then, using impression material, the dentist takes an "index" locating the implant with respect to the neighboring teeth and sends this information to the dental lab. Using this as reference, a skilled technician builds a wax model of the abutment by hand, and then casts it in metal using the lost-wax casting technique.
Atlantis uses CAD and CNC milling to create customized abutments for affixing a crown to a patient's jawbone that more precisely fits into the gap left by the original tooth. (Images courtesy Atlantis Components.)




However, this process also presents limitations. Not only does it take several weeks to complete, but also, creating dental implants by hand is a highly specialized skill.

With the Atlantis abutment, the index can be taken when the implant is first placed, and the abutment can be placed when the soft tissue is first opened, eliminating a visit to the dentist's office and providing the patient with a tooth weeks earlier. This is possible because the shape of the Atlantis abutment is very close to the shape of a natural tooth, so the gum tissue looks natural as it heals.

The process begins with a plaster model of the teeth created by the dentist from an impression of the patient's mouth. To re-create the exact location of the fixture in the plaster model, a separate impression also is taken of the implant itself. Atlantis scans the plaster models using an off-the-shelf optical 3D scanner and then imports the resulting point cloud of data into Raindrop Geomagic's (Research Triangle Park, NC) Studio software, running on an NT-based PC, where the data is turned into a 3D surface model representing the teeth. Atlantis then digitally modifies a 3D abutment-model template so that the abutment model fits the 3D surface model of the patient's mouth precisely. "The surface model we create in Geomagic Studio is used only as reference geometry so that we can modify our 3D abutment model to fit it," explains Bethany Grant, Atlantis engineering program manager.

The final step involves creating a tool path and machining the abutment in titanium on a CNC milling machine. Atlantis then delivers the abutment to the dentist. Although the process currently takes about a week today, Grant says Atlantis is working to reduce the turnaround time to hours.

For adults who require orthodontic work on their teeth but don't relish the thought of wearing metal braces, Align Technology offers Invisalign, a system that uses a series of customized plastic "aligners." The aligners exert enough pressure on the patient's teeth so that the teeth are gradually repositioned. Nearly invisible, the aligners fit snugly over the teeth and are replaced every other week with new, recalibrated aligners until the patient's teeth are in the final desired position.

The Invisalign treatment begins with an orthodontist taking an impression of the patient's mouth. The orthodontist sends the impression, along with a diagnosis and treatment plan, to Align, which casts a mold from the impression and, using a 3D scanner, scans the mold into an NT-based workstation. From there, the company uses proprietary software called Treat to generate a 3D animation that represents the patient's teeth moving from their original state to their final, treated state. After the orthodontist approves the treatment based on the animation, Align uses 3D Systems' (Valencia, CA) SLA-7000 stereolithography machine to produce a series of customized, clear polymer orthodontic aligners.
Using digital design techniques, Align Technology creates customized orthodontic devices for adults. (Images courtesy Align Technology)




The patient wears each aligner for about two weeks before switching to the next one in the series. Because the desired tooth movement is built into each customized aligner, patients need to revisit the orthodontist only once every six weeks to make sure the aligners are working properly. With traditional braces, patients must visit the orthodontist once every two weeks to have the braces tightened. -A.D.
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