Moving to Monolithic
Inside Dental Technology delivers updates on digital workflows, materials, lab techniques, and innovation in dental technology through expert articles and videos.
January 1, 2011
Volume 2, Issue 1, January 2011
Page(s):
Chip-proof, esthetic, strong, biocompatible, durable, price-competitive—these are just a few of the qualities that have many laboratories turning to full-contour crowns and bridges produced from CAD/CAM-milled monolithic materials. These restorative solutions have no porcelain overlay material to risk sheer or fracture, nor do they require specialized pressing techniques and equipment. They are constructed of solid monolithic zirconia, solid ceramic, or a combination of the two materials fused into a solid structure that can withstand the chewing forces of the posterior regions of the mouth and yet exhibit the esthetic qualities demanded in the anterior. Better yet, they bring laboratories a step closer to full automation and can be sold at a price that is competitive with offshore products.
“I think the ability of manufacturers to provide customers with a material that, when finished, is price-competitive with offshore products makes milling full contour so attractive,” says Rita Acquafredda, vice president and general manager of Zahn Dental. The company is now introducing a full-contour solution (www.zahndental.com) that gives dental laboratories two options—they can outsource it to the CMC Milling Center in Arvada, Colorado, by sending their .STL scan files or a model, or they can purchase the material as a milling disc for in-house production. Any CAD/CAM system that accepts a 100-mm milling disc can mill the material, with each disc producing anywhere from 33 to 44 single crowns. Soon, the discs will also be available in a 98-mm disc size.
Full-contour zirconia restorations first appeared on the market in February 2010 with Glidewell’s BruxZir® Solid Zirconia crowns and bridges (www.bruxzir.com). Designed for patients who grind their teeth and typically destroy conventionally processed crowns or bridges, BruxZir restorations claimed indestructibility with their high strength and durability. Now the rest of the market is following suit, with a host of new indications for zirconia and all-ceramic products.
What held the market in limbo for nearly a year were concerns that zirconia had the potential to wear down natural dentition at a much faster rate than conventional indirect materials if it were not treated properly. But it seems those fears have been abated as several manufacturers have or are planning to get into the full-contour zirconia milling game.
“Before we release a new product or new indication, we make sure it will perform to the expectations of the dentists and patients,” says Colin Norman, director of Digital Materials at 3M ESPE. Keenly aware of industry concerns about zirconia’s abrasiveness to natural dentition, the company extensively tested its newest indication for the market—the Lava™ All Zirconia restoration (www.3MESPE.com). “We ran a battery of in vitro tests before we felt confident enough to release Lava All Zirconia restorations,” Norman says. Indicated for milling full-contour single crowns, three- to six-unit bridges, long-span bridges, and cantilever bridges, Lava All Zirconia is a new indication for the existing Lava zirconia milling material.
Acquafredda says tests of highly-polished, full-contour zirconia restorations have suggested they are actually less abrasive than conventional porcelain-fused-to-metal crowns and bridges. 3M ESPE’s in vitro tests have shown that the wear characteristics exhibited by polished zironia is surprising low, according to Norman. “It does not cause drastic wear of opposing enamel,” he explains.
Unlike glass-based materials, chewing forces have not been found to make zirconia rougher. However, polishing of monolithic zirconia is required. One question that still remains unanswered is what happens to that restoration in the hands of the dentist. If the delivered final crown or bridge requires chairside adjustment, excessive grinding with the wrong instruments could change the bulk properties of the zirconia, leading to strength deterioration.
For laboratory owners whose clients request full-contour zirconia restorations, the anatomically contoured crowns and bridges can be surface-polished using standard polishing burs, followed by a polishing paste such as Zircon-Brite (DVA, Inc, www.dentalventures.com) and a fine-coated diamond bur to achieve the high shine. Norman claims the wear of a zirconia restoration is long lasting when a high-shine polish has been applied to its surface.
While there are surely more full-zirconia solutions to come, several are currently available or soon will be, including a new offering from Zahn Dental, 3M ESPE’s Lava All-Zirconia, Zenostar® crowns and bridges by Wieland (www.wieland.com), and One Source Dental’s Millenium Concept restorations (www.one-sourcedental.com).
At next month’s Midwinter Meeting in Chicago, Ivoclar Vivadent (www.ivoclarvivadent.com) will launch the IPS e.max CAD-on, a new processing technique that enables dental laboratories to create zirconia-based IPS e.max bridges. This restorative solution will round out the IPS e.max line by offering a definitive esthetic solution for the posterior region that does not require veneering porcelain and is stronger than a pressed overstructure to zirconia.
The substructure is milled from IPS e.max ZirCAD zirconia and the veneering structure is milled from IPS e.max CAD lithium disilicate. “Instead of layering or pressing a feldspathic or fluorapatite glass, which is only about 100 MPa in strength, we now can mill out a much stronger full anatomical lithium disilicate overstructure for the top that is 360 MPa,” says Jason Obrokta, marketing manager of All Ceramics at Ivoclar Vivadent. The two halves are fused with Crystall./Connect, a ceramic slurry that bonds the lithium disilicate and zirconia into a strong, solid structure. The company has developed a 40-mm lithium disilicate milling block for the Sirona inLab® MC XL milling unit (www.sirona.com). The newest Sirona software enables expanded applications for IPS e.max CAD and IPS e.max ZirCAD, which can be combined to create up to four-unit bridges.
Obrokta says it takes about 30 minutes to mill a three-unit bridge, depending on morphology, and a single 40-minute firing cycle to simultaneously crystallize the blue state IPS e.max CAD and fuse the ZirCAD framework. “What makes this solution efficient is that you can mill the IPS e.max CAD bridge while you are sintering the IPS e.max ZirCAD framework,” Obrokta explains. “This gives laboratories the opportunity to turn around a three- to four-unit bridge in a few hours, if they are using the fast sintering Programat S1.”
Midwinter attendees will also get the first look at an innovative way to characterize milled full-contour restorations without applying stains and glaze. Zahn Dental plans to introduce Hilla, a digital transfer “decal” film that fuses to any milled monolithic material to give the crown or bridge the depth of a highly esthetic, multilayered porcelain product. “It is a digital transfer on a printed porcelain film,” Acquafredda explains. “The film is applied to the restoration, providing predictable characterization every time. It can be applied to porcelain, zirconia, or any all-ceramic material.”
Now all dental laboratories need is to custom digitize each film by integrating the manufacture process with a digital shade device, and the ability to automate the application and fusing process for full automation.
