By Dr. Michael DiTolla
My first exposure to solid zirconia was while I was the director of clinical education and research at Glidewell Laboratories, where we began to experiment with the material in 2007 and made it available to dentists in the summer of 2009. From the first time I placed a solid zirconia crown in a patient’s mouth, I have been fascinated by the material. The rapid growth of the material within the laboratory indicated that solid zirconia was filling a void that existed among indirect restorative materials: the need for a high-strength, cementable, tooth-colored crown. At the 2016 Chicago Midwinter meeting, Dentsply Sirona introduced CEREC Zirconia, a single-visit solid zirconia crown that could be fabricated in about the same amount of time as other chairside crown materials.
When speaking to the dentists who acted as beta testers for the CEREC Zirconia product, one of the most frequent comments I heard was how much they enjoyed rediscovering the joy of conventional cementation. Since zirconia oxide has the highest flexural strength of the all-ceramic crown materials, it can be cemented regardless of how thick the restoration is. In my opinion, cementing solid zirconia is preferable to bonding it into place because of the challenge of cutting off these crowns. I have found that cutting off a cemented solid zirconia crown takes half as long as cutting off a bonded crown. My advice to dentists is to cement their solid zirconia crowns routinely, only adhesively bonding them when needed due to a short, overtapered, or otherwise nonretentive prep.
The high flexural strength of solid zirconia allows dentists to fabricate crowns with a minimum material thickness of 0.6 mm when clinical conditions dictate minimal reduction. However, solid zirconia crowns that are fabricated at 0.6 mm cannot be adjusted; although if the occlusion is high, the opposing tooth needs to be adjusted instead. So while the minimum material thickness of solid zirconia is 0.6 mm, a more ideal reduction for solid zirconia is 1.0 mm. At this thickness, you can make an occlusal adjustment or 2 to the crown without having to worry about violating the minimum material thickness. Furthermore, CEREC Zirconia is stronger at 1.0 mm than it is at 0.6 mm, and gets even stronger at 1.5 mm (if clinical conditions allow this much reduction).
Perhaps the biggest advantage of solid zirconia is that it can tolerate featheredge margins. This ability allows laboratory teams to fabricate solid zirconia crowns in cases where otherwise they would have to ask the dentist to re-prepare and re-impress the tooth, such as with a lithium disilicate crown. Solid zirconia can adapt to nearly any preparation without dentists having to change what they do.
In February 2016, Dentsply Sirona introduced the CEREC SpeedFire furnace, making it possible for dentists to do a single-visit solid zirconia crown in about the same time as a lithium disilicate crown. The Table compares the workflows for the 2 restorative materials mentioned in this article.
In addition to being a sintering furnace, the SpeedFire also acts a staining and glazing oven, allowing a dentist to have one furnace that can sinter solid zirconia, lithium disilicate, and stain/glaze these restorations as well. The accompanying case report and Figures illustrate all the steps required to fabricate a single-visit solid zirconia crown.
Diagnosis and Treatment Planning
The patient presented with an amalgam filling in his mandibular right second molar (No. 31) (Figure 1). He had symptoms of cracked tooth syndrome and was very tender to biting on a Tooth Sloth Tongue Cleaner (Pearson Dental) on both the mesiobuccal and the distolingual cusp. Upon examination, cracks were evident in the lingual, mesial, and distal tooth structure. It was decided that a full-coverage option was the best choice, and the patient agreed to proceed with a single-visit in-office CAD/CAM crown. In this case, a high-strength zirconia crown was planned using the new CEREC Zirconia mono L block and CEREC SpeedFire furnace (Figures 2 and 3).
|Figure 1. Tooth No. 31 had a previously placed occlusal amalgam with cracks in the tooth structure in 3 separate locations. A full-coverage option was indicated.
|Figure 2. CEREC SpeedFire (Dentsply Sirona), an induction furnace that is controlled completely by software that allows crystallization of high strength zirconia in roughly 15 minutes. It also can also glaze most glass ceramics, making it the first “all-in-one” furnace.
|Figure 3. The CEREC SpeedFire is run by the software and included in the software as a device. When the zirconia is milled, the software will automatically transfer the proper program to the CEREC SpeedFire based on shade and volume of zirconia used.
Clinical and In-Office Laboratory Protocol
After the administration of the mandibular block for anesthesia (Septocaine [Septodont]), No. 31 was prepared (Medium/Fine Chamfer bur 856-012/856F-012 [Meisinger USA]) for the full-coverage crown. Zirconia is much more tolerant of thin chamfer and even knife-edge preparations, allowing the dentist to be ultraconservative with the tooth reduction. In this case, the axial reduction was less than 0.5 mm (Figure 4). Occlusal reduction was at roughly 1.5 mm, allowing enough reduction for proper anatomic form of the final crown (Figure 5). Because of the strength of zirconia, we can prepare the tooth less and still get proper anatomic form because it can tolerate thinner occlusal thicknesses. There is also no need to prepare the tooth to the gingiva or subgingival if enough retention form is present.
|Figure 4. Facial aspect of final preparation. In this case, axial reduction was less than 0.5 mm.
|Figure 5. Occlusal view of the final preparation. The occlusal reduction was at roughly 1.5 mm.
|Figure 6. After imaging with the CEREC Omnicam (Dentsply Sirona) and marginating the preparation, one can observe the conservative 0.5-mm axial reduction.
|Figure 7. Occlusal view of the preparation from the CEREC software. Note the clean, conservative, and accurate visualization of the margins.
|Figure 8. Initial proposal with the new CEREC 4.4.3 biogeneric software (Dentsply Sirona) (note: this software should be released by the time of publication). Note the positioning of the tooth, cusps, and proper adaptation to the opposing dentition forming proper occlusion. Using the Biojaw Algorithm, the global analyzing of the arch gives proposals needing very minimal adjustments.
|Figure 9. Because of the strength of zirconia, one can prepare using conservative anatomic reduction. Final fissure thickness is only 0.99 mm.
|Figure 10. The software will prompt you to choose the final CEREC Zirconia shade (Dentsply Sirona). This has a direct influence in the sintering times.
|Figure 11. CEREC Zirconia mono L block. You can see there is a quality label on top of the picture as well so that the software can verify it is an approved zirconia to use with the CEREC SpeedFire.
|Figure 12. Dentsply Sirona’s milling units now come with the option to add dry milling for zirconia.
|Figure 13. The dry-milled CEREC Zirconia crown as immediately seen after the milling process. Note the smaller sprue area from carbide milling in comparison to the larger sprue when using wet diamond grinding.
|Figure 14. The finishing kit (Meisinger USA) from which the fine pink twist polisher was used to carefully accomplish pre-sintering finishing and polishing at low RPMs. It allows removal of finer bur marks and increased definition of the final product.
|Figure 15. CEREC Zirconia crown after fine prefinishing with the pink twist polisher (Meisinger USA).
The preparation was then imaged (CEREC Omnicam [Dentsply Sirona]) and digitally marginated (Figure 6). The tooth was then designed using the CEREC 4.4.3 biogeneric software (Dentsply Sirona) (Figures 7 to 9). (Note: This software was not released yet at the time of this case preparation, but should be by the time this article has gone to press.) A shade was determined before preparation. The software will prompt the clinician to choose the final CEREC Zirconia shade (Figure 10) as this will have a direct influence on the sintering times in the furnace.
After the design process was completed, the dry milling of the restoration using the CEREC Zirconia mono L block was done (Figures 11 to 13). Dry milling zirconia offers the advantage of not having to worry about predrying it before the sintering process, thus decreasing its processing time. It’s also milled 23% larger with smaller finisher burs (10 finisher), which will help develop more precise fits, occlusion, and detail.
Pre-sintering finishing and polishing was then carried out carefully at extremely low RPMs using appropriate finishing instruments (Meisinger USA) (Figure 14). The pink twist polisher (found in the middle area of the finishing kit) delivered a fine prepolish without damaging the zirconia (Figure 15).
Next, to help internalize color, and to increase the aesthetics of this restoration, the zirconia was infiltrated with water-based infiltration liquids before sintering (Figures 16 to 22). The easiest way to apply these fine infiltration details is with a Zig Detailer Watercolor Brush (available at amazon.com). You can load these brushes with the water-based infiltration liquids and apply to the pre-sintered zirconia. The finished detailing can be seen in the sintered zirconia crown, as removed from the furnace, in Figure 23.
After sintering, the final polish was imparted to the zirconia crown using all 3 Meisinger twist polishers (green, blue, and pink, as seen in Figure 14). Polishing the post-sintered zirconia to a mirror finish will create a smooth surface that is very kind to the opposing dentition (Figure 24). After final polish, one has the option to do additional staining and glazing if desired, or one can simply cement the polished restoration, if preferred. In this case, Lustre Pastes (GC America) were also used to add a little more color and glaze to the final restoration (Figure 25).
|Figure 16. When desired, to internalize color and increase aesthetics of zirconia restorations, the zirconia was infiltrated with water-based infiltration liquids before sintering using a Zig Detailer
|Figure 17. Water-based incisal blue Aquarell infiltration (Zirkonzahn). This is used over the cusp tips and marginal ridges to help simulate translucency.
|Figure 18. Water-based incisal violet Aquarell infiltration. This is used to further help simulate the enamel translucent effect in the zirconia.
|Figure 19. Water-based Okklusal Amber shade effect (Whitepeaks Dental Solutions). This coloring liquid helps create occlusal depth and dentin show-through effect.
|Figure 20. Zirkonzahn incisal blue and incisal violet loaded into the Zig Detailer Watercolor Brushes.
|Figure 21. Painting incisal blue infiltration liquids on to the pre-sintered zirconia restoration.
|Figure 22. Final CEREC Zirconia crown with incisal blue, incisal violet, and Okklusal Amber infiltrated and ready for final sintering.
|Figure 23. Crown immediately out of the sintering furnace (after 15 minutes of speed-sintering). Note that the infiltrating colors are very subtle in their effects of creating depth.
|Figure 24. After sintering, the zirconia crown was polished using all 3 of the Meisinger USA twist polishers (green, blue, and pink, as seen in Figure 14).
|Figure 25. After final polishing, Lustre Pastes (GC America) were used to add a little more color and glaze to the final restoration.
|Figure 26. The completed restoration was luted into place using a glass ionomer cement (FujiCEM 2 [GC America]).
Finally, the completed restoration on tooth No. 31 was luted into place using a conventional glass ionomer cement with (FujiCEM 2 [GC America]) (Figure 26). One of the great advantages of zirconia with retentive preparations is the option to conventionally cement the restoration. This increases both predictability and efficiency, especially with second molars. The final aesthetics are a little bit opaque with ultra-high strength zirconia, but having a cementable and tooth-colored restoration, with exquisite fit and minimal preparation on a second molar, is a decided advantage for the patient. Our patient was extremely pleased with the final results and, in addition, with the latest technologies and techniques used for the single-visit in-office preparation, fabrication, and delivery.
Dr. Skramstad, a 2000 graduate of the University of Minnesota School of Dentistry, maintains a restorative practice in Orono, Minn, focusing on aesthetic, implant, and CAD/CAM dentistry. He is an alpha/beta tester for Sirona Dental Systems and works closely with the engineering team to help with hardware/software development. A product consultant for multiple dental companies, he tests and evaluates many products prior to market launch and has published numerous articles on materials and CAD/CAM. He is a certified advanced CEREC trainer and has lectured internationally on technology, implantology, and digital dentistry. He is a resident faculty member in the CAD/CAM department of Spear Education and cerecdoctors.com, a nationally renowned website and continuing education center in Scottsdale, Ariz. He can be reached at firstname.lastname@example.org.
Disclosure: Dr. Skramstad is a CEREC educator at Spear Education.
Dr. DiTolla was in private practice for 15 years before becoming director of clinical education for Glidewell Laboratories. In 2015, he became director of clinical affairs for Sirona Dental. He is also director of SIROWORLD, Sirona’s official community for fans of its digital technologies. He has lectured on behalf of Clinicians Report, presenting its iconic “Dentistry Update” lectures from 2012 to 2015. In 2011, he received the “Most Effective Dentist Educator” award in a national survey of dentists. He can be contacted via email at email@example.com or through his website, located at drditolla.com. Listen to his podcast Accidental Geniuses, which can be found at accidentalgeniusespod.com.
Disclosure: Dr. DiTolla is director of clinical affairs for Dentsply Sirona.