Bonding discoveries
For a few years now, I’ve bonded all of my crowns. In one office, I used Multilink Automix Next Generation (Ivoclar Vivadent), and in another I used RelyX Unicem 2 (3M). Both resin cements worked consistently as long as I followed the manufacturer’s instructions.
Before I go on, I should tell you that my dental philosophy is to take a few extra steps now to reduce treatment headaches later. This applies to lubricating provisional restorations with Vaseline, for example, and wrapping floss with a double knot in embrasures of pontics prior to temporary cementation to make excess cement removal a breeze.
To avoid dealing with the headache of an unhappy patient, I do what I can to ensure my crown doesn’t debond. But why not just use a luting cement and forget about trying to bond? Personally, I think luting cements are so white and opaque that they destroy any lifelike translucency we can achieve with glass and zirconia restorations. In other words, don’t worry about taking a stump shade if you’re using a luting cement because all of the natural shading of the prepared tooth will be blocked out by the luting cement.
Say you are using a resin cement to bond an indirect restoration. After trying in the crown and preparing to insert, you’re typically supposed to do the following:
• Clean the tooth (i.e., use pumice).
• Decontaminate the intaglio of the restoration.
• Prepare the intaglio with some type of adhesive or priming agent to link the cement to the crown. (Some brands of cement, like Unicem, don’t require a separate primer because it’s already built in.)
• Cement and tack cure, and then clean up the excess.
I began to realize, however, that the step I wasn’t paying much attention to was one of the most important ones: decontaminating the intaglio of the restoration, which is the true source of the zirconia debonding issue.
Bear with me as we discuss the chemistry involved: The zirconia we use in dentistry is zirconium oxide, and the oxide part is where all the bonding happens. It is the functional phosphate groups that bond to the zirconia. But wait, what else contains tons of phosphate? That’s right, saliva. Therefore, once a crown is tried in the patient’s mouth, all of the potential bonding sites on the zirconia crown are now irreversibly occupied by phosphate groups. (3,4) Once this happens, we need to remove these phosphate groups to make the oxide sites ready to bond again.
This is easier said than done. Wiping with alcohol doesn’t do anything. Sandblasting can do it—however, most manufacturers recommend you leave the sandblasting to your laboratory. Some claim that excess microetching with aluminum oxide can increase future fracture risk. However, microetching is an important step because it’s theorized to increase bond strength by increasing surface energy of the intaglio surface. (5,6) Porcelain etch (hydrofluoric acid), although helpful for glass restorations like lithium disilicate, doesn’t do anything clinically relevant to zirconia. Oh, and phosphoric acid etch? Big no-no. Phosphoric etch just makes the problem worse by flooding whatever oxide groups aren’t already occupied by phosphate with, you guessed it, phosphate groups.
How do we remove the phosphate from the oxide sites we need to bond to? There are two reliable methods: Ivoclean (Ivoclar Vivadent) and Sodium hypochlorite (NaOCl).
Ivoclean is superconcentrated zirconia oxide in a liquid suspension (so make sure to shake well before using). It acts like a magnet to remove all the phosphate from the crown by creating a concentration gradient. Rinse off after 20 seconds, and now you’ve got a zirconia crown with all of its oxide groups ready to accept a bond from your cement. (7)
Another recently discovered method is to just wipe the intaglio of the crown with gauze that is soaked in 5% sodium hypochlorite. The theory is that NaOCl breaks down the phosphate groups so they can no longer bond, and it can be removed by rinsing with water followed by air drying.8 Bleach, then bond—voilà!
In conclusion, it’s possible to bond zirconia as long as you (1) effectively decontaminate the tooth and the crown and (2) bond following the instructions of the cement manufacturer.
Author’s note: The author would like to thank Mark Hartslief, BSc, RDT, and the New York Center for Digital Restorative Solutions (NYCDRS) laboratory and knowledge center for fabricating the pictured restorations and their contribution to this case study. NYCDRS can be contacted at (646) 757-5840 or [email protected]. Visit them at nycdrs.org.
References
1. Seghi RR, Denry IL, Rosensteil SF. Relative fracture toughness and hardness of new dental ceramics. J Prosthet Dent. 1995;74(2):145-150.
2. Christel P, Meunier A, et al. Mechanical properties and short term in-vivo evaluation of yttrium-oxide-partially-stabilized zirconia. J Biomed Mater Res. 1989;23(1):45-61.
3. Yang B, Lange-Jansen HC, Scharnberg M, et al. Influence of saliva contamination on zirconia ceramic bonding. Dent Mater. 2008;24(4):508-513.
4. Kweon HK, Hakansson K. Selective zirconium dioxide-based enrichment of phosphorylated peptides for mass spectrometric analysis. Anal Chem. 2006;78(6):1743-1749.
5. Blatz MB, Alvarez M, Sawyer K, Brindis M. How to bond to zirconia: the APC concept. Compend Contin Educ Dent. 2016;37(9):611-617.
6. Kern M. Bonding to oxide ceramics—laboratory testing versus clinical outcome. Dent Mater. 2015;31(1):8-14.
7. Wolfart M, Lehmann F, Wolfart S, Kern M. Durability of the resin bond strength to zirconia ceramic after using different surface conditioning methods. Dent Mater. 2007;23(1):45-50.
8. Rosentritt M, Behr M, Hahnel S, Preis V. Surface treatment on shear bond strength of high translucent zirconia. Poster presented at: International Association of Dental Research General Session (IADR); March 22-25, 2017; San Francisco, USA.
Alec J. Ganci, DDS, FICOI, attended Stony Brook School of Dental Medicine, completing an oral surgery externship in Madagascar. He continued his training as a resident at North Shore University Hospital, serving as chief resident, and completed a two-year fellowship in advanced prosthodontics and implant dentistry. He is an industry consultant and lectures extensively on various topics, including implants, dental materials, digital dentistry workflows, and esthetics. He can be reached at [email protected].
