3D Printed Permanent Crown Materials

Inside Dentistry (ID): How do the current 3D printable materials that are indicated for permanent crowns and bridges perform in terms of bond strength?

Nathaniel Lawson, DMD, PhD (NL): In our laboratory, we have achieved bond strengths with 3D printed crowns that are similar to those that we have achieved with glass-ceramic lithium disilicate. Of course, in the laboratory, we can also get similar bond strengths with zirconia, but even though there have been clinical trials with good zirconia bonds, the clinical trust in the zirconia bond is not what it is for lithium disilicate. Nonetheless, the results that we've seen for 3D printed crown materials in the laboratory are promising.

ID: What protocol do you utilize to bond these 3D printable materials?

NL: Most 3D printed crown materials consist of some form of resin with filler particles inside, so they are very similar to resin composites. Although there can be some slight variations between these materials based on their filler content or lack thereof, I would suggest the same bonding strategy for all of the currently available ones in this area. Start by sandblasting with aluminum oxide particles to roughen the intaglio surface of the restoration. Then, apply a methacrylate-based adhesive; most of the leading brands of universal adhesives on the market qualify. Finally, use a resin cement. This protocol is based on research that we have conducted in our laboratory, which we will be publishing as an abstract at the annual meeting of the American Association for Dental, Oral, and Craniofacial Research later this month. We tested several combinations, and some other elements can be added to this protocol based on preference, but in my opinion, the most efficient bond to a 3D printed crown material is achieved with sandblasting, a bonding agent, and a resin cement.

ID: Why not use a silane primer?

NL: When bonding 3D printing materials, some clinicians might consider using a silane primer like what is used with glass-based ceramic materials. The filler particles in these 3D printing materials are typically silica (ie, glass), so the theory is that you could bond to some of those particles by silanating the composite first. However, in our studies with 3D printed crowns, using a silane primer was not as effective as using an adhesive as the primer. We also examined the results of using them together—silane and then adhesive—but that did not make a significant difference.

ID: What do the manufacturers typically recommend in that regard?

NL: At least one company that I am aware of produces a special adhesive primer for its 3D printed crown material. I do not know its composition, but its consistency looks like that of an adhesive, so I would guess that it does not contain silane. Unfortunately, many manufacturers do not provide instructions for bonding their materials; they leave that to the cement companies. But the cement companies are not focused on 3D printed crown materials, so it is somewhat of a no-man's-land. That being said, the few manufacturers that I am aware of who provide those types of instructions do recommend using silane. I do not know the basis for those recommendations; however, based on what I have tested in the laboratory, I would put my money on sandblasting, adhesive, and resin cement.

ID: You mentioned that one company offers a special adhesive primer. Given the propensity for 3D printing materials to be part of closed systems, will we see more specific bonding materials from those manufacturers?

NL: I could envision that happening.

ID: Would there ever be a reason to etch these materials?

NL: Etching a 3D printed material with an acid would be a huge waste of time. Moreover, when we have tested it, at least with bonding composites, etching can actually be detrimental because it pulls out filler particles, which decreases the bond strength. To effectively roughen a 3D printed crown material for bonding, you really need to sandblast it.

ID: Are there any significant differences between the materials within the category that might make bonding to some easier than others?

NL: The resin composition of 3D printed crown materials can vary substantially. We have done chemical analyses of certain resins, including some that are Bis-GMA, some that are UDMA, and some that are unique resins not used in other dental composites. However, the commonality is that they are all methacrylate-based, which means that the sticky end of the molecule is similar for all of them. That is how they are all able to polymerize with each other, and it is the basis for the theory that they should all be able to be bonded with a bonding agent.

ID: Do the surface characteristics of 3D printed restorations make a difference?

NL: We have not studied that, but I would speculate that sandblasting makes that irrelevant. You might have different surface characteristics based on the orientation of the crown in the printer, but that should not be distinguishable after sandblasting.

ID: How does the post-curing process for 3D printed crowns impact their ability to be bonded?

NL: Of course, the curing process for 3D printed crowns is different from that of milled composite crowns, which are cured with heat while under pressure. We have theorized that 3D printed crowns might not achieve the same degree of convergence after curing as milled ones and that this difference may lead to better bond strengths because some unpolymerized parts of the resin—parts that have not joined up with chains of resin—might be available for bonding with adhesives and cements. However, this is speculative; we do not have data to support this theory. We also had a theory that the length of time between when a crown was printed and when it was bonded might impact the strength of the bond that could be achieved. We thought that if a crown was bonded immediately after it was printed, there might be more uncured resin present to improve the bond. However, when we tested that theory, we did not see any difference in the bond strengths achieved between crowns that were bonded within an hour of being printed and those that were bonded a couple of days later.

ID: Can the cleaning protocols used for 3D printed restorations impact bond strength?

NL: Following manufacturers' instructions for washing 3D printed crowns is important because not doing so can negatively impact their strength. For example, we have tested certain materials that actually become weaker when completely submerged in the washing stations. The strength of the bonds that can be achieved with these materials, however, should not be directly impacted by their washing protocols.

About the Expert

Nathaniel Lawson, DMD, PhD, is the director of the Division of Biomaterials at the University of Alabama at Birmingham School of Dentistry. He has published more than 200 articles, abstracts, book chapters, and periodicals related to dental materials.