Smaller Particles, Big Potential

A Q&A With Julian Conejo, DDS, MSc, assistant professor of clinical restorative dentistry and director of chairside digital dentistry for the Division of Restorative Dentistry at the University of Pennsylvania School of Dental Medicine

Inside Dental Technology (IDT): What constitutes a nano-hybrid?

Julian Conejo, DDS, MSc (JC): It is basically a type of resin with very small particles. The smaller the particles in any resin, the better the esthetic outcomes we can achieve because we are able to polish it at a higher degree. What is the compromise to achieve this high level of luster? In a way, it is the flexural strength. That is why we have gone from composites that have bigger particle size that were mostly indicated for the posterior region, to give us more longevity in areas with high masticatory forces, to now getting the smaller components to be able to get what we want in the esthetic zone.

IDT: How important is the bondability factor?

JC: We have invested a lot of time answering this question. When we are talking about milled restorations, we have created in our classification a group called resin matrix ceramics. Within this group, we have two divisions. One, this type of microhybrid/nanohybrid composites that are machines, and two, the hybrid ceramics. So we need to differentiate between a resin block and a hybrid ceramic that also contains resin, because these resins we are focusing on today, these restorations milled from these resin blocks, are not etchable.

With veneers, inlays, and other indirect restorations, we are used to etchable materials such as feldspathic ceramic and lithium disilicate. But these types of resin are not affected by acid etching. So we need to understand this and switch to air particle abrasion. The effect of air particle abrasion with small aluminum oxide particles in an average of 50 microns will create a nice micromechanical retentive surface. This air abrasion will also create or elevate the area where we are going to bond. Once this is done, we can achieve mechanical interlocking. But we want to combine the mechanical features with chemical bonding as well. That is why the following step would be an application of a silane coupling agent. Silane has been used for many years for silica-based ceramic, but it also has an effect on these types of resin. So, in summary, we have to air abrade and apply a silane coupling agent, and that is the way we treat these resins before bonding.

I do not want to extend too much, but it is also important to understand that bond strength will be dictated by both surfaces. We are talking about the surface of the restoration, but also we need to be very detailed with the treatment of the abutment tooth. If you are working on a veneer where you have a lot of enamel, you need to etch your enamel with phosphoric acid and then apply your bonding agent. If you are working on a more intracoronal restoration design, such as an inlay, where you have more dentin within your preparation, you are not going to etch, you are going to apply your bonding agent. So we need to be very clear on how to treat both surfaces, to make sure we do not have a weak link in our bond strength.

IDT: So it is similar to bonding to zirconia?

JC: The first step is the same, but the second part varies. For composites, you want to use a silane. For zirconia, you must use a primer that contains MDP—the phosphate monomer that has the ability to bond to oxides.