An Interview with Russell Giordano, DMD, CAGS, DMSc

Russell Giordano, DMD, CAGS, DMSc, an Associate Professor in the Department of Restorative Sciences & Biomaterials at Boston University’s Henry M. Goldman School of Dental Medicine, discusses the future of indirect restorative materials.

Inside Dental Technology: Are any new indirect restorative materials on the horizon to challenge the domination of zirconia and lithium disilicate?

Russell Giordano: Ideally, any restorative material should be reliable, reproducible, and damage resistant. With respect to millable indirect materials, they should machine quickly, be "bur kind," and require minimal post machining finishing — polishing only is ideal. Chipping should be minimal. The material should also allow for minimal tooth reduction and have a range of translucency to either match natural dentition or work in cases where some opacity is needed to mask discoloration.

Hybrid ceramic materials such as Enamic (VITA North America, vitanorthamerica.com) hold promise to meet these criteria. It is easy to machine (4 minutes for a crown); is bur kind (140 blocks per one bur set); and requires only polishing after machining. It is chip resistant and has some resilience that conventional ceramics lack. It also is more fracture resistant at lower thicknesses than conventional glass ceramics and has a minimum thickness of only 1.0 mm, almost matching zirconia’s approximately 0.7 mm. I think there will be new combinations and formulations based on that ceramic structure.

With respect to conventional ceramics, alumina is an overlooked material. It can have better translucency and lacks all of the potential handling problems that exist with zirconia. There are no problems with veneer chipping or with excessive phase transformation and degradation. Conventional composite resin block-type machinable materials may also further improve with additional research and are currently replacing indirect composite resins chairside. The density is high, contacts may be designed better, and overall fabrication time can be less than packing composite by hand, layer by layer.

Current and new blocks with layered "enamel/dentin" structures will also be important in furthering the esthetics of machinable ceramics.

IDT: How much impact do you expect the new partially and fully sintered zirconias will have chairside and in the laboratory?

RG: Zirconia for chairside applications is too challenging for most dentists and will not easily fit into their workflow — even though fast firing does now allow a single zirconia restoration to be ready in approximately 20 minutes. Zirconia restorations must also be carefully polished, and adjustments chairside may be time-consuming.

Fast fire technology will help laboratories improve productivity. However, sintering protocols for the maximum number of units must be followed carefully; otherwise, the density and microstructure will not be correct, which affects the translucency, strength, and stability.

IDT: Do you believe that in a majority of cases all-ceramic materials are being used for their proper indications, or would other materials sometimes serve the patient better?

RG: Most professionals in the dental community look at strength as the sole criterion for material selection. That is completely wrong. Strength values, which are determined under perfect conditions in the research laboratory, bear little relation to how the material will react in the mouth. The strength of a material changes over time, especially in the oral environment.

Indirect restorative materials, on the other hand, rely on a variety of factors regardless of how resistant to damage and fracture they are. Dentists must execute proper tooth preparation, the right amount of reduction, and good bonding techniques. Of these factors, the biggest problem is adequate reduction. The indirect restorative materials we currently use require from 2.0 mm down to 0.7 mm minimum thickness. Again, it is not just strength. If adequate thickness is not achieved, then load-bearing capacity is too low and the restoration will fail.

IDT: Do you foresee 3D-printed crowns becoming a reality in the near future?

RG: I was fortunate enough to work with developers of 3D printing technology at the Massachusetts Institute of Technology. Development of 3D printing/rapid manufacturing technology for the production of fully dense all-ceramic restorations is several years away. Issues remain due to the low density of printed ceramics. Density issues relate back to particle sizes and packing of powder beds for printing and/or suspension density for ink jet-type systems. The relatively low density requires proper compensation for shrinkage, and small differences in density often cause warping.

The ultimate manufacturing outcome for this technology would be incorporating spatial shade data from a digital shade device and feeding this information into a 3D printer. The printer would build up the crown layer-by-layer in a shade that exactly matches the patient’s natural dentition.