by Ariel Raigrodski, DMD, MS
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CAD/CAM technology has opened many doors to the dental community in terms of what can be provided for patients in support of function, longevity, and esthetics. What is important is that it enables us to use esthetic and durable materials, such as alumina, zirconia, and titanium, which we otherwise would not be able to use. It also allows us to process homogenous copings, bridge frameworks, and implant abutments at a level of quality we would not be able to achieve without this technology.
The fundamentals of design
As we know, marginal integrity is of paramount importance to the success of indirect restorations. Research has shown that adequate marginal integrity can be achieved with CAD/CAM restorations with high levels of success. Some might think that marginal integrity depends primarily on the type of material used, but a combination of factors contribute to the quality of a restoration.
The software used to design the restoration is important, as is the operator of the system. It is vital to have a technician who understands the concept of marginal integrity when designing a framework. Also, the communication between the dentist and dental laboratory is critical for clinical success.
During the past few years, we've gained the ability to customize copings for crowns, bridges, and implant abutments at a high level. While we used to wax-up to full contour for crowns and bridges, then cut back and create the framework, today we can do it digitally.
We have the capability to take full-contour wax-ups and master casts, scan them, and then superimpose the images, giving us a better ability to design the framework. In a full-mouth reconstruction, we can now scan and design the copings in a way that facilitates optimal support for the veneering porcelain, which is often the weak link of a restoration.
Successful complete coverage restorations
Five years ago, we started a study on posterior three-unit fixed partial dentures, which we placed in 16 patients. It was our first opportunity to use zirconia-based bridges.
These restorations had a minimal connector surface area of 9 square millimeters. All restorations were fabricated with zirconia margins and cemented conventionally with RMGI.
Our initial results were published in 2006 in the Journal of Prosthetic Dentistry, and demonstrated that in the short term the restorations did well. It was encouraging to see no failures in the zirconia itself.
An additional concern during this study was the bond between the porcelain and zirconia, but we did not observe any delamination between the zirconia and veneering porcelain.
Few patients experienced a minor chipping of the veneering porcelain, mainly on second molars, but this did not require replacement of the restorations.
After five years, the results continue to be promising, although we have seen two fractures of the veneering porcelain on the second molars, which were considered as failures. We have not seen any problems with marginal integrity, discoloration, or radiographic data.
The interesting thing is that zirconia-based restorations were introduced to respond to the need for strong posterior restorations. The thought of achieving esthetics with zirconia-based restorations was initially farfetched — no one thought it would be a good material for the esthetic zone.
At the University of Washington, we've learned that we can use a zirconia coping in the anterior segment with a thickness of 0.3 mm in the cervical area, and then design it anatomically to provide adequate support to the veneering porcelain in other areas.
There was some concern that a coping that thin might increase the likelihood of fractures, but all of the restorations we have studied are doing well so far.
The marginal integrity has been acceptable, and we have not had problems with marginal discoloration, recurrent decay, or periapical lesions. We are encouraged about using this technique in the esthetic zone, as the restorations appear to be highly esthetic.
Custom implant abutments
Custom implant abutments may be critical in the esthetic zone, and zirconia is showing promising capabilities for this application as well. We use the same concepts to design custom implant abutments as we use for crowns and bridges. This allows us to provide patients with implant-supported restorations with custom zirconia implant abutments.
With this technology, we now have the ability to control the design of esthetic copings, frameworks, and custom abutments. In the future, we will see increasing interaction between dental labs and dental practices regarding concepts of design and applications.
As software evolves, the dental lab community must recruit and train skilled technicians capable of mastering this sophisticated technology at the highest level. The results of improved skills among dentists and dental technicians will be more esthetic and functional restorations for our patients.
If dental professionals and dental technicians focus on quality design and techniques, the materials and technologies will continue to evolve and facilitate better patient care and a brighter future for dentistry.
Note: Dr. Raigrodski presented this information at 3M ESPE's Global Symposium. To watch his complete presentation, visit www.3MESPE.com/symposium.
Dr. Ariel Raigrodski is associate professor and director of graduate prosthodontics at the Department of Restorative Dentistry at the University of Washington. A diplomate of the American Board of Prosthodontics, he serves on editorial boards of several dental journals. He lectures nationally and internationally on implants, all-ceramics, and CAD/CAM technology, and holds private practice in Kenmore, Wash. Reach him at [email protected].
