May 2010, Volume 6, Issue 5
Published by AEGIS Communications
Question: Which do you prefer, high-strength ceramics or porcelain-fused-to-metal?
The esthetic revolution is moving full speed ahead, sort of. We have experienced a bit of pushback from the clinical community in the adaptation to ceramic veneered zirconia-supported restorations. The keys to our results are proper case selection (adequate occlusal reduction, no sharp line angles on preparations), proper framework design (properly supporting the veneering porcelain), maintaining the use of high-strength material (many aftermarket products have been introduced with lower cost and lower flexural strength) and high-fusing ceramics properly fired and cooled. The documentation on these topics is significant. The proper firing and cooling rates are crucial to long-term clinical performance. The unknown factor is how many clinicians are adjusting these restorations occlusally and re-firing incorrectly, creating stresses within the porcelain leading to clinical failure. The proper firing procedure for the specific ceramic veneered over zirconia should be communicated to the laboratory clients.
We are experiencing a new level of interest in metal-free restorations with IPS e-max® (Ivoclar Vivadent) and full-contour zirconia (Drake Zircast). The lithium disilicate product particularly has captured the attention of clinicians and laboratories with thin veneers at 0.3 mm. This was previously only available through a labor-intensive, high-skill stacked porcelain restoration to refractory or foil.
Full-contour zirconia crowns as a substitute for cast gold on second molars has generated a major level of interest from clinicians. Many patients are resistant to the esthetic limitations of cast gold. The material does not exhibit great vitality due to high opacity compared to conventional ceramics, but provides an option for high-strength clinical performance with acceptable esthetics in the most posterior region.
The attachment to porcelain-fused-to-metal (PFM) has remained strong, largely due to a lifetime of success from clinicians in practice 20 years or more. Coupled with more frequent fracture of all-ceramic restorations of any type, it is easy to understand the hesitation of many. The generational adaptation to newer technology combined with improvements in materials and methods will certainly bring change to the type of restorations prescribed and delivered, in due time.
Conventional PFM restorations still comprise about 70% of the indirect crown-and-bridge restorations prescribed by dentists in the United States. They have a long track record of service and performance in the profession since appearing as the “esthetic alternative” to full-gold crowns. When using veneering porcelain with a properly matched coefficient of thermal expansion, the long-term esthetic and functional result can be quite good if the proper amount of space is prepared by the dentist to allow the technician to create a sufficient thickness of ceramic that will have the strength to withstand occlusal forces and be naturally esthetic as well. Herein lies the challenge. Traditional feldspathic porcelains have a strength rating of about 80 MPa to 110 MPa. At an optimal thickness of 1 mm to 1.5 mm in an occlusal stress-bearing area, this can be quite sufficient. The problem is the resistance to shear stress. In cases where either the thickness of the material is compromised or the porcelain is not supported adequately by the substructure, this may not be enough to withstand flexural forces of mastication resulting in fracture of the restoration. From an esthetic perspective, it has always been a challenge for the ceramist to create “lifelike” esthetics in a conventional PFM, particularly in the cervical third of the restoration because of the lack of light transmission through the metal substructure.
High-strength ceramics, such as IPS e.max, present clinical advantages in both of these areas of strength and esthetics. With a strength rating of around 400 MPa, these materials have four times the flexural resistance as conventional feldspathic porcelains. This can be huge when ideal thickness for support and/or substructure support cannot be achieved. Also, as an all-ceramic material that allows light transmission through the core, lifelike esthetics is far easier to achieve than with conventional PFM restorations.
For almost 20 years now, we have seen high success rates with the alumina-based ceramics for all-ceramic crowns for single-unit restorations from the bicuspid forward as the failure rate has been similar to PFMs. For first molars, we have seen a slightly higher failure rate than PFMs, and for second molars we have seen about a 2% failure rate, which is roughly double that of PFMs. Thus, alumina-based restorations clearly can be recommended as an alternative to PFMs for anterior teeth and bicuspids. In the last few years there has been discussion about some chipping and cracking problems with the porcelain veneered to zirconia copings; this has been a significant problem with our clinical cases as well, especially with our early cases. We believe most of the problem is because of a thermal conductivity problem with the porcelain and zirconia. We have been working for several years with very modified firing schedules to combat this problem. Since we have altered our technique, our remakes for chipping have been reduced to less than a percent annual failure rate. Thus, we feel comfortable recommending zirconia-based restorations if all design and firing parameters are followed.
The reality is that many dentists’ comfort zone is still with metal-ceramic restorations, and so a good place to start would be anterior all-ceramic, perhaps up to the first bicuspid, and then do posterior PFM restorations. If your comfort zone is to use a PFM restoration for posterior teeth (assuming the tooth needs to be crowned) you can do just as good esthetics with the PFM in the posterior as any all-ceramic restoration, assuming proper preparation and framework design. We have had a lot of experience with Captek restorations; they have proved to be an excellent alternative to conventional PFMs.
I have no reservation in recommending single all-ceramic crowns—either zirconia- or alumina-based—and, with the proper adhesive technique, using the e.max material for single-tooth restorations. For small bridges with proper connector design I would recommend zirconia from good, and for larger fixed partial dentures I would use conventional metal ceramics.
About the Authors
David Avery, CDT
Mr. Avery is the Director of Professional Services at Drake Precision Dental Laboratory in Charlotte, North Carolina.
Robert A. Lowe, DDS
Dr. Lowe is a Diplomate of the American Board of Aesthetic Dentistry and has a private practice in Charlotte, North Carolina.
Edward A. McLaren, DDS, MDC
Dr. McLaren is the founder and Director of the UCLA Center for Esthetic Dentistry Residency Program, and the founder and Director of the UCLA Master Dental Ceramist Program in Los Angeles, California.