By Robert S. Wisler, CDT, and John W. Farah, DDS, PhD
Journal of Dental Technology, May 2008
Digital dentistry is the future. To be viable and succeed as a dentist and a laboratory owner, both of these professionals need to embrace digital dentistry and, more specifically, CAD/CAM dentistry.
Some of the concerns that laboratory owners have about digital dentistry are working with and selecting new materials and techniques along with the costs associated with these changes. The rising cost of gold and other alloys is adding to the concerns of both laboratory owners and dentists.
Dentists have concerns as well. So many CAD/CAM systems that are advertised all use some type of Zirconia. As a result, dentists wonder, "Is all zirconia the same? What is the difference between all-ceramic materials and zirconia? Will all-ceramics replace PFMs?" In addition, dentists have questions about how restorations should be treated and cemented.
This article will address some of the concerns and present information for all-ceramic restorations that have been compiled by The Dental Advisor over a 25-year period. The Dental Advisor provides dental professionals with evidence-based and clinically relevant information about dental products and equipment and reports objective clinical evaluations, comprehensive long-term clinical performance studies and unbiased laboratory testing in its reports. We will highlight the flexural strength of different types of traditional ceramics and high-strength ceramics. We will present data that will show that the fracture rates of alI-ceramic restorations are similar if not better than PFM restorations. Lastly; we will share our four-year LAVA data and address some of the questions and concerns as they pertain to zirconia.
The Dental Advisor Laboratory Survey conducted in 1987 illustrated challenges that laboratories experienced in working with dentists (Figure 1). One can see that the main issues that faced laboratories 20 years ago are currently very much the same. Communication between the laboratory and dentist was definitely a major roadblock 20 years ago.
Communication and productivity can be vastly improved with digital workflow (Figure 2). Steps can be completed simultaneously rather than sequentially. A milling center can receive the scan and begin working at the same time the laboratory is designing the crown. Scanning intra-orally instead of impressioning provides instant feedback to the dentist, laboratory and milling center. Once the scan is received, the die is digitally cut and margins are marked. The milling center can create the coping and a laboratory technician can begin the plan fur finishing. Numerous steps both by the dentist and laboratories are omitted and time is saved using digital workflow.
How have ceramics changed over the past 20-30 years? Ceramic cores have gotten at least 10 times stronger over the past 20 years. In addition to the strength of ceramic, bonding agents have improved greatly. When used properly, bonding agents can actually strengthen the tooth/ceramic entirely. Bonding agents have been very reliable during the past 15 years. This improvement is evident from ceramic failure data compiled at The Dental Advisor during the past 25 years. The data shows a low fracture rate of all-ceramic restorations (Figure 3).
A total of 5,816 restorations representing nine different ceramics have been monitored and tracked by The Dental Advisor using the following ceramics:
• Cerpress (Leach and Dillon) - 42 percent.
• OPC (Penrron) - 16 percent.
• IPS Empress (Ivoclar Vivadent) - 13 percent.
• 3G (Pentron) - 9 percent.
• IPS e.max (Ivoclat Vivadent) - 5 percent.
• Oprec HSP (Pentron) - 5 percent.
• LAVA (3M ESPE) - 4 percent.
• Lumineers (Den-Mat) - 4 percent.
• Cerinate (Den-Mat) - 2 percent.
Figures 4 and 5 represent the years of service and the type of restorations. Figure 6 depicts the fracture rate of ceramics during the 25-year period and compares the all-ceramic fracture rate to that of PFM crowns. It is clear that all-ceramic restorations are as successful as metal-supported restorations. Dentists and laboratories can therefore use all-ceramic restorations with confidence.
Some clinical unknowns still face dentists. The teeth need to be prepared properly with an adequate reduction and avoidance of sharp line angles. In addition to tooth preparation, dentists need to know the best way to cement these restorations. Zirconia is very hard and not etchable. Sandblasting does help in improving retention by about 3 percent. It is recommended that the restoration be cemented with self-adhesive resin cement such as RdyX Unicem (3M ESPE), Maxcem (Kerr Corporation), GC Fuji Cern Automix (GC America) or Clearfil Esthetic Cement (Kuraray America).
Dentists associated with The Dental Advisor, in partnership with two laboratories, have placed more than 300 restorations made from Lava™ crowns and bridges during the past four years. No failures of the zirconia substructure have been experienced in any of the restorations. However, we have documented the fracture of the veneering material in 10 restorations. At this time, studies are currency underway to determine the causes of fracture. Inadequate coping design, firing temperatures, differences in coefficient of thermal expansion between the ceramic and coping, handling of the coping, and mixing and matching of layering porcelains with zirconia can all contribute to inconsistent results and failure.
Figure 7 shows that Lava™ crowns and bridges performed well in fracture resistance, lack of micro leakage, and minimal wear. The lower rating of the esthetics is based on the fact that early in the technology, the veneering ceramic had not evolved enough in color matching with zirconia cores and technicians had a learning curve in using the ceramic.
A laboratory has many factors to consider before incorporating CAD/CAM restorations into their services. Tracking PFM versus all-ceramic restorations fabricated in your laboratory will provide information about your current volume and future growth patterns. With the rising cost of gold, many laboratories are choosing all-ceramic materials as an alternative solution because of the fixed costs associated with all-ceramic restorations.
One critical factor in this decision is whether or not outsourcing will be profitable. Due to the high expense and training costs of integrating a milling system in your own laboratory, many are choosing to outsource to milling centers. Before partnering with a milling center, it is important to consider the type of support you will receive. Technical support is important, but having marketing and knowledgeable local representatives to assist your business are key ingredients for success as well. The milling center you choose should have strong clinical data and technical research to support the use of their particular brand and provide confidence to you and your dentist clients. Laboratories need to partner with a milling center that has the expertise to provide exceptional restorations that have precise fit of the coping to the die, marginal integrity and uniform porcelain support. A working relationship should be established with a milling center to clarify the following: bridge span length, framework design, preparation requirements, type of mold work needed, type of stone, turnaround time, coping thickness, scannable blockout wax, zirconia frame color charts and shipping costs. The relationship between the laboratory and the milling center must be profitable for both parties.
How does a laboratory convert doctors to the new technology?
• Provide data, articles and information on CAD/CAM and zirconia-based ceramics.
• Provide educational seminars to clients and roundtable discussions on adopting new technology.
• Offer trial programs to your customer dentists in order for them to see the finished product of zirconia ceramic restorations.
• Utilize study modes of preparation designs before and after photographs for education.
• Share that zirconia-based restorations offer several key advantages to dentists: metal-free restorations with comparable strength to PFM, bio-compatibility and positive tissue response, fixed costs and consistent pricing per unit, excellent esthetics and simplicity of cementation.
The future of CAD/CAM is very positive. Zirconia-based ceramics provides an intimacy of fit and superior esthetics that has excellent vitality and translucency. Costs are very competitive with other types of all-ceramic restorations. Long-term clinical data now exist on Lava™ crowns and bridges, as well as other all-ceramic restorations. Additional studies are needed on full-contour zirconia crowns in contact with enamel (wear rates), as well as the shading of zirconia. All ceramic restorations have proven their efficacy.
It is important to be knowledgeable and remember that all zirconia ceramics and CAD/CAM systems are not created equal. Mixing and matching copings with veneering ceramics is not recommended. Work within a system of manufacturer guidelines for success. Zirconia-based ceramics and CAD/CAM, along with digital dentistry, are the future due to labor costs and consistency of automation. Laboratories can get involved with minimal investment and choosing the right outsourcing partnership.
Together with dentists, laboratories can learn and grow with this new technology as it changes and develops into the future. CAD/CAM smart sourcing involves a thorough review of the way you do business and how to successfully partner with others. Decisions need to be made by the laboratory about how to deliver consistent esthetic results to dentist clients with a profitable return on investment. Future success depends on your ability to view technology and outsourcing as a positive solution to your daily challenges.
The Authors Answer Common Zirconia Questions
Q: Does zirconia ceramic degrade intra-orally in strength after 5·10 years?
Yes, all-ceramic materials degrade over time, however, some do so more than others. The strength of zirconia starts at 1200 MPa; even if it degrades 5- 10 percent. the strength is still significantly higher than other ceramics.
Q: Is it harder to cut through a zirconia crown than a PFM crown?
Not necessarily, using fine- and medium-grit diamond burs one can cut through a zirconia crown almost as fast as a PFM crown. In fact, some base metal alloys are very difficult to cut through.
Q: Should the dentist silanate the restoration before cementing?
No. Silanating does not add to the bond strength.
Q: Should I use water-spray when grinding on zirconia?
Absolutely, the white light or hot spot you see between the diamond and coping has been measured at 1500°C. If the temperature of zirconia rises above 1000°C, a phase change can occur which will Induce cracking and surface defects.
Q: How important is preparation design to the strength of zirconia?
Preparation design Is critical. Rounded line angles and a circumferential sloping shoulder are ideal. The zirconia coping should be designed anatomically to support the veneering porcelain. The thickness of the overlaying ceramic should not exceed 2mm.
Q: Is the fit of zirconia crowns and bridges as good as PFM crowns and bridges?
Yes, the marginal fit of zirconia crowns and bridges can be as goad or better than PFM crowns and bridges, due to the advanced parameters of the design software.
About the Authors:
Robert S. Wisler, CDT
Wisler owns Alpha Dental Studio, Inc., eDL, in Farmington Hills, Mich. He graduated from Ferris State University's Dental Technology program in 1979 and became a Certified Dental Technician in 1982. He spent the first 10 years of his career as an in-house technician for three general dentists who specialized in crown and bridge and implants.
John W. Farah, DDS, PhD
Farah is a graduate of the University of Michigan. His degrees include a dual PhD in dental materials and aerospace engineering, received in 1972, and a DDS in 1978. Farah taught at the University of Florida and the University of Michigan. He initiated THE DENTAL ADVISOR in 1983. He and Dr. John Powers are co-editors of the publication and work closely with approximately 300 dental professionals to evaluate and rate new products and equipment.