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.
Figure 1
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.
Figure 2
New Ceramics
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).
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:
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.
Figure 6
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.
Figure 7
Laboratory Considerations
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
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.
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.
Should the dentist silanate the restoration before cementing?
No. Silanating does not add to the bond strength.
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.
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.
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.
