16
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CERECDOCTORS.COM
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QUARTER 1
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2016
M AT E R I A L S
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B Y D E N N I S J . FA S B I N D E R , D . D . S . , A . B . G . D .
a recent innovation by dentsply sirona has introduced the fabrication and delivery
of full-contour zirconia
crowns in a single appointment. This is a great opportunity to review the current status of zirconia as a restorative material and
consider how it will complement the existing restorative material options with CEREC.
Chairside Full-contour
Zirconia Restorations
A Strong, Fracture-resistant Alternative Restorative Material
Zirconia is different from all other chairside CAD/CAM mate-
rials in that it is a polycrystallinematerial that does not contain glass
particles. Thequalityof zirconia is a functionof theprocessingof the
raw materials by the manufacturer to minimize porosity, enhance
density, and ensure a homogenous material. The crystalline struc-
ture of zirconia also has a unique ability to resist crack development.
Zirconia is stabilized to be a tetragonal crystal at room tempera-
ture. If a crack is initiated in the zirconia, the energy at the leading
edge of the crack causes a phase shift to monoclinic crystals. The
crystal phase shift results in a localized volumetric expansion that
creates a compressive force on the developing crack, preventing it
from propagating. This process is called Transformation Tough-
ening, and is a significant contributor to thehigh strengthof zirconia
and its ability to resist chipping and fracture under function.
Early zirconia crowns were veneered with feldspathic porcelain
to improve the esthetic appearance of the crown because zirconia is
opaque and less translucent thanglass containing ceramics.However,
the chipping of the veneer porcelain ledmany dentists to be discour-
aged with their use. Full-contour zirconia crowns have become
preferred due to the simple fact that the surface of a full-contour
zirconia crown is resistant to chipping or fracture. This avoids the
primary failuremechanismof porcelain veneered zirconia crowns.
Zirconia is generally too hard to mill in the sintered, or processed
state. Itwouldbeverydestructive tomillingdiamondsaswell as result
in excessive mill times. Instead, zirconia is machined in a partially
fired, or pre-sintered form. Pre-sintered zirconia is easily milled
with very good margin fidelity. And a post-milling oven sintering
process at very high temperature, ~1,350 C to 1,500 C, is required to
fully crystalize the zirconia. The oven sintering process also results
in a volumetric shrinkage of zirconia between 20 percent and 25
percent. Manufacturers bar code the zirconia blocks to record the
specific shrinkage percentage of the block. The bar code is input to
thedesign software tomathematically expand thedesignby the block
shrinkage factor. The milled restoration is obviously considerably
larger than the desired final restoration. It is tempting to perform
contour adjustments to the softer, pre-sintered restoration, however,
there is a significant risk of damage if it is not done very carefully. The
milled restoration shrinks to the correct volumetric formduring oven
sintering to create the fully crystallized zirconia restoration.
STRENGTH
Strength of ceramics is oftenmeasured by either flexural strength or
fracture toughness. The veneering porcelain on the surface of PFM
crowns has a flexural strength in the range of 90-100 MPa. High-
strength ceramics such as lithium disilicate or zirconia-reinforced
lithium silicates have flexural strengths in the range of 350-425
MPa with a fracture toughness of 3.2 to 3.5 MPa/m2. Zirconia has
a flexural strength in excess of 900 MPa and a fracture toughness
of 5.5 to 7.4 MPa/m2. The strength of the ceramic material is often
cited as a critical element in the success of the restoration. Obvi-
ously, thickness of the material as a result of appropriate occlusal
reduction and adhesive bonding of glass-containing ceramics have
shown current CEREC materials as very successful in high-stress
posterior restorations. However, zirconia does allow for a thinner
occlusal reduction and the opportunity for conventional cementa-
tion, benefits many dentists have found desirable.
SURFACE WEAR
Occlusal wear is often thought of as a function of hardness. However,
this is a misperception as wear is actually a function of smoothness.
Zirconia is averyhardmaterial that has been thought tobevery abra-
sive to opposing tooth structure. However, this is a result of creating
a rough surface for the zirconia, not the hardness of the zirconia.
There is some discussion as to the optimum surface for a zirconia
restoration. Oven firing a ceramic glaze to the zirconia creates a very
smooth surface, however the glass-containing glaze is a thin layer of
about 100 microns and will wear in function over time. This would
expose a potentially rough surface of the zirconia, possibly leading
