Dentaltown March 2018 - 68

Fig. 2: Crown removed and monoclinic
transformation being measured using
X-ray diffraction.

Fig. 3: Opaque zirconia crown on mandibular
first molar. (Issaquah Dental Laboratory)

Zirconia evolves

eliminated; as long as the tetragonal form was
present, the positive effect-transformation
toughening-created a transformation zone
around forming cracks that slowed crack
growth, producing few chips and fractures
in the frame material.
An accelerated aging test for measuring
transformation from tetragonal to monoclinic
phase is conducted by autoclaving the
specimens under steam pressure.7 This
in vitro test demonstrates that grain size
affects transformation and, within limits,
small grains transform more slowly. An hour
of autoclaving represents approximately
3-5 years of intraoral function. X-ray
diffraction of aged tetragonal containing
zirconia, translucent-containing zirconia
and cubic-containing zirconia demonstrates
reduced low-temperature transformation
to monoclinic phase in cubic-containing
zirconia.
Recently8 the tetragonal-to-monoclinic
transformation was tested by measuring the
change of partially stabilized (tetragonalcontaining) zirconia crowns (Lava Plus
from 3M) cemented in vivo and worn for
12-14 months (Fig. 1). After that period
the crowns were removed and transformation was measured using X-ray diffraction
(Fig. 2). The monoclinic transformation
was 10.6 percent-a small change.8
Opaque (3 mole-percent yttria) dental zirconia has high fracture toughness
(5-9 MPa*m1/2), with a flexural strength

Unt i l 2014, z irc on ia c ont a i ned
3 mole-percent yttria, or approximately
5 weight-percent. The original frame or
opaque zirconia materials (Lava, BruxZir,
Cercon, IPS E.max ZirCad, DC-Zirkon) were
composed of yttria partially stabilized tetragonal polycrystals larger than 1 micron (1u)
with a small cubic component. Partially
stabilized tetragonal crystals spontaneously
revert to the monoclinic crystal with a
3-5 percent expansion by low-temperature
transformation. Low-temperature transformation to the monoclinic phase begins
at the restoration surface and produces a
rougher occluding surface.3 The fear was,
because zirconia was three times harder
than the occluding enamel, that opposing
enamel wear would increase, and continue
to increase as the restoration aged and
the zirconia continued to transform. To
reduce this potential wear and reduce the
opacity of the restoration, the zirconia
frame was covered with veneering porcelain.
Unfortunately the veneering material was
porcelain with low flexural strength and
fracture toughness, creating chipping of
the veneered restoration.5
After 2014, the zirconia crystal size
decreased to approximately 0.2-0.5u and
increased yttria was added to stabilize
zirconia by forming the cubic phase, which
decreased low-temperature degradation. With
these changes the veneering material was

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