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16 | cerecdoctors.com | quarter 2 | 2013
prostheses having porcelain chipping and
cracking (e.g., 25 percent to 50 percent).12
Of potential concern is the propensity for
partially stabilized zirconia to auto-cata-
lytically transform at surface grain bound-
aries due to an interaction with water
(discussed momentarily) that may create
major structural issues in the future.
Many “authorities” have offered
numerous explanations for porcelain
chipping that simply do not withstand
critical thinking or analysis, including:
1) non-anatomic substructure design; 2)
unsupported porcelain; 3) weaker porce-
lain; 4) thermal expansion/contraction
mismatches, and; 5) poor porcelain-
zirconia bonding. More well-considered
hypotheses have included; 1) residual
stresses arising from thermo-mechanical
parameters;15 2) auto-catalytic transfor-
mation during porcelain firing;16 and; 3)
enhanced auto-catalytic transformation
of green-machined structures at mouth
temperature.17 As yet-unpublished
research from our laboratory strongly
supports, transient stresses within the
porcelain developing as a result of too-
rapid cooling acting on flaws present
from too rapid heating.
In addition, many clinical authorities
report that they stopped having porce-
lain problems when they enforced a
slow-cooling protocol (including Drs.
Avi Sadan and Ed McLaren – personal
communications). One paper given at
the 2010 meeting of the International
Association for Dental Research (IADR)
on clinical data from four private prac-
tices reported only 2 percent porcelain
chipping in two years to three years for
702 prostheses (authors; D. Nathanson,
S. Chu, H. Yamamoto and C. Stappert).
These clinicians reported that their
laboratories were aware of the need to
cool slowly. Numerous dental materials
companies now include a caution to slow
cool in written and web-based informa-
tional materials. This continues to be an
active area of research.
All ceramics are susceptible to
subcritical crack growth and corrosion
effect cause by water, which breaks
the bond between atoms at the crack
tip. This leads to slow growth of cracks
and resuls in a decrease of materials’
strength. Partially stabilized zirconia-
basedmaterials are uniquely susceptible
to auto-catalytic transformation of the
crystals, from tetragonal to monoclinic,
at relatively low temperatures, called
low-temperature degradation (LTD).18
While generally studied at autoclave
temperatures of a few hundred degrees
centigrade, significant percentages of
transformation can be extrapolated
as being possible at oral temperatures
using activation energy data from
ceramics literature (Chevalier).
Another striking clinical study
presented at the 2010 IADR exam-
ined partially stabilized zirconia discs
embedded in the flanges of mandibular
partial dentures, demonstrating a much
higher rate of transformation for a dental
zirconia in two years to three years than
predicted for an engineering zirconia by
Chevalier (author; Tomaž Kosmac, Jožef
Stepfan Institute).
Water can “catalyze” the process at
surface grain boundaries, and the trans-
formation of crystal continues from
layer to layer through the entire body,
leading to micro-cracks formation, grain
pullout and a decrease in strength.13 This
phenomenon was particularly impor-
tant in causing the failure of zirconia
hip prostheses submitted to autoclave
sterilization. Although the informa-
tion presented above raises concern,
both published clinical trials and those
presented at international research
meetings evidenced no bulk fracture in
dental restorations under the observa-
tion time — indicating that LTD has no
major influence on the clinical behavior
of dental restorations.19
Clinical Indications
Based on Clinical Research
There is much supporting data fromclin-
ical studies of most important ceramics
being used in dentistry today.20-52 The
general message is that for a single crown
on any single-rooted tooth, numerous
systems are indicated – some need to be
bonded and some can be cemented or
bonded. For single-unit posterior teeth,
the choices involve the veneered alumina
or zirconia systems, veneered or full-
thickness lithium disilicate, or full-thick-
ness zirconia. For three-unit anterior
FDPs, veneered alumina and zirconia and
full-thickness (non-veneered) lithium
disilicate have good track records. For
posterior three-unit FDPs only veneered
or un-veneered zirconia are considered
to be suitable.53
Comparison of Resin-Based
Composites vs. Ceramics
Two groups of materials can be consid-
ered for fabricating indirect esthetic
restorations from manufactured blocks
via chairside CAD/CAM. Data from rela-
tively long-term clinical studies is demon-
strating the superiority of ceramics as the
most durable option with the longest-
lasting esthetics as well for all types of
restorations.
The first generation of laboratory-
fabricated composite resins, introduced
| | | k e l ly
Many clinical authorities report that they
stopped having porcelain problems when they
enforced a slow-cooling protocol.