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quarter 2 | 2013 | cerecdoctors.com | 17
in the 1980s, had several limitations
such as low fracture and wears resis-
tance, and color instability.54-56 This poor
clinical performance drove develop-
ment of enhanced materials. Changes
in the composition, polymerization
and improved particulate reinforce-
ment resulted in more durable direct
and laboratory-fabricated composites.
Improved formulations in filler size,
shape, composition and concentration
resulted in superior mechanical charac-
teristics.56 Changes in the polymerization
system led to a more uniform cure and an
enhanced level of polymerization, and
this has resulted in increased flexural
and tensile strength, increased resistance
to abrasion and fracture, and improved
color stability.54-58
Unfortunately, neither the physical
properties nor, more importantly, the
clinical performance distinguishes indi-
rect composites from direct compos-
ites. One five-year follow-up and one
11-year study of direct versus indirect
resin-based composite restorations
revealed no significant differences in
wear, morphology, fracture or secondary
caries.59,60 By 11 years, indirect restora-
tions had a slight, but statistically insig-
nificant, advantage in these categories,
indicating, “… the more time-consuming
and expensive inlay technique may not
be justified.”60
Most clinicians would likely assume
that composite cured under laboratory
conditions would be superior to mate-
rials cured intraorally. This is not the case
from a materials viewpoint, given that
properties of set resin-based composites
are based on their “degree of cure,”which
is not much different whether they are
cured in the mouth or the laboratory.
“Degree of cure” or “percent conversion”
means the number of double bonds (i.e.,
monomers) that have reacted to form
polymer. Essentially a conversion of 75
percent means that the set composite
still has 25 percent unreacted monomer.
Today’s systems achieve as good as 75
percent to 80 percent, and extra heat,
pressure or light does little if anything
to improve conversion. Compositionally,
laboratory and direct resins are virtually
identical as well (i.e., same filler particle
sizes and concentrations).
Four popular indirect resins were
recently examined for their degree of
conversion, which was found to vary
from63 percent to 81 percent, right in the
range of direct-cure systems.61,62 Unlike
ceramics, improved properties are not
found with resin-based composites
cured under industrial conditions as in
the manufacturing of CAD/CAM blocks.
Alarmingly, in one head-to-head
comparison of CAD/CAM resin and
ceramic single-tooth restorations, 200
CAD/CAM restorations were followed
for three years.63 For the first 120
restorations, patients were randomly
assigned to each material. Vanoorbeek
et al (2010)63 reported that, “Due to early
occurring complications and inferior
results with the composite resin resto-
rations, only all-ceramic crowns were
placed thereafter until the required
number of restorations for the study
was achieved (n = 200)”. Cumulative
success rates after three years were 55.6
percent for the composite resin, and
81.2 percent for the ceramic.
Even with nano-scale filler particles,
these materials are still particle-filled
resins – not “resin nano ceramics”! They
are simply not ceramics. Disturbing early
failures of the CAD/CAM composite
crowns reported by Vannoorbeek et al
(2010)63 likely reflect: 1) the flexibility of
this material (low elastic modulus), and;
2) a poor bond (as discussed above). This
is not a new class of material andmay not
even be an incrementally improved over
the previous resin-based CEREC block.
Complete Prosthesis
Automation
Figures 6 and 7 present a very exciting
next step in CAD/CAM processing
of esthetic and structural prostheses.
Within the CEREC design software is
the capability of designing simultane-
ously the structural zirconia substruc-
ture and the overlying esthetic porce-
lain or lithium disilicate. These two
components are then joined either by
a special firing step (Ivoclar Vivadent,
Inc.) or by bonding (Vita Zahnfabrik).
While no clinical data yet exists for
either system, the concept seems funda-
mentally sound.
6
Figs. 6-7: Ivoclar Vivadent CAD-on
restoration
7