PARTIAL CERAMIC CROWNS. ESTHETIC AND TISSUE CONSERVATIVE
RESTORATIONS – PART I: POSTERIOR TEETH
276
Critical thickness of ceramic = 2 mm
Figure 11. Different preparation designs, from a prospective clinical study.
influence of retentive, partially retentive and non-
retentive preparation designs on marginal quality
was investigated (Fig. 9) after thermo-mechanical
loading. 26
In general, no significant differences of the
marginal quality could be found between the three
preparations. However, few parallel walls facilitate
the placement of the PCCs, because such walls are
used for guidance to secure proper seating during
luting. However, no sharp edges are allowed,
which impair proper seating and correct fit of the
restorations. Furthmore, increased shear forces may
arise and compromise the strength and longevity of
the entire restoration.
The retention rate of PCCs using a defect-oriented
preparation design as described above was studied
in a number of clinical investigations, and loss of
retention was found to be low and mainly dependent
upon the luting material and its correct handling (see
below). 10,27
It can be concluded that retentive cavity designs with
rather parallel walls are not needed for ceramic partial
crowns and a more defect oriented preparation
design with only few parallel walls is recommended.
Approximal box depth
The approximal cavity floor with a margin located in
dentin has long been considered to be a problem
for adhesive restorations in general. Insufficient
bonding to dentin and insufficient cavity access
with the consequence that the proper technique
could not be correctly performed were reasons
for bond failure resulting e.g. in secondary caries.
However, new bonding systems (see below) have
improved the bond to dentin dramatically. Anyhow,
it is important that the required steps for good
adhesive bonding can properly be executed; thus
excellent accessibility also to approximal cavity
floors is necessary, especially during luting. Recently,
the “proximal box elevation technique” has been
introduced as an alternative method to restore large
cavities with proximal margins below the cement-
enamel junction by sealing the dentin margin with
an adhesive/direct composite prior to placement of
a direct or indirect restoration in a second step (28).
The use of self-adhesive resin cements may not be
suitable in this case. Little clinical experience exists
with PCCs and the proximal box elevation technique.
Preparation/polishing Instruments
Cavity preparation is usually performed using
diamond burs (Fig. 10) with a cylindrical or conical
shape and a flat head and rounded edges. Fine
grit instruments are recommended for finishing the
cavity margins, which – by the way – may also improve
bonding of SE adhesives (see below), because the
created thin smear-layer allows for better permeation
for these substances. Ultrasound preparation
instruments can also be used for finishing approximal
boxes. Fine grit diamond instruments can also be
employed for occlusal and approximal adjustments
of ceramic partial crowns. Important is that this
adjustment must be performed avoiding heat and
crack initiation; water coolant is recommended.
Furthermore, ceramic surfaces must be polished
following adjustments, in order to prevent/reduce
plaque adhesion, increased abrasion of opposing
teeth and crack propagation. 18,29 Achieving smooth
surfaces depends on a sequential application of all
polishing steps. 30 Examples for ceramic partial crown
preparations in posterior teeth are shown in Fig. 11.
7. Which adhesive luting material?
Main problems of these materials are the washing
out and wear of the luting materials in the luting
space, the discoloration and eventually debonding
of the restoration. Generally, PCCs fabricated from
different silicate based ceramics must be adhesively
luted. Suitable materials are composite resins (only
light or dual curing materials) in combination with
dental adhesives (E&R), self-adhesive cements or
compomers (Fig. 12). Resin modified glass ionomer
cements (RMGIC) have been marketed for this
purpose, but in vitro we have observed problems
with one of these materials, leading to fractures of
the PCCs after thermo-mechanical loading. This
Stoma Edu J. 2017;4(4): 270-281
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