a
b
c
d
Figure 10. Preparation and polishing instruments for ceramic partial crowns: (a) cylindrical diamond burs (course and fine
grit) with rounded edges, (b) conical instruments with rounded edges and a stop at the frontal part, in order to keep the
cavity depth, (c) ultrasound preparations tips for finishing approximal boxes, (d) silicone or rubber based instrument for
polishing ceramic surfaces after occlusal/approximal adjustments.
glaze and strength. In this step the lithium-meta-
silicate is converted to lithium disilicate, then having
its final mechanical strength (360-400 MPa).
6. Which Preparation?
Problems of ceramic fractures related to its
mechanical properties and the resulting failures have
been outlined above. Rules for a suitable preparation
must first of all take care of these material properties.
Ceramic thickness
The necessary thickness of the ceramic to avoid crack
propagation or fracture on loading was investigated
in an in vitro study simulating repeated subcritical
loading and thermocycling. 21 PCCs (Vita Mark II,
Cerec3 System) were fabricated with 0.5-1.0 mm and
1.5-2.0 mm ceramic thickness. PCCs were adhesively
luted to the cavities with either Excite/Variolink II or
RelyX Unicem. After thermo-mechanical loading
15 PCCs of group 1 (0.5-1.0 mm) and two PCCs of
group 2 (1.5-2.0 mm) fractured. The difference was
statistically significant. Although the test material
(Vita Mark II) is a feldspatic glass ceramic with less
strength than the current lithium disilicate or zircon
oxide reinforced lithium silicate ceramics, we still
recommend – being on the safe side – a minimum
thickness of the ceramic of 1.5 to 2.0 mm (Table 2).
Inlay or Partial Crown
The decision, whether the preparation design should
include the cusps (partial crown) or not (inlay), should
be based on both, the size of the defect and the
luting technique (adhesive/non-adhesive). Tooth
fractures or crack formation as a possible precursor
of fractures may occur if the remaining tooth structure
is too weak (Fig. 8). For non-adhesively luted/placed
dental restorations, the generally accepted rule was
that if the occlusal cavity is larger than 1/3 of the
oral vestibular distance of the tooth, the cusp had
to be covered. However, information concerning
adhesively luted ceramic restorations was lacking.
Therefore, in an in vitro study, 22 cavities were
prepared for PCCs with the non-functional cusps not
covered and adjusted to wall thicknesses of 1.0 mm
and 2.0 mm. Ceramic restorations were fabricated
and adhesively luted to the cavities with Excite/
Variolink II. After thermo-mechanical loading the
specimens with 1.0 mm of remaining wall thickness
revealed statistically significant more cracks after
TCML than the group with 2.0 mm of remaining
cusp wall thickness. (Table 3). In another study, 23
restorations with 1 mm thin cuspal wall with and
without coverage were compared using the same
method as described above. Horizontal reduction
of thin non-functional cusp walls showed a tendency
of less enamel crack formation and better marginal
sealing than thin (= 1 mm) non-functional cusp walls
without coverage.
Although the clinical relevance of cracks for the
functioning of teeth was questioned, it was shown
that enamel cracks may progress toward a complete
loss of the whole tooth wall, which would require a
new restoration or even tooth extraction. 24,25
From these studies it can be concluded that – to be
on the safe side – a remaining cusp wall thickness of
less than 2 mm should be protected by coverage with
an at least 1.5 to 2 mm thick ceramic layer to avoid/
reduce enamel cracks and marginal deficiencies.
Preparation design
Traditionally, the preparation design for partial
crowns using metal alloys was “retentive” with
artificially created rather parallel box walls in order
to support the retention of the metal restoration
by friction. However, ceramic partial crowns are
adhesively luted, by which bond strength between
restoration and tooth is significantly improved.
Therefore, the question was, if PCCs still require
a retentive preparation. In an in vitro study, the
PARTIAL CERAMIC CROWNS. ESTHETIC AND TISSUE CONSERVATIVE
RESTORATIONS – PART I: POSTERIOR TEETH
Table 3. In vitro increase of crack formation in enamel for 1 mm and 2 mm residual dental wall thicknesses after luting and
after thermo-mechanical loading; numbers of samples (teeth); modified according to (Krifka S, et al. Ceramic inlays and partial
ceramic crowns: influence of remaining cusp wall thickness on the marginal integrity and enamel crack formation in vitro. Oper
Dent. 2009;34(1):32-42).
BL vs. luting
Thermo/mech loading vs. BL
Natural teeth
1 mm 2mm 1 mm 2 mm No changes 6 8 0 5 11
Increase of cracks 7 4 13 7 1
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