All Ceramic Crown Preparation
Authored by:
Dr. Grace De Souza
Crowns are an example of indirect
restorations, which are recommended to restore
and strengthen weakened tooth structure.
All-ceramic crowns offer the potential for
superior esthetics in comparison to
porcelain-fused-to-metal (PFM) crowns, although
the latter may also result in a very good
esthetic outcome. All-ceramic crowns can be made
from numerous materials, but lithium disilicate
and zirconia are amongst the most popular ones,
due to the combination of excellent esthetics and
very desirable mechanical properties. Here we
will present an anterior tooth preparation (#11)
to receive a monolithic lithium disilicate
crown.
The materials needed
are:
- Scalpel blade and
handle
- Silicone putty impression
material
- Metal matrix
band
- Wooden
wedges
- High-speed
handpiece
- Coarse-grit flat-end tapered diamond
bur
- Coarse-grit football-shaped
diamond bur
- Fine-grit tapered flat-end
(torpedo) diamond bur
- Small diameter round end (or
long needle) diamond
bur
-
End-cutting diamond
bur
The preparation of an anterior
tooth for a lithium disilicate crown should
follow this sequence of steps
- Make a putty index prior to
tooth preparation which will serve as a guide
for the amount of tooth reduction. For this
purpose, a silicone putty is properly mixed,
and an impression is taken of the tooth to be
prepared. This should include approximately two
teeth mesial and distal to the tooth that will
be prepared.
- Protect the tooth adjacent
to the proximal wall that you will be preparing
(tooth #21) with a metal matrix band stabilized
with a wooden wedge. Under high-speed and
water-cooling, slice the enamel off the mesial
interproximal contact of tooth #11 using the
small diameter round end diamond bur. Keep the
tip of the bur away from the gingival
tissue.
- After protecting tooth #12
as previously described, slice the enamel off
the distal interproximal contact of tooth #11
using the same small diameter diamond
bur.
- For the incisal reduction,
make 2 or 3 depth orientation grooves
(approximately 1-1.2 mm deep) using a flat-end
tapered diamond bur. Using the same bur,
complete incisal reduction by connecting the
grooves until the incisal edge is smooth and in
an angle of approximately 45 degrees towards
the lingual aspect of the
tooth.
- The labial reduction is
initiated with the tapered diamond bur. Make
three depth orientation grooves in two
different planes: the first in the gingival
aspect parallel to the long axis of the tooth,
and the second in the incisal aspect, slightly
inclined towards the incisal plane of the
tooth. The initial depth of these grooves
should be approximately 0.5 mm. Gradually
connect the grooves to complete the labial
reduction and ensure that the labial surface is
smooth. This includes rounding off all line
angles, including the incisal-labial,
proximal-incisal and proximal-labial
angles.
- For the lingual reduction,
start with the same flat-end tapered diamond
bur in the gingival third of the tooth. Make
three depth orientation grooves approximately
0.5-0.7 mm deep, parallel to the tooth long
axis. Connect the grooves while establishing
the shoulder finishing line around the gingival
margin. Continue moving the bur towards the
proximal aspect of the tooth to connect the
lingual surface to the proximal walls keeping
the depth consistent. Then, with the football
shaped bur reduce the lingual concavity
(incisal edge to the cingulum) to the same
depth as the gingival aspect, approximately
0.5-0.7mm. To avoid over reducing, a good
strategy is to reduce one half of the incisal
portion, and then reduce the second half
following the same
depth.
- To finish off the
preparation, use the fine grit tapered bur to
smooth and blend the totality of the crown
preparation, rounding off any remaining sharp
line angles.
- The circumferential shoulder
margin is defined and finalized with the
flat-end tapered diamond bur
and the end cutting diamond
bur. The
labial aspect should be at the gingival margin,
while the lingual aspect may be 0.5 mm
supragingival.
The final
dimensions of the preparation vary according to
the restorative material to be used, either
lithium-disilicate or zirconia. For lithium
disilicate, the overall reduction is 1.0 mm with
an incisal reduction of 1.5 mm and a 1 mm deep
shoulder finishing line. For zirconia crowns,
incisal reduction stays the same (1.5 mm), and
overall reduction is approximately 0.7-0.8 mm. A
bilayered zirconia crown will require more
reduction. Consult the preparation quickguide and
flowchart in the restorative manual for the
recommended amount of reduction
for all crown
preparations.
Article Reviewed By
Dr. Greg Anderson & Dr. Joel
Rosenbloom