Page 27 - CEREC Q3|2013

quarter 3 | 2013 | cerecdoctors.com | 25
avoid rounding our line angles because
we want to maximize retention. This will
often leave minimal occlusal reduction on
the external cusp incline, and can create a
weak point in our restoration.When plan-
ning a high margin, don’t forget to check
the occlusion and make sure there is
enough reduction on the functional cusp,
and rely on enamel bonding for reten-
tion. Don’t place a margin near occlusion
on the buccal of mandibular cusps or the
lingual of maxillary cusps. Figures 3 and
3a show a slice of a crown designed with
limited functional cusp bevel and high
occlusion; adjusting this occlusion will
sacrifice restoration thickness.
Rough preps and sharp angles will
require over-milling of the restoration
to fully seat (Fig. 4). This can lead to a
significant reduction in the restoration’s
thickness. Minimal over-milling is not a
problem if you plan for it and understand
when and where it will happen. Avoid
significant over-milling by rounding line
angles, smoothing a rough preparation
surface and eliminating thin dentin peaks.
On occasion, it is necessary to place a
build-up to control the milling around
these thin areas of tooth structure.
Spacer and occlusal milling offset can
also play a role, but it is a small one. Spacer
will reduce the thickness of your crown
by the amount you have set in parameters
(usually around 100-150 µm). Occlusal
milling offset will not affect the amount
of space you need to reduce a tooth, but it
can affect the perceived thickness of your
crown during the design process.
My standard reduction for an e.max
crownwith aBiogeneric individual design
is 2.0 mm; I approach tooth preparation
by reducing the occlusal surface first
using an occlusal reduction bur with 2.0
mm cutting surface. Figure 5 shows the
reduction of the tooth in Figure 1. Next,
I will remove the existing restoration,
break contact with the adjacent tooth,
then round my margins and line angles
(Fig. 6). I routinely check the restoration
thickness using the blue minimal thick-
ness bubble (Fig. 7). To do this, I keep
my minimal thickness global parameter
set at 700 µm. After my initial proposal,
I will select restoration parameters and
change the minimal occlusal thickness to
my desired restoration thickness plus the
occlusal milling offset. If I want a restora-
tion thickness of 1200 µm andmy occlusal
milling offset is 175 µm, then I will round
up the minimal thickness to 1400 µm. The
minimal thickness bubble will consider
spacer and over-milling in its calculation
but not occlusal milling offset. Using the
minimal thickness bubble, it is easy to
adjust thin areas and finish with a great
restoration (Fig. 8).
With every restoration, we should start
with a plan and knowwhat material thick-
ness we need to achieve. Paying attention
to these common pitfalls will help us avoid
surprises and compromises.
For questions and more information,
Dr. Davis can be reached at
mcddds@me.com.
5 6
7 8
Avoid significant over-
milling by rounding
line angles, smoothing
a rough preparation
surface and eliminating
thin dentin peaks.
Fig. 5: Initial reduction
Fig. 6: Final prep
Fig. 7: Minimal thickness checked
Fig. 8: Final restoration